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SCREWS. 417
SCREWS.
"Pitch," "Inch Pitch" and "Lead" of Screws and Worms.
The term " pitch of a screw," as commonly used, means its
number of threads per inch, while the " inch pitch " is the dis-
tance from the center of one thread to that of the next. For
instance, a one-inch screw of standard thread is usually said to
be an " eight pitch" screw, because it has eight threads per inch of
length ; but it might more correctly be said to be a screw of
>s-inch pitch, because it is >£-inch from the center of one thread
to the center of the next
The " lead " of a worm or a screw means the advancement
of the thread in one complete revolution ; therefore, in a single-
threaded screw, the inch pitch and the lead is the same thing, but
in a double or triple-threaded screw the inch pitch and the lead are
two different things. The " lead " in a double-threaded screw will
be a distance equal to twice the distance from the center of one
thread to the center of the next, but in a triple-threaded screw the
lead is three times the distance from the center of one thread to
the center of the next.
Screw Cutting by the Engine Lathe.
When the stud and the spindle run at the same speed
( which they usually do) the ratio between the gears may always
be obtained by simply ascertaining the ratio between the num-
ber of threads per inch of the lead-screw and the screw to be
cut.
Example.
The lead-screw on a lathe has four threads per inch and
the screw to be cut has 11)4 threads per inch ( one-inch pipe-
thread). Find the gears to be used when the smallest change gear
has 24 teeth and the gears advance by four teeth up to 96.
The ratio of the number of threads per inch of the two screws
is as 4 to 11^.
As the smallest gear has 24 teeth and the gears all
advance by four teeth, this ratio of the screws must be mul-
tiplied by a number which is a multiple of 4 and which, at least,
gives the smallest gear 24 teeth. For instance, multiply by 8
and the result is 8 X 11 J^ = 92 teeth for the gear on the lead-
serew ; 8 X 4 = 32 teeth for the gear on the stud.
Cutting riultiple=Threaded Screws or Nuts by the
Engine Lathe.
Calculate the change gears as if it was a single-threaded
screw of the same lead. Cut one thread and move the tool the
proper distance and cut the next thread.
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