How to use the spindle RPM calculator
Tooling catalogs publish surface speed, not spindle speed. Surface feet per minute describes how fast the cutting edge sweeps across the material, so the same SFM turns into a completely different RPM once the tool diameter changes. Enter the recommended SFM and the diameter and this tool returns the spindle speed and the matching feed rate.
The formula is RPM = 12 x SFM / (pi x D), commonly rounded on the shop floor to RPM = 3.82 x SFM / D with D in inches. Running 100 SFM on a 1/2 in cutter gives roughly 764 RPM. Table feed in inches per minute is then the chip load per tooth multiplied by the number of flutes and the spindle speed.
As starting points for carbide tooling, mild steel runs near 350 SFM, alloy steel near 250, stainless near 200, cast iron near 300, and aluminum around 1,000 SFM. High speed steel tooling typically runs at about a third of those numbers, while coated carbide with through-tool coolant can go higher.
Machine rigidity, tool overhang and depth of cut all shift the usable window, so treat the output as a starting recipe rather than a final answer. Watch for chatter, edge wear and surface finish and trim the speed or feed accordingly, and start below the calculated values on thin walls and long reach work.
Frequently asked questions
RPM equals 12 times the surface speed in SFM divided by pi times the tool diameter in inches, which shop floors shorten to 3.82 times SFM divided by diameter. At 100 SFM on a 0.5 in tool that works out to about 764 RPM.
For milling, enter the diameter of the rotating cutter. For turning, enter the diameter of the workpiece being cut and recalculate at each step, because surface speed falls as the part diameter shrinks.