When cutting threads on the lathe we need to look at the change gears and pitch. The change gears on the back of the headstock that connect the headstock rotation to the leadscrew rotation along with the leadscrew pitch determine the pitch you will cut.
The gears shown here on a Hobbymat MD65.
The spindle is driven with a belt and pulley system with an intermediate reduction pulley that can be seen on the left hand side of the image.
A gear is located on the spindle and this has a key to locate it on the main shaft.
The set of gears here have a large reduction as the system is set up for a fine traverse.
The pillar at the bottom of the image has a lever that operates the dog-clutch to engage and disengage the leadscrew.
The number of teeth on these four gears determines the ratio between the spindle and the leadscrew.
The pitch of the leadscrew determines how far the saddle moves for each rotation of the leadscrew.
The calculators have been gradually moving online rather than embedded in an excel workbook. Let us know whether this works better or if you would like both options.
The workshop calculations download now has a worksheet for change gears. This offers the online calculator as per above, but also a combinations calculator.
Enter the thread pitch you want to cut, leadscrew pitch and then 3 gear options for each of the positions in the change gear train. This then looks at all combinations and highlights the exact match in green. Matches with 2% are highlighted amber. This also finds options for TPI threads with a metric leadscrew and vice versa.