eaded
to correspond with the pitch for which the tool is required, and then it
is fluted to form cutting edges, and is hardened. The hob is then used
like a milling cutter for forming the end of the thread tool. The tool
is sharpened by grinding on the top. The method of cutting a Whitworth
thread is, of course, similar to that followed for a U. S. standard or
V-thread, in that the tool is set square with the unthreaded blank and
at the same height as the lathe centers, in order to secure a thread of
the proper form. Care should be taken to turn the blank to the right
diameter so that the top of the thread will be fully rounded when the
screw is the required size.

[Illustration: Fig. 19. United States Standard Thread]

[Illustration: Fig. 20. Standard Sharp V-thread]

[Illustration: Fig. 21. Acme Standard Thread]

[Illustration: Fig. 22. Square Thread]

[Illustration: Fig. 23. Whitworth Standard Thread]

[Illustration: Fig. 24. Standard Worm Thread]

=Worm Threads.=--The standard worm thread has an angle of 29 degrees
between the sides, the same as an Acme thread, but the depth of a worm
thread and the width of the flat at the top and bottom differ from the
Acme standard, as will be seen by comparing Figs. 21 and 24. The whole
depth of the thread equals the linear pitch multiplied by 0.6866, and
the width of the thread tool at the end equals the linear pitch
multiplied by 0.31. Gages notched for threads of different pitch are
ordinarily used when grinding worm thread tools.

When it is necessary to cut multiple-threaded worms of large lead in an
ordinary lathe, difficulty is sometimes experienced because the
lead-screw must be geared to run much faster than the spindle, thus
imposing excessive strains on the gearing. This difficulty is sometimes
overcome by mounting a belt pulley on the lead-screw, beside the change
gear, and connecting it to the countershaft by a belt; the spindle is
then driven through the change gearing from the lead-screw, instead of
_vice versa_.

=Coarse Threading Attachment.=--To avoid the difficulties connected with
cutting threads of large lead, some lathes are equipped with a coarse
screw-cutting attachment. The arrangement of this attachment, as made by
the Bradford Machine Tool Co., is as follows: On the usual reversing
shaft, and inside of the headstock, there is a sliding double gear, so
arranged as to be engaged with either the usual gear on the spindle, or
with a small pinion