Reference
Feeds and speeds basics
Four numbers describe a milling cut. Get the units straight, start conservative, and check the tool maker before the first chip.
The four numbers
- Surface speed
- How fast the cutting edge passes the work. Inches use SFM (surface feet per minute). Metric uses m/min. 1 SFM = 0.3048 m/min.
- Spindle speed
- RPM. Same number in both unit systems. A bigger diameter at the same surface speed turns slower.
- Chip load
- Thickness of the chip each tooth takes. Inches per tooth, or millimeters per tooth. Too little rubs and work-hardens. Too much breaks the edge.
- Feed rate
- How fast the table moves. Inches per minute, or millimeters per minute. It is RPM × chip load × number of flutes.
Formulas
RPM = (SFM × 12) / (π × diameter in inches)
RPM = (m/min × 1000) / (π × diameter in mm)
Feed (in/min) = RPM × chip load (in/tooth) × flutes
Feed (mm/min) = RPM × chip load (mm/tooth) × flutes
SFM = RPM × π × diameter (in) / 12
m/min = RPM × π × diameter (mm) / 1000
Carbide starting bands
Coated carbide, a rigid setup, and a cut that is not a full-width slot. High-speed steel is often about one third to one half of these surface speeds. Prehard and hardened stock is slower than annealed. These are classroom bands, not a catalog.
| Material | SFM | m/min | Note |
|---|---|---|---|
| 6061 aluminum | 800–1200 | 240–370 | Watch for chip welding. ZrN or uncoated is common. |
| 1018 steel | 400–600 | 120–180 | Annealed mild steel, flood or a light oil. |
| 4140, annealed | 300–450 | 90–140 | Prehard 4140 (about 28–32 HRC) is often closer to 200–300 SFM (60–90 m/min). |
| 304 stainless | 200–350 | 60–105 | Keep the tool loaded. Rubbing work-hardens it. |
| Gray cast iron | 400–700 | 120–210 | Often cut dry. Mind the dust. |
Chip load by diameter
Peripheral milling, with radial engagement under about half the diameter. In a slot, cut the chip load roughly in half and listen. Axial depth depends on stickout and the holder; a stub end mill in steel often starts near half a diameter deep, and that is still a guess until the tool maker's chart says otherwise.
| Diameter | in/tooth | mm/tooth |
|---|---|---|
| 1/8 in (3.18 mm) | 0.0005–0.0010 | 0.013–0.025 |
| 1/4 in (6.35 mm) | 0.001–0.002 | 0.025–0.051 |
| 3/8 in (9.53 mm) | 0.0015–0.003 | 0.038–0.076 |
| 1/2 in (12.7 mm) | 0.002–0.004 | 0.051–0.102 |
| 3/4 in (19.05 mm) | 0.003–0.006 | 0.076–0.152 |
If the question is really a gear — diametral pitch, module, center distance, backlash — use gearcalculators.com. If you need the gear made, talk to Evolvent Design.