Calculate spindle speed with n = 1000 × Vc ÷ (π × D), using cutting speed Vc in metres per minute and diameter D in millimetres. The answer is revolutions per minute, or rpm; the table below converts example cutting speeds into rpm and does not select a speed for your material.
The metric cutting speed formula
n: spindle speed in rpm.
Vc: cutting speed in m/min.
D: drill diameter in mm; for milling, use the effective cutting diameter at the depth of cut.
π: the mathematical constant pi.
The drilling formula uses drill diameter. The milling formula uses cutting diameter at the actual depth of cut, which needs particular attention for ball-nose and round-insert cutters.
Example cutting-speed-to-RPM chart
Every value is calculated with 1000 × Vc ÷ (π × D) and rounded to the nearest whole rpm. The column headings are example inputs, not recommendations for HSS, carbide, steel or any other tool or material.
Diameter D (mm)
At 20 m/min (rpm)
At 40 m/min (rpm)
At 60 m/min (rpm)
5
1273
2546
3820
6
1061
2122
3183
8
796
1592
2387
10
637
1273
1910
12
531
1061
1592
16
398
796
1194
20
318
637
955
25
255
509
764
Worked example: 10 mm at 40 m/min
Enter 10 for D and 40 for Vc. The calculation is 1000 × 40 ÷ (π × 10) = 1273.2395… rpm, giving 1273 rpm after rounding. If the diameter becomes 20 mm at the same cutting speed, the calculated result is 636.6198… rpm, giving 637 rpm.
RPM and feed are separate inputs
For drilling, feed in mm/min equals feed per revolution in mm/rev multiplied by rpm. For milling, table feed in mm/min equals feed per tooth multiplied by rpm and the number of effective teeth.
As a drilling arithmetic example, 0.10 mm/rev at 1000 rpm gives 100 mm/min. Those inputs demonstrate the calculation only. Obtain the feed recommendation for the actual tool and work material before using it on a job.
What to check before choosing a tool
Start with the required diameter and operation. For a listed 10 mm indexable milling tool, see the 10 mm APMT Indexable Endmill. Check its insert requirements and obtain the cutting data for the actual insert and material.
Confirm whether your input is cutting speed, spindle speed or feed.
Confirm the diameter required by the tool maker's calculation.
Compare the result with the machine and tool assembly limits.
Keep the approved cutting speed and feed source with the setup sheet.
