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SCREWS. 419
flat at the top and bottom for a distance equal to one-eighth of
the pitch, thus the depth of the thread is only three-fourths of
a full, sharp thread. (See Fig. 1.)
Fig. 2 shows the shape of the Whitworth (the English) sys-
tem of thread. As compared © ^
with the American system, ^""".^^^,-552
the principal difference is in
the angle between the sides
of the thread, which is fifty-
five degrees, and one-sixth of
the depth of the full, sharp
thread is made rounding at
FlG " 2
the top and bottom. There is also a difference in the pitch of
a few sizes.
The common V-thread screws have the angle of thread of
sixty degrees, the same as the United States standard screws,
but the thread is sharp at both top and bottom. This style of
thread is rapidly, as it should be, going out of use. The prin-
cipal disadvantages of this thread are that the screw has less
tensile strength, and it is also very difficult to keep a sharp-
pointed threading tool in order.
Diameter of Screw at Bottom of Thread.
The diameter of screws at the bottom of thread is obtained
by the following formulas :
—
United States Standard Screws
:
n
For V-threaded screws
:
n
For Whitworth screws
:
d— D 1.281
n
d= Diameter of screw at bottom of thread.
D = Outside diameter of screw.
n = Number of threads per inch.
1.299 is constant for United States standard thread.
1.733 is constant for sharp V-thread.
1.281 is constant for Whitworth thread.
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