Fig. 1263.
Fig. 1264.
It will be observed that this can be done in any lathe having a slide rest, and that the grooves cut in one piece will be an exact duplicate of that in the other, or guide groove, save such variation as may occur from the thickness of the tracer point, which may be allowed for in forming the guide or originating groove. From the wear, however, of the tracer point, and from having to move the cutting tool to take successive depths of cut, this method would be undesirable for continuous use, though it would serve excellently for producing a single cam. An arrangement for continuous use is shown in [Fig. 1263], applied to a lathe having a feed spindle at its back, with a cam g upon it. This cam g may be supposed to have been produced by the method already described. A tracer point h, or a small roller, may be attached to the end of the slide-rest and held against g by the weight w, which may be within the lathe shears if they have no cross girts, as in the case of weighted lathes. The slide-rest may be arranged to have an end motion slightly exceeding the motion, caused by the cam, of the tracer h. Change gears may then be used to cause the cam g to make one rotation per lathe rotation, cutting four recesses in the work; or by varying the rotations of g per lathe rotation, the number of recesses cut by the tool t may be varied. Successive depths of cut may then be put on by operating the feed screw in the ordinary manner. In this arrangement the depth and form of groove cut upon the work will correspond to the form of groove upon the cam-roller g; or each groove upon g being of a different character, those cut on the work will correspond. The wear on the cross slide will, in this case, be considerable, however, in consequence of the continuous motion of the tool-carrying slider, and to prevent this another arrangement may be used, it being shown in [Fig. 1264] as applied to a weighted and elevating slide rest. The elevating part of the slide rest is here pivoted to the lathe carriage at i, the weight w preventing play (from the wear) at i. A bracket j is shown fast to the elevating slide of the rest, carrying a roller meeting the actuating cam g. In this arrangement the cut may be put on by the feed screw traversing the slider in the usual manner, or the elevating screw k may be operated, causing the roller at the end of j to gradually descend as each cut is put on into more continuous contact with g as the latter rotates. The form of groove cut by the tool does not, in this case, correspond to the form on g, because the tool lifts and falls in the arc of a circle of which pivot i is the centre of motion, and its radius from i being less than the radius of g, its motion is less. But in addition to this the direction of its motion is not that of advancing and receding directly toward and away from the line of lathe centres, and the cam action is reduced by both these causes.
Fig. 1265.
The location of pivot i is of considerable importance, since the nearer it is to the line of centres the less the action of the cam g is reduced upon the work. As this is not at first sight apparent, a few words may be said in explanation of it. It is obvious that the farther the pivot i is from the tool point the greater will be the amount of motion of the tool point, but this motion is not in a direction to produce the greatest amount of effect upon the work, as is demonstrated in [Fig. 1265]; referring to which, suppose line a b c to represent a lever pivoted at b, and that end a be lifted so that the lever assumes the position denoted by the dotted lines d and e, then the end of c will have moved from circle f to circle g, as denoted by arc h; arm c of the lever being one-half the length of arm a b, and from circle f to circle g, measured along the line h, being one-half the distance between a and the end of the line d, the difference in the diameters of circles f and g will represent the effect of the cam motion on the tool under these conditions. Now, suppose a j is a lever pivoted at k, and that end a is raised to the dotted line d, then arm j, being one-half the length of a k, will move half as much as end a, and will assume the position denoted by dotted line l, and the difference in the diameter of circles f and m will represent the cam motion upon the tool motion under these conditions. From this it appears that the more nearly vertical beneath the tool point the pivoted point is, the greater the effect produced by a given amount of cam motion. On this account, as well as on account of the direction of motion, the shape of the actuating cam may be more nearly that of the form required to be produced in proportion as the pivoted centre falls directly beneath the tool point. But, on the other hand, the wear of the pivot, if directly beneath the tool point, would cause more unsteadiness to the tool; hence it is desirable that it be somewhere between points k and b, the location being so made that (b representing the pivoted point of the rest) the line b c forms an angle of 50° with the line b a. It is obvious that when the work is to be cam-grooved on a radial face the pivoted design is unsuitable, and either that in [Fig. 1262] or [1263] is suitable.
Similar cam motions may be given to the cross feed of a lathe: thus, the Lane and Bodley Company of Cincinnati, Ohio, employ the following method for turning the spherical surfaces of their swiveling bearings for line shafting.