Tenmetalman
The special gear for converting imperial screw-cutting to metric screw-cutting is a 100/127 tooth compound pair i.e. 2 gears fixed together on a common centre.
The stud gear drives the 127 and the 100 drives the gearbox input gear.
However you will also need a range of stud gears to get accurate conversions using a limited sub-set of the range of threads available on the imperial screw-cutting box.
For example the full set for the Heavy 10 gives all the ISO ratified pitches from 6 down to 0.20 mm using stud gears of tooth count 48, 44, 40, 36, 32, 28 and 26 and imperial TPI settings of 4, 8, 16, 32, 64 and 10, 20, 40, 80. If you are mathematically inclined you can also dig out some of the odd balls.
There are other gear sets able to give the necessary results using different combinations of gear box setting and drive gears. Some require the gearbox driver to be changed as well as the stud gear. The SouthBend set uses more gears than strictly necessary but if you look at the way the listing and setting pans out its pretty logical and easy to keep straight what's going on. Basically for all pitches divisible by 0.25 you leave the second lever in plunger hole 1 and start with the first lever in A, run down through the gears in order move lever to B and carry on. For the other pitches not divisible by 0.25 you put the plunger in hole 3 and work as before to fill in the gaps, obviously you don't use all possible combinations so there are two ways of generating some threads.
Some other makers set-ups can get hairy. Try telling the difference between a 0.55 mm pitch and a 0.65 mm pitch before the job is scrap when you have got a two gear change fuddled up!
Clive
