Have decided this is a simple enough project that I can do.
But how many teeth should I put in the gear, in order to cut a variety of metric threads?
(The leadscrew is metric, 1.5mm/thread; Busybee Craftex 7x8)
It can be calculated, and the probable best reference would be _Machinery's Handbook_ -- but for metric threads, no single gear will do the job. The threading dials on metric lathes have a sliding gear cluster connected to the dial, and the threading chart tells you which gear to use for each thread pitch -- so it is a bit more complex than it at first seems.
Aside from that, you either have to cut each gear with sloped teeth to match the lead angle of the leadscrew, or make a mounting for the dial holder which holds the axis of the gear cluster parallel to the lead angle (and thus tilted somewhat relative to vertical on the lathe).
If you want an example, and have access to the old South Bend _How to Run a Lathe_ book, turn to the section on metric threading, that is page 85 (in my 1944 edition (edition 50)), and you will see the drawing of the gear cluster on the bottom of the page. For whatever pitch leadscrew that one has, the gears are 26, 28, 30, and 33 teeth. The dial itself is also more complex, with lots of marks with different letters from A through D, and the threading chart shows which letters may be used with which gear step for which pitch. There is not a clear enough close-up of the dial to make it totaly clear just what the division is, but it looks somewhat irregular.
So -- this is a bit more involved a project than it would be for an imperial leadscrew and imperial threads.
Good Luck, DoN.
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