degrees (see Fig. 21) and the depth is made
equal to one-half the pitch plus 0.010 inch to provide clearance and
insure a bearing upon the sides. The thread tool is ordinarily ground to
fit a gage having notches representing different pitches. An improved
form of Acme thread gage is shown in Fig. 17. The tool point is first
ground to the correct angle by fitting it to the 29-degree notch in the
end of the gage, as at _A_. The end is then ground to the proper width
for the pitch to be cut, by testing it, as at _B_. The numbers opposite
the shallow notches for gaging the width represent the number of threads
per inch. With this particular gage, the tool can be set square by
placing edge _D_ against the turned surface to be threaded, and
adjusting the tool until the end is in line with the gage, as at _C_. By
placing the tool in this position, the angle between the side and the
end can also be tested.

[Illustration: Fig. 17. Gage for grinding and setting Acme Thread Tools]

In case it should be necessary to measure the end width of an Acme
thread tool, for a pitch not on the regular gage, this can be done by
using a vernier gear-tooth caliper, as indicated in Fig. 18. If we
assume that the caliper jaws bear on the sides of the tool at a distance
_A_ from the top, equal to 1/4 inch, then the width of the tool point
equals the caliper reading (as shown by the horizontal scale) minus
0.1293 inch. For example, if the caliper reading was 0.315 inch, the
width at the point would equal 0.315 - 0.1293 = 0.1857 inch, assuming that
the sides were ground to the standard angle of 29 degrees. The constant
to be subtracted from the caliper reading equals 2 _A_ tan 14 deg. 30' or,
in this case, 2 x 0.25 x 0.2586 = 0.1293.

[Illustration: Fig. 18. Measuring Width of Acme Thread Tool with Vernier
Gear-tooth Caliper]

=The Whitworth Thread.=--The Whitworth (or British Standard Whitworth)
thread, which is used principally in Great Britain, has an included
angle of 55 degrees, and the threads are rounded at the top and at the
root, as shown in Fig. 23. The shape of the tool used for cutting this
thread is also shown in this illustration. The end is rounded to form
the fillet at the root of the thread, and the round corners on the sides
give the top of the thread the required curvature. Every pitch requires
a different tool, and the cutting end is given the curved form by
milling or hobbing. The hob used for this purpose is accurately thr