Tawas,
Original motor recommendations for the 10" was 1/3 or 1/2 HP, 1725 to 1750 RPM.
The speed range of the Atlas 10" and 12" machines when fitted with a nominal 1725 RPM motor is 28 to 2072 RPM. The speed range in direct drive is 164 to 2072. With back gears engaged, the speed range is 28 to 345 RPM. For most turning work on a 10" or 12", you wouldn't use back gears. However, it depends upon the diameter of the workpiece and the cutter speed you need for the material that you are working. For a table of cutting speeds versus diameter versus RPM, look in the MOLO around page 47 to 49, depending upon which edition you have. A couple of pages earlier, you will find a chart of various types of steel and other materials versus recommended cutting speeds with HSS (High Speed Steel) cutters.
Robert D.