Cutting speed is the surface speed at the cutting edge, not simply the spindle RPM. RPM changes with tool or workpiece diameter. A small tool and a large tool can run at very different RPM values while using the same cutting speed.
The basic relationship
For metric milling and drilling work, the common relationship is:
RPM = cutting speed x 1000 / (pi x diameter)
Cutting speed is usually shown as meters per minute. Diameter is in millimeters. This formula explains why a smaller diameter tool needs higher RPM to reach the same surface speed.
Why RPM alone is not enough
RPM only tells you how fast the tool spins. It does not tell you chip thickness, cutting force, or heat. For milling, feed per tooth and number of flutes are just as important. For turning, feed per revolution and depth of cut strongly affect surface finish and tool load.
In practice, formulas give the starting point. The final setting must still consider material hardness, tool coating, holder rigidity, overhang, coolant, and whether the operation is roughing or finishing.
Example checks before changing parameters
- If chips are blue or the edge wears fast, cutting speed may be too high for the setup.
- If rubbing and built-up edge appear, cutting speed or chip load may be too low.
- If chatter starts, reduce overhang and improve clamping before only lowering RPM.
- If the machine load is high, reduce depth of cut, feed, or width of cut.
HEYI’s cutting parameter calculator can help with first-pass values, but the actual setup still needs engineering judgment. For carbide tools, start from the relevant carbide tooling family and send the operation details through the RFQ form when the parameter window is unclear.