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SCREW CUTTING CHANGEWHEEL COMBINATIONS FOR A LATHE WITH AN IMPERIAL LEAD SCREW Harold
The Data is for a lathe having an 8TPI lead screw and with 14 changewheels of 20
to 75 teeth in increments of 5, plus gears of 21 and 38 teeth. The lists show that
in almost all cases there are many combinations that produce the same result, if
therefor the viewer does not have the full set it is very likely that there will
still be a suitable combination using those available.
Other lathe specifications
If your lathe is not supplied with gears in increments of five, or the leadscrew
is other than 8TPI, then the list can still be used but a little calculation will
be required as follows.
When the leadscrew is other than 8TPI, say 10TPI, multiply the required pitch by
8/10 and look up that value. Say a thread of 15 TPI is required, this will give 15
x 8/10 = 12 look up twelve and use one of the values listed.
If say your gears are in increments of four, then, typically, in the lists read 16
for 20, 20 for 25, 24 for 30, etc. and the ratios will be maintained.
If having carried out these calculations you may wish to be sure that you have made
the correct choices. You can always check your chosen values from basics using the
Resulting TPI = Leadscrew TPI x (N1 x N2 x N3)/(R1 x R2 x R3) where N and R are the
driveN and driveR gears.
Do take note that the numbers 1, 2 and 3 in no way indicate the order that the gears
are to be fitted. This will have to be chosen to ensure that the required clearances
are achieved, see explanatory drawing
Metric pitches on an Imperial lathe.
As the lists detail each value in terms of TPI the easiest way to use them for metric
threads is to convert the metric pitch to TPI. This is easy, just divide 25.4 by
the metric threads pitch and look up the resulting value. For example, take a required
pitch of 3mm. 25.4 divided by 3 gives a value of 8.4667TPI, look for this value in
the appropriate list. Not surprisingly, an exact match is not available but there
are 11 combinations that provide a value of 8.4656 TPI. This is an error of just
under two one hundredths of one thou between adjacent threads.