MakerKiln

Cutting Speed to RPM Calculator

Spindle RPM from a cutting speed and a diameter, for drilling, turning or milling, with a table of starting speeds by material.

Local onlyThe math runs in JavaScript in this tab. Your figures are remembered in this browser's local storage, and nothing is uploaded.

Spin a tool too fast and the edge burns; too slow and it rubs instead of cutting. The right speed comes from how fast the cutting edge should move through the material, divided down for the diameter you are running. Enter the diameter and a cutting speed and this gives you the RPM to set on the drill press, lathe or mill.

The formula

Cutting speed is how fast the edge travels, in surface feet per minute or meters per minute. A big tool covers that distance in fewer turns than a small one, so RPM is the speed divided by the circumference. In inches it comes out to 3.82 times the surface speed over the diameter; in metric, a thousand times the speed over pi times the diameter in millimeters. Drilling, turning and milling all use the same relationship.

Starting speeds by material (HSS)

For a high-speed-steel tool: aluminum around 250 to 300 SFM, brass about 120 to 150, mild steel 80 to 100, cast iron 60 to 80, stainless a slow 40 to 60, and most plastics 200 to 400. Carbide runs two to four times faster than these. In metric, multiply SFM by about 0.3 for meters per minute. These are starting points from the Machinery's Handbook ranges; your machine's rigidity and whether you use coolant move them up or down.

Reading the chips

The chips tell you if the speed is right. Nice curls or a steady ribbon means you are close. Blue or brown chips, a squeal, or a burnt smell mean too fast, or too little feed and coolant. Dust and a rubbing tool that dulls fast usually means too slow. Adjust the speed first, then the feed, and keep notes on what worked for each material.

Drilling runs slower than you think

A drill's cutting edge is out at its rim, so a big bit reaches its surface speed at a low RPM. A half-inch bit in steel wants only a few hundred RPM, not the thousands a drill press idles at. Slow down for big bits and hard metal, feed firmly enough to make a chip rather than rub, and peck deep holes so the chips clear instead of packing and grabbing.

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