Other lathe specifications
If your lathe is not supplied with gears in increments of five, or the leadscrew
has a pitch of other than 2mm, then the list can still be used but a little calculation
will be required.
When the leadscrew is other than 2mm, say 3mm, multiply the required pitch by 2/3
and look up that value. Say a pitch of 1mm required, this will give 1 x 2/3 = 0.6666
look up this value and use one of the combinations 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. However, this
does not apply to combinations that have the 21 tooth gear in them.
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
Pitch Required = Leadscrew pitch x (R1 x R2 x R3)/(N1 x N2 x N3) where N and R are
the driveN and driveR. DO note this is different to when threads are considered in
terms of TPI where the formula has driven over drivers
Do take note that the numbers 1, 2 and 3 in no way indicate the order the gears
are fitted. This has to be chosen to ensure that the required clearances are achieved,
see explanatory drawing
Imperial pitches on a Metric lathe.
As the lists detail each value in terms of metric pitch the easiest way to use them
for Imperial threads is to convert the imperial TPI to a metric pitch. Just divide
25.4 by the TPI required and look up the resulting value. For example, take a requirement
of 10TPI . 25.4 divided by 10 gives a metric pitch of 2.54mm, look for this value
in the appropriate list. Not surprisingly, an exact match is not available but there
are 21 combinations that provide a value of 2.5397 mm an error of just 0.0003mm between