Fig. 1246.
In cutting a taper thread in a lathe, it is preferable that the taper be given by setting over the lathe tailstock, rather than by operating the cross slider from a taper-turning attachment, because the latter causes the thread to be cut of improper pitch. Thus, in [Fig. 1246] is a piece of work between the lathe centres, and it will be readily seen that supposing the lathe to be geared to cut, say, 10 threads per inch, and the length a of the work to be 2 inches long, when the tool has traversed the distance a it will have cut 20 threads, and it will have passed along the whole length of the side b of the work and have cut 20 threads upon it, but since the length of line b is greater than that of a, the pitch of the thread cut will be coarser than that due to the change wheels. The amount of the error is shown by the arc c, which is struck from d as a centre; hence from c to e is the total amount of error of thread pitch.
Fig. 1247.
But if the lathe tailstock sets over as in [Fig. 1247], then the pitch of the thread will be cut correct, because the length of b will equal the length of tool traverse; hence at each work revolution the tool would advance one-twentieth of the length of the surface on which the thread is cut, which is correct for the conditions.
| VOL. I. | METHODS OF BALL TURNING. | PLATE XIII. | |||
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| Fig. 1248. | Fig. 1249. | Fig. 1250. | |||
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| Fig. 1251. | Fig. 1252. | ||||
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| Fig. 1253. | |||||
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| Fig. 1254. | Fig. 1255. | ||||