Theoretical values for estimating only. Confirm against the mill certificate or a stamped calculation before ordering or fabricating.
Cutting speed comes from the material and the tool coating; RPM is just arithmetic on top of it. Get the arithmetic right and tool life, surface finish and cycle time all follow.
From cutting speed to RPM
RPM = (cutting speed × 1,000) ÷ (π × tool diameter). A 20 mm cutter at 120 m/min runs at 1,910 rpm. Note that the same cutting speed gives a much higher RPM on a small tool.
From RPM to feed rate
Feed rate = RPM × number of teeth × feed per tooth. Feed per tooth is set by the tool supplier and depends on the material, the radial depth of cut and the rigidity of the setup.
Typical cutting speeds
Mild steel 100–150 m/min with carbide, stainless 60–100, aluminium 300–600, cast iron 80–150. Coated carbide and high-pressure coolant allow the upper end; HSS runs roughly a third of these figures.
What limits the number
Machine maximum spindle speed, tool runout, workpiece rigidity and chatter. If the calculated RPM exceeds the machine limit, reduce the diameter or accept a lower surface speed.
Frequently asked questions
How do I calculate RPM from cutting speed?
RPM = cutting speed (m/min) × 1,000 ÷ (π × diameter in mm). For 120 m/min and a 20 mm cutter that is 1,910 rpm.
What cutting speed should I use for mild steel?
100 to 150 m/min with carbide tooling, or about a third of that with high-speed steel. Start at the lower end and increase until chip colour and tool life tell you to stop.
What is feed per tooth?
The thickness of material each cutting edge removes per revolution. It is set by the tool manufacturer — typically 0.05 to 0.15 mm for a carbide end mill in steel.
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