There is a simple way to cut metric threads on the South Bend 9C change gear lathe at no additional cost and little work. It is possible to combine two of the change gears to make a compound gear that is accurate to within two parts per thousand acting as a metric transposing gear. That is an accuracy of two threads per meter, close enough for most work.
Here is the setup: stud gear is 32 tooth driving 56 tooth screw gear. 56 tooth screw gear is compounded with 44 tooth screw gear. 44 tooth screw gear drives 80 tooth screw gear on the lead screw. This gives 25.45 threads per inch.
This pic is the change gear setup.
In this next picture are the compounded change gears. They are fastened together with a couple of 8-32 socket head cap screws with a washer between the gears for clearance in the train. I bolted them together and drilled two holes, then tapped one gear and screwed them together. It takes only a few seconds to separate them for normal use. Also in the pic is the bronze bushing I made for the gears to run on since the change gears have a different bore (7/16") than the idlers. Easy part to make, could be steel, or brass as well. Make it about .020 longer than the thickness of the change gear stack. The last item is a test piece, 12mm X 1mm threads.
Here is a chart showing the possible metric threads and the gears to use.
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