Feed Rate
Feed rate is the linear speed at which a CNC tool advances through the workpiece, typically expressed as mm/min or in/min in milling/router work, and as mm/rev or in/rev in turning. In G-code it is the F value, a primary control on chip load, surface finish, cycle time, and tool life. It directly determines how aggressively the cutter engages material, influencing material removal rate and heat generation.
On the shop floor, feed rate dictates cutting aggressiveness. Higher rates boost material removal but risk insufficient chip evacuation or spindle torque, causing recut chips, heat buildup, and accelerated tool wear. In milling, feed is set per unit time as the tool moves across the workpiece; in turning, it is set per revolution because chip thickness ties directly to spindle rotation. For millwork and edgebanding, feed governs line speed and affects edge quality, adhesive performance, and cut consistency. Too fast overloads cutters or adhesive systems; too slow overheats panels, smears glue, or creates chatter. Proper feed calculation—Feed Rate = RPM × chip load × number of flutes—ensures efficient cutting, good surface finish, and tool longevity. Practical signs of wrong feed include rubbing and burnished cuts (too low) or chatter and scalloped walls (too high).
- Rubbing instead of shearing: The cutter rubs the workpiece, generating excessive heat and dulling the tool, leading to poor surface finish and built-up edge on metals.
- Excessive tool deflection and chatter: Feed rate exceeds spindle torque capacity, causing deflection, chatter, and poor dimensional control, often resulting in broken cutters or scalloped walls.
- Chip load mismatch after RPM change: Increasing spindle speed without adjusting feed reduces chip thickness, causing rubbing, heat, and accelerated tool wear.
Is feed rate the same as cutting speed?
No. Feed rate is the tool’s forward motion through the material, while cutting speed refers to the speed of the cutting edge relative to the workpiece surface and is commonly tied to spindle RPM.
Why is feed rate critical in CNC machining?
It controls chip thickness, cutting forces, heat generation, surface finish, and throughput, directly affecting both part quality and tool economy.
What does “feed per tooth” mean?
It is the amount each flute removes per revolution; feed rate equals spindle speed times chip load times flute count, so it is the bridge between spindle RPM and actual cutting aggression.