Milling speeds and feeds

RPM, feed rate and metal removal for end mills, with chip thinning.

Spindle speed is 12 × surface speed ÷ (π × diameter) in inch units, or 1000 × speed ÷ (π × diameter) in metric. When the radial width of cut is under half the diameter, the chip is thinner than the programmed feed per tooth; the chip-thinning factor raises the feed so the tool still cuts a real chip instead of rubbing. Starting speeds are conservative — use your tool maker's data when you have it.