spindle speed?

Feed, Speed, and Depth of Cut

Cutting speed is defined as the speed at which the work moves with respect to the tool (usually measured in feet per minute). Feed rate is defined as the distance the tool travels during one revolution of the part.

Cutting speed and feed determines the surface finish, power requirements, and material removal rate. The primary factor in choosing feed and speed is the material to be cut.

However, one should also consider material of the tool, rigidity of the workpiece, size and condition of the lathe, and depth of cut.

For most Aluminum alloys, on a roughing cut (.010 to .020 inches depth of cut) run at 600 fpm. On a finishing cut (.002 to .010 depth of cut) run at 1000 fpm.

To calculate the proper spindle speed, divide the desired cutting speed by the circumference of the work.

Experiment with feed rates to achieve the desired finish. In considering depth of cut, it's important to remember that for each thousandth depth of cut, the work diameter is reduced by two thousandths

Hopefully you may also find the attached useful? :-)

I don't think chip per tooth comes into play in the calculation.

RPM = SFM * π / 12 / Diameter

RPM = 250 * π / 12 / 0.75

RPM ≈ 1273