Surface Speed Calculator

A spinning wheel's RPM does not tell you how fast its edge is really moving, and the edge speed is what sands, grinds, buffs or drives. A big wheel and a small one at the same RPM have very different rim speeds. This gives you the surface speed from the diameter and RPM, in feet per minute, meters per second and miles per hour, or the RPM to hit a surface speed you want.

The rim moves, not the shaft

What does the work on a grinder, sander or buff is the speed of the surface flying past, not how many times a minute the shaft turns. That surface speed is the circumference, pi times the diameter, times the RPM. Because it scales with diameter, a ten inch wheel throws its rim nearly twice as fast as a six inch wheel at the same RPM. So when you compare or swap wheels, compare surface speed, not RPM, or you will be running one of them far too fast or too slow.

Why grinding wheels have a max RPM

A grinding wheel is held together against the force trying to fling it apart, and that force depends on rim speed. Every wheel has a maximum safe surface speed, and the maker prints a maximum RPM for its size to stay under it. As a wheel wears smaller, its rim speed at the same RPM drops, so it gets safer, but never spin a wheel faster than its rating, because past the limit a wheel can burst. This is one place surface speed is a safety number, not just a performance one.

Every abrasive has a sweet spot

Sanding belts, flap wheels and buffs each work best in a band of surface speed: too slow and they load up and cut poorly, too fast and they glaze, burn the work or sling off compound. Belt sanders are often rated in feet per minute for this reason. When you change a drum or wheel diameter, use this to find the RPM that keeps the surface speed in the band the abrasive likes, and the finish and the belt life both improve.

It is just a wheel speed, anywhere

The same math covers anything round that rolls or drives: a robot or cart wheel turning your drive speed into ground speed, a lathe turning diameter and RPM into cutting speed, a pulley or roller moving a belt. Give it the diameter and the RPM for the travel speed, or a target speed to get the RPM. For a vehicle wheel the surface speed in miles per hour is the road speed, which is why that is one of the outputs.

More MakerKiln tools

• cutting speedCutting Speed to RPM CalculatorSpindle RPM from a target cutting speed and a diameter, for drilling, turning or milling.→
• pulley rpmPulley RPM CalculatorOutput RPM, drive ratio and torque change between two pulleys.→
• gear ratioGear Ratio CalculatorReduction, output RPM and torque multiplication from two tooth counts.→

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