Fig. 238.
The return or backing cam obviously depends for its shape upon the forward cam, and the latter having been determined, the requisite form for the return cam may be found as follows. In [Fig. 238] let a represent the forward cam fastened in any suitable or convenient way to a disc of paper, or, what is better, sheet zinc, b. The cam is pivoted by a pin passing through it and the zinc, and driven into the drawing-board. A frame f is made to carry two rollers r and r′, whose width apart exactly equals the extreme length of the forward cam. The faces d d of the frame f are in a line with a line passing through the centres of the rolls r r′, and the cam is also pivoted on this line, so that when the four pins p are driven into the drawing-board, the frame f will be guided by them to move in a line that crosses the centre of the cam a. Suppose then that, the pieces occupying the position shown in the engraving, we slide f so that roller r touches the edge of cam a, and we may then take a needle and mark an arc or line around the edge of r′. We then revolve cam a a trifle, and, being fast to b, the two will move together, and with r against a we mark a second arc, coincident with the edge of roller r′. By continuing this process we mark the numerous short arcs shown upon b, and the crowns of these arcs give us the outline of the return cam. It is obvious that, while the edge of the cam a will not let roller r (and therefore frame f) move to the right, roller r′ being against the edge of the backing or return cam as marked upon b, prevents the frame f from moving to the left; hence neither roll can leave its cam.
Fig. 239.
We have in this example supposed that the frame carrying the rollers is guided to move in a straight line, and it remains to give an example in which the rollers are carried on a pivoted shaft or rocking arm. In [Fig. 239] we have the same cam a with a sheet of paper b fastened to it, the rollers r r′ being carried in a rock shaft pivoted at x. It is essential in this case that the rollers r and r′ and the centre upon which the cam revolves shall all three be in the arc of a circle whose centre is the axis of x, as is denoted by the arc d. The cam a is fastened to the piece of stiff paper or of sheet zinc b, and the two are pivoted by a pin passing through the axis e of the cam and into the drawing-board, while the lever is pivoted at x by a pin passing into the drawing-board. The backing or return cam is obviously marked out the same way as was described with reference to [Fig. 238].
Fig. 240.