Part 7 (2/2)
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 finis.h.i.+ng 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_
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