Feeds and Speeds Basics: RPM and Feed Rate Formulas
Two numbers decide whether a cutter lasts or dies: spindle RPM and feed rate. Both come from one formula pair you can run by hand.
The two numbers
Speed is how fast the spindle turns — RPM. Feed is how fast the tool moves through the material — mm/min or in/min. Get both right and the cutter removes chips cleanly and runs cool; get either wrong and you rub, heat, burn or snap the tool. Every CNC job starts with these two values.
The RPM formula
RPM = Vc × 1000 ÷ (π × D), where Vc is the cutting speed in m/min and D the tool diameter in mm (in imperial: RPM = SFM × 3.82 ÷ D in inches). So a 6 mm carbide end mill in aluminium at Vc 300 runs about 15,900 RPM — machine caps and rigidity decide the rest.
The feed formula
Feed rate = RPM × flutes × chip load. Chip load is the thickness each tooth bites — typically 0.05–0.2 mm in wood and plastics, 0.01–0.08 mm in metals. At 15,900 RPM, 2 flutes and 0.05 mm chip load, feed = 1,590 mm/min. Feed and RPM move together: change one and the other follows.
Reading the cut
Too low a chip load and the tool rubs instead of cutting — heat, glazing, work hardening in metals and melted edges in plastics. Too high and the spindle loads, the cutter deflects or breaks. Listen for a steady chip sound: squealing means rubbing, chattering means too much engagement.
Start from tested presets
The Feeds & speeds bundles starting values for common materials — softwood, plywood, MDF, hardwood, acrylic, aluminium, ABS — for routers and mills, and computes RPM and feed rate from your tool geometry in both unit systems.