PROCESS AND IMPLEMENT - LIMITATIONS
M. L. Puri
This is a technical analysis of process and implements limitations based on empirical data gathered through actual workshop observation
Effect of limitations of Engineering processes and implements on the choice of Mechanical profiles and forms and their methods of production
In this note it is intended to study the effect of limitations of Engineering processes and implements on the choice of Mechanical profiles and forms and their methods of production. The study is intended to be analytic and an endeavor will be made to keep description to a minimum.
Profiles: The most common profiles selected for machine construction are:
1. Plane surface
2. Surface of revolution
Helical surface is perhaps equally important. But it requires a special treatment and as such will be dealt with separately along with other special profiles.
1. Plane surface – may have to conform to any of the following specifications:
(a) Where the principal property is the quality of being plane e.g. top of a surface table,
(b) Where in addition to (a) an angular location is also desired e.g. side of an angle plate,
(c) Where along with (a) and (b) a dimensional location is also imposed e.g. side of a slip gauge.
In general a plane surface may be completely defined if its surface quality is specified is specified and its angular and dimensional locations stated.
Note: With obliquity other than zero, the dimensional location has to be stated with reference to a selected spot.
2. Surface of revolution – Only surfaces swept by straight lines coplaner with the axis of revolution are considered. Externally such a surface is located by its axis. For convenience, the axis is obtained by the right intersection of two virtual planes. These planes are usually selected so that they are parallel or perpendicular to certain other locating planes. Like plane surface, a revolved surface too has to conform to specified quality. Its other aspects can be stated by:
(a) Obliquity of the generating line to the axis,
(b) distance of the generating line from the axis, measured in a selected transverse plane.
Note: This distance is constant when the generating line is parallel to the axis. This means that it is only for tapers that size has to be stated with respect to a definite axial location.
Comment – Surface quality depends upon the process employed for finishing. Angular location depends upon the method of layout and setting. Dimensional control is a function of the feed of the cutting tool and the method of gauging. It should be noted however, that degree of accuracy in location, whether angular or dimensional cannot exceed that incorporated in surface quality. In addition it will be influenced by the errors of gauging and setting instruments.Surface Quality – is judged by:
(a) the conformity of the finished surface with the predetermined profile,
(b) representative character of the surface, measured by the percentage of the absolute surface area, constituted by the actual metal at the surface level.
Its specification is essential, because it is an important factor in the life of mating detail.
A machined surface is produced by the sweep of the point where the chip parts. Its quality is influenced by: -
(c) deflection of work on its being clamped for machining
(d) deflection of work, tool and machine under pressure of the finishing cut,
(e) variation of the distance between the tool point and the point of chip-parting, during the finishing cut,
(f) wear of the tool point during the finishing cut,
(g) accuracy with which the tool point is guided relative to work, when not cutting or in order words accuracy of machine slides, spindles etc.
Each of these items will be considered in detail at a later stage.Layout.
In leveled plate work, layout is two dimensional and ordinary drawing instruments e.g. straight edge, dividers etc. can be successfully employed.
Work on solids is complicated by the entry of the third dimension. This necessitates co-ordinate reference planes (a maximum of three) to define locations in space. On easily workable materials such as wood, the starting point is the production of these planes on work itself. After laying out the respective projections on these prepared planes, shaping is carried out by actual carving out of material.
With metals this process is extremely tedious and expensive. As such its use is restricted to rare cases like die-sinking, where alternatives are not successful. Usually the work is first approximately brought to its shape and dimension by casting or forging and then finished by machining (if required). A bare minimum called machining allowance is left in the first instance to cover the discrepancies of manufacture and to provide for cleaning up.
With such work there is hardly any scope for applying plateworker’s or even pattern maker’s laying out tools (not of course the entire equipment). Hence external planes of reference are employed for marking off. There is no relative movement between work and the reference planes during the process of marking and as such the same may for the time being be considered part of the work it self.
In practice, only one plane aided by a squaring implement serves the purpose of three. Marking during a particular configuration produces lines or plane boundaries on the work which are parallel to the reference plane. If a second marking be imparted after holding the first perpendicular to the reference plane, the conditions are as if it were taken from the second co-ordinate plane with work in the first configuration. In a third configuration with the previous two markings perpendicular to the same plane, references from the third co-ordinate plane may also be obtained.
A practical substitute for this external plane is the top of a surface table. Scribing is done by the point of a surface gauge adjusted with a graduated scale or other means such as slip gauges. Squaring implement is a try square or another instrument of this family.
Accuracy of marking depends upon the quality and extent – a factor in the steady movement of the surface gauge – of the reference surface. Since the surface table forms part of the work only temporarily, the limitations of surface extent and quality imposed upon a particular piece of work are not shared by it. Its use not only makes marking off of solids possible, but also offers scope for improving the quality of layout.Setting
This term is usually applied to work in which more than one surfaces have to be produced in specified relationship. Ordinary bar work is, for instance, excluded.
Setting consists in aligning work with machine feeds so that sweep of the tool point would produce one of the surfaces required in specified relationship. It does not control the extent of cut and as such has no influence on size. Its scope is directional rather than dimensional.
By aligning work, is meant, aligning its axis of co-ordinates. This can be achieved by a maximum of three translations at right angles and three rotations at right angles, provided each movement is free or unaccompanied by any auxiliary movement. In partly finished work a surface of revolution is a substitute for one axis of co-ordinate and a plane surface for two.
For accurate setting the indicating point should be guided by the same constraints, which would guide the tool.Note: It is usual in machine tool construction to finish the faces and edges of work tables as also those of some slides, so that they lie in alignment with the planes containing the feed lines. Thus, they may be used as substitutes for feed planes generated by the actual sweep of the tool point. Principal convenience of such a substitution lies in its permitting the use of less elaborate setting instruments, such as try squares, surface gauges etc.
Accuracy of setting made in this manner will however be limited due to:
i. initial error of substitution
ii. loss of surface quality through use,
iii. Further discrepancies when slides and spindles are adjusted and reconditioned.
As such, for important work, this approximate setting should always be followed by a final setting by a properly guided indicating point.
Interposition – On account of the desired sensitiveness and accuracy, the contact area of indicating instruments is usually limited. Again the representative character of similar areas on the work surfaces decreases with inferior surface quality. If settings be made with reference to such spots, there is the risk of misleading indications. In order to obviate this risk , surfaces which may be safely assumed to be absolutely perfect for the work in question, are interposed so as to make simultaneous contact with sufficient area of work surface and the indicating point (interposed surfaces are assumed parallel), so that the indication may correspond to the average level of work surface.Measurement
The process of profiling entails, measurement of distances and angles. Distances can be measured more accurately and conveniently than angles. As such it is usual to specify angles also in terms of distances, by making use of the properties of triangles, (except in cases like tool making where it is simpler to measure angles as such).
The simplest method of measuring distance is to read it against a graduated scale. But direct application of such a scale is not always possible. So instruments of caliper family have been introduced, to facilitate transference of linear dimensions for measurement. Protractors or their modifications are employed for direct angular measurement.
Accuracy of measurement depends upon:
(a) reliability of transference,
(b) accuracy of the standard of comparison,
(c) precision of comparison.
Practically all the improvements and refinements, incorporated in the modern measuring instruments – which incidentally made it possible to record Engineering Tolerances – are with the intention of improving one or more of the above factors.
Because of its special importance, this subject will be considered in greater detail along with fits, tolerances and limit gauges.
Process and implement limitationsIntroduction
In the following lines it is intended to develop an idea of the degree of approximation to which a hypothetical conception is approached in finishing practice and the consequent effects of such limitations on the choice of profiles and their method of production. In this development when nothing specific is mentioned only ordinary cases are implied. For the sake of convenience the reference planes will usually be horizontal and vertical. Again it is assumed that machine beds are set level as also surface tables. But it must be remembered that this choice of setting is only for the sake of convenience and as such need not be considered absolute.
The following are the usual profiles adopted in machine work.
1. A plane surface.
2. A surface of revolution.
3. A helical surface.
4. Special cam profiles etc.
Of these the first three are those most commonly employed.
A plane surface.
A plane surface may have to conform to any of the following specifications: - (a). where the principal property is the quality of being a plane e.g. the top of a surface table.
(b). where in addition to (a) an angular location with respect to another existing surface – assuming both the surfaces to belong to the same solid – is also desired, e. g. the side of an angle plate. (c) where along with (a) and (b) a dimensional location is also imposed, e. g. the opposite faces of a gauge block. In general a surface may be completely defined if its quality is specified and its angular and dimensional locations are stated. With obliquity other than zero the dimensional location will have to be stated with reference to a given spot.Comment.
The surface quality depends upon the process employed for finishing.
The angular location will depend upon the method of layout and setting. The dimensional control is primarily a function of the feed of the cutting tool, assisted by the method of gauging
But it must be remembered all the same that the degree of accuracy in location, whether angular or dimensional, cannot exceed that incorporated in surface quality, nor can the inherent errors in gauging and setting implements be ignored.Setting (with reference to the planer for simplicity) implies a process of aligning the work with the machine feeds; namely the stroke line, the cross-feed and the tool head feed; so that after the operation is complete the rotational freedom of the work relative to the tool point is constrained, whereas a translational freedom within the range of the feeds is still possible.
Interposition:
On account of the desired sensitiveness and accuracy, the contact area of indicating instruments is usually limited. Again the representative character of similar areas on the work surfaces decreases with inferior surface quality; and if settings be made with reference to such spots there is the risk of a misleading indication. In order to obviate this risk; surfaces which, compared with the quality of the work surface itself, may safely be assumed to be absolutely true, are interposed so as to make simultaneous contact with a reasonably sufficient area of the work surface and the indicating point. As no irregularity in the interposed surface is assumed the indication will correspond to the average level of the work surface in question.Finishing sequence: - As noted above, a finished surface is located and produced to quality. Corresponding to the degree of accuracy desired, methods of layout and production are selected to yield pre-determined results. In the usual run of work, the following gives the scope of selection:
1. Marking off the job, and machining to the boundary lines. Here, the accuracy will be that of the scribing and the accuracy to which it is followed in machining. If the scriber is adjusted in comparison with a rule, the accuracy will be influenced by the thickness of the graduation line, and that of the scribing point. Incidentally, the former has to be visible to the naked eye and the latter has to be maintained during the operation, and thus leaving aside other factors, the limit may be taken as equal to + .002” on machined coppered surface to well within 0.010” on a chalked casting, assuming good marking.
Superimposing upon this the approximation, with which the scribed line is followed in machining, the limit of error may roughly be doubled.
2. Layout for compensation followed by subsequent marking; as certain selected primaries are finished.
3. No. 2 as assisted by “measuring and setting instruments of feel”.
4. The employment of external agencies to ensure accuracy of (a) specified relationship and (b) dimensions.Comment:
This amplification is made with reference to existing JMP loco-practice.
No. 2 has an advantage over No.1 in as much as the final error is substantially reduced by progressive compensation. As the maximum discrepancy, assuming a usual maximum of three markings cannot be greater than about 1/16” , the bare machining allowance of 1/16” can always be obtained if the minimum initial was 1/8”. Actually this is an exaggerated figure, as in practice, the error as it progresses is usually to a certain extent self-compensatory and not cumulative in the same direction.
The locomotive cylinder is machined by this method.
The employment of gauges improves dimensional accuracy if the surface quality falls well within the gauge limit (say within 20-30% of the limit).
The last method is usually reserved for tool –room practice, though it can be applied in case of intricate work employing special fixtures or by making use of the geometrical freedom allowed in the machine itself.Aids of accuracy
As stated above, the final accuracy of work is considerably influenced by the accuracy of layout and setting, when either or both of them are employed. As such it is worth while considering methods which improve their quality.
In plate work the layout is usually two-dimensional, as the thickness part seldom comes into consideration. Thus after leveling up the plate, the usual drawing instruments e.g. the straight edge, the divider, protractor etc. can be successfully employed.
The work on solids is however complicated by the entry of the third dimension, and this necessitates the existence of co-ordinate reference planes (a maximum of three) to define locations in space. On material such as wood, which is easily workable, the starting point is the production of those planes on the work itself. After laying out the respective projections on the prepared planes, the shaping is achieved by actual carving out – entailing a very considerable removal of material.
With metals, however, this process would be extremely tedious and expensive. Besides, it will not lend itself to the present day requirements of accuracy. As such, the work is first approximately brought to its final shape and dimensions by the processes of casting and forging, and only a bare minimum called the machining allowance is left to cover the discrepancies of manufacture and to provide for cleaning up.
With the work produced in this manner, there is hardly any scope for applying the plate workers’ or even the pattern – makers’ (not of course the entire equipment) layout tools. Thus external planes of reference are employed for marking off. During the marking process, there is no relative movement between the reference planes and the work itself. This is equivalent to the work having those planes as a part of itself.
In practice, however, only one plane of reference usually serves the purpose of three, when aided by a squaring implement. The marking during a particular configuration produces lines or plane boundaries parallel to the reference plane, and if a second marking is imparted after holding the first perpendicular to the reference plane, the conditions are as if it were taken with reference to a vertical co-ordinate plane with the work in its first configuration. With the work in a third configuration, with the previous two markings, perpendicular to the reference plane; the references from the third co-ordinate plane are also obtained. This simple process will locate projections of selected points, on the work external surface, as well as the boundaries of planes parallel to the co-ordinate planes.
This extent of right location generally suffices for ordinary cases which have no oblique planes or axes. With obliquity, however, the conditions are somewhat different in as much as the distance location refers to a selected point and a practicable specification of obliquity has to be given.The oblique plane:
A plane is completely defined if a straight line contained in it can be conveniently located and its obliquity with respect of the parent plane, usually horizontal during marking off – is stated , taking the given line as the ground line for the generation of co-ordinate planes. It will be obvious that the angle of obliquity will be contained in the third co-ordinate plane (vertical). For ordinary cases, it will be helpful to so position the ground line that the third co-ordinate plane comes approximately parallel to a generous face on the work. Where greater accuracy is needed, the creation of a virtual third co-ordinate by other means may be necessary.
Note: – Usually in cases like cylinders, the error involved in assuming the rough casting face in question to be vertical can safely be neglected.The external guide.
The practical substitute for the external reference plane is the top of a leveled surface table. The scribing implement is the point of a surface gauge, and the squaring tool is a try square or some other instrument of this family. The distances are locating in the vertical plane by the adjustment of the scribing point in comparison with rule markings (rule vertical). With special surface gauges, vertical adjustments are assisted by verniers and gauge blocks.
The accuracy of marking depends, besides other factors, on the quality of the guiding surface. The extent of the guiding surface also matters as it limits the base area of the surface gauge. (A generous base area is an aid to the steady movement of the scribing block). Since the surface table forms part of the work only temporarily, the limitations of surface extent and quality imposed upon a particular job are not shared by it, and thus the use of a surface table not only makes the marking off of solids possible, but also offers scope for the improvement in the quality of layout.Surfaces of revolution.
In this treatment Bar work is excluded for the time being. Again only parallel and taper surfaces of revolution, internal or external, are considered – irregular profiling being omitted. The general cases treated will be those of such surfaces produced in specified relationship with their own counterparts or plane surfaces.
The truth of a parallel or taper bore considered by itself consists in 1, its maximum surface irregularity and 2, the accuracy of its geometrical profile. The first aspect of the latter consideration is the roundness of cross section, the second being conformity with the specified taper (where parallelism is a special case with zero taper) straightness of the axis. As this special case is the one most commonly employed in practice, we understand from a surface of revolution a parallel one, unless otherwise specified. As regards the external relationship, the surface of revolution is defined by its axis obtained (usually) by the intersection of two planes at right angles. As for plane locations, the conditions have already been stated. In the case of tapered surfaces the size which varies along the axis has to be defined with reference to a selected cross section on the axis.
The measuring instrument for these surfaces usually belongs to the caliper family, the solid plug gauges and ring gauges are also employed for smaller sizes, say up to six inches. For tapers, the calipers, though employed, are not the correct instrument, as they have to bear on an incline; while their own jaws are parallel. In addition the exact spot of their application cannot be conveniently located. A more accurate way of gauging these surfaces is to feel a known diameter on the checking gauge from a location cross section (presented as a plane surface perpendicular to the axis) at a calculated distance. As the tapers generally employed are not very severe, the limits of axial feel for a corresponding diametral accuracy can be much coarser. Thus, given an accurate method of radial tool feed, and assuming the presence of requisite surface finish the dimensional tolerance can be made much finer than the axial feel tolerance (the accuracy improving with the reduction in taper). The cross section of reference is sometimes substituted by an axis, represented by a test bar, perpendicular to the axis of the surface being gauges (as in X-class cross-head socket bore machining).Supporting work on the machine. (some random observations)
a. When work is supported on the machine for machining, the machine, through the medium of the supporting tackle, withstands the following:
1. Forces due to the dead weight of the work.
2. A centrifugal force (when work revolves) depending on the eccentricity of the neutral axis of work, relative to the axis of rotation.
3. Forces due to the cutting action. Also inactive force on reciprocating cutting action.
b. For accurate machining, it is essential that there should be no give of the work relative to the tool in the form of spring or actual displacement. The former affects the depth of cut and when variable, over the area under the cut, gives a finished profile different to that intended. The latter directly alters the setting, and is no less injurious.
c. A force is resisted with least give when the resistance is that of bearing on a generous area. In tension, the screw thread with its imperfections is always liable to stretch. The worst is friction where efficiency of grip depends on the tightening force – a variable factor.
d. In machine tools, the guiding is done only by a selected number of faces, which are simultaneously designed to withstand direct forces successfully. If the mode of support shifts the aspect of forces so that they have to be resisted at spots, not designed for that purpose, the consequence is usually localized wear and misalignment, which in its turn impairs the quality of machining.
e. An excessive overhang means a divergence between the lines of weight and support respectively and hence the necessity of an opposing couple to resist the bending tendency caused by the eccentric loading.
Note:– If the support is a faced flange, clamped on to a parent face, then the sliding tendency along the flange face due to weight will be resisted by friction, while that the due to bending will be resisted by bearing on the bottom side and clamp tension at the top. (For rotation each clamp will be alternately relieved and loaded if clamps lie away from the axis of rotation).
In spigoted supports, (assuming a reasonable percentage of spigot bearing surface, at top and bottom, to be in contact with the corresponding surface of the recess, after permissible canting of the work and consequent yielding at the surface) the shear (equal to the weight of the overhang) will be practically supported in its entirety by the spigot, whereas the bending will be shared by the spigot and the clamps – the portion of bending going to clamps decreasing with the tightness of the spigot fit and the effective area of the spigot surface in contact.
A similar job held by chuck jaws will have its weight supported directly by the jaws, whereas the bending action will be resisted by the frictional couple (assuming the jaw contact to be small).
f. When work is fixed relative to the machine, it occupies a space equal to its own volume, but when it revolves, it needs a clear space in the shape of a cylinder with its length equal to the maximum length of the work along the axis of rotation and radius equal to the maximum distance of any point on the work from the same axis. It is assumed that space divisions are made by right planes, and no advantage is taken of the space left unswept by the work profile in the cylinder defined above.
g. Setting up is most conveniently done when the work rests in contact with the adjusting points (usually three) by virtue of its own weight. Thus, for awkward and heavy jobs, a horizontal machine table on to which the job will be subsequently held, is an advantage.
h. The strength of the portion of the work employed to transmit to the supports the various forces discussed above, has also to be considered. Thus, for extra heavy jobs, the presence or absence of a convenient spot to perform the above function, will also influence the mode of support.
Comment: – As in machining, it is the relative motion between work and the tool that matters, we can distribute the motions of the work and the tool relative to the machine structure in a fashion which suits our convenience, but at the same time, retains the essential features of the relative motion between the work and the tool. As the machine is supposed to be adjustable within its range, it has to incorporate units such as slides, gear boxes, clutches, etc. Now, when two or more of these units are combined, and the combination itself moves as a whole, practical difficulties of construction are frequently met with.
The following may be cited as typically illustrative: –
1. When a rotating spindle with variable speeds has to be incorporated in an adjustable slide, difficulties of providing a positive drive for the spindle through the range, as also those of supporting the speed changing mechanism (when placed alongside the spindle) have to be encountered.
Note: – The introduction of independent motor drives has, to a reasonable extent, solved the problem, as in modern radial drills and tool room gauge grinders.
2. Another important example is the limitation of machine tools intended to produce circular profiles in which the tool rotates, while the work is given a translation; in the matter of producing tapers.
Note: – If the work undergoes rotation relative to the tool point, without any relative translation, the result is the production of a surface of revolution with negligible length along the axis. This surface can be extended to any profile depending upon the natures of the radial and axial relative movement between the work and the tool. The nature of the profile (instantaneous) will be governed by the resultant of radial and axial feeds. If both of them are maintained uniform, the slope of the resultant remains constant and the resulting profile is a taper. In short, the taper results from a resultant oblique tool feed relative to the axis of rotation. If, this obliquity could be arranged by a setting operation so that the tool can be conveniently fed along this oblique path by a simple direct translation the geometry of this motion will depend entirely on the setting and will be independent of the rate of feed. If, on the other hand, it is the result of a composition, it is closely influenced by the relation between the axial and the radial feeds.
The rates of feed can only be limited (for practical reasons of construction) and thus the number of tapers that can be produced by the various combinations (neglecting for a while the influence of such a selection on surface quality) cannot be indefinite. Thus, fine adjustment – so necessary for accurate work – may become a practical impossibility if the above method of taper production were contemplated.
Now, the provision for adjustment within a given range, as usually incorporated in machine tool slides is unrestricted for the range, and thus does not delimit the alignment of tapers with standards.
From the above, we gather that taper cutting cannot be the domain of a machine in which the path of feed (for such a machining) cannot be independent of feed rates. Now, in machines where the tool revolves relative to the work along with a provision for right feeds only, an oblique feed can only be achieved by composition, which brings in the rate factor. As such, these machines are incapable of taper production, (excepting, of course, short lengths produced by form tools given axial and radial feeds).
3. The following refers to bores only but is equally applicable to external surfaces of revolution.
Note: – the term aspect implies the configuration of a straight line or a plane when referred to coordinate planes without taking into consideration the dimensional locations of a selected point therein from the ground line.
In the production of bores, the work is aligned with the axis of rotation of a spindle which rotates with it, either the work or the tool. If an axial feed of the work relative to the tool be arranged, a parallel bore will be produced with its radius equal to the distance of the tool point from the axis of rotation. ( No radial movement of the tool during the operation is assumed). If it be desired to produce another bore in the same work a second alignment of the work with the axis or rotation will be essential, if the axis of the second bore be different. The accuracy of relationship between the two axes will depend upon the accuracy with which the settings are made.
Now by reason of the initial accuracy incorporated in machine units, as also due to the convenience with which they can be manipulated and locked, it is usually an advantage to secure the second setting at least in aspect by the use of such units; (generally slides). With work revolving, it may be necessary to mount such slides on the revolving spindles. This will present the following difficulties:-
1. For securing large work the slides will have to be somewhat massive. Again their adjustability will always bring in the question of considerable unbalanced masses.
2. With limited sweep, in extreme positions of the slides the range of adjustability will also be limited.
3. The varying transmission of motion to these slides, and the maintaining of aligned bearings may substantially handicap the design.
As such, the provision of such slides or other units, as aids to boring in specified relationship is reserved for machines where the tool revolves. In such a case, the only slide mounted on the spindle is that which varies the distance of the tool from the axis of rotation: - a provision for size adjustment. This slide is comparatively lighter and does not present extraordinary difficulties, in its incorporation.The horizontal boring mill.
In the common patterns of this machine the following are provide:-
1. A radial tool feed for size adjustment and facing.
2. An axial feed for achieving the length of the bore.
3. A cross feed for work and a vertical for the tool head to facilitate the second setting for bores with the same aspect as that of the first bore. (Surfaces omitted for the present).
4. A rotation of the work table about a vertical axis to enable oblique settings in the horizontal plane.
The boring mill can be employed for the production of flats as well, if the axial feed be stopped after providing the requisite depth of cut and the radial or the cross feed be worked in conjunction with revolution of the tool. The former will produce a surface with a circular outline, whereas the latter will give a channel if the tool is made to approach the work from clear space and the feed be continued till the tool cuts no more. The surface so produced will lie perpendicular to the axis of rotation of the tool. The sides, whether of the circular recess or the channel, will have the same inclination to the tool-periphery taking the cut. (to the axis rotation). If successive cuts be taken, by making use of the axial feed, then part of the side measured from the tool and equal to the axial feed given after the tool has just made contact with the job, less the length of tool profile taking part in the cut (measured along the feed line) will lie perpendicular to the bottom surface.
The sides of the circular recess will disappear when the greatest diameter of sweep envelops the work area presented. Similarly, one or both sides of the channel can be made to disappear when the diameter of sweep exceeds the width of the “presented area” in the vertical plane.Special fixtures.
Special fixtures usually have a substantial weight, as such it is advisable to confine them (unless they are reasonably light) to machines where they can be directly supported; as when the surfaces on which it is intended to carry them are horizontal. (Here the line of weight and that of support coincide, thus, no bending tendency due to the weight of the fixtures is experienced at the bearings). With fixtures which have to revolve, the question of balance about the axis of rotation (with work in position) as also that of swept volume is an extra consideration.Vertical boring mill: -
Is essentially a lathe with its axis of rotation vertical. Its use offers the following advantages:
1. The weight of the work as also a substantial component of the cutting force is practically directly supported by thrust bearings, and as such the bending strain on bearings when heavy work is machined is considerably reduced.
2. The setting operation is very much facilitated when a considerably difficult / intricate work has to be set on a face plate on a lathe.
3. When fixtures are employed, the primary locating face remains in contact with the table face by virtue of its own weight, and does not depend so much for location on the clamping force.
4. For large diameter work, the accommodating area is horizontal as against vertical on a lathe (where height of centers above ground level puts a limit on the work diameter, even after gapping the bed).
On the other hand, the range of work for its height is limited by the vertical travel of the cross slide. Again the cross slide is supported as a cantilever, as against the dead-end support of a lathe carriage. The weight of these slides calls for a provision for balancing and the effect of vertical lead-screw slackness (in the form of the slide dropping at the end of the cut) is injurious.Setting on the machine table.
Note: - The term “right” when applied to ordinary cases of layout or machining, implies that the axes or plane surfaces on the work covered by the term will be either mutually parallel or perpendicular.
In order to set a job, so laid out, on a machine provided with perpendicular translational feeds, (sometimes a right indexed table as well); for all possible machining in one setting, the maximum that is necessary is to align the axes of co-ordinates of lay out on the work, or their parallel substitutes, with the feed lines of the machine.
Comment: The co-ordinate axes are usually themselves virtual, but they are represented on the external surface of the work by the boundaries of corresponding co-ordinate planes. It is important that these boundaries should be compared with the feed lines only in planes perpendicular to the co-ordinate planes providing the boundaries. The irregularity of a surface carrying the scribed line will not produce an error, if the aligning point (assuming a feed line to be represented by the motion of a point carried in the tool post, moved solely by the feed in question) be sighted with the boundary line, along the parent plane of the boundary.
A machined surface with negligible surface irregularity, when aligned with one of the planes containing a pair of feed lines, is a substitute for two axes of co-ordinate contained in its own plane. The third axis can be substituted by a second surface perpendicular to the first, when aligned with another feed plane (it is assumed that these surfaces are right located from the co-ordinate planes of lay out.)
The first surface has to be set by comparing two lines contained in it in planes perpendicular to it, and again mutually perpendicular (for convenience and magnification of error), with two feed lines of the machine. Whereas, for the second surface, only one line contained in a plane parallel to the first surface, is necessary for comparison. In fact, the second surface may as well be a strip with its length parallel to the first surface.
In the above method of setting, the indicating point is given the same movement relative to the work as would be imparted to the tool employed to perform the machining. As such, the finished surface will correspond to the effective sweep of the point (on the tool) penetrating the deepest; assuming that at the end of the cut the sides of the tool leave contact with the material.
Indirect setting: – It is usual in machine tool construction to finish the faces and edges of the work table as also those of some slides, so that they lie in alignment with the planes containing the feed lines. Thus, they may be used as substitutes for the feed planes generated by the actual sweep of the tool point. The principal convenience of such a substitution lies in their permitting the use of less elaborate setting instruments such as try-squares, surface-gauges etc.
Note: – It should be appreciated, however, that the accuracy of such a setting will depend upon the accuracy of substitution, which is influenced by:
1. The surface quality of such surfaces after being in service for some time.
2. Tables which themselves remain fixed after a setting as those of certain shapers and are liable to lose alignment, with the feed lines;
a. due to the straining action of the load,
b. when guides are reconditioned, but the locations for these are not touched,
c. by reason of their cantilever support, which encourages them to cant, depending upon the backbolt tightening pressures.
As such, after an approximate setting in this manner, it is always advisable to have a final check with the feed lines.Slides: –
If the rotational freedom of a slide be constrained, then any straight line fixed relative to the slide will maintain its aspect during the range of travel. If the slide maintains an one plane surface contact with its guide, then it loses two degrees of its rotational freedom, namely those about two perpendicular axes contained in the plane of the surface. The third will also be lost, if it makes a second plane surface contact with another surface of the guide inclined to the first. The line of motion of the slide will then be a straight line defined by the travel of any fixed point on the slide, and it will have the same aspect as the junction line of the constraining planes of the guides.The lathe.
Broadly speaking lathe work may be classified as work incorporating surfaces of revolution, plane or circular surfaces or portions thereof so that all the axes of revolved surfaces and centers of plane surfaces capable of being machined during one given setting of the work will lie on a common axis. In order to produce the required surfaces, the work is rotated about the common axis and the tool given a translational feed motion in a plane containing the axis of rotation. The motion is either parallel to the axis – longitudinal feed; or perpendicular to it – cross feed. An oblique feed can also be arranged:
1. By altering the aspect of the axis in the common plate of the axis and the feed concerned, as by offsetting the dead centre for work produced between centers.
2. By providing an auxiliary feed for the tool, as by a taper attachment or by an auxiliary slide. Generally one feed alone is worked at a time, excepting for profiling work. The depth of cut is obtained by the temporary working of a feed perpendicular to the principal feed, to the extent of the desired penetration. (Only the single point tool is considered).
Note: – In this type of machining the sizing perpendicular to the principal feed is done by the adjustment of the machine slide and as such, the same tool can be employed to produce different sizes within the range provided.
When however, the cross adjustment is dispensed with and size tools are employed with axial feed, only one particular size from a given tool can result, as in drilled or reamed holes.Tool mounting methods:–
For different operations on work in a given setting it is frequently necessary to use different types and shapes of tools. Again, each tool has to be set (size setting excluded), prior to its presentation to the work , and each takes some time to finish its operation on the work. Thus, work would be finished in a shorter time if we could:
a. cut down the setting time of the tool,
b. cut down the operation time either by an alteration in the tool itself or by presenting more than one tool to the work at the same time.
a. can be achieved by increasing the number of tool berths on the machine, as by the employment of turret tool holders. If all the required tools can be accommodated on the turret then setting time will be eliminated for each job done after the first; assuming that the tool edge lasts for the jobs to be finished.
b. can only be possible if
i. tools are mounted on more than one slide, and all of them can be worked together, as in capstan or turret combination lathes.
ii. More than one tool is carried by a mounting in the line of presentation to the work. This is usually the case for size tools carried on special tool holders.Some points in quantity production, to gauges.
1. A major portion of the gauge limit should be reserved for adjustment of tool penetration.
2. The remaining portion may be shared
a. by surface and size irregularity caused by the particular mode of finishing.
b. Geometrical irregularity of profile caused by the inherent error in the machine.
2b. should not be more than a fraction, say, about 20% of the total limit. Usually the machine tools are so designed that the mode of finishing they employ keeps 2a. even less than 2b. – provided, of course, that proper care is taken regarding the choice of correct tools and they are used under proper cutting conditions only. No overstraining of the machine can be allowed, which permanently impairs the alignments or at least causes localized excessive wear resulting in non-compensatable back-lash.The effect of mode of finishing is influenced as follows –
1. Extent of variable spring between the work and the tool.
2. Depth of cut employed for the finishing operation and its variation during the cut.
3. Keenness and endurance of the cutting edge employed for finishing.
4. The guiding of the finishing tool.
5. Material machinability under a given set of conditions.
6. The influence of cutting forces on the distribution of bearing pressures in as much as a tendency towards inducing the guiding surface to lose contact is concerned.Tolerances for machining operations
The following tolerances for different classes of machine work are based upon the experience and practice of the Pratt and Whithey Co. in making equipment for rifle manufacture. These figures are subject to variation, and are given as a guide. It is assumed that the machines in each case are in good condition. The figures are also intended to apply only to the manufacture of duplicate parts on an interchangeable basis, and are not given as representing the greatest degree of accuracy obtainable.
Lathe work.
Rough turning: minimum tolerance of .005” for diameters from 1/4”– 1/2”; .010” for diameters from 1”– 2”, and .015” for larger diameters.
Finish turning: tolerance of .002” for diameters from 1/4”– 1/2”; .005” for diameters from 1”– 2”; and .007” for larger diameters. The most accurate as well as most economical method of finishing many classes of cylindrical work is by grinding, so that accurate lathe work prior to grinding is not necessary.
Automatic screw machine work –
Tolerances of .003” for turning with boy tools; tolerance of .003” for forming tools less than 3/4” wide; and .004” for widths between 3/4” and 1/2”. Tolerances of .006”for hollow milling from 3/16” to 1 /2” diameters; .008” from 1/2” – 3/4” dia. and 0.10” from 3/4” – 1” dia. For drilling, tolerances range from .002” for drills from No. 60 to No. 30, to about .007” for drills from 3/4” – 1” dia. Realing enables the tolerance to be reduced to .001” for sizes up to 1/2” dia. and to .0015” for sizes from 1 / 2” – 1”.
Milling operations.
While a tolerance of .002” is feasible, it should, if possible, be increased to .004” or .005” to secure greater economy in manufacturing. If a single surface is to be milled, the tolerance may be from .002” to .003”, but when there are two or more surfaces to be milled, all but the most important one should be given tolerance of about .005”, if practicable. The tolerance for end milling when the slot is little deeper than the mill diameter is .004” for widths from 1/4” – 1/2”, .006” for widths from 1/2”– 3/4” and .008” for widths from 3/4” – 1”; for straddle milling the tolerance may be .003”, for form milling .005”. Somewhat greater tolerances should be allowed for hand milling than for power milling, because the feeding motion is not so even.
Drilling
Tolerances for drills no. 60 to 30 .002 from No. 30 to 1 .003”. For drill diameters from 1/4” – 1/2” .004”; for diameters from 1/2” – 3/4” .007 and for diameters from 1”-2” .010”
Grinding
Cylindrical and surface grinding, .0005 tolerances. Vertical surface grinding machine, .0002”, which tolerance may be reduced to .0001” under favourable conditions.
Planning operations.
Tolerances varying from .005” to .010” may be maintained in planing comparatively large parts, such as machine tool slides.
Thread cutting.
When threads are cut in a lathe, tolerances on the pitch diameter of from .0015” – .002” may be maintained. When milling screw threads, it is possible, with a machine extremely well maintained, and when using a very accurate form of cutter, to maintain a tolerance of .001” for short pieces on the pitch diameter, and a tolerance of .002” on the outside and bottom diameter; but it is impracticable to give tolerances for interchangeable manufacture more accurate than .002” on the pitch diameter and .004” for the outside and bottom diameter. Tolerances on the outside diameter refer only to Whitworth or other threads with a formed top of thread. The larger tolerance given for the outside diameter does not affect the accuracy or working of the thread, because the apex of the thread is of little value and the important dimension is the pitch diameter.
Hand and machine reaming.
For diameters up to 1”, a tolerance of .0004” may be maintained for hand reaming; for diameters above 1”, the tolerance may be .0005”. These tolerances are increased somewhat for machine reaming. For diameters up to 1/2” the tolerance may be about .0005”, for diameters from 1/2” – 1” from .00075” – .001; and for diameters above 1” .0015.
Complex dimensions.
For clarity we shall call a dimension “elementary” when it connects two consecutive locations in a particular assembly.
It will be called complex when it is made up of two or more elementary dimensions.
In a machine construction, locations are usually obtained from finished surfaces with the assistance of measuring implements. In practice, neither surface quality nor measurement can be perfect. As such, the location and hence the elementary dimension incorporates errors due to both these factors.
In a complex dimension, the errors of the constituting elementary dimensions may accumulate or be compensative; but all the same, the possibility of their accumulation on either side of the standard (assuming a bilateral tolerance for each elementary dimension) cannot be overlooked.
Now, by standardising each elementary dimension, the tolerance of the complex will have to be stated as an algebraic sum total of the tolerances of all the elementary dimensions involved. If it be desired to produce the complex dimensions within fine limits, and the constituting elementaries be several, the permissible tolerance for the elementaries may become too fine for actual production, if both the factors of desirability and practicability are taken into account.
Under the circumstances, a convenient solution is to measure the total error of the complex dimension on completion of assembly, and to compensate it at a selected spot within it. With this procedure the accuracy of this dimension will be influenced by two factors only; the method of recording the error and the approximation with which compensation is carried out.
If the mode of adjustment is made fine enough, and the accuracy of measurement detects an adjustment of that grade; we can if we desire, produce the complex dimension within the limits of adjustment irrespective of the tolerances of its constituting elementaries.
Note: In the above treatment, only the error in locating distances was considered; but if the dimension incorporated moving joints such as pin joints, the effect of working clearances (for pin joints, diametrical clearances) on the dimensions cannot be compensated. In that case, the sum total of all relevant clearances forms an additional bilateral tolerance (the limit being obviously equal to double the sum) on either side of the standard dimension, obtained by holding the clearances symmetrical, for example, in case of pin joints by holding the pins central with their corresponding bearings.
In complex assemblies, for example, the side rod – coupling-rod-pin assembly in a locomotive, a sport adjustment is not practicable. Under such circumstances, the maximum anticipated error is distributed in the form of extra allowance between mating surfaces. If such an allowance were not given then there is always the possibility of impairing the desired symmetry of working clearances. If the location error goes beyond the amount provided as clearance, even successful assembling may be prevented. It will be noted that such a provision reduces the serviceable life of bearings providing the compensation.Influence of misalignment on size.
When a detail is guided by more than one bearing, it is necessary to keep the alignments well within the size tolerances; or conversely if an alignment of a particular grade cannot be produced, and subsequently maintained, between such guides; then, it is not possible to achieve a finer fit between the detail and at least one of the guides, either. Difficulty in producing accurate alignments increases when guides are mounted on members separately fixed to the foundation, especially when the guides themselves are offset. In such cases, it is advisable to recognise these inherent limitations and allow for them in the form of courser allowances on size. When this allowance can be distributed the proportion of distribution will be governed by the convenience of assembly.Introduction:
When two surfaces press together, then due to friction, a certain minimum tangential force depending upon the pressure and the coefficient of friction between the pair is needed to cause relative motion between the two. If the force value is kept well below this minimum then for all practical purposes the pair behaves as a single rigid component.
Again if a material be elastically strained, the straining agency, in that state, has to withstand reaction depending upon the degree of strain and the area over which it is spread, at the surface of contact. Now, force is needed to bring about a deformation, whether elastic or plastic, and thus the mere absorption of force is not a sure indication of an elastic deformation being produced.
(In the following treatment only circular matings are considered.)
When an oversized rod is forced into a corresponding under – sized ring, the following makes the assembly possible:-
1. A superficial flow of material at the contact faces, depending in extent on the nature of the materials and the quality of the mating surfaces. The process is one of breaking up the ridges and filling up the valleys, thus reducing the average rod size and increasing the size of the bore. This is a somewhat plastic deformation.
2. An elastic state of strain is also produced to a reasonable depth, which stretches the ring and compresses the rod.
If the irregularities of contact be great, and the maximum limit of interference be high, the penetration of plastic deformation may become so great that there is a substantial reduction in the effect of the elastic strain. In addition, the stress distribution, being irregular with localized concentration, may become a potent source of starting fatigue cracks.
The mutual elastic strain of the above description will result in a bearing force at the faces of contact, the value of which for a given bearing stress will depend upon the effective area of contact. Now, the force needed to cause a longitudinal movement or twist between the two mating members depends upon the bearing force and the coefficient of friction. If we assume the value of the latter to be fixed, and the bearing stress to be limited then the load carried by such an assembly as a rigid component will be determined by the amount of interference – which determines the strain and thus the bearing stress. - In order to practically realize the full load carrying value for a given interference, we have to set limits for the measurement of maximum interference in order to keep the strain within limits. Again, a fair estimation of the effective contact area will have to be made with a given gross area of contact. The former depends upon the surface quality and geometrical accuracy of the mating details. This indirectly implies that the gross area of contact will have to be computed with due regard for the limitations of the finishing processes. Now, area is a function of length and diameter the ratio of which cannot be abnormally high so as to produce assembling difficulties such as buckling etc.It can be easily appreciated that specification of interference fits on the basis of assembling tonnage alone, with no reference to surface quality and thereby effective contact area cannot be relied upon, inasmuch as the maximum interference is uncontrolled and can thus take the strain out of the elastic range; nor can the risk of local stress concentration be completely obviated.
Some practical aspects:-
The mechanical forcing is usually achieved in a hydraulic press, where the provision of an initially aligned start is a definite practical assistance. In fact, without it the ordinary press may fail to give a suitable assembly. The extent of the start varies with conditions and may be as low as a slight chamfer at the starting ends. But with slack and poorly aligned apparatus it has to be somewhat more – an increase being an advantage.
With longer engagements, and poor starts, a tendency to wobble during the operation is observed. This increases the forcing tonnage, and scores the metal surfaces, which in addition to producing a mis aligned assembly makes dismantling very difficult.
Taper vs. the parallel fit: With tapers the measurement of interference is accurately and conveniently carried out by the simple measurement of draw. Again the quality of mating, when nothing else is provided can be actually tried out with colour. In addition, a generous start and a small interference length afford an extra important advantage.Metal cutting.
Metal cutting as applied to finishing practice incorporates two essentials, namely: -
1. the penetration of the tool.
2. a subsequent splitting caused by wedge action.
In wedge action, the forces transmitted by the two sides of the wedge bearing against the sides of a vee-groove formed in the material to be machined, tend to cause a split in the metal. If the material on the two sides of the groove be of uneven thickness as it always is, the weaker side is deformed and the wedge is thus permitted to proceed further. Again, if the deformed metal, or chip as it is usually called, is sufficiently stiff, then the splitting spot, with a wedge continuously feeding, will run ahead of the edge of the wedge. It will be noted that the extreme edge of the wedge is put out of action as soon as parting starts.
Anyhow, prior to parting, the wedge action could not have been possible without the existence of a vee-groove in the metal. Such a vee-groove is produced by a tool taking its own start by actual compression. Now, for a given force, at the back of the wedge, the intensity of compression tending to produce the groove will increase with the decrease in area of the penetrating surface of the tool. Or in other words, it will increase with the keenness of the cutting edge. As will be explained later, the keenness of the edge is a more important property of a tool which has to penetrate frequently, e.g., a hand scraper, which has to make a finer cut, than if it is employed on heavy roughing.
Metal cutting differs from a similar operation employed on say splitting fuel wood, inasmuch as –
Cutting has to proceed along predetermined paths within specified limits. This brings in the necessity for maintaining a positive relation between tool and work, in the direction of the depth of cut. Again, the cutting edge has to sweep a surface relative to the work. Assuming that the tool makes a line contact with the work, (or point contact) a relative motive of some sort between the tool and work will have to take place before the requisite surface can be successfully produced.
In practice such a condition which provides motion along desired paths and prevents it in the undesired directions has been achieved to a fair degree of approximation by guiding the tool with surface slides.
Comment: All the same, it must not be forgotten that the above achievement is only an approximation, and positiveness of relationship will be destroyed if details concerning the following are not attended to:–
1. An actual displacement of the supports.
2. An elastic spring of the tool, work or supports (to a lesser extent).
3. A displacement of the cutting edge caused by wear.
A complete elimination of these discrepancies is not possible in practice. Even an approximation entails elaborateness and expense. Therefore, it is not attempted beyond what appears absolutely necessary to successfully achieve the desired grade of machining.Cutting tools are compared on the basis of:-
1. The power they consume to remove a given amount of metal.
2. The surface quality they produce.
3. The rate at which they part metal.
4. The duration for which they can remain in service between grindings.
Saving in power is a direct economy. The rate of removal of metal, on the hand, influences the productivity of the machine tool. Surface finish is usually a governing factor, and to meet the specification of surface quality, the other factors may have to be subordinated. Endurance of the cutting edge, in certain cases, e.g., in long finishing cuts, is the deciding factor. In others, it leads to saving
a. of setting times
b. tool material lost in grinding.The mechanism of cutting.
The profile of a machined surface is produced by the sweep of the line where parting of the chip occurs. If this line maintains a constant relationship with the tool edge during the particular cut, then the sweep of the parting line, or in other words, the machined surface left after the cut will correspond to the sweep of the tool edge. If by the employment of external agencies, the tool is positively guided relative to the work, (when the strain of the cut exists) to sweep a predetermined surface, then the machined surface will correspond to a similar predetermined profile to the extent to which the relationship between the tool edge and the parting line is maintained uniform during the cut. (No shift of the tool edge from its predetermined path is assumed). From the above we find that neglecting other influences two essentials of accurate machining are:
1. A successful guidance of the tool (its effective portion being implied) along its predetermined path relative to the work with the cut on.
2. Maintenance of a constant relationship between the tool and the parting line throughout the cut.
It may not be possible to completely realize the above conditions in practice, but all the same, the degree of achievement in these directions will always influence the quality of machining.
It is found that to maintain a constant relationship between the tool and the parting line; it should be possible for the tool to deform the chip in order to maintain its own feed. As such, the chip after a certain distance from the parting line, will leave contact with the tool; and throughout the cut, only a constant length of it will be engaged in the splitting operation. Thus, so far as the chip is concerned, it will not tend to destroy the above relationship, provided, of course, the material is homogeneous and the chip section uniform (or the chip stiffness constant).The mode of deformation.
The results of certain experiments and observations stand to show that the parted chip is actually made up of sections adhering to one another. Each of these sections is individually compressed and subsequently sheared. Thus, the machining operation is a localized one which keeps on repeating itself during the cut. As such, the cut is uniform but for the variation of conditions which occur between successive partings of sections. Considering the effect of these periodic variations on the regularity of the machined surface, we find that the range of irregularity is governed by the extent of penetration of the tool required to part individual sections of the chip. Again, it can be shown that this necessary tool penetration is a function of the depth of the cut taken. From this, we can reasonably infer that surface irregularity will increase with an increase in the depth of cut, other conditions remaining the same.
As we have seen above, the cut is somewhat uncontrolled over a distance governed by the depth of cut. Now, a surface will have at least a periodic regularity if it is possible to repeat similar conditions of rupture during the progress of the cut. If, however, such a repetition is not possible during the entire cut, the cut will be more or less uncontrolled (the loss of control varying with conditions):
As for the standing metal, it is not supposed to undergo any deformation during machining. If deformation is inevitable, it should be an elastic one, and uniform. To successfully cover this aspect, the tool should be so presented to the work that under the strain of the cut, only the least portion of it is pressed against the standing metal, and the extent of contact does not vary during the cut. Such a condition is reasonably achieved in practice by providing a clearance between the tool face and the standing metal.Tool elements.
We have already seen what clearance in a tool is for. It is measured by the angle formed between the tool and the tangent to the surface of the standing metal at a particular point of contact of the tool with that surface. Besides clearance, there are two other important elements in a cutting tool – they are Rake and Cutting Angle.
Rake is measured by the angle formed between the tool face in contact with the chip and the normal to the standing metal surface at a particular point of contact of the tool with that surface.
Cutting Angle or the wedge Angle of the tool is obtained by subtracting from 90°, the Clearance and Rake Angles.
Note: – All these elements have been defined with reference to a particular point on the tool only, as their values generally differ at different points on the contact line between tool and work. These elements are always measured in a plane, containing the particular point to which they refer and also perpendicular to the standing metal surface.Object of Rake.
The parted chip has a tendency to follow a course, usually curved, which is inclined to the normal to the standing metal surface at the parting point (if we confine ourselves to a particular point only for simplicity) towards the tool. Now, if the face of the tool be placed tangential to that course, the chip will leave the tool without exerting any pressure on the face. But, if the tool be placed in a manner which tends to obstruct the free movement of the chip, the chip course will have to be diverted by its actual deformation before it can get clear off the tool. This necessary deformation will increase with the deviation of the aspect of the tool face from the condition of its being tangential to the chip course. The deforming force so employed causes an extra strain on the tool and the machine; besides the operation consumes useful power which consumption in its turn becomes a source of tool wear and heat generation. As such, it is desirable to keep this deformation to a minimum if it cannot be completely eliminated. The provision of rake is a step in this direction – its extent measuring the approach to the ideal condition of “no chip deformation at the tool face”.Cutting Angle.
An important factor in tool design is the capacity of the tool to resist the cutting forces successfully near the cutting edge. Equally important is the ability of its cutting edge to penetrate the metal with the application of the least penetrative force, without undue crumbling. The cutting angle is a measure of the above properties. When large, it makes the tool strong to resist the cutting forces and saves its edge from crumbling. If, however, penetrating convenience be sought, a smaller cutting angle is preferable. In practice, its value is compromised between keenness and strength.
Note: – It will be noted that we cannot simultaneously provide a large cutting angle and a large rake angle, and therefore here again a compromise is necessary. As for clearance, it should be the least we can get along with. It can be shown that, given necessary strength, a smaller cutting angle will enable the tool to remove more metal for the same amount of power consumed.
Elements of the cutting force. (Chip sections considered individually) The cutting force may be supposed to be made up of the following
1. A force needed to compress the section.
2. A force needed to slide the section over the standing metal.
3. A force needed to slide the back of the section over the tool face.
The least two elementary forces may be assumed to remain constant during particular cut, but the first will vary from zero at the start to a maximum when the section is about to shear off.Heat factor in cutting.
The power consumed for the operation of cutting is converted into heat. If the conditions of cutting do not alter, the rate of heat generation will remain constant. If, at the cutting spot the rate of heat dissipation is not the same as that of heat generation, the result will be a rise of temperature in the locality of the cutting spot. The usual modes of heat dissipation while cutting are:
1. A dissipation by conduction to the bodies of the work and the tool.
2. A dissipation by direct radiation, from the spot as also from the bodies of the work and the tool.
3. A dissipation due to heat carried away by the chip.
4. A dissipation due to the presence of an external agency such as a coolant.
It will be noted that the rate of heat dissipation by the first two methods is dependent upon the difference of temperature existing between the cutting spot and the surroundings, and as such, will be more pronounced at elevated temperatures. Similarly, the heat carried away by the chip is a function of temperature. Item four, however, is influenced by other factors also. For instance, it depends upon the specific heat of the coolant as also its quantity employed to dissipate a given amount of heat.
Neglecting item four, for a while, we gather that the temperature of the cutting spot will continue to rise till the rate of heat dissipation equalises that of heat generation (assumed constant).
Now, a cutting tool needs to be hard before it can cut and we know that practically all tools loss their hardness when sufficiently heated, that is, when heated to the critical temperatures pertaining to their materials. As such, to maintain cutting, one factor of importance is to keep the above stated equilibrium temperature below the corresponding critical point of the tool steel.
To arrange such a condition we can proceed as follows:
1. Either reduce the rate of heat generation by reducing the rate of metal removal, or
2. to reinforce the rate of heat dissipation by the employment of a coolant, or
3. to substitute a tool steel with a higher critical point.
The first solution is the least desirable of all, and is avoided if it can be helped. The third is limited by the available – grades of tool steel and their cost. The last is employed as far as it is practicable.
It should be appreciated, however, that the tool cannot be allowed to soften, and if the last two solutions are insufficient, the rate of metal removal has to be decreased.Some causes of tool breakdown.
A tool is supposed to have broken down when, instead of cutting it merely rubs, leading to the generation of excessive heat, resulting in the oxidization or “burning out” of the tool.
It is seen that the loss of keen edge is the primary cause of tool breakdown and as such, factors which tend to destroy the keen edge are also conducive to tool breakdown. The following have marked effect on the endurance of the keen edge:
1. Heat. We have already seen that excessive heat destroys hardness and thus deprives the edge of its strength to successfully perform cutting.
2. Rigidity, of supports. Lack of rigidity between the tool and the work in the direction of the depth of cut causes the keen edge to frequently bang against the standing metal and thus get crumbled.
3. Wear. This is mostly caused by the hip sliding on the tool face. It is usually in the form of cavity near the cutting edge. By its progress, it reduces the effective depth of the tool section which resists cutting forces, and when excessive, it causes the tool portion in front of it to snap off.Influence of cutting edge profile.
When a chip is parted, it tends to take a course normal to the cutting edge at all points on the contact line (an observation). If its course is diverted at any point of parting, it is done only by deforming the chip; and the area of the tool face executing the deforming is itself subjected to an extra pressure, depending on the extent to which it contributes to the deformation.
Note: This deformation should be differentiated from that needed to alter the natural curved course of the chip, when the tool face does not lie tangential to the natural chip curvature. In fact, the two deformations take place in planes mutually perpendicular.
Coming back to the chip; if the individual particles in a chip cross section tend to follow parallel paths, they will not encounter any obstruction from the surrounding particles (even frictional resistance will be absent if they proceed at the same speed). But there will be definite fouling of particles if their paths converge, the extent of fouling increases with the acuteness of convergence. This will lead to some sort of deformation resulting in the concentration of pressure at selected areas of the tool face.
It will not be difficult to see that a straight line is the only cutting edge profile which will make the chip particles take parallel courses and thus allow the chip itself to proceed undeformed. Such a profile, however, is only possible where the cutting edge can be made wider than the work, as in plate planer tools. Here, the tool clears the work on either side of the line of penetration. With the work wider than the tool edge, machining is completed by the employment of feed. Under these conditions, the tool makes contact with the work, both across the line of penetration and across the feed line. These contacts may join on a point or they may join on to a curve. In any case, if we consider the parting line to cover both the contacts, it will not be difficult to appreciate that the parting line cannot be straight, and as such, the deformation of the chip is inevitable. But the injurious pressure concentration will be less severe if sharp points (providing areas of extreme convergence) are avoided, and the merging curve is broadened out (to approach a straight line).Tool Chatter.
A tool is said to chatter when it produces periodic undulations on the finished surface. The following are the common causes of such a phenomenon:
1. As we have already seen, the tool pressure fluctuates during the formation of the chip.
“These fluctuations have a fixed period of time under a given set of conditions (such as the nature of metal to be cut, depth of cut, form of tool). If this period synchronises with the natural period of vibration of tool or work, or some part of the machine, then the vibrations may be much magnified and cause chatter. Such a condition of sychronisation is avoided by constructing the tool in such a manner that the chip is broken up in different ways at different points of the cutting edge”, Taylor.
Note: It is partly for this reason that Mr. Taylor recommends his round nose tools.
2. The second cause arises from the fact that the cutting edge, by reason of the shape of the ordinary tool shank, tends to dig further in to the metal when the tool undergoes a spring due to the pressure of the cut. The net result is a gradual digging in of the tool followed by its sudden jumping out. This phenomenon periodically repeats itself during the cut, and thus produces chatter. This tendency to chatter is more pronounced with wider cuts. The remedy of such a chatter lies in the employment of “spring tools” (to be explained later).Influence of placing on tool elements.
From the definition of tool elements we find that the tool elements “clearance” and “Rake” can only be measured when the placing of the tool relative to the work is given – it being possible to increase one at the expense of the other by mere shifting to the tool aspect. As such, in order to make these terms more understandable, it is desirable to standardize the tool placing - atleast for tools of universal application whose placing is subject to variation at the hands of individual operators. It must, however, be borne in mind that the standardized placing is selected for convenience only, and is as such not binding when more efficient alternatives are presented.
A common illustration of this standardization is the standard lathe tool. In our definition for clearance and rake we make use of a tangent and a normal to the standing metal surface at the point of contact of the tool and this surface. Now, these two lines are definitely related to each other (by an inclination of 90°), and as such, for purpose of reference, only one is sufficient.
In a lathe the axis of work is horizontal, and as such, at one point lying on the curved (round) surface (that is, the point lying in a horizontal plane containing the main spindle axis) the tangent to that surface is vertical and the normal horizontal. Thus, we can refer to a horizontal or a vertical plane for measuring rake or clearance angles, if we support the tool point at centre height. To go a bit further, we can employ any face on the tool shank for this purpose if that face is at the time of supporting placed horizontal or vertical.
It should be appreciated that tool angles are produced when the tools are away from the work and if a simple location can be found on the tool itself which can be made use of both when grinding and when setting the tool relative to the work, it is a great practical advantage.
As for horizontal or vertical (more often horizontal) locations for the reference plane on the tool itself, it will be seen that the usual aspect of support tables on machine tools is either horizontal or vertical. As such, their provision is not extra botheration.
Thus, the only care needed to set a correctly ground tool placed standard, is to set it correctly to the centre.
Note: – Considering the adverse effects of carelessness in this respect, it can be shown by geometry that a tool losses in clearance (at the same time gaining in rake correspondingly) when set above the centre. It is vice versa for a setting below centre. Again the effect is more marked with small diameters than it is with larger ones. (This is so for external turning; for boring the effect is reversed).Mathematically:
If the shift above or below centre = X
and radius of standing metal = r
and corresponding loss or gain of Clearance or rake = Ө
Then Ө = sin -1x/r (approx.)The production of holes.
The following are the important specifications of a hole or bore (enlarged hole). The permissible tolerances for the various items stated, usually govern the choice of the cutting tool and the method of guiding it.
1. The hole should have a round cross section.
2. The axis of the hold should be a straight line.
3. The axis of the hold should be at its proper location and in its proper alignment.
4. The size of the hole should be correct and uniform over the entire length of the hole, (parallel holes only are considered).
5. The quality of hole surface should have a tolerance finer than the finest tolerance specified for the previous items. (This is a logical corollary when measurement is made with reference to the hole surface.)
Comment: - 1. The roundness of cross section is best ensured when the work revolves. With the tool revolving, an ovality depending upon the extent of misalignement between the feed line and the axis of rotation is liable to occur.
2. Assuming the guide lines in the machine tool to be straight, a curved axis can only be the result of a variable cramp of the tool, under the strain of the cut. It should be noted that a curved axis cannot result when the work revolves, unless the work is itself cramped at a variable rate.
Note: Cramp may be visualised as an elastic straining under constraint, of the revolving member so that the instantaneous axis of rotation for points lying in a particular cross section is the tangent to the strained neutral axis at the point where the latter meets the particular cross section.
A simple case of cramp may be obtained by revolving a test bar between lathe centers and by straining it in the middle by a vee support held in the tool post. If other causes of error be absent, a dial gauge fixed to the machine structure will not indicate eccentric running at any cross section; providing thereby that the test bar is rotating about its own cramped axis.
3. The accuracy of size is governed by the accuracy of tool setting (with single pointed tools), or by the accuracy of size of the tool itself when the tool is a multi-edged one, as in the case of a drill.Note. In a multi-edged tool of the above type the cutting edges are located from a common axis. As such the size of the hole will correspond to the tool size only when the said axis of the tool is kept aligned with the axis of relative rotation between the tool and the work.
As for the uniformity of cross section it will depend upon the accuracy of alignment of the effective feed line (that is feed line after considering relative spring between tool and work) with the axis of rotation.
Another factor influencing size uniformity is tool wear, occurring during the cut. It can cause a direct loss of size, as also bluntness, which results in a greater spring of the tool. In fine finishing cuts the usual result of this spring is that the tool leaves the cut altogether.
4. The desired grade of surface quality influences the choice of the finishing process. The usual modes of finishing are:
1. Drilling – for small size clear holes (for bolts rivets etc.)
2. Reaming – for small size (or when specially designed for medium also) holes, needing an accuracy of say U tolerances.
3. Grinding – for accurate Work of variable sizes, where the quantities do not warrant costly one-size tools.
4. Lapping – for high precision work.
5. Compression rolling – for small and medium sized holes in comparatively soft materials, e.g. bronzes where a compressed skin of metal is supposed to give good wearing properties (The Fessler-Maupin Process).Drilling: –
The production of a round hole in solid is called drilling. The minimum requirement for the drilling operation is a cutting edge revolving about one of its ends, fed into work along a direction perpendicular to its plane of rotation. If the axis of rotation be given a shift so that the given sense of rotation is opposed to the cutting direction of part of the edge, then that part will have to drag on – its penetration being made possible by crushing only. The result will be a breakdown of that part, at least. If the entire cutting edge lies at a distance from the axis due to this shift, then a projection of material will be left in the centre, which will tend to foul with the feed of the tool, and again the feeding further will depend upon a successful crushing of this projection.
In actual practice, the difficulties of achieving pure cutting in drilling are recognized. As such, in drill designs, provision is actually made for a successful crushing of the material in the vicinity of the centre – care, of course, being exercised to keep the unavoidable crushing to a minimum. Again, two edges are used instead of one for the following reasons:
1. Given correct grinding, the total amount of tooling is equally shared by both the cutting edges; or for a given feed, the strain on the cutting edge is reduced to half.
2. As a spring of one cutting edge will throw even more work on the other, the tendency is self-compensated if other influences are absent. These absences of spring will lead to a greater positiveness of size.
Note: If more than two edges be employed, then the cutting edges, or lips as they are called, cannot be brought sufficiently close together and thus the dead area in the centre becomes excessive – a handicap which cannot be tolerated in drilling.
Other problems in drilling are:
1. A successful ejection of the chip.
2. A practical method of feeding the drill point with coolant.
We have already seen elsewhere that the only cutting edge profile which makes a chip proceed without deformation or fouling is a straight line. Now, in drilling tough materials the only practical way of clearing the chip is to make it continuous, and then to suitably guide it out. Considering, therefore, the continuity of the chip alone, the cutting edge profile in a drill should be a straight line. As for guiding out the chip a screw path is a convenient guide in a restricted space. But, while providing for these two aspects, we cannot ignore the elements of the cutting edge itself; that is, it must always have rake, and clearance. Another consideration is the guiding of the drill (considering that it is supported at one end only). But then, the guiding action should not be a source of excessive friction, leading to a loss of power, and also jamming of the tool.The Twist Drill.
The most efficient design of a universal drill, which reasonably incorporates the above requirements and is at the same time, not too expansive, is the standard twist drill. To start with, the body (barrel) of the drill is a cylinder (of diameter equal to the desired hole size) surmounted by a cone. Two similar spiral grooves, 180° out of phase, are then cut in the body with their axes coinciding with the axis of the cylinder. Their lines of breaking through the surmounted cone, form the cutting edges. (Of two such lines per groove, the one providing rake is selected for cutting). The inclination of the spiral to the drill axis gives the rake of the cutting edge. Incidentally, for the provision of the rake, the sense of the spiral is such that it forms a convenient way out for the chip, and an effective way in for the coolant. The clearance of the lip is formed by relieving the metal behind it, (this can be done by grinding the drill point about an axis inclined to the axis of the drill, the inclination measuring the clearance angle).
Note: – The net effect of such a grinding is that successive sections on the lip surface form parts of cones increasing in steepness as we recede from the cutting edge. To guide the drill in the hole, with the least friction, but to maintain the size of the cutting edge at the same time, the body of the drill is relieved but for a narrow land, which covers the cutting edge. The crushing is performed by the central core left in the body after cutting out the grooves. To resist bending, this core is made thicker towards the shank.
To guard against the drill binding in the hole, the barrel is made tapering towards the shank (.001” in diameter for every inch of length).
The location of a drill in the machine spindle (or an equivalent attachment, when the work revolves), and its subsequent drive involves a consideration of the following:
1. In the working position the drill axis should be coincident with the axis of relative rotation, if a drag of the cutting edge, as also an enlarged hole, are to be prevented.
2a. The drill, should receive a positive feed pressure along its axis,
2b. and a torque to do the necessary cutting.
3. Considering the wide application of the tool, it’s mounting and dismantling from the machine should be quick and simple.
4. As more than one size of drill may have to be used, on the same spindle, some simple but effective provision for such adaptability is also essential.
In a twist drill the drive is obtained through the shank – partly by friction, and the rest positively. The positive driving action is obtained by the engagement of the flat end of the shank known as the tang – in the slot of the spindle socket.
Note: – A location is best obtained when the locating surfaces are in contact with one another under pressure. Again, for matings, a hard contact is not convenient when the surfaces are parallel. In such a case, interference commences from the very start, and continues for the entire length of the contact; the same holds when the mating surfaces have to be separated. A taper, however, has the advantage that practically the entire length of engagement is obtained without interference, and only an insignificant draw is needed to pull the two surfaces together. In dismantling too, just that draw has to be overcome and the release is practically instantaneous.
As for the selection of the profile of the mating surface itself, a surface of revolution is the most convenient and effective, when the desired location is that of the axis; the surface of location, by its very nature, being symmetrical about the axis.
Thus, for the location of axes, (when no relative motion is desired) a convenient location when practicable, is a tapered shank circular in section – the axis of the shank coinciding with the axis to be located. Incidentally, a shank mating as such is capable of transmitting an axial pressure as well.
Coming back to the drill shank, it is made tapered (in the standard machine – tool variety) and circular (in section). Knowing the advantages of this profile, let us consider them in relation to the drill requirements given above.
1. We have already shown that this profile is the most efficient to ensure items 1 and 3.
2. It is practicable for 2a; it also enables a portion of the cutting torque (2b) to be transmitted by friction, depending in extent upon the force with which the shank is forced into its socket, that is, upon the feed pressure. It should be appreciated that the necessary cutting torque varies directly as the feed pressure, which is exactly what this type of shank provides.
3. Adaptability is ensured by the interposition of taper sleeves of varying sizes between the shank of the drill and the socket in the machine spindle.
Note: - The drill is put on the machine by hand, and dismantled by the use of a taper drift, through a slot in the socket, bearing on the top of the tang on one side and the end of the slot on the other.Twist drill grinding.
The rake of the cutting edges is fixed by the angle of inclination of the flute spiral to the drill axis. As such, the only items to be controlled in drill grinding are:
1. The angle of the drill point, the correctness of which ensures a straight cutting edge. For standard steel drill this angle is 118°.
2. Lip-clearance. This should be from 12-15°.
Note: - Given correct lip-clearance, the dead centre line makes an angle of 120-135° with the cutting edge. This may be taken as an indirect way of checking lip clearance.
3. The two cutting edges should be equal and equally inclined to the axis of the drill.Drilling feed and speed.
As no clearance is provided on the land part of a drill, efficient cutting on the side is not possible with this tool. As such, it is advisable to concentrate the major portion of cutting on lips only. This can be achieved if the rate of feed is light. To compensate for the decreased amount of metal removed per revolution, the speed of the drill may be increased. Thus, we see that a drill will generally yield the best results with high speed and light feed.Forces on a drill.
Theoretically, a drill transmits to its cutting point a compressive force and a torque – compression to provide the feed pressure and torque to do the splitting. But in practice, it also experiences a certain amount of bending due to some cramp resulting from a certain misalignment between.
1. the starting point on the work and the axis of the machine spindle, (for drilling machines)
2. axis of the drill as supported and the axis of rotation of the work, (for lathes)
Note: – The drill itself is assumed to be straight.Drill breakdown.
The breakdown of the drill may be classified
a. breakdown of the cutting edge, and
b. breakdown by a fracture.
Breakdown of the cutting edge may be taken as a condition where the drill needs regrinding, substantially before its span of life between grindings. This is generally the result of either an injudicious selection of feed or speed or excessive clearance. Another cause for a drill blunting too quickly is the lips being of unequal length, or asymmetrically inclined to the axis. (In this case one of the lips will have to do more than its allotted share of the work.)
An excessive speed will be indicated by the burning out of the cutting edge. If the drill chips out at the cutting edge, it is a sure indication of too heavy a feed, or too much lip clearance.
A fracture, in a drill, occurs when the stresses induced in its body are greater than those which the drill is designed to carry. These are produced either due to excessive torsion, or excessive bending, or both. As a rule, the fracture will occurs at the weakest section covered by the stresses. The worst case of torsion occurs when the drill binds in the hole. If the shank can successfully transmit the torque, the fracture will occur at some section of the barrel. If, however, during such a binding (or even in normal working conditions) the shank gets loose in the socket, the entire driving strain is thrown off the tang, which is liable to twist off, except in the case of small size drills. Bending can cause a direct failure. Incidentally, it causes the drill to bind, and thus assists in increasing torsional strains. As such, it is quit possible that failure under these conditions, is caused by bending and torsion combined.Note: - The injurious effect of cramp is most marked in deep drilling because, for a given misalignment, bending strains go on increasing as the length of the drill outside the hole decreases.
Some causes of drill binding.
1. Chip clogging: When a broken chip, instead of a continuous chip, is formed, the tendency of the chip is (specially with tough materials) to clog itself between the drill body and the hole. The natural remedy lies in investigating the cause of a broken chip and then providing for its continuity.
2. Backlash in the machine spindle: If there be excessive backlash between the machine spindle and its feed mechanism, there is a tendency for the drill to punch the material instead of cutting it gradually when nearing the end of a through hole. The cause of such a tendency is the weight of the spindle (quite a substantial one), tendening to fall through a distance equal to the backlash. The drill thus caught in the end material is virtually jammed, thus bringing in the liability of a torsion failure.
3. The effect of cramp on binding has already been given.
Note: - Another type of failure in the form of splitting up the centre is caused by insufficient lip clearance, when feed pressure is applied.Digging in of the tool.
The pressure of the chip on the tool face has a component along the line of penetration of the tool. With a tool having positive rake this component tends to pull the tool further into the metal (with negative rake the sense of this component is opposite and with no rake the component itself vanishes). This pulling-in or digging-in tendency of the tool is resisted by the relative rigidity of tool and work along the line of penetration. If this rigidity is insufficient, the depth of cut will increase to the extent to which the rigidity can be destroyed.
An important factor which influences the above rigidity is the placing of the body of the tool relative to the line of penetration. If the body of the tool lies along this line then the tool resists the digging in tendency mostly in tension. But if the body lies across it, the resistance will be mostly of bending. For an intermediate position, the resistance is partly of bending and partly of tension – the proportion of bending increasing as the placing approaches the across position.
Now, under similar condition of loading, the deflection of the tool point due to bending will be much more than its corresponding displacement due to a tensile stretch. As such, other influences being identical, a placing of the tool with its body along the line of penetration prevents the tendency to dig-in more successfully than one with the body of the tool across the same line.
Note: - Incidentally, an increase in the depth of cut causes a direct increase of the cutting force. Besides, due to the thickening of the chip, the chip pressure also increases. An increase of chip pressure in its turn increases the value of the digging-in component, which causes the depth of cut to increase even further. As such, a loss of rigidity of the above nature, if not prevented in the first instance, can go on developing the cutting forces at an increasing rate till an actual breakdown of the tool occurs.Other effects of bending.
Another adverse influence of bending is that it alters the initial relationship between the tool and the work. The result is generally:
1. An alteration in the cutting elements, namely clearance and rake. Loss of clearance, when it occurs will make penetration more difficult. As for rake, its decrease will increase the chip pressure, whereas its increase can lead to a digging – in of the tool.
2. An alteration in the geometrical relationship between the cutting edge and the work. This may cause either an increase or a decrease in the depth of cut. This influence is most adverse when the depth of cut tends to increase with an increase in bending deflection; in which case the tool can produce chatter or it may even break down.Some factors in bending:-
Bending occurs when the lines of action and reaction lie apart. The deflection due to bending increases with the load and also with the unsupported length. It decreases with an increase in the section modulus. Again, for the same load acting at a point at a given distance from a support, the deflection at that point is less with a beam than with a cantilever of the same section modulus.Elimination of bending.
From the above treatment we gather that bending of the tool becomes a serious handicap when taking heavy cuts. As such, to secure the advantage accruing from heavy cuts, it is necessary to eliminate, or at least minimize bending in cutting tools. One way of eliminating bending would be to so arrange the cutting action, that at any instant the various bending tendencies acting on the tool cancel one another. In tools bending due to cutting goes by placing two similar cutting edges along a diameter – the placing being symmetrical about the axis of rotation and the sense of cutting identical.
If bending becomes inevitable, then its extent should be reduced by:
1. Employing the maximum permissible section modules for the tool.
2. Reducing the unsupported length of the tool to a minimum.
3. If possible, using a beam (instead of a cantilever) support for the tool.
Note: - It should be appreciated, however, that if bending cannot be successfully eliminated or minimized, then a reduction of the cut becomes inevitable.Boring.
In the conventional sense, boring implies the enlargement of existing holes by a cutting operation – the finished surface being a surface of revolution.
The boring tool has to cut inside the body of the work. As such, its cross section is limited by the size of the existing hole. Again, this reduced cross section has to be of sufficient unsupported length so that the tool completes the cut without its body fouling with the work. The handicap of cross section is acutely felt with small diameters, and that of unsupported length with long holes. As for the cutting elements, a boring tool resembles an external turning tool except in that it needs an extra clearance of the end facing the standing metal of the hole. The amount of this extra clearance, which is needed to clear the heel of the tool, increases with a decrease in bore diameter, if the tool section remains the same. Considering the influence of discrepancy in setting the tool relative to the centre (standard placing of the tool assumed), a setting above the centre decreases rake and increases clearance correspondingly; the effect being vice-versa for a setting below centre. It will be noted that this effect of setting discrepancy is opposite to that produced by a similar discrepancy for external turning.
As already stated, the length and cross section (and hence section modulus), of a boring tool are determined by the bore. Therefore, the only other convenient methods of reducing bending are:
1. A multi-edged tool
2. A beam support provided with single or multiple edges.
For a proper functioning of the multi-edged tool, the axis of the tool from which its edges are located, has to be aligned with the axis of rotation. This means:
1. We lose the advantage of the cross adjustment of the machine for size. The tool in this case will produce only one size, that fixed by the distance of the cutting edges from the axis of the boring bar. (This distance being the same for every cutting edge).
2. To comply with the assumption of design, the cutting edges will have to be accurately located from the axis of the bar, which itself will have to be precision-made. This will considerably increase the cost of the tool.
3. The tool will have to be mounted in a special fixture, such as a turret head. This will necessitate the employment of a special machine or at least a special fixture.
The above remarks apply to a reasonable extent to a two and support also.
From the above, it will be appreciated that the employment of multi-edged tools, as also of two end support, though it reduces bending (and offers certain other advantages) is only economical when the quantity to be produced justifies the expenditure on precision tool manufacture and costly special machine tools or fixtures.Note: Cases can arise, however, where the very specification of desired accuracy and the dimensions of the work warrant expenditure in this direction. In such cases, the alternative is not between costly and cheap manufacture, but rather between job and no job. But, in jobbing work, the standard single pointed cantilever tool is employed as far as practicable. Here, the advantage of obtaining a range of sizes from the same tool, overcompensate the advantage of improved cutting capacity.
Locomotive Boiler Plate Work
In locomotive boilers, or in any other similar fabrication, the individual plates frequently have profiles – internal or external – cutting them, which are required to register with similar profiles on related neighbouring plates when the fabrication is complete. Many a time the cutting of profiles has to be guided by a scribed marking on the plate itself. To ensure the registration of profiles, it is necessary that the markings, too, should register. A convenient and accurate way of achieving this, we know, is to obtain the markings on all the related plates from common planes of reference with the plates placed in their proper relative positions.
Now, in plate work of this type the dimensions of the plates are usually large. Again, the placing of the plates in the assembly is generally awkward. As such, the employment of externally prepared plane surfaces of reference for marking off becomes difficult, if not sometimes actually impossible. The plane surfaces available in the structure itself, many a time do not stand at an appropriate aspect for reference, and then sometimes projections, such as stay heads on them further render them unfit for guiding the laying out instruments such as the surface gauge.
In view of the above it will be obvious that in plate work of this nature, laying out with an adjustable point guides from a plane surface of reference, internal or external, cannot be considered with advantage.
Examining the scope of alternative methods of layout, we find that our plate has a plane surface (omitting curved surfaces for the present) of reasonable regularity, at least after it has been levelled. Therefore, all the profiles marked off on the plate surface are plane figures, their containing plane being the plane of the plate, (neglecting the thickness of the plate).
To locate the datum points of a plane figure, we need a reference to two- co-ordinate planes which intersect the plane of the figure – in this case the plane of the plate. If measurement be made in the plane of the plate, we can refer to the traces of the co-ordinate planes in the plane of the plate, instead of referring to the co-ordinate planes themselves. The locations in both the cases will be identical in dimensions, if the plate plane is perpendicular to both the co-ordinate planes, (in which case it will itself be a third co-ordinate plane) be other than 900 , then an obliquity correction will have to be applied to locations obtained from the trace of that plane, in order to make them, identical with similar locations obtained from the plane itself. In any case, the measurement when referred to the traces can be made directly in the plane of the plate, and simple drawing instruments, such as dividers, trammels, set-squares etc., can be conveniently and effectively employed. Again, if the traces of the same pair of co-ordinate planes be employed on all the related plates, for reference, the requirement of a common reference would also be satisfied.
But even with this facility, an important difficulty that remains is the obtaining of the traces themselves. To obtain them with the plates in position, again calls for a scribing point guided by a surface of sufficient extent to trace the boundary of a co-ordinate plane. This, as we have seen, is not practicable under the circumstances. Another method would be to fix the traces of the co-ordinate planes on individual planes arbitrarily, in the first instance; and then in positioning the plates, to so align the corresponding traces on the related plates that they lead to the generation of virtual planes of common references. This is in effect, the reverse of the first method, but in practice we find it much more convenient.
Now, if we are to follow the procedure outlined above, we must devise means to carry out the following within specified limits:
1. This fixation of the plane of the plate at its proper aspect.
2. The alignment of the related traces to generate the requisite co-ordinate planes.
TRACE: – means a line in which a plane meets a co-ordinate plane.
In practice, it is usual, to take the plane surface of one of the plates as the datum plane of reference. In the case of an approximately rectangular box type fabrication, to which we shall confine ourselves for the present, the surface of the bottom plate is the one generally selected. For convenience of designation, we shall consider its plane to be placed horizontal, so that the other co-ordinate planes become vertical. The vertical planes are generally so disposed that the same one of them intersects all the related members (usually two opposite to each other) of a set of sides, which need inter registration of profiles cut in them. As such, the very same plane leaves a vertical trace on each of the related plates. Again, all the side plates intersect the bottom plate. Therefore, each one of them carries in its plane a horizontal trace, - the trace being the meeting line of the plate plane with the horizontal. Thus we see that this disposition of co-ordinate planes makes a common reference (in the form of traces) for all the related plates possible.
Now, the traces of co-ordinate planes in any given plane are mutually perpendicular (traces of perpendicular planes in the same plane are also perpendicular). Therefore, on fixing one of the traces, on the plate surface arbitrarily, the other is conveniently but definitely located for its aspect in that plane by a simple geometrical construction for perpendiculars.
Our horizontal plane of reference is a real plane surface (as against the vertical co-ordinate planes, which are virtual); therefore, the traces of this plane of reference are contained by two real surfaces, namely the side and bottom surfaces, simultaneously. As such, the alignment of the horizontal-plane-traces reduces itself to making the mating edges (which are the equivalents of these traces) of the bottom and side plates coincide. If the edges themselves become virtual, due to the provision of say radiused flanges, parallel substitutes thereof, in the form of scribed lines can also serve the purpose.
With the horizontal traces so aligned, the traces of the vertical co-ordinate plane generated in the plane of the plates by geometrical construction, will lie in parallel verticals planes. To make them co-planner, all that is necessary is an adjustment of the side plate relative to the bottom, along the horizontal-plane-trace, till the two ends of the traces of the vertical co-ordinate plane on the two plates meet.
To fix the plane of the side plate, reference to one co-ordinate plane only is sufficient. This plane, should preferably be in the form of a plane surface in order to permit the use of common setting and measurement instruments. In our case, the horizontal co-ordinate plane serves this purpose, and is therefore selected for reference.
The angle between the side and the bottom plates will be measured in a plane perpendicular to the ground line of the two planes. If this ground-line be perpendicular to a vertical co-ordinate plane, the traces of that plane in the two plates can guide the measurement, or else, traces of an appropriated plane may be scribed on the plates as described above to serve the same purpose.
By generating a triangle in the plane of measurement with one of its apexes on the ground line of the plates, the apex angle, that is the inclination of the side plate, may be fixed by fixing the ratio of the three sides of the triangle. This procedure will substitute a distance measurement for a direct angular measurement – rather awkward under the circumstances.
The following method is even simper when the aspect of two adjoining plates relative to the bottom has to be fixed simultaneously.
From geometry, the junction line of the adjoining plates or a parallel substitute thereof, if the junction line be itself virtual, is scribed on each of the adjoining plates with reference to their respective traces of reference. In assembling, in addition to aligning the traces, the two counterparts of the junction line are also made co-incident; and that fixes the aspect of both the sides positively. If the other three side-junction-lines too, be fixed in this manner, the five plates of the box (assuming the lid to be absent) will stand in correct angular relationship.
It will be obvious that the procedure elaborated above satisfies both the important conditions of laying out the trace reference – the degree of approximation depending upon the accuracy of scribing and that of subsequent aligning – and thus makes such as lay out possible.Screw threads
General
Helix: A helix is a curve traced out by a point which rotates uniformly about an axis, at a fixed radius, and at the same time translates uniformly along it.
Screw surface: A screw surface is generated by a straight line making a constant angle with an axis, which uniformly rotates about that axis and at the same time translates uniformly along it.
Screw thread: A screw thread is a helical projection formed around the curved surface of a cylinder, so that the groove that accompanies it agrees with it in profile.
The cylinder so formed is called a screw and its female mate is a nut, and the two form a screw pair.
Lead: The lead of a helix is the axial distance traversed by its tracing point in one complete revolution.
Pitch: when referred to a screw thread, pitch implies the axial distance between a certain point on a particular thread, and a similar point on the adjoining one. For a single thread pitch equals the lead.
Inclination: The inclination of a helix is measured by the complement of the angle which the tangent to the helix at any point makes with its axis. Mathematically.
tan (inclination) = Lead/circumference of rotation of the tracing point.
Comment: It will be observed that all the points on the generating line of a screw surface trace helices. Every such helix has the same lead, but its inclination increases as the tracing point approaches the axis. As such the screw surface as a whole cannot be said to have any particular inclination; the inclination must always be referred to a particular helix constituting the screw surface.
The development of a helix is a straight line, but its projection in an axial plane is a curve which grown steeper on moving away from the axis on its either side. (This can be verified from the geometry of slope at various points in the projection of the helix in that plane).
Between the screw and its mating nut, two degrees of freedom are necessary for any relative motion to take place.
The freedoms are:
1. A rotation about the axis of the screw pair.
2. A translation about the same axis.
These movements are definitely related for a given pair, and neither of them can be isolated.Multiple threads
When more than one projection of identical profiles and within equal leads are formed on the same cylinder, so that the grooves which accompany them have the same profile as that of any projection the resulting cylinder becomes a screw with multiple threads.
The lead of a multiple threaded screw is the same as that of any contributing thread. The definition of pitch holds here also, but in this case pitch equals lead: number of starts.
For the successful formation of a multiple thread it is necessary that the starts should be evenly spaced at any cross section of the screw. To achieve this it is sufficient to space them evenly at one cross section only.Other elements of a screw.
1. Full diameter – is the maximum diameter of the screw or the nut measured at right angles to the axis.
2. Core diameter – is the minimum diameter of the screw or the nut measured at right angles to the axis.
3. Root – the bottom of the thread groove.
4. Crest – the top of the thread on full diameter of a screw and core diameter of a nut.
5. Flank – the side of a thread joining the root and the crest.
6. Angle – the angle between the flanks.
7. Simple effective diameter – is mean diameter at the working surface of the screw. For the usual form of the vee thread, this diameter equals the length of a line passing perpendicularly through the axis and terminating at each end in the straight part of the flank of the thread outline.
8. Compound effective diameter – is the effective diameter over a specified length of the thread, and is greater than the simple effective diameter due to pitch error (which nearly always exists). It can be measured only by a full form gauge.
Broadly speaking, screws may be classified as:
1. Those which are employed as components of the final products.
2. Those which are employed in the form of tools or as components of appliances. Here, the screw is employed as a means of production rather than the product itself.
The most general use for the first class of screws is as a fastening for other components. As such the screw is subjected to considerable strains while performing its function. From the first principles of machine design the two parts, that is the core and the thread, should be of equal strength. The strength is determined by its diameter, but the thread, which is subjected to wear, should be stronger then is theoretically necessary.
A screw must bear a certain load, and the only place where such a load can be successfully taken is the flank of threads. A screw of good mechanical quality must have flank contact; its threads must bear on the flanks and may with convenience clear at the root and the crest.
It would appear a simple matter to obtain flank contact, but this is affected by the most common of thread troubles, namely the pitch error.
It is usual when thinking of screws, to regard errors of diameter as being solely errors of dimensions at right angles to the axis, and the pitch error as being one of length alone. This is correct when full or core diameters are referred to, but by effective diameter is meant the size of a screw at a point on the flank halfway between the full and the core diameters. If the flank is displaced axially, an increase in the effective diameter occurs due to the pitch error.
The combination of the effective diameters of screw and nut determine the amount of flank contact between the two, and the flank thickness of the threads halfway down the threads is the determining factor in the strength of the component to resist load – in other words it is the mechanical quality of the thread, or the amount of the metal that must give way when the thread strips under load.
The simple effective diameter controls the mechanical strength of the thread, the compound effective diameter defines its virtual size over a given length of engagement – it decides the assembling property of the thread.
The screw with pitch error bears on the flanks at each end and clears at the middle. All metals can be deformed, however, and the threads of each screw and nut, when tightened together, are distorted till flank contact takes place. This deformation is permissible if it takes place within the elastic limits of the related material, but its exceeding the above limit will render the threads which are plastically deformed, practically useless. Even with elastic deformation, more load is borne by threads which come into contact first. It is in view of this statement that the control of pitch error, which in its turn, defines the necessary distortion prior to flank contact; becomes essential.
Incidentally, it may be stated that an excessive length of engagement does not improve the mechanical strength of the nut. For a given pitch error only a particular length of engagement will satisfy the above conditions. Any superimposed engagement will bring about a plastic deformation of an equal length of the same from the starting end, thereby bringing the effective engagement to its original value.
Another point worthy of consideration is that due to pitch error, the flank contact on the opposing ends of a nut is not on the same side of the thread section, The effect is that when the bolt or stud is thrown into tension, only contact on one end is effective in resisting load, the other end relieving its contact under the stress. As such, the first thread must deform equal to the maximum axial clearance between any pair or flanks, before all the threads make contact.
The only way of assembling screw pairs with pitch error is by reducing the simple effective diameter, (for 550 threads) by twice the total pitch error over the given length of engagement. This is brought about by:
1. Reduction of the effective diameter as a result of the reduction of the core diameter of the screw (or vice verse for the nut), while maintaining the original form of the thread.
2. By thinning the thread, in which case the profile of the groove will be wider and different from that of the thread.
Cutting with a vee tool, the first can be brought about by an extra radial feed of the tool, whereas for the second, an axial displacement of the tool relative to its original position is made.Forge limitations
The blacksmith’s job is subject to certain limitations. A few of them are:
1. The rough material for the smith generally comes from the rolling mill, where the most convenient sections are square and round in the bar form. On special requisitions, rectangular sections may also be obtained. As such, the forging process has to be based on a bar of the above sections, as the starting point.
2. The easiest operation in hot shaping is the squeezing of metal between two parallel flat surfaces, either by percussion or by steady pressure. Therefore, a sequence of operations which consistently employ this mode of shaping will be economical and convenient.
Note: Rounding blocks of suitable size, too, are not costly.
Besides, fullers of various shapes can be cheaply and effectively employed, (usually for jobbing work).
3. Intricate blocks are both costly and weak, and should only be resorted to when the economy obtainable in production by their use over compensates their initial cost.
4. Material is weakened by upsetting. Hence, unless there exists some special advantage in producing by upsetting, as for example in bolt manufacture (where the bar employed gives the correct size of the body without further machining), the process of drawing down should be preferred. It has been established that a scientific execution of the drawing down process makes the material more compact and strong.
5. Metal on squeezing is displaced in two directions perpendicular to the line of squeezure, more in the direction of the grain and less across it. Again, the displacement depends upon the mode of squeezure. If, for example, the force is localized as by the intervention of a fuller, most of the metal will tend to be displaced across the fuller. To this, a third condition of constraint may be added, In the case of a hoop for instance, if the metal is locally heated and squeezed, it will not be displaced along the grain to any appreciable extent (assuming the hoop to be formed by bending); but practically all of it, will undergo a lateral displacement. This is due to the resistance offered by the cold metal to elongation along the grain. As a rule, when hot metal comes into conflict with cold, the latter always wins. So it will be clear, that under a unidirectional squeezure, the amount of displacement in a particular direction depends upon more than one factor. To anticipate such an amount of displacement is a matter of experienced judgment.
Note: The case is different, however, in rolling, where squeezure fixes the cross section positively, and as such, the third dimension that is the length of the rolled bar, can be anticipated.
6. It is possible to forge a particular cross section into another cross section of the same area, even neglecting the burning loss if length is constrained, because the material will flow along the length and that too considerably: As such, the area of the billet cross section should be greater than the maximum cross section desired in the finished work, if the maximum cross section desired in the finished work, if the operation has to be finished by drawing down only. When, however, a billet of equal or reduced cross section has to be used, up-setting at a certain stage will become necessary. As a buckling tendency will develop when upsetting with reduced cross section and increased length, it is advisable to do the upsetting with the greatest cross section and the minimum length.
7. A change of cross section occurs only when the central core of the metal is displaced. If the blows are light, and most of the central core is not touched, then the cross section can undergo a change of shape only, the area remaining practically the same. This effect is noticeable in rounding an octagonal cross section at full heat, with light blows. After rounding up, the diameter of the finished job actually exceeds the distance across octagon flats. (The area of the inscribed circle of the octagon is less than the area of the circumscribing octagon by the amount left in the corners, a superficial flow of which the finished diameter, greater than the diameter of the inscribed circle, that is, greater than the distance across octagon flats). .
Note: The choice of the octagon was purely arbitrary, the same effect will be noticed with any other cross section, under similar conditions.Forging design
Perhaps the most important point in designing a particular forging is the avoidance of thin sections. Thin sections chill quicker than thick ones, and as they chill they offer more resistance to the hammer blow and prevent proper forging of the sections which are still hot enough to forge. This may result in forgings which are both oversized and overweight. Then thin sections in the forging necessitate raised sections in the die and these wear quickly.
Variations in the thickness of sections result in uneven strains during forging, and may even cause trouble when the pieces are hardened.
While flash clearance in the dies is provided primarily to carry away excess metal, it also greatly influences the filling of the die impression. Too thin a flash chills too quickly, and holds the dies apart so that the impression cannot be properly filled. Then, the resistance of the metal flowing out through thin flash clearance after the impression fills results in over sized forgings. When the flash is too thick, it allows the metal to flow out before the impression fills. Incidentally, it also throws more work on the trimming die. The designer will not always get the flash clearance correct in the first instance, but it can be changed to suit after a try out in the forge shop.
The gutter, which is an enlargement of the flash clearance, is usually on the top die, so that the bottom lies flat on the trimming die. The gutter is employed to receive excess metal, the same as the flash; but if only as deep as the flash, it would be so wide that there would not be enough striking face for the die. It is customary to employ a flash clearance on the finishing impressions only, and not on the break-down or blocker impressions. The breakdown varies so much that there is no given formula for figuring it exactly. The usual method is to divide the forging into sections a quarter or half or an inch apart. The cross sectional area is figured at each of these and the breakdown is made the same area with a slight safety allowance. The excess metal flows out into the flash. The metal will stretch endways when being broken down, the amount of this stretch being determined by experience.
Note: The breakdown in dies is resorted to when the manufacture is straight from the bar. If, however, the feed is derived from scrap material of irregular shape, a preliminary forging may be more conveniently substituted for the breakdown operation.
In case of a difficult forging, or forgings in quantity, a blocker or rougher impression can be sunk in the same die. This impression preshapes the metal in order to get the best results in the finished impression. It also takes much of beating from the finish impression, and thus increases the life of the latter.
The draft on forgings is an allowance added to the finished size to assist the forging in leaving the die. It varies from about 1 in 12, to about 1 in 8.
Replaceable inserts are advisable when the protruding parts are comparatively thin in section. They are liable to excessive wear and even breakage, and the replacement of the whole die is a costly affair.
Parting line, usually comes in the centre for symmetrical parts, but it is more a matter of convenience. In details where the job is finished by turning it between the dies, the operator should be able to do so conveniently between each successive contact of the blocks. In case of details, where certain shoulders have to be more accurately located, the parting line may be offset the centre.
As a general rule, the depth in either block of the impression should not exceed two-thirds of the width of the section at that spot.
Hints in die design.
1. It should be remembered that metal flows with the grain rather than across it.
2. It is necessary to use a large enough die block, so that the forge cavity is well supported, and there should be enough flat surface left on the die block to withstand continued blows without sinking.
3. When the forging has filled, both the upper and the lower impressions and the excess metal forced into the flash, the dies will seemingly pound together, but the forging will measure larger than the combined upper and lower depths. What happens is that the resistance to the flow of the metal is sufficient to bounce the upper die and leave the forging too large. It is then necessary to face off the dies to compensate for this over size if the manufacturing limits demand it.Design of forgings
4. Hard die steels do not wear or sink, but are liable to crack under heavy wedging action.
5. The greatest enemy of the die is heat. The sharp corners are exposed to more of the heat and have less area to carry it away. When they become softened they easily wear out, or even get upset.
6. Two other important factors which influence the life of the die are shock and abrasion. Shock in the result of pressure in forging machines and blows in a hammer. Abrasion is the wearing of the die as the metal rubs past it. This wearing, or wash of the die as it is called, makes deep grooves at points of most constant contact.
7. An important consideration in die design should be a convenient ejection of the finished piece on the completion of the forging operation.
8. In carving out the die, it should be seen that the fiber runs along the line of greatest stress. In a die which is long and narrow or oval shaped, the greatest stress is usually across the narrowest part.
9. Obliquity of the face of the die leads to a kicking over tendency during contact of the blocks. This may be avoided by the provision of a proper kicking step, which will resist the lateral component of the blow. A better method of overcoming kicking over is to balance the dies by making a double stamping in the same block.
10. It is a generally recognized rule that high parts such as pegs or bosses be placed in the top die, while parts of the stamping which require consolidating and spreading out may be placed in the bottom die.
11. In rounding blocks, where work is finished by rolling it between the blocks, the joint ends of either block-cavity should to well rounded off, so that the fin formed in the initial stages is triangular in shape and can be successfully forced into the parent metal without folding.
12. In forked stampings, such as spanners, it is advisable to leave a thin connecting piece in the fork cavity which can then be removed in the trimming operation. This will control the profile and size of the fork end better, and will obviate the risk of cracking during stamping.
Note: Flash has an important function inasmuch as it serves as a cushion between the blocks during final blows, and such, reduces shock.Die material.
For large stamps of fairly plain shape, high-carbon steel is generally used in the normalized condition. But, for dies of intricate shape, and for such dies which place great strain on the steel owing to the way in which the metal of the stamping has to be forced into them, alloy steels are used. These are heat-treated and oil-toughened after the dies have been cut.
Note: Hardness in a die resists abrasion and sinking, and thus maintains size and shape of the mould. Toughness, on the other hand, resists shock, and thus prevents the die from cracking during pounding. As the two properties are in opposition, in practice a compromise has to be sought.
The Drop Forger’s Association specifies the following materials:
%
Carbon steel
Nickel-chrome steel
Carbon
0.55-0.65
0.3-0.36
Silicon
0.3
0.3
Manganese
0.8
0.4-0.7
Nickel
1.0
3.0-3.6
Chromium
-
0.6-1.0
Sulphur and Phosphorus
0.05 maxm.
0.05 maxm.
Brinell
196-255
269 maxm.
Izod
10 ft. lbs. minm.
40 ft. lbs. (On piece from 3 " bar forged hardened and tempered)
Note: 1 Nickel chrome steel to be supplied oil hardened and tempered.
Note: 2 The steel for die blocks should be sufficiently forged to toughen it. The forging down may be carried to about 50-70% of the initial cross section.Hand forgings vs. Stamping.
As we have already seen, we cannot definitely anticipate the shape and dimension of work during forging when the squeezure is unconstrained, because the course of the metal flowing out cannot be positively controlled.
In the above method of working – the one generally followed in hand forging – the mode of manufacture is localized unidirectional squeezure assisted by constant gauging till the work is approximately brought to the desired shape and size. The degree of approximation will depend on the skill of the worker and the equipment he possesses.
If the limits of approximation are not close enough, (as is many a time the case) then the finish is obtained by cutting off or cutting out, as the case may be, (assuming that an allowance for this operation was left in the first instance). The cutting may be performed on the forge with the work hot, or it may be carried out by cold cutting tools.
It will be obvious that the above process is tedious and costly, and as such, not quite suitable for quantity production on an interchangeable basis.
If on the other hand, the metal be forged to fill a given cavity or mould, (while in the plastic state) completely, and if it be possible to take out the filling from the cavity without any further distortion the filling will correspond to the mould shape and dimension (with metal at the forging temperature).
A practical difficulty can arise in providing the exact amount of material to fill the mould completely. But, if the cavity, includes a receptacle of volume sufficient to carry some excess material, within limits; and if the initial volume of the metal forced into the mould be deliberately kept somewhat in excess of the volume of the parent cavity, within the same limits, a complete impression of the parent mould on the metal can always be ensured provided the excess enters its corresponding receptacle only after filling the parent mould. If, we could provide means of conveniently trimming off the excess material from the filling proper, the result would be a filling corresponding to the parent mould only.
Now, if the mould cavity were a female impression of a particular product, our filling would be the product itself; produced as regards shape and size within definite limits – definite inasmuch as the same limits would be obtainable on all the pieces turned out by the mould, so long as the initial conditions could be maintained. Discrepancies are, of course, liable to occur due to various causes, but what is important is that these can be allowed for and as such, their limits controlled.
It will be noted, however, that a mould of the above nature is a costly single purpose tool. Again, its function usually calls for some special equipment to manipulate it. As such, this procedure of producing by forcing metal into moulds, or stamping as it is usually called can only be economically employed when there is quantity production. If the number of pieces required be small, it will usually be found cheaper to produce them by hand forging, even after including the extra finishing costs.
Upset forging
With long, thin bodied stampings which have only a small length of an enlarged section, it is not economical to produce by drawing down from the largest section. The work that would be involved in such a structure in the finished work does not justify it.
In such cases, it has been found more economical to manufacture such details from a section which the details embody for their part; the enlarged section being formed by a local upsetting operation. Some important applications of this method of manufacture are found in the making of bolts, rivets and other work of this family.
It has found that upsetting is better done with steady pressure than with percussion. In upsetting, the line of pressure is along the length of the work. Again, the material used in this mode of manufacture is usually in the bar form – which for convenience in handling is worked with its length horizontal. Therefore from the above considerations, at least, an upset forging machine should, in the first instance, be in the form of a press, mechanical or hydraulic and its line of pressure should be horizontal.
The drop stamp is not an appropriate appliance for such a purpose; firstly because it forges the metal by percussion and not by steady pressure, and secondly due to the very nature of its source of power, namely gravity, it cannot give a horizontal line of pressure.Hydraulic Forging
The useful function of drop stamps are chiefly limited to producing intensified surface pressures, which are admirably calculated to cause the metal to take fairly sharp impressions on comparatively thin sections; or to deforming blocks of steel, where the volume of the metal is not large. But the outstanding deficiency of the drop stamp is its lack of penetrating power. The energy expended by the fall of the drop is principally absorbed on the surface of the metal operated upon; hence in cases where the actual displacement of the metal is large, the drop stamp is not a suitable tool.
A hydraulic forging press, on the other hand displaces the metal to the very core (that it is so, will be observed from the convex shape of the end of a pressed bar; under light percussion the end is usually concave). As such, the press can be effectively employed where a considerable displacement of the metal is necessary. There exists a large and comparatively undeveloped field for the applications of the hydraulic forging press, in another direction also, that is as an adjunct to the drop stamp battery – the press being used for penetrative displacement, and the drop stamp for completing the surface formation.
Other uses of the hydraulic press, in forge operations will be found in heavy piercing, punching, drawing, extrusion etc.Piercing
Piercing implies the production of a cavity in plastic metal by its actual flowing out under pressure – no metal being removed. Cases frequently occur in stamping, where through or partial cavities have to be incorporated. One way of producing these cavities is to carve them out with cold cutting tools, after the hot operation is over. If, however, they could be produced in the hot operation itself, it would be more economical, and therefore desirable. The through cavies can either be punched or pierced. As for the blind ones, the usual method is piercing.
Coming back to the through cavity, it will be noted that punching involves the actual removal of material, which is mostly wasted. This waste is considerable when the size of the cavity is comparatively large. So a substitution of piercing in such cases, will lead to economy. Another incidental advantage of piercing is that when metal is forced out in this manner, it takes a better impression of the mould.
A complete piercing of a through cavity will bring the piercing punch end to end against some hard part of the mould, and as such, will involve a potent risk of damage to the tools of the machine, with even small discrepancies in the travel of the punch. To guard against this risk, it is usual to stop piercing short of the complete length of the hole, and the disc of the scrap left is subsequently punched out.Castings vs. forgings
Certain materials, such as cast iron, can only be cast. Again, certain shapes, such as steam housings with intricate passages, fall naturally in the domain of castings. But there are details, made of material such as mild steel, which lend themselves to both casting and forging equally well. Examples of these may be found in brackets, bell-crank levers, handles etc.
Now, materials like mild steel cannot be easily cast. Again, the soundness of the casting, for stress-carrying members, cannot be very much relied upon – there always being the imminent risk of holes, sponginess etc. In forging, on the other hand, even the existing defects of the billet are removed, to a reasonable extent, the material being rendered more compact. Also, in forgings, dealt with in a scientific manner, it is possible to make the material grain run along the stress lines. Thus, a forging may be expected to withstand stresses better than a casting. Hence, if a forging costs the same as a casting, or even slightly more, it would be advisable to give it preference.Flame cutting
Theory
If a jet of oxygen be directed upon wrought iron or mild steel previously well cleaned and heated to redness, the metal will ignite and acting as its own fuel, it will burn away rapidly, forming iron oxide.
Theoretically, this burning should continue so long as sufficient oxygen is supplied, without additional heat from outside; but the oxide, unless removed will stop the action. Fortunately, the oxide melts at a lower temperature than wrought iron or mild steel, and being in liquid form when the metal is molten is blown away by the oxygen jet provided that sufficient heat is applied to keep it fluid.
The process of cutting by gas is only really economical when the oxide has a melting point lower than that of the metal. High carbon steels have appreciably lower melting points than mild steel and in the neighborhood of the melting point of the oxide, do not cut very well, and cast iron can only be cut with great difficulty and with a relatively large gas consumption.
Most of the other metals including copper, brass, bronze and aluminium, do not burn in oxygen in the manner described and therefore cannot be cut by gas.
A special blowpipe is used for cutting, with two orifices, in the nozzle – one in the centre for the oxygen igniting jet, and the other, an annular orifice, for the oxy-acetylene heating flame.
As far as the heating flame is concerned, the blow pipe is of the injector type, and it can be used, on both low pressure and high pressure systems (low pressure in Jamalpur). The central oxygen jet is separately controlled.Scope of the process
Introduction
In the fabrication of structures, and construction of machine parts, cutting off or cutting out often comes in. Then large sized stocks need to be cut into smaller pieces; while in certain modes of repair, for example boiler work, cutting is sometimes preferred to dismantling.
We may further analyze the subject as:
1. Straight line cutting; usually of plates, bars or other rolled sections to convenient dimensions prior to their shaping or direct fabrication.
2. Cutting to definite profiles demanding a certain degree of accuracy, and a certain quality of surface left after the cut.
Until not very long ago, we were using saws, shears, drilling machines (assisted by slotters), set under percussion, for the former type of cutting; and milling machines, shapers etc. followed by swaging for the latter.
Note: in fact in many cases, the mode of forging which eliminated cutting out had to be deliberately arranged.
For internal cutting, the operation of power punching was, and still is, employed.
Now we shall study the possibility of flame cutting being applied to these operations. For this purpose, we shall discuss the limitations of the process itself in the first instance. The limitations to be considered are:1. Depth of cut.
2. Width of cut.
3. Quality of surface finish.
4. The guidance of the cutter.
5. The nature of the power supplied.
6. Cost etc. of the machines itself.1. Depth of cut.
As indicated in the pamphlet of the British Oxygen Co., machines have been manufactured to give a cut as deep as 15". This covers the entire range of work likely to be met in locomotive practice, even including the deepest parts of the rolled sections. The most important point in this connection is that the machine can conveniently take a cut over irregular sections, both rolled and forged (when set for the greatest depth). A plane shear fails here, and cold saws (when heating is not desirable or when large capacity saws are not available) will be slow and of insufficient range.
It will be noted that this cutting operation does not depend upon the usual wedge action – no cutter being in contact with the job itself. As such, there is no necessity for a positive grip to withstand the reactions of the cut, nor are any massive sections needed in the machine construction to bear the strain of the cut.
2. Width of the cut.
This may be considered as regards the amount of the material wasted in the cut and the uniformity with which the cut is maintained. The usual width of the cut, taken on universal machines is 1/8" – 5/32", and this with self centering nozzles will give a fairly square cut, say 1/32" on a 6" depth.
3. Surface finish.
As observed, the surface regularity approaches the one on a neat stamping – machining for ordinary profiling alone, not being necessary.
4. Guidance of the cutter.
As observed, the type of guide employed on vertical machines, cannot introduce any appreciable error in the surface quality obtained from the cutter itself. (Provided, of course, that the work is leveled prior to cutting).
There are no slides on the machine, except for vertical adjustment – the nozzle being swung on an elbow mechanism moving on ball–bearings. There are no reactions transmitted to the machine, and the question of distortion or localized wear does not arise.
The Guide is universal, inasmuch as it follows any profile automatically when a template is available, the advancement of the cutting jet being actuated by a half-inch magnetized roller moving in contact with the template profile, at predetermined speed. Due to the cutter following its course automatically, the operator is left free to concentrate on the cut proper, specially in the vertical adjustment for irregular sections.
Note: The machine should be erected level, and at a place free from vibrations, to avoid gravity effects on the sensitivity movement of the arm.
5. Power and cost.
The cutter gets its power from the cutting oxygen, and the heating oxygen and acetylene. The former is obtained in steel cylinders at pressures of 120 atmospheres, and after throttling, is employed at pressures of 25-62 1bs. per sq. in. Acetylene is generated at low pressures from calcium carbide. The cutter is electrical derived from 220 V. mains. The Universal machine costs £ 364/- in England (pre-war).
6. Range of work
1. Straight line cutting. This type of work has to be carried out under varying conditions – favorable and awkward, and it is actually these conditions which influence the method of cutting.
Taking plate work for instance, thin template profiling is equally cheap, if not cheaper, on the standard profile slotter. For thicknesses up to ½", requiring straight trimming, shears are quite satisfactory. But, in awkward places, such as boiler cutting up, or other constructional work, flame cutting is preferable.
Greater thickness, such as 1" locomotive frame plate, can be conveniently profiled by this process. The important point here is that the plates are not subject to any bending or buckling during profiling, and as such, maintain their original level.
Sections up to 5” square are easily cut on cold shears. Larger sections for forging purposes are readily parted under the hammer when hot. But when the same sections have to be cut away from such equipment, portable flame cutting apparatus is the one most handy.
Similarly, rolled channel, tee, etc. sections for engineering work are best sized by flame cutting. Sometimes drilling at awkward spots may be replaced by a flame cutting cut.
But, the most important use to which is this process is put in a locomotive works, is its being an aid to forging. Intricate profiling on the forge can be substituted by slab production followed by flame cutting. In other cases, awkward welds are easily avoided by the assistance of this process.