## Turning Archive 2004

Subject:
Measuring spindle speed and other rotating things

Rick R, So Cal
>Hi All,
If you ever wanted to measure the RPM of something spinning, and you have a bicyle with a speedometer, read on:

I put together a homemade slow-speed grinder, and since it's powered by a DC motor with no nameplate information, I was curious what RPM I was running this thing. So I grabbed my road bike, touched the front wheel to the spinning grinding wheel, and noted the MPH reading on the speedometer. It read 15 MPH, so that was the surface speed of the grinding wheel. Given that the grinding wheel diameter is 5.5", I converted suface speed to RPM, which came out at 917.

You could use the same method to measure your lathe spindle speed, for example. Just chuck up something with about a 5" diameter, measure surface speed with your bike, then do the conversion to RPM. I have done the 'rithmetic for you:

RPM = MPH/d x 336.14 (in hr/min mi)

Where d is the diameter of the spinning object in inches, and (in hr/min mi) are the units of 336.14 (for the sake of mathematical correctness).

In my example,
15MPH/5.5 in x 336.14 = 917 RPM

Oh yeah, if you try this on a grinding wheel, give your tire a good spin before touching it to the grinding wheel so you don't grind your tire like I did!

Rick