Most machinists have seen a drill chart. Fewer actually know how to read one — or why the numbers mean what they mean. Here's the practical version.
SFM stands for Surface Feet per Minute. It's a measure of how fast the cutting edge of the bit is moving against the workpiece material — not how fast the spindle turns, but how fast the actual metal-on-metal contact is happening at the outer edge of the bit.
Why does this matter? Because heat generation at the cutting edge is driven by surface speed. Too fast → the edge overheats, the metal softens, wear accelerates. Too slow → the bit rubs instead of cuts, also generating heat.
The key insight: SFM is the target. RPM is what you dial into the machine to hit that target.
RPM = (SFM × 3.82) / Diameter (inches)
The 3.82 is a constant derived from unit conversion (12 / π ≈ 3.82).
Example: Drilling 1/2" diameter into mild steel. Target SFM for HSS in mild steel: ~80 SFM. RPM = (80 × 3.82) / 0.5 = 611 RPM.
Example 2: Drilling 1/4" diameter into aluminum. Target SFM: ~300 SFM. RPM = (300 × 3.82) / 0.25 = 4,584 RPM. If your machine tops out at 3,000 RPM, run it at the top. You won't hit ideal SFM, but you're cutting in the right direction.
| Material | SFM Range (HSS) | Notes |
|---|---|---|
| Aluminum (6061, 2024) | 200–400 | HSS loves aluminum — push speed |
| Brass / Bronze | 100–200 | Easy to drill, decent SFM |
| Mild Steel (1018, A36) | 70–100 | Most common shop material |
| Alloy Steel (4140, 4340) | 40–70 | Back off speed, use cutting oil |
| Stainless Steel (304, 316) | 30–50 | Run slow, use sulfurized oil, no dry drilling |
| Cast Iron (gray) | 50–80 | Dry is fine — no coolant needed |
| Titanium | 20–40 | Slow, flood coolant, sharp bit mandatory |
| Plastics (ABS, Delrin) | 100–300 | Varies — check for melting chips |
| Drill Size | Mild Steel (~80 SFM) | Aluminum (~250 SFM) | Stainless (~40 SFM) | Cast Iron (~65 SFM) |
|---|---|---|---|---|
| 1/8" | 2,445 RPM | 7,640 RPM | 1,222 RPM | 1,988 RPM |
| 3/16" | 1,630 RPM | 5,093 RPM | 815 RPM | 1,325 RPM |
| 1/4" | 1,222 RPM | 3,820 RPM | 611 RPM | 994 RPM |
| 3/8" | 815 RPM | 2,547 RPM | 407 RPM | 663 RPM |
| 1/2" | 611 RPM | 1,910 RPM | 305 RPM | 497 RPM |
| 3/4" | 407 RPM | 1,273 RPM | 203 RPM | 331 RPM |
| 1" | 305 RPM | 955 RPM | 153 RPM | 248 RPM |
Set your machine to the nearest available speed. Close is good enough — you're not going to destroy bits for being 50 RPM off.
| Bit Diameter | Feed Rate (IPR) |
|---|---|
| Under 1/8" | 0.001–0.002 IPR |
| 1/8" – 1/4" | 0.002–0.004 IPR |
| 1/4" – 1/2" | 0.004–0.007 IPR |
| 1/2" – 1" | 0.007–0.015 IPR |
| Over 1" | 0.015–0.025 IPR |
In practice on a manual drill press, you're feeling this rather than measuring it. The bit should cut steadily, chips should come out cleanly, and you shouldn't have to lean on the quill to make progress. If you're forcing it, back off feed. If it's squeaking and not moving material, increase feed.
The SFM recommendations are written for properly ground bits with correct point angles, lip relief angles, and a centered chisel edge. If your bit has been run past its service life, no RPM setting fixes that. Keep the bits sharp, the math works the way it's supposed to.
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