Cutting speed and RPM calculator
Spindle RPM and feed rate for milling, drilling and turning, with starting cutting speeds for carbide and HSS.
How it is calculated
Cutting speed is how fast the cutting edge moves along the metal, in metres per minute. The spindle RPM that gives that speed depends on the diameter: a small tool has to spin faster.
Milling feed (mm/min) = feed per tooth × number of teeth × RPM
Drilling and turning feed (mm/min) = feed per revolution × RPM
Worked example
A 10 mm carbide end mill in mild steel at 120 m/min runs at 1,20,000 ÷ (3.1416 × 10) = 3,820 rpm. With 4 teeth at 0.05 mm per tooth, the table feed is 0.05 × 4 × 3,820 = 764 mm/min.
The cutting speeds in the calculator are common starting values, not limits. Coated grades, rigid machines and good coolant allow more; long overhangs and old machines need less.
Questions
How do I calculate spindle RPM?
RPM = cutting speed (m/min) × 1,000 ÷ (π × diameter in mm). For a 10 mm tool at 120 m/min that is about 3,820 rpm.
What cutting speed should I use for mild steel?
A common starting point is about 120 m/min with carbide and about 30 m/min with HSS. Adjust from your tool maker's chart and from the chips and finish you see.
Which diameter do I use for turning?
The diameter of the work at the point of cut, not the tool. As the part gets smaller, the RPM must go up to keep the same cutting speed.
How do I calculate milling feed rate?
Feed rate in mm/min = feed per tooth × number of teeth × RPM.
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