=Operation of Special Crankshaft Lathe.=--The total equipment of this machine (see Fig. 27) is carried on a three-tool turret tool-block. The method of turning a crankshaft is as follows: A round-nosed turning tool is first fed into a cross stop as ill.u.s.trated in the plan view at _A_, which gives the proper diameter. The feed is then engaged and the tool feeds across the pin until the automatic stop lever engages the first stop, which throws out the feed automatically. The carriage is then moved against a positive stop by means of the handwheel. The roller back-rest is next adjusted against the work by the cross-feed handwheel operating through a telescopic screw, and the filleting tools are brought into position as at _B_. These are run in against a stop, removing the part left by the turning tool and giving the pin the proper width and fillets of the correct radius. If the crankshaft has straight webs which must be finished, two tools seen at _b_ are used for facing the webs to the correct width. During these last two operations, the crank is supported by the roller back-rest, thus eliminating any tendency of the work to spring.

[Ill.u.s.tration: Fig. 28. (A) Spherical Turning with Compound Rest. (B) Concave Turning]

After one pin is finished in the manner described, the back-rest is moved out of the way, the automatic stop lever raised, the carriage shifted to the next pin, and the operation repeated. The tools are held in position on the turret by studs, and they can be moved and other tools quickly subst.i.tuted for pins of different widths. This machine is used for rough-turning the pins close to the required size, the finishing operation being done in a grinder. It should be mentioned, in pa.s.sing, that many crankshafts, especially the lighter designs used in agricultural machinery, etc., are not turned at all but are ground from the rough.

=Spherical Turning.=--Occasionally it may be necessary to turn a spherical surface in the lathe. Sketch _A_, Fig. 28, shows how a small ball-shaped end can be turned on a piece held in a chuck. The lathe carriage is adjusted so that the pin around which the compound rest swivels is directly under the center a. The bolts which hold the swivel are slightly loosened to allow the top slide to be turned, as indicated by the dotted lines; this causes the tool point to move in an arc about center _a_, and a spherical surface is turned. Light cuts must be taken as otherwise it would be difficult to turn the slide around by hand.

[Ill.u.s.tration: Fig. 29. Spherical Turning Attachment for Engine Lathe]

Sketch _B_ ill.u.s.trates how a concave surface can be turned. The cross-slide is adjusted until swivel pin is in line with the lathe centers, and the carriage is moved along the bed until the horizontal distance between center _b_ of the swivel, and the face of the work, equals the desired radius of the concave surface. The turning is then done by swinging the compound rest as indicated by the dotted lines. The slide can be turned more evenly by using the tailstock center to force it around. A projecting bar is clamped across the end of the slide at _d_, to act as a lever, and a centered bar is placed between this lever and the tailstock center; then by s.c.r.e.w.i.n.g out the tailstock spindle, the slide is turned about pivot _b_. The alignment between the swivel pin and the lathe centers can be tested by taking a trial cut; if the swivel pin is too far forward, the tool will not touch the turned surface if moved past center _c_, and if the pin is too far back, the tool will cut in on the rear side.

=Spherical Turning Attachments.=--When spherical turning must be done repeatedly, special attachments are sometimes used. Fig. 29 shows an attachment applied to a lathe for turning the spherical ends of ball-and-socket joints. The height or radius of the cutting tool and, consequently, the diameter of the turned ball, is regulated by adjusting screw _A_. The tool is swung around in an arc, by turning handle _B_ which revolves a worm meshing with an enclosed worm-wheel. As will be seen, the work is held in a special chuck, owing to its irregular shape.

[Ill.u.s.tration: Fig. 30. Attachment for Turning Spherical End of Gasoline Engine Piston]

Another spherical turning attachment is shown in Fig. 30. This is used for machining the ends of gasoline engine pistons. The cross-slide has bolted to it a bar _A_ carrying a roller which is pressed against a forming plate _B_ by a heavy spring _C_. The forming plate _B_, which is attached to a cross-piece fastened to the ways of the lathe bed, is curved to correspond with the radius required on the piston end, and when the tool is fed laterally by moving the cross-slide, it follows the curve of plate _B_. The piston is held in a special hollow chuck which locates it in a central position and holds it rigidly.

In connection with lathe work, special attachments and tools are often used, especially when considerable work of one cla.s.s must be turned; however, if a certain part is required in large quant.i.ties, it is usually more economical to use some semi-automatic or automatic turning machine, especially designed for repet.i.tion work.

=Turning with Front and Rear Tools.=--In ordinary engine lathe practice, one tool is used at a time, but some lathes are equipped with tool-holders at the front and rear of the carriage so that two tools can be used simultaneously. Fig. 31 shows a detail view of a lathe in which front and rear tools are being used. These tools are of the inserted cutter type and the one at the rear is inverted, as the rotary movement of the work is, of course, upward on the rear side. This particular lathe was designed for taking heavy roughing cuts and has considerable driving power.

[Ill.u.s.tration: Fig. 31. Front and Rear Tools used for Roughing]

The part shown in this ill.u.s.tration is a chrome-nickel steel bar which is being roughed out to form a milling machine spindle. It is necessary to reduce the diameter of the bar from 5-7/16 inches to 3-3/4 inches for a length of 27 inches, because of a collar on one end. This reduction is made in one pa.s.sage of the two tools, with a feed of 1/32 inch per revolution and a speed of 60 revolutions per minute. The use of two tools for such heavy roughing cuts is desirable, especially when the parts are required in large quant.i.ties, because the thrust of the cut on one side, which tends to deflect the work, is counteracted by the thrust on the opposite side.

[Ill.u.s.tration: Fig. 32. Lo-swing Lathe for Multiple Turning]

Sometimes special tool-holders are made for the lathe, so that more than one tool can be used for turning different surfaces or diameters at the same time, the tools being set in the proper relation to each other. The advantage of this method has resulted in the design of a special lathe for multiple-tool turning.

=A Multiple-tool Lathe.=--The lathe shown in Fig. 32 (which is built by the Fitchburg Machine Works and is known as the Lo-swing) is designed especially for turning shafts, pins and forgings not exceeding 3-1/2 inches in diameter. It has two carriages _A_ and _B_ which, in conjunction with special tool-holders, make it possible to turn several different diameters simultaneously. At the front of this lathe there is an automatic stop-rod _C_ for disengaging the feed when the tools have turned a surface to the required length. This stop-rod carries adjustable stops _D_ which are set to correspond with shoulders, etc., on the work. The rod itself is also adjustable axially, so that the tools, which are usually arranged in groups of two or more (depending upon the nature of the work), can be disengaged at a point nearer or farther from the headstock as may be required, owing to a variation in the depth of center holes. For example, if it were necessary to feed a group of tools farther toward the headstock after they had been automatically disengaged, the entire rod with its stops would be adjusted the required amount in that direction.

[Ill.u.s.tration: Fig. 33. Lo-swing Lathe arranged for Turning a Steering Knuckle]

The gage _G_, which is attached to a swinging arm, is used to set the stop bar with reference to a shoulder near the end of the work, when it is necessary to finish other parts to a given distance from such a shoulder or other surface. The use of this gage will be explained more fully later. Cooling lubricant for the tools is supplied through the tubes _E_. The lathe shown in the ill.u.s.tration is arranged for turning Krupp steel bars. A rough bar and also one that has been turned may be seen to the right. The plain cylindrical bar is turned to five different diameters, by groups of tools held on both carriages.

[Ill.u.s.tration: Fig. 34. Plan View showing Method of driving Steering Knuckle and Arrangement of Tools]

=Examples of Multiple Turning.=--Figs. 33 and 34 show how a Lo-swing lathe is used for turning the steering knuckle of an automobile. Four tools are used in this case, three cylindrical surfaces and one tapering surface being turned at the same time. For this job, the four tools are mounted on one carriage. The taper part is turned by the second tool from the headstock, which is caused to feed outward as the carriage advances by a taper attachment. This tool is held in a special holder and bears against a templet at the rear, which is tapered to correspond with the taper to be turned. This templet is attached to a bar which, in turn, is fastened to a stationary bracket seen to the extreme left in Fig. 33. This part is finished in two operations, the tool setting being identical for each operation, except for diameter adjustments. As the ill.u.s.trations show, three of the four tools employed are used for straight turning on different diameters, while the fourth finishes the taper.

These pieces, which are rough drop forgings, are first reduced to the approximate size. When it becomes necessary to grind the tools, they are reset and those parts which have been roughed out are turned to the finished size. The average time for the first operation, which includes starting, stopping, turning and replacing the piece, is one minute, while for the second operation with the finer feed, an average time of two minutes is required. The work is driven by sleeve _S_, which fits over the spindle and is held in position by the regular driver, as shown. This sleeve is notched to fit the knuckle, so that the latter can easily and quickly be replaced when finished.

One of the interesting features of this job lies in the method of locating the shoulders on each knuckle, at the same distance from the hole _H_ which is drilled previously, and which receives the bolt on which the knuckle swivels when a.s.sembled in a car. As soon as the knuckle has been placed between the centers, a close-fitting plug _P_ (Fig. 33) is inserted in this hole and the indicator arm with its attached gage or caliper _G_ is swung up to the position shown. The stop-rod on which the stops have been previously set for the correct distance between the shoulders is next adjusted axially until the gage _G_ just touches the plug _P_. The indicator is then swung out of the way, and the piece turned. If the next knuckle were centered, say, deeper than the previous one which would, of course, cause it to be located nearer the headstock, obviously all the shoulders would be located farther from the finished hole, provided the position of the stops remained the same as before. In such a case their position would, however, be changed by shifting the stop-rod until the gage _G_ again touched the plug thus locating all the stops with reference to the hole.

As the adjustment of the stop-rod changes the position of the taper templet as well as the stops, it is evident that both the shoulders and the taper are finished the same distance from the hole in each case. The connection of the bracket (to which the templet arm is attached) with the stop-rod is clearly shown in Fig. 33. This bracket can either be locked to the ways or adjusted to slide when the stop-rod is moved.

[Ill.u.s.tration: Fig. 35. First and Second Operations on Automobile Transmission Shaft--Lo-swing Lathe]

The part ill.u.s.trated in Fig. 35 is an automobile transmission shaft. In this particular case, cylindrical, tapering and spherical surfaces are turned. The upper view shows, diagrammatically, the arrangement of the tools and work for the first operation. After the shaft is "spotted" at _A_ for the steadyrest, the straight part _C_ and the collar _B_ are sized with tools _S_ and _R_ which are mounted on the left-hand carriage. A concave groove is then cut in collar _B_ by tool _R_, after which spherical end _D_ is formed by a special attachment mounted on the right-hand carriage. This attachment is the same, in principle, as the regular taper-turning attachment, the subst.i.tution of a circular templet _T_ for the straight kind used on taper work being the only practical difference.

[Ill.u.s.tration: Fig. 36. Axle End turned in One Traverse of the Five Tools shown]

After the surfaces mentioned have been finished on a number of pieces, the work is reversed and the tools changed as shown by the lower view.

The first step in the second operation is to turn the body _E_ of the shaft with the tool _T_ on the left-hand carriage. The taper _F_ and the straight part _G_ are then finished, which completes the turning. It will be noted that in setting up the machine for this second operation, it is arranged for taper turning by simply replacing the circular templet with the straight one shown. When this taper attachment is not in use, the swiveling arm _M_, which is attached to a bracket, is swung out of the way.

The method of driving this shaft is worthy of note. A dog having two driving arms each of which bears against a pin _N_ that pa.s.ses through a hole in the spindle is used. As the ends of this pin, against which the dog bears, are beveled in opposite directions, the pin turns in its hole when the dog makes contact with it and automatically adjusts itself against the two driving members of the dog. The advantage of driving by a two-tailed dog, as most mechanics know, is in equalizing the tendency to spring slender parts while they are being turned.

[Ill.u.s.tration: Fig. 37. Lathe Knurling Tool having Three Pairs of Knurls--Coa.r.s.e, Medium and Fine]

In Fig. 36 another turning operation on a lathe of this type is shown, the work in this case being a rear axle for a motor truck. The turning of this part is a good example of that cla.s.s of work where the rapid removal of metal is the important feature. As the engraving shows, the stock, prior to turning, is 3-1/2 inches in diameter and it is reduced to a minimum diameter of 1-1/16 inch. This metal is turned off with one traverse of the carriage or by one pa.s.sage of the five tools, and the weight of the chips removed from each end of the axle is approximately 12 pounds. The time required for the actual turning is about 9 minutes, while the total time for the operation, which includes placing the heavy piece in the machine, turning, and removing the work from the lathe, is 12 minutes. The axle revolves, while being turned, at 110 revolutions per minute and a feed equivalent to 1 inch of tool travel to 60 revolutions of the work is used. It will be noticed that the taper attachment is also employed on this part, the taper being turned by the second tool from the left. As the axle is equipped with roller bearings, it was found desirable to finish the bearing part by a separate operation; therefore, in the operation shown the axle is simply roughed down rather close to the finished dimensions, leaving enough material for a light finishing cut.

=Knurling in the Lathe.=--Knurling is done either to provide a rough surface which can be firmly gripped by the hand or for producing an ornamental effect. The handles of gages and other tools are often knurled, and the thumb-screws used on instruments, etc., usually have knurled edges. A knurled surface consists of a series of small ridges or diamond-shaped projections, and is produced in the lathe by the use of a tool similar to the one shown in Fig. 37, this being one of several different designs in common use. The knurling is done by two knurls _A_ and _B_ having teeth or ridges which incline to the right on one knurl and to the left on the opposite knurl, as shown by the end view. When these two knurls are pressed against the work as the latter revolves, one knurl forms a series of left-hand ridges and the other knurl right-hand ridges, which cross and form the diamond-shaped knurling which is generally used.

If the surface to be knurled is wider than the knurls, the power feed of the lathe should be engaged and the knurling tool be traversed back and forth until the diamond-shaped projections are well formed. To prevent forming a double set of projections, feed the knurl in with considerable pressure at the start, then partially relieve the pressure before engaging the power feed. Use oil when knurling.

The knurls commonly used for lathe work have spiral teeth and ordinarily there are three cla.s.ses, known as coa.r.s.e, medium and fine. The medium pitch is generally used. The teeth of coa.r.s.e knurls have a spiral angle of 36 degrees and the pitch of the knurled cut (measured parallel to the axis of the work) should be about 8 per inch. For medium knurls, the spiral angle is 29-1/2 degrees and the pitch, measured as before, is 12 per inch. For fine knurls, the spiral angle is 25-3/4 degrees and the pitch 20 per inch. The knurls should be about 3/4 inch in diameter and 3/8 inch wide. When made to these dimensions, coa.r.s.e knurls have 34 teeth; medium, 50 teeth; and fine knurls, 80 teeth.

[Ill.u.s.tration: Fig. 38. Hendey Relieving Attachment applied to a Lathe]

The particular tool ill.u.s.trated in Fig. 37 has three pairs of knurls of coa.r.s.e, medium and fine pitch. These are mounted in a revolving holder which not only serves to locate the required set of knurls in the working position, but enables each knurl to bear against the surface with equal pressure. Concave knurls are sometimes used for knurling rounded edges on screw heads, etc.

=Relieving Attachment.=--Some lathes, particularly those used in toolrooms, are provided with relieving attachments which are used for "backing off" the teeth of milling cutters, taps, hobs, etc. If a milling cutter of special shape is to be made, the cutter blank is first turned to the required form with a special tool having a cutting edge that corresponds with the shape or profile of the cutter to be made. The blank is then fluted or gashed to form the teeth, after which the tops of the teeth are relieved or backed off to provide clearance for the cutting edges. The forming tool used for turning the blank is set to match the turned surface, and the teeth are backed off as the result of a reciprocating action imparted to the toolslide by the relieving attachment. The motion of the toolslide is so adjusted that the tool will meet the front of each tooth and the return movement begin promptly after the tool leaves the back end of the tooth.

[Ill.u.s.tration: Fig. 39. Relieving a Formed Cutter]

These attachments differ somewhat in their construction and arrangement but the principle of their operation is similar. Fig. 38 shows a Hendey relieving attachment applied to a lathe. A bracket carrying the gearing _A_ through which the attachment is driven is mounted upon the main gear box of the lathe, and the special slide _B_, which is used when relieving, is placed on the cross-slide after removing the regular compound rest. The gears at _A_ are changed to suit the number of flutes or gashes in the cutter, tap or whatever is to be relieved. If we a.s.sume that the work is a formed milling cutter having nine teeth, then with this particular attachment, a gear having 90 teeth would be placed on the "stud" and a 40-tooth gear on the cam-shaft, the two gears being connected by a 60-tooth intermediate gear. With this combination of gearing, the toolslide would move in and out nine times for each revolution of the work, so that the tool could back off the top of each tooth. (The gearing to use for various numbers of flutes is shown by an index plate on the attachment.) The amount of relief is varied to suit the work being done, by means of a toothed coupling which makes it possible to change the relative position between the eccentric which actuates the toolslide and the cam lever, thereby lengthening or shortening the reciprocating travel of the tool.

[Ill.u.s.tration: Fig. 40. Relieving Side of Angular Milling Cutter]

=Application of Relieving Attachment.=--Some typical examples of the kind of work for which the relieving attachment is used are shown in Figs. 39 to 42, inclusive. Fig. 39 shows how a formed milling cutter is relieved. The toolslide is set at right angles to the axis of the work, and the tool moves in as each tooth pa.s.ses, and out while crossing the s.p.a.ces or flutes between the teeth. As the result of this movement, the tops of the teeth are backed off eccentrically but the form or shape is the same from the front to the back of the tooth; hence, a cutter that has been relieved in this way can be ground repeatedly without changing the profile of the teeth, provided the faces are ground so as to lie in a radial plane.

When relieving, the cutting speed should be much less than when turning in order to give the toolslide time to operate properly. A maximum of 180 teeth per minute is recommended, and, if wide forming tools are used, it might be advisable to reduce the speed so low that only 8 teeth per minute would be relieved. It is also essential to use a tool having a keen edge, and the toolslide should work freely but be closely adjusted to the dovetail of the lower slide. Before beginning to back off the teeth, it is a good plan to color the work either by heating it or dipping into a strong solution of copper sulphate. This will enable one to see plainly the cutting action of the tool in order to stop relieving at the proper time.

[Ill.u.s.tration: Fig. 41. Relieving a Right-hand Tap]

Fig. 40 shows a method of relieving the teeth of an angular cutter. For an operation of this kind the toolslide is swiveled around at right angles to the side that is to be relieved. By the use of an additional universal joint and bearing to permit the toolslide to be swung to a 90-degree angle, the teeth of counterbores, etc., can be relieved on the ends. When the attachment is used for relieving inside work, such as hollow mills and threading dies, the eccentric which controls the travel of the toolslide is set so that the relieving movement is away from the axis of the cutter instead of toward it. This change is made by the toothed coupling previously referred to, which connects the cam lever and oscillating shaft, the latter being turned beyond the zero mark in a clockwise direction as far as is necessary to obtain the desired amount of travel. For internal work it is also necessary to change the position of the opposing spring of the toolslide, so that it will press against the end of the slide and prevent the tool from jumping into the work.

[Ill.u.s.tration: Fig. 42. Relieving a Hob having Spiral Flutes]

Fig. 41 shows how a right-hand tap is relieved. The ordinary practice is to first set the tool the same as for cutting a thread. The motion of the toolslide is then adjusted so that the tool on the forward stroke will meet the front of each tooth, and start back as soon as the tool leaves the end of the land or top of the tooth. Taps having a left-hand thread can be relieved by two different methods. With the first method the cut starts at the cutting edge of each tooth, and ends at the "heel," the tool moving in toward the center of the work. With the second method, the cut begins at the heel and discontinues at the cutting edge, the tool being drawn away from the work during the cut.

When using the first method the tap must be placed with the point toward the headstock, the shank end being supported by the tailstock center.

This is done by providing an extension or blank end at the point of the tap long enough to hold the driving dog. With the second method, the tap is held between centers the same as one having a right-hand thread, but the travel of the toolslide is set the same as for inside relief.

=Relieving Hobs or Taps Having Spiral Flutes.=--With this attachment, taps or hobs having "spiral" or helical flutes can also be relieved. (A spiral flute is preferable to one that is parallel to the axis, because with the former the tool has cutting edges which are square with the teeth; this is of especial importance when the lead of the hob or tap thread is considerable.) When relieving work having spiral flutes (as ill.u.s.trated in Fig. 42), the lead of the spiral and the gears necessary to drive the attachment are first determined. After the attachment is geared for the number of flutes and to compensate for the spiral, the lead-screw is engaged and the backing-off operation is performed the same as though the flutes were straight. The carriage should not be disengaged from the lead-screw after starting the cut, the tool being returned by reversing the lathe.

When gearing the attachment for relieving a tap or hob having spiral flutes, the gears are not selected for the actual number of flutes around the circ.u.mference but for a somewhat larger number which depends upon the lead of the hob thread and the lead of the spiral flutes. Let us a.s.sume that a hob has 6 spiral flutes and that the attachment is geared for that number. The result would be that as the tool advanced along the thread, it would not keep "in step" with the teeth because the faces of the teeth lie along a spiral (or helix which is the correct name for this curve); in other words, the tool would soon be moving in too late to begin cutting at the proper time, and to compensate for this, the attachment is geared so that the tool will make a greater number of strokes per revolution of the work than the actual number of flutes around the circ.u.mference.

With this attachment, the two gears listed on the index plate for the actual number of flutes are selected, and then two compensating gears are added, thus forming a compound train of gearing. The ratio _R_ of these compensating gears is determined as follows:

_r_ + 1 R = ------- _r_