Spindle speed
| Units | Formula | Inputs |
|---|---|---|
| Inch | RPM = SFM × 12 ÷ (π × D) | SFM in feet/min; D in inches |
| Metric | RPM = Vc × 1,000 ÷ (π × D) | Vc in metres/min; D in millimetres |
Milling feed rate
Feed rate = RPM × effective tooth count × feed per tooth. With feed per tooth in inches, the result is inches per minute. With millimetres per tooth, the result is millimetres per minute.
Use the toolmaker’s effective tooth count for the cutter. Do not mix feed per revolution with feed per tooth.
Worked arithmetic example
Suppose the inputs are a 0.5-inch cutter, four effective teeth, 300 SFM, and 0.002 inch per tooth. These values illustrate the calculation; they are not a recommendation for a particular material or tool.
- RPM = 300 × 12 ÷ (π × 0.5) = approximately 2,292 RPM.
- Feed = 2,291.83 × 4 × 0.002 = approximately 18.33 inches/min.
When the spindle reaches its limit
If the machine is limited to 2,000 RPM, use 2,000 in the feed calculation. At the same 0.002 inch per tooth, the feed becomes 16 inches/min. The actual surface speed is approximately 262 SFM.
Check before using the result
The formulas convert the inputs you supply. They do not select a suitable cutting speed or chip load. Check the toolmaker’s recommendations for the exact cutter, material condition, engagement, coolant, and setup. Tool stickout, workholding, and available spindle power also matter.
Use the calculations in Kinzel
The shop-floor portal calculates RPM and feed in either unit system, applies an optional spindle limit, and saves a cutting recipe with the job. The public 4140 example explores additional model estimates for a fixed setup.
Formula reference
For metric milling notation and equations, see Sandvik Coromant’s milling formulas and definitions. The worked example above is Kinzel’s arithmetic illustration.