This tracing point should be, if paper be used to trace on, a piece of the hardest pencil obtainable, and should be filed so that its edge, if flat, shall stand as near as may be in the line of motion when rolled, thus marking a fine line. If sheet zinc be used instead of paper a needle makes an excellent tracing point. Several of the curves, c, should be struck, moving the position of the generating segment a little each time.
Fig. 119.
On removing the segments from the paper, there will appear the lines shown in [Fig. 119]; a representing the pitch circle, and o o o the curves struck by the tracing point.
Fig. 120.
Cut out a piece of sheet zinc so that its edge will coincide with the curve a and the epicycloid o, trying it with all four of the epicycloids to see that no slip has occurred when marking them; shape a template as shown in [Fig. 120]. Cutting the notches at a b, acts to let the file clear well when filing the template, and to allow the scriber to go clear into the corner. Now take the segment a in [Fig. 118], and use it as a guide to carry the pitch circle across the template as at p, in [Fig. 120]. A zinc template for the flank curve is made after the same manner, using the rolling segment in conjunction with the segment b in [Fig. 115].