Most of the drill bit damage we see at MachinistPost comes from one of two things: running too fast in hard material, or running too slow in soft material. Both kill edges faster than correct speeds. Both are avoidable.
Here's what you need to know to set speeds correctly and stop burning up tooling.
Everything starts with surface feet per minute (SFM) — the speed at which the cutting edge moves through material. SFM is material-dependent. RPM is what you actually set on the machine. The relationship between them depends on drill diameter.
The formula:
RPM = (SFM × 3.82) ÷ Diameter (inches)
The 3.82 is a constant derived from unit conversion (12 ÷ π). Some shops round it to 4 for mental math.
Example: Drilling 3/8" (0.375") in mild steel (SFM = 80):
RPM = (80 × 3.82) ÷ 0.375 = 813 RPM
If you're on a drill press with speed steps, pick the closest step at or below 813.
These are conservative starting points for HSS (M2 grade) with adequate chip clearance and cutting fluid where noted.
| Material | SFM (HSS) | Notes |
|---|---|---|
| Free-machining aluminum | 200–300 | High SFM — goes fast, use coolant |
| General aluminum (6061) | 150–200 | Back off on thin sheet |
| Brass (free-cutting) | 150–200 | Similar to aluminum |
| Cast iron (gray) | 50–80 | Dry is fine — chips are powder |
| Mild steel (1018, A36) | 70–100 | Most common job shop material |
| 4140 steel (annealed) | 50–70 | Back off if it work-hardens |
| Stainless 304/316 | 30–50 | Slow and steady — use cobalt if possible |
| Tool steel (D2, H13) | 20–40 | Slow, rigid setup, cutting fluid |
| Titanium | 20–30 | HSS works but cobalt is better |
| Plastics (nylon, HDPE) | 150–250 | Slow feed to avoid melting |
Key note: Stainless needs slow SFM and constant feed pressure. Dwelling (stopping mid-hole without chip evacuation) work-hardens the surface and kills edges fast.
For the most common drill sizes and materials. Print this and put it on the drill press.
| Drill Size | Mild Steel (80 SFM) | 6061 Aluminum (175 SFM) | Stainless 304 (40 SFM) |
|---|---|---|---|
| 1/8" (0.125") | 2445 | 5355 | 1222 |
| 3/16" (0.1875") | 1630 | 3570 | 815 |
| 1/4" (0.250") | 1222 | 2678 | 611 |
| 3/8" (0.375") | 815 | 1786 | 407 |
| 1/2" (0.500") | 611 | 1338 | 306 |
| 5/8" (0.625") | 489 | 1071 | 244 |
| 3/4" (0.750") | 407 | 893 | 203 |
| 1" (1.000") | 306 | 670 | 153 |
These are starting points — dial them in based on chip color, sound, and how the machine feels.
The chips coming out of the hole tell you everything.
HSS in mild steel:
HSS in aluminum:
HSS in stainless:
RPM without feed rate is incomplete. Feed rate (how fast the drill advances into the material) determines how much material each cutting edge removes per revolution.
Rule of thumb for hand-fed drill press:
For CNC, typical feed per revolution (IPR) by material and size:
Running correct speeds dramatically extends drill life. A properly run HSS bit in mild steel will stay sharp for hundreds of holes before needing resharpening. The same bit run too fast in the same material might need attention after 20 holes.
Geometry also matters after resharpening. A bit with perfect point angle and lip symmetry runs correctly at spec speeds. A bit with unequal lip heights effectively becomes a smaller effective cutting diameter — one edge does all the work, heat concentrates there, and you're back to a dull edge faster than you should be.
If resharpened bits are running hot or wearing fast at correct speeds, check the geometry. It's almost always a lip height issue.
Get a resharpening quote →