Super simple spindle RPM set up

Tom Griffin has lots of helpful references on his web site but the one that caught my eye was the simple way he explains how to find the proper RPM needed for common cutting materials (SFPM) like:

• Mild Steel - 100
• Tool Steel - 70
• Cast Iron - 60
• Aluminum - 250
• Brass - 300

Using any bit diameter you like.

The formula is as simple as 4 x CUTTING SPEED (number from the list above) divided by the diameter of the bit being used.

So for example I like using a 1/8th (.125") bit to cut Aluminum.

So to figure out what RPM to set my spindle to.
I would multiply the Aluminum setting listed above (250) x 4 = 1000
and then divide that by the bit size I am using the 1/8th (.125") This leaves me with 8,000 RPM .

I have listed the video below (watch it its good)


Here is Toms Chart so you can experiment with other materials as well.

Toms Charts