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screws 433
Flat head screws have an included angle of 82 degrees, which
is a maximum angle for this style head, but a reduction of this
angle of not more than one or two degrees, due to the wear of tools
in their manufacture, may be tolerated.
Round heads, so called, are, however, in axial cross section, a
semi-ellipse, hence formula B and C cover all the practical details
for determining their form; B being the major diameter and C
the minor radius of the ellipse forming the head of the screw.
International Standard for Metric Screw Threads.
An international standard for metric screw threads was dis-
cussed at a congress which met for that purpose at Zurich, in
October, 1898. The form of thread adopted is based on the
Sellers thread, which it will be remembered has the shape of an
equilateral triangle truncated one-eighth of its height at top and
bottom.
To insure interchangeability, and to reduce the wear on taps
and dies, the congress recommended that the bottom of the
thread should be rounded off by any suitable curve, which should
not deepen the cut more than an amount equal to one-sixteenth
of the pitch beyond the standard Sellers type. The top of the
thread is to be left flat, as in the Sellers system. The standard
sizes and pitches decided upon are given in Table No. 72.
The taps to be used in this sys-
tem of threads, have a flat at the
bottom of the thread of one-eighth
of the pitch, and are truncated
one-sixteenth of the pitch on the
top of the threads. This will then,
as is seen in Fig. 2, give a clear-
ance between the nut and its screw,
both at the top and at the bottom of
the threads and secure a bearing
against the slant side of the thread.
Table No. 72 was calculated by
the following formulas: (See Fig. 2)
D = Diameter of screw in milli-
meters.
P — Pitch in millimeters Fig. 2
t = D — 1.299 P = Diameter of tap at the bottom of the
thread in millimeters.
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