A while ago I picked up a little Haighton Cadet 2.5" centre height modeller's lathe.
I have been fiddling about fixing up the machine, replacing the motor and generally renovating it and felt I could improve on the rather simple change-gear arrangement. In particular I wanted to add metric threading.
The Cadet has a chain of five or six gears that are driven from a permanent 24T on the spindle to a fixed spindle (carrying a 60T gear), one adjustable gear spindle which slides in a radial slot a fixed distance from the fixed-spindle (allowing only two sets of second-gears as standard, 50:40 and 30:60) that enables a range of lead-screw gears to be used. While the arrangement works OK, it requires a different lead-screw gear for each thread.
My simple mod it to fit an eccentric spindle in place of the sliding spindle to allow a greater range of second-gears, in particular a 37:47 metric translations pair.
The Cadet uses 30dp 14.5 PA gears with a 1/4" spindle hole . Bizarrely, the gears are coupled with a pair of holes & pins (fixed in the 60T) or, in the case of the 60:20 feed rate pair, permanently riveted together. Each lead-screw gear has an unusual boss just to add to the complexity!
I chose to stick with the holes and couple my gears with removable pins (the downside of this, is it limits the gear to a minimum 30T ) .
After some juggling with a spread-sheet I finally settled on eight additional gears (61, 60, 47, 45, 43, 37, 35 and a 20 with hub) that allow me to cut M2 - M16 and 0BA - 10BA at good accuracy , with a passable 30dp & 32dp as a bonus.
By good fortune the 61T gear will fit on the fixed spindle in place of the standard 60T (the 24:61 ration provides an alternative metric translation for some threads that require additional divisions/multipliers ( M12 x 1.75 and some BA sizes)
I'd like to thank Peter Colman, for making a superb job of hobbing my blanks in to gears.