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Milling speeds and feeds calculator

Spindle speed comes from the cutting speed the tool can take. Feed rate comes from the chip load each tooth can take. Here are the formulas, a worked example, and when to correct for chip thinning.

Spindle speed (RPM)

Cutting speed is how fast the cutting edge moves across the material. In inch shops it is given in surface feet per minute (SFM); in metric shops it is Vc in meters per minute.

3.82 is 12 ÷ π, which turns feet into inches around the circumference. Take the starting SFM or Vc from the tool maker's data for the material, then adjust from what you see and hear in the cut.

Feed rate (IPM or mm/min)

With chip load in inches per tooth the result is inches per minute (IPM); with mm per tooth it is mm/min.

Worked example

A 1/2" three-flute carbide end mill in 6061 aluminum at 800 SFM and 0.004" per tooth:

Radial chip thinning

When the width of cut (radial engagement, ae) is less than half the tool diameter, the chip is thinner than the feed per tooth. Running the book chip load then rubs instead of cuts, which wears tools and causes heat. Raise the feed per tooth to get the chip thickness you want:

Example: a 1/2" end mill at 0.050" width of cut (10% step-over). √(0.5 × 0.05 − 0.05²) = 0.15, so the factor is 0.5 ÷ 0.30 = 1.67. A 0.004" target chip needs about 0.0067" per tooth.

Metal removal rate

Multiply MRR by the material's unit power to estimate spindle horsepower, and check it against what the machine can deliver at that RPM.

Formulas give a starting point. Tool maker data, machine rigidity, workholding and stick-out decide the final numbers.