I need a new pulley for my 1940s-era drill press. I found a four-step pulley of similar size to the original. How can I calculate the drill-bit speed for each setting?

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Q:

I need a new pulley for my 1940s-era drill press, but the manufacturer has gone out of business. At the hardware store, I found a four-step pulley of similar size to the original, but slightly different in diameter for each step. How can I calculate the drill-bit speed for each setting?
—James Allen, Dallas

A:

To calculate spindle speeds, James, you need to know the diameter of the two pulleys the belt runs on, plus the motor speed. The equation looks like this:
d1 × s1 = d2 × s2
*d1 = diameter of the motor pulley
*s1 = motor speed (found on the motor plate)
*d2 = diameter of the spindle pulley
*s2 = spindle speed

Example: The motor pulley measures 118 ", or 1.125", and the motor runs at 1,725 rpm. The spindle pulley measures 4". So to find the spindle speed:
1.125 × 1,725 = 4 × s2
1940 ÷ 4 = s2
s2 = 485 rpm

Use the same formula to calculate all the other pulley combinations, and keep them on a chart at your drill press.

To find the proper speed for different types and sizes of drilling accessories, download our free speed chart at woodmagazine.com/drillpresschart.