Harold Hall

METRIC PITCH typically 18 = 1.8mm pitch

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.

Double Check

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 following formula.

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

Changewheel Setup

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

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 adjacent threads.

Metric

Screw Cutting a square thread lead screw

The lathe specification.

The Data is for a lathe having a lead screw pitch of 2mm and with changewheels of 20 to 75 teeth in increments of 5, plus a 21 tooth gear. 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 of gears it is very likely that there will still be a suitable combination using those available.