Speeds and feeds calculator

RPM and feed rate, with the chip thinning correction most charts leave out.

The formulas

RPM = (SFM × 12) ÷ (π × Ø)   imperial
RPM = (Vc × 1000) ÷ (π × Ø)   metric
feed = RPM × chip load × flutes

Radial chip thinning, and why light passes burn tools

When radial engagement drops below half the cutter diameter, the tooth no longer removes a chip as thick as the programmed feed per tooth. The chip thins, and if you do not raise the feed to compensate, the edge rubs instead of cutting. Rubbing generates heat without removing material, which work-hardens the surface and kills the tool.

factor = Ø ÷ (2 × √(ae × (Ø − ae)))

At 10% radial engagement on a 1/2" cutter the factor is about 1.67, so the feed should be roughly two thirds higher than the chart says. This is the single most common reason a finishing pass sounds wrong.

Every number here is a starting point. Surface speed depends on coating, coolant, rigidity and how worn the spindle is. Confirm against your tooling supplier's data and take a test cut.
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