Cutting Tools Reference Library · Engineering & Failure Analysis
Cutting Speed Basics — SFM, RPM & Feed
The formulas that connect surface speed (SFM or m/min), spindle speed (RPM) and feed, with a worked RPM table, so a counter or field rep can turn a manufacturer's recommended speed into a machine setting.
Draft, open for review. Check every value against the governing standard and the manufacturer’s data before use.
What it is
Cutting tool makers publish a recommended surface speed — how fast the cutting edge moves past the material — rather than a spindle speed. Surface speed is given in surface feet per minute (SFM) in inch units or meters per minute (m/min) in metric. Because the edge of a large tool travels farther per revolution than a small one, the same surface speed needs fewer RPM on a larger tool. Feed is how far the tool advances: per revolution for drills (IPR or mm/rev) and per tooth for milling cutters (chip load).
How to identify
Find the starting surface speed and feed in the tool manufacturer's data for the tool material, coating and workpiece material. Then convert with the formulas below.
- RPM (inch): RPM = (SFM × 12) ÷ (π × D), where D is tool diameter in inches. The shop shortcut is RPM ≈ (SFM × 3.82) ÷ D.
- RPM (metric): RPM = (m/min × 1000) ÷ (π × D), where D is in millimeters.
- Drilling feed rate: IPM = RPM × IPR.
- Milling feed rate: IPM = RPM × chip load per tooth × number of flutes.
RPM at a given surface speed
Computed from the formula above and rounded to the nearest 10 RPM. Choose the SFM from the tool maker's data — this table only does the arithmetic.
| Tool dia (in) | 50 SFM | 100 SFM | 200 SFM | 400 SFM |
|---|---|---|---|---|
| 1/8 | 1530 | 3060 | 6110 | 12220 |
| 1/4 | 760 | 1530 | 3060 | 6110 |
| 3/8 | 510 | 1020 | 2040 | 4070 |
| 1/2 | 380 | 760 | 1530 | 3060 |
| 3/4 | 250 | 510 | 1020 | 2040 |
| 1 | 190 | 380 | 760 | 1530 |
Key points
- Surface speed is set mainly by the workpiece material and tool material. Harder, tougher and heat-resistant materials need lower speeds; aluminum and brass can run much faster.
- Carbide and heat-resistant coatings generally allow higher speeds than plain HSS — but only in a rigid setup.
- Speed drives heat and tool wear; feed drives chip thickness and productivity. Too low a feed can be as harmful as too high a speed, especially in work-hardening stainless steels.
- Start at the manufacturer's recommended value, then adjust based on chip color and shape, finish, noise and tool wear.
- Small tools often need more RPM than a machine can deliver; run at the machine's maximum and keep the feed per tooth or per revolution correct.
Common mistakes
- Running every drill at the same RPM regardless of diameter.
- Confusing IPM (feed rate) with IPR (feed per revolution) or chip load.
- Using HSS speeds for carbide (wasting the tool) or carbide speeds for HSS (burning it).
- Ignoring the extra speed reduction most makers call for in deep holes and with long-series drills.
Governing standards: No single standard sets cutting speeds; use the tool manufacturer's published data. Tool-life test methods: ISO 3685 (turning), ISO 8688 (milling).
Reference only. Verify against the current governing code or standard, the manufacturer's data and the project documents before use. Apps for Distributors disclaims liability arising from reliance on this reference.