When boring a hole larger than the tool by helical interpolation, the tool center follows a smaller circle than the cutting edge. This finds the center path radius and the feed rate to program so the cutting edge runs at your intended feed.
Center path radius = (hole diameter – tool diameter) / 2. Programmed feed = desired edge feed x (center radius / hole radius).
How it works
G-code drives the tool center, not the cutting edge. For an internal hole, the center traces a smaller circle than the edge, so programming the edge feed rate directly into the control would run the edge faster than intended. Scaling by the radius ratio keeps the edge at the speed you actually want.
FAQ
Does this apply to circular pockets too? Yes, any time a tool is interpolating in a circle smaller than the finished feature, the same center-to-edge radius ratio applies.
Related Guides
CNC Calculators · Circular Interpolation Calculator · SFM Calculator