9x3 . Where is the serial number normally located? Also I need instruction about how to use the gears etc and thread cut.
It has two drives. One threaded , one slotted. There is a thread counter/ index my father made. As a kid I remember using this lathe and cutting threads using the counter. But I can't remember enough... are manuals available?
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In general, if the spindle and leadscrew are geared together 1:1 the lathe cuts the same pitch as the leadscrew. For other pitches you need gear ratios the same as the pitch ratios. For example to cut 16 TPI with an 8 TPI leadscrew the leadscrew has to be reduced to 1/2 of spindle speed. Gears can't have too few teeth, so the gear for 13 TPI has 13 * 4 = 54 teeth and would be matched with 8 * 4 = 32 teeth.
A spreadsheet can be useful if you are reconstructing the gear train ratios.
The thread counter dial on my South Bend allows disengaging the half nuts to move the carriage quickly back to the thread start, without stopping the lathe. It's not really necessary, you can back out the bit and reverse the lathe instead.
The 1942 edition of "How to Run a Lathe" may be better than newer editions for a lathe from the 1920's: My 1965 South Bend preserves many relics of that age such as the option for overhead leather belt drive.