ad Tool]

Very often the size of a threaded hole can be tested by using as a gage
the threaded part that is to fit into it. When making such a test, the
tool is, of course, moved back out of the way. It is rather difficult to
cut an accurate thread in a small hole, especially when the hole is
quite deep, owing to the flexibility of the tool; for this reason
threads are sometimes cut slightly under size with the tool, after which
a tap with its shank end held straight by the tailstock center is run
through the hole. In such a case, the tap should be calipered and the
thread made just small enough with the tool to give the tap a light cut.
Small square-threaded holes are often finished in this way, and if a
number of pieces are to be threaded, the use of a tap makes the holes
uniform in size.

=Stop for Thread Tools.=--When cutting a thread, it is rather difficult
to feed in the tool just the right amount for each successive cut,
because the tool is moved in before it feeds up to the work. A stop is
sometimes used for threading which overcomes this difficulty. This stop
consists of a screw _S_, Fig. 16, which enters the tool slide and passes
through a block _B_ clamped in front of the slide. The hole in the block
through which the stop-screw passes is not threaded, but is large enough
to permit the screw to move freely. When cutting a thread, the tool is
set for the first cut and the screw is adjusted until the head is
against the fixed block. After taking the first cut, the stop-screw is
backed out, say one-half revolution, which allows the tool to be fed in
far enough for a second cut. If this cut is about right for depth, the
screw is again turned about one-half revolution for the next cut and
this is continued for each successive cut until the thread is finished.
By using a stop of this kind, there is no danger of feeding the tool in
too far as is often done when the tool is set by guess. If this form of
stop is used for internal threading, the screw, instead of passing
through the fixed block, is placed in the slide so that the end or head
will come against the stop _B_. This change is made because the tool is
fed outward when cutting an internal thread.

[Illustration: Fig. 16. Cross-slide equipped with Stop for Regulating
Depth of Cut when Threading]

=The Acme Standard Thread.=--The Acme thread is often used, at the
present time, in place of a square thread. The angle between the sides
of the Acme thread is 29