How do I calculate the bit speed for my drill-press pulleys?
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
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 11⁄8 ", 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.