Calculate Drill or Lathe Speed

If the pulley RPM on your tool is not labelled you can calculate it.

Measure the radius or circumference of each pulley. It doesn't matter which, as long as you stay consistent and the inside and outside diameters of all the pulleys are proportional. It is the ratio that matters here.

The motor should state the RPM. Use this to make your calculations.

Divide the motor pulley measurement by the measurement of the pulley the belt connects to. This will be your ratio. Then multiply this ratio by the RPM of the motor.

Example:

motor pulley radius=2 inches

follow up pulley radius=4 inches

motor RPM=1000

2/4=0.5

0.5x1000=500

500rpm

If three or more pulleys are used the same calculation can be made.

Example:

motor pulley radius=2 inches

second pulley radius=4 inches

third pulley radius=8 inches

motor RPM=1000

2/4=0.5 (first ratio)

0.5x1000=500

second pulley RPM=500

4/8=0.5 (second ratio)

0.5x500=250

third pulley RPM=250

Make a chart of all pulley positions and calculate each RPM.

The appropriate speed for the wood lathe can be calculated based on the diameter of the turning piece. When you multiply the diameter of the wood in inches by the RPM of the lathe the number should be between 6000 and 9000. I created a graph to help visualize the diameter and speed relationship.

If the diameter is 1 inch the speed should be between 6000rpm and 9000rpm.

6000/1=6000, 9000/1=9000.

If the diameter is 3 inches the speed should be between 2000rpm and 3000rpm.

6000/3=2000, 9000/3=3000.

Heavy roughing cuts should however be made at lower speeds.

Many drill speed charts can be found online with varying ranges.

Drill speed can depend on:

type of material being drilled

material hardness

hole size

drill bit material

bit sharpness

whether cutting fluid is used or not

etc.