Part 7 (1/2)
=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 s.h.i.+fted 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 finis.h.i.+ng 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 mes.h.i.+ng 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 s.h.i.+fting 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.