Formula
RPM is surface speed times 12, divided by π times the diameter in inches. In metric that is the same spindle: cutting speed in metres per minute times 1000, divided by π times the diameter in millimetres.
RPM = SFM × 12 / (π × D)
feed = RPM × chipload × teeth
Milling multiplies by the flute count. Turning uses one tooth, so the feed per minute is RPM times the inches per revolution. Drilling uses the flute count, usually two. The starting speeds below are midpoints for this site. Chip for a mill or a drill scales from the 1/2 inch reference by diameter / 0.5 and is clamped between 1/4 and 2 times that reference. Turning does not scale. None of this is a substitute for the chart that came with the tool. The 3D print cost calculator is a different process.
Worked examples
100 sfm, 1 in diameter. 1200 / π = 382 rpm.
1/2 in end mill, 400 sfm, 0.003 in per tooth, 4 flutes. RPM = 400 × 12 / (π × 0.5) = 3,056. Feed = 3,056 × 0.003 × 4 = 36.67 ipm. The starting chipload for mild steel and carbide at 1/2 inch is 0.004, not 0.003. The 0.003 is a number you typed.
The same mill at 1/8 in. The scaled chipload hits the floor, one quarter of the 1/2 inch reference, which is 0.001 in for mild carbide.
Reference
Starting surface speed in feet per minute, then chipload in inches at a 1/2 in cutter. HSS is the lower column.
| Material | HSS sfm | Carbide sfm | HSS chip | Carbide chip |
|---|---|---|---|---|
| Aluminum | 300 | 1000 | 0.0030 | 0.0050 |
| Mild steel | 100 | 400 | 0.0020 | 0.0040 |
| Stainless | 50 | 200 | 0.0015 | 0.0030 |
| Brass | 250 | 600 | 0.0030 | 0.0050 |
| Cast iron | 70 | 300 | 0.0020 | 0.0040 |
| Plastic | 300 | 800 | 0.0040 | 0.0080 |
Questions
Are these Machinery’s Handbook numbers?
No. The surface speeds and chiploads are starting windows chosen for this site, in the range a tooling chart usually opens with. They are not a transcription of Machinery’s Handbook. The insert or end-mill manufacturer’s SFM and chipload take precedence.
What is the difference between milling, turning, and drilling?
RPM is the same identity for all three. Milling feed is RPM × chipload × flutes. Turning uses one tooth, and the chipload is inches per revolution. Drilling usually has two flutes, so inches per revolution is chipload times two. You can type a different tooth count.
Does chipload scale with diameter?
For milling and drilling, the starting chipload scales with diameter divided by 1/2 inch, and it is clamped between one quarter and twice the 1/2 inch reference. Turning does not scale. This is a starting scale, not chip thinning.
Sources
- RPM = SFM × 12 / (π × diameter in inches). The metric form, Vc × 1000 / (π × mm), is the same spindle speed.
- Surface speeds and chiploads are starting points chosen for this site, in the band a tooling catalog usually opens with. They are not a copy of Machinery’s Handbook or of any one manufacturer’s card. That card takes precedence.