Surface Speed to RPM Calculator

In machining, the speed that really matters is the surface speed, the rate at which the cutting edge passes over the material, and getting it right is the single biggest factor in tool life, surface finish and how fast you can work. The trouble is that machines are set in revolutions per minute, not surface speed, and the conversion between the two depends on diameter, so it changes every time the part or tool size changes. This calculator does that conversion for you in an instant. Enter the recommended cutting speed in metres per minute and the diameter in millimetres, the workpiece diameter for turning on a lathe or the cutter diameter for milling and drilling, and it returns the spindle speed in RPM to dial into the machine. The relationship is straightforward once you see it: the circumference, pi times the diameter, is the distance the edge travels in one revolution, so dividing the surface speed by that circumference gives the revolutions per minute. Because larger diameters cover more distance per turn, they need lower RPM to keep the same surface speed, which is why a big lathe job runs slowly while a small drill spins fast. Using the right surface speed for your material and tool combination, the value manufacturers publish for, say, carbide on steel or high-speed steel on aluminium, protects your tooling, gives a cleaner finish and avoids burning the edge or work-hardening the material. This tool is a daily companion for machinists, engineers, toolmakers and anyone running a lathe, mill or drill, in the workshop or the classroom. The formula and a worked example are set out below.

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