“Calculating Spindle Speed”

In machining, spindle speeds, also called “cutting speeds”, are extremely important for tool life, part finishes and over all part quality.  

This is especially true in CNC machining where the programmer must be able to calculate the spindle speed based on recommended “cutting speeds” usually given in either “Feet Per Minute” (FPM, U.S. Units) or in “Meters Per Minute” (MPM, Metric Units).

Below are instructions on how to calculate spindle speeds using cutting speeds.  I did not put the instructions in “Formula Form” but instead just in a Step by Step instructional form.  For most people (myself included) it is easier to follow.

Cutting Speed to RPM ( U.S. Units):

(1) Obtain Recommended Cutting Speed in FPM (Feet Per Minute)

(2) Multiply FPM (Cutting Speed in Feet Per Minute) by 12

(3) Multiply the Stock or Tool Diameter by pi (3.1416)

(4) Divide Step 2 by Step 3 to obtain the RPM


Example:

We want to calculate the RPM for a .750” cut diameter or .750” diameter tool with the recommended cutting speed of 300 FPM (Feet Per Minute)

300 FPM x 12 = 3600 (Ref. Step 2)

.750” Diameter x 3.1416 = 2.3562 (Ref. Step 3)

Divide 3600 by 2.3562 = 1528 RPM (Ref. Step 4)

Cutting Speed to RPM ( Metric Units):

(1) Obtain Recommended Cutting Speed in MPM (Meters Per Minute)

(2) Multiply MPM (Cutting Speed in Meters Per Minute) by 1000

(3) Multiply the Stock or Tool Diameter by pi (3.1416)

(4) Divide Step 2 by Step 3 to obtain the RPM


Example:

We want to calculate the RPM for a 22mm cut diameter or 22mm diameter tool with the recommended cutting speed of 125 MPM (Meters Per Minute)

125 MPM x 1000 = 125000 (Ref. Step 2)

22mm Diameter x 3.1416 = 69.1152 (Ref. Step 3)

Divide 125000 by 69.1152 = 1809 RPM (Ref. Step 4)

RPM to Cutting Speed ( U.S. Units):

(1) pi (3.1416) x Stock or Tool Diameter in Inches x RPM

(2) Divide Step 1 by 12 to obtain the Cutting Speed in FPM (Feet Per Minute)


Example:

We want to calculate the Cutting Speed for a .750” cut diameter or .750” diameter tool with a RPM of 5000

3.1416 x .750” x 5000 = 11781 (Ref. Step 1)

Divide 11781 by 12 = 982 FPM (Ref. Step 2)

RPM to Cutting Speed ( Metric Units):

(1) pi (3.1416) x Stock or Tool Diameter in Millimeters x RPM

(2) Divide Step 1 by 1000 to obtain the Cutting Speed in MPM (Meters Per Minute)


Example:

We want to calculate the Cutting Speed for a 25mm cut diameter or 25mm diameter tool with a RPM of 5000

3.1416 x 25mm x 5000 = 392700 (Ref. Step 1)

Divide 392700 by 1000 = 392.7 MPM (Ref. Step 2)

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