Full resolution (JPEG) - On this page / på denna sida - Screws - International standard metric screw threads - To gear a lathe to cut metric thread when the lead screw is in inches - To calculate the change gear wihen cutting metric screws by an English lead-screw
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436 SCREWS
ard threads, simply by using a tap drill that is a little over size
(which usually is done anyhow and using a sizing tap having its
external diameter so much larger that the truncation or flat of the
top of its threads would be only one-sixteenth of the pitch in-
stead of one-eighth of the pitch as it is on U. S. Standard taps.
This would prevent the screws from squeezing either at the
top or at the bottom of the thread but it would take a bearing
against the slant side of the thread.
Such taps could then be made according to the following
formula:
0.1083
D 1
=Outside diameter at top.
D = Outside diameter at U. S. S. screw.
n = Number of threads per inch.
For instance: The outside diameter of a tap for a 1 inch screw
0.1083 .
8 threads per inch would be 1 + —g
— = I-OI35 lnch.
The flat on the top of the tap would then, of course, be only
0.0078 instead of 0.0156 inches.
To Gear a Lathe to Cut Metric Thread when the
Lead=Screw is in Inches.
Use two intermediate gears, one having 100 teeth and the
other 127 teeth, fasten these two gears together on the same hub,
and gear the 100-tooth gear into the gear on the lead-screw
and the 127-tooth gear into the gear on the stud (see Fig 3).
The lathe will then cut, practi-
cally, one-half the number of threads
per centimeter that it originally cut
per inch with a common intermediate
gear. For instance, the stud gear has
24 teeth, the screw gear has 48 teeth
and the lead-screw has four threads
per inch ; the lathe will then, with a
common intermediate gear, cut eight
fig. 3 .
"^^^ XS^V threads per inch, but by using such a
double intermediate gear as is shown
in Fig. 3 the lathe will cut four threads per centimeter, which is
the same as one-fourth times 10 and equals 2^ millimeters pitch,
which corresponds to a metric standard screw of 18 millimeters
diameter.
To Calculate the Change Gear when Cutting Hetric
Screws by an English Lead= Screw.
Divide 20 by the pitch in millimeters, and the quotient is the
corresponding number of threads per inch to which the lathe
must be geared.
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