Why Radial Chip Thinning Happens
A milling tooth only cuts the full programmed chip thickness when it engages a workpiece across roughly half the tool's diameter or more. At lighter radial engagements, common in high-efficiency or trochoidal toolpaths, the tooth sweeps through a shallower arc and the actual chip it removes is thinner than the feed-per-tooth value programmed into the control. Left uncompensated, this means the tool is running at less than its safe chip load and could go faster.
Using the Compensation Factor
The chip thinning factor here is the ratio between actual chip thickness and programmed feed per tooth at a given radial engagement. Divide your target chip thickness by that factor to get the programmed feed per tooth that will actually deliver it, then multiply by flute count and RPM to get a usable feed rate in inches per minute.
Where the Effect Disappears
Once radial engagement reaches about half the tool diameter, the tooth is cutting close to its full programmed thickness and the compensation factor approaches 1, meaning no adjustment is needed. This is part of why slotting (full-width) cuts use the programmed feed per tooth directly while light high-feed passes need this correction to avoid leaving performance on the table.