October 23, 2026 · DIY & Workshop · 6 min read

Cutting Speed and Feed Rate: Where Every Machining Job Starts

Ask five machinists for the right speed and feed and you'll get five answers — but they'll all be conversions of the same two starting values. Cutting speed and chip load are properties of the material and tool; RPM and feed rate are just the machine-friendly forms of those numbers.

Cutting speed versus RPM

Cutting speed (V) is how fast the tool edge moves through the material, measured at the cutting point — in meters per minute or surface feet per minute. Spindle RPM is what you dial in. They're linked by the workpiece or tool diameter:

A 100 mm shaft turned at 150 m/min needs RPM = 150,000 ÷ 314 ≈ 477 RPM. The same speed turning a 25 mm shaft needs about 1,910 RPM — which is why lathes with constant surface speed control exist.

Feed rate

Feed rate is how fast the tool advances. For turning and drilling it comes from the feed per revolution:

Typical feed per revolution: 0.1–0.3 mm for turning with carbide, 0.05–0.15 mm for drilling depending on diameter. For milling, the key value is chip load per tooth, so the formula becomes F = RPM × feed/tooth × number of flutes.

Starting values that actually work

• Aluminum: cutting speed 200–400 m/min with carbide; HSS tools far less, around 90–150.
• Mild steel: carbide 100–200 m/min; HSS 20–40 m/min.
• Stainless steel: carbide 60–120 m/min; keep feeds steady to avoid work hardening.
• Cast iron: carbide 100–200 m/min; brittleness forgives a lot.

These are starting points, not gospel. The honest procedure is to start conservative, listen, and adjust: chattering means reduce speed or feed; a burnt or rapidly dulling edge means you're too slow for carbide or too fast for the material.

Mistakes worth avoiding

• Confusing diameter with radius in the RPM formula — a factor-of-two error that snaps small drills.
• Feeding too slowly in stainless, letting the tool rub and harden the surface.
• Using HSS tables for carbide tools and abandoning metal cutting as "impossible."

Setting up your machine room? Essential Workshop Tools covers the measuring instruments that keep all this honest.