Why stainless steel breaks ordinary drill bits
Stainless steel — especially 304 and 316 — work-hardens the moment the cutting edge rubs instead of cuts. The surface jumps from roughly 180 HB to 300+ HB in seconds, and a standard HSS bit that was cutting fine suddenly stops cutting and starts rubbing. Heat builds, the edge dulls, and the bit snaps.
Two more properties make it harder: stainless conducts heat poorly, so heat stays in the cutting edge rather than flowing into the workpiece; and it is gummy, so chips tend to stick and weld to the flutes.
1. Use the right drill: cobalt, not basic HSS
M35 cobalt HSS (5% cobalt) keeps its hardness at elevated temperature — roughly up to 600°C — while standard HSS starts to soften far earlier. In practice, M35 bits last several times longer than HSS on 304 stainless, and they resist the rubbing that triggers work hardening.
- Material: M35 cobalt HSS (HSS-Co 5%)
- Point: 135° split point — self-centering, so the bit does not walk on the curved or deburred surface
- Range: Ø1.0-14 mm covers most sheet, tube, flange and bracket work
2. Slow the speed down (this is where most bits die)
The single biggest mistake is running stainless at carbon-steel speeds. As a starting reference for M35 on 304/316:
| Hole Ø | Speed (rpm) | Feed (mm/rev) | Notes |
| Ø3 mm | 900-1200 | 0.05-0.08 | Rigid setup, short pecks |
| Ø5 mm | 700-900 | 0.08-0.10 | Peck every 1×D |
| Ø6-8 mm | 500-700 | 0.10-0.15 | Coolant flow must reach the tip |
| Ø10-13 mm | 350-500 | 0.15-0.20 | Pilot hole or step drilling |
Treat these as starting values: listen to the cut, watch the chip colour, and adjust. Blue chips mean too much heat.
3. Coolant and chip evacuation
Use a generous flow of cutting fluid or a water-miscible coolant — never drill stainless dry. For deep holes, peck drill (retract every 1× diameter) so chips clear; if chips stay in the flute, the bit re-cuts them, heat spikes, and the edge breaks down.
4. Rigidity: the invisible cause of breakage
- Clamp the workpiece — any movement loads the edge laterally
- Use a short, rigid bit or reduce overhang in the chuck
- A pilot hole (or step drilling with two sizes) reduces load on large diameters
- Sharp, centred start: 135° split point avoids a wandering pilot
Frequently asked questions
Do I need a pilot hole? For Ø8 mm and above in stainless, yes — step drilling reduces load and heat noticeably.
Can I use the same bit on stainless and mild steel? Physically yes, but keep stainless bits separate. Once a bit has rubbed on stainless it is usually dulled, and it will then cut poorly everywhere.
What surface finish can I expect? A properly fed cobalt bit produces a clean, near burr-free hole; light deburring with a ceramic fiber stone removes the remaining edge in seconds.
Need the right bit for your material? We supply M35 cobalt drills Ø1-14 mm factory-direct, with application support and free sample evaluation.