When you adjust the spindle speed override on a CNC machine, the feed should change proportionally to maintain chip load. This calculator tells you the correct feed override for any speed change.
Calculate āMaintaining constant chip load when spindle speed changes.
A shop running a Haas VF-2 with a 10k spindle in aluminum uses an 8 mm end mill programmed at 10,000 RPM and 1,500 mm/min feed. The operator hears chatter and drops the spindle override to 70% ā now the spindle is running at 7,000 RPM. But the feed stays at 1,500 mm/min. The chip load per tooth increases from 0.047 mm to 0.067 mm ā a 42% increase. On a 4-flute end mill in aluminum, this chip load is still acceptable, but barely. In steel or stainless, the same override would overload the tool and risk breakage.
The correct action: reduce feed rate proportionally. At 70% spindle speed, feed should also be at 70% ā 1,050 mm/min. This maintains the programmed chip load and keeps cutting forces at the designed level. The calculator above computes the corrected feed rate automatically for any spindle override setting.
Operators adjust spindle override for three reasons: chatter (reduce RPM to find a stable speed), tool wear (increase RPM to compensate for dulling tools ā a stopgap), or material variation (hard spots in castings require lower speeds). In all three cases, the feed override should match the spindle override to maintain the programmed chip load. Running 80% spindle speed with 100% feed increases chip load by 25%.
The solution is simple: when you change the spindle override dial, change the feed override dial to the same percentage. The override knobs are adjacent on most Haas, Mazak, and Okuma controls, but operators often forget the second adjustment. This calculator quantifies the cost of that forgetfulness.
Why should feed rate match spindle speed override? To maintain constant chip load. Chip load = feed per tooth. If spindle speed drops but feed stays the same, the tool cuts more material per tooth, increasing cutting forces and potentially causing tool failure.
What happens to chip load when spindle speed drops 20% without feed adjustment? Chip load increases by 20%. The tool takes 20% more material per tooth, increasing cutting forces by approximately 15-25% depending on the material's specific cutting force.
Can I increase feed override to improve cycle time? Only if the spindle speed is also increased proportionally. Increasing feed override without increasing RPM overloads the tool. The Milling Force Calculator can help determine the maximum safe chip load for your specific tool and material.
Is feed rate override bad for tool life? Not if used correctly. Matching feed override to spindle speed override maintains tool life. Mismatching them ā especially running higher feed at lower speed ā accelerates tool wear by overloading the cutting edges.