Cutting Speed, RPM & Feed Rate Calculator
Spindle speed and feed from surface speed and chip load — milling, turning, drilling.
Starting points, not a recipe. Rigidity, coolant, tool overhang and workholding all move the numbers, usually downward on a light machine.
How it works
Two numbers drive everything on a machine tool. Surface speed is how fast the cutting edge passes through the material, and it is a property of the material and the cutter, not of the machine. Chip load is how much each tooth takes per revolution, and it is a property of the cutter's size and rigidity. Spindle RPM and feed rate are both derived — outputs, not choices.
RPM = surface speed × 1000 ÷ (π × diameter) in metric, or SFM × 12 ÷ (π × diameter) in inches. Feed rate is then RPM × flutes × chip load.
The failure mode nobody expects is cutting too slowly. A cutter at correct speed with too fine a feed rubs instead of cutting, work-hardens the surface — catastrophic in stainless — and wears the edge by friction rather than by shear. A chipped tooth is usually too much feed; a rounded, polished tooth is almost always too little.
Radial chip thinning is the other thing that trips people up. When radial engagement drops below half the cutter diameter, the actual chip is thinner than the programmed feed per tooth, because the cutter is in the cut for only part of its arc. At 10% engagement the real chip is about 60% of the commanded feed, so the feed has to rise to compensate — which is exactly why high-speed toolpaths run feeds that look reckless on paper.
Common questions
The RPM is higher than my machine can do — now what?
Run the machine flat out and drop the surface speed to match. Recalculate the feed at the actual RPM, because feed follows spindle speed. An undersized spindle is the normal reason a small mill cuts aluminium poorly.
What chip load should I use?
Roughly 1% of cutter diameter per tooth in steel and 2% in aluminium — 0.05 mm on a 6 mm cutter, 0.12 mm in aluminium. Deflection and holder rigidity matter more than the material once the cutter is long and thin.
Why does my stainless job go hard and stop cutting?
Feed too light. Austenitic stainless work-hardens under a rubbing edge, and the next pass then has to cut through the hardened layer. Take a deliberate chip, keep the cutter moving, and never dwell in the cut.
Does this cover turning and drilling?
Yes — set flutes to 1 and use the work diameter for turning, the drill diameter for drilling. Chip load then means feed per revolution, and material removal rate is still valid.