Cutting Metric Threads on the Mini-Lathe

As supplied the Mini-Lathe is equipped to cut Imperial threads. There is a metric conversion kit available, consisting of a new leadscrew, and halfnut. However thanks to the work of quite a number of people, change gear combinations have been worked out that allow the cutting of metric threads with the standard Imperial leadscrew and change gear set.

The results are very close to the required thread sizes, well within the normal working tolerances. You will see from the table that the errors are very small.

Below is a table of change gear combinations that I have reformatted to be very similar to the change gear information that is stuck to the side of the Mini-Lathe

Notes:

A , B, C, D = Teeth on Changewheels
Convert Metric pitch to inch: MT x 0.03937 Result in MT_to_Inch
Calculate Inch TPI:1/MT_to_Inch Result in InchTPI
Calculate ideal ratio: Leadscrew pitch / Inch TPI Result in Ideal Ratio
Calculate actual ratio of gears: (A/B)x(B/C) Result in Actual Ratio
Calculate error: Ideal Ratio / Actual Ratio Result in Error