What Are M35 & M42 Cobalt Drill Bits?
M35 and M42 are High Speed Steel (HSS) grades with Cobalt added to improve heat resistance (“red hardness”). This matters most when drilling low-thermal-conductivity metals like stainless steel, where heat stays at the cutting edge.
- M35: ~5% Cobalt (balanced toughness + heat resistance)
- M42: ~8% Cobalt (maximum heat resistance, but more brittle)
M35 vs M42: Key Differences
| Item | M35 (5% Co) | M42 (8% Co) |
|---|---|---|
| Heat resistance | High | Very high |
| Toughness (anti-snap) | Better | Lower (more brittle) |
| Best for | Hand drills, general stainless, shop work | Drill press, hard alloys, production drilling |
| Typical failure mode | Overheating if RPM too high | Chipping/snapping if drill wobbles |
Which One Should You Choose?
- Choose M35 if you drill by hand, need durability, or want the safest all-round cobalt option.
- Choose M42 if you use rigid setups (drill press/CNC), drill very hard alloys, or need maximum heat resistance.
If your target material is very hard (e.g. hardened steel), technique matters more than grade — keep RPM low, feed pressure high, and use coolant.
How to Drill With Cobalt
- Center punch to prevent walking.
- Low RPM (too fast is the #1 cause of failure). Use a Speeds & Feeds Chart.
- High feed pressure so the bit cuts chips (not dust).
- Use coolant (oil for stainless, cutting fluid for alloys).
- Peck drill for deep holes to clear chips.
Common Failures & Fixes
- Blue / burnt tip: RPM too high or no coolant. Slow down immediately.
- Chipped cutting lips: drill wobble or side-load. Improve rigidity or switch from M42 to M35.
- Snapped bit: chips jammed in the flutes. Peck drill and clear chips often.
Need OEM M35/M42 cobalt drill bits or private label packaging? Contact Zhonghuan Tools for a quotation.
