The designation printed on a drill bit shank — M2, M35, M42 — isn't marketing. It's a material specification that tells you the alloy composition, the hardness range, and the heat resistance of the cutting tool. Understanding what these grades mean helps you choose the right bit for the material you're drilling and justify — or avoid — the cost premium of cobalt grades.
The AISI M-Series: What the Numbers Mean
HSS drill bits fall under the AISI (American Iron and Steel Institute) M-series classification for molybdenum high-speed steels. All M-series HSS shares a base of tungsten, molybdenum, chromium, vanadium, and carbon — the combination that gives high-speed steel its ability to maintain hardness at elevated cutting temperatures. What varies between grades is primarily cobalt content and the balance of other alloying elements.
| Grade | Cobalt % | Hardness (HRC) | Hot Hardness | Cost vs M2 |
|---|---|---|---|---|
| M2 | 0% | 63–65 | Baseline | Baseline |
| M35 | 5% | 65–67 | +15–20% over M2 | +30–50% |
| M42 | 8% | 67–70 | +25–35% over M2 | +60–100% |
M2: The Standard Grade
M2 is the baseline HSS grade for most commercial twist drills. It contains no cobalt, and its alloying package — roughly 4% chromium, 2% vanadium, 5% molybdenum, 6% tungsten — provides excellent toughness and wear resistance for general-purpose drilling.
M2 performs well in:
- Low carbon and mild steel
- Aluminum, brass, copper
- Cast iron
- Plastics and composites
- Low-alloy steel under 35 HRC
The limitation of M2 is its hot hardness — the temperature at which the steel begins to soften. M2 starts losing meaningful hardness above approximately 900°F (480°C). In applications where chips are coming out straw-colored or blue, the cutting edge is approaching that range and dulling faster.
For the majority of drilling work in fabrication, maintenance, and general machining, M2 is entirely correct. The cobalt grades exist for specific material challenges, not as universal upgrades.
M35: The 5% Cobalt Grade
M35 adds 5% cobalt to the M2 base alloy. Cobalt dissolves into the steel matrix and raises the temperature at which the steel retains its hardness — known as hot hardness or red hardness. M35 retains meaningful cutting hardness at temperatures 15–20% higher than M2.
In practice, this means M35 handles heat-generating materials better. The grades where M35 shows a meaningful advantage over M2:
- Stainless steel (300 and 400 series): Stainless work-hardens at the cutting zone and generates significant heat. M35 survives longer in stainless without edge softening.
- Alloy steel 35–45 HRC: Harder steels generate more heat per cut and require an edge that maintains hardness at elevated temperature.
- Titanium alloys: Titanium has poor thermal conductivity — heat stays in the cutting zone rather than dissipating through the chip. M35 handles this better than M2.
- High-temperature alloys (Inconel, Hastelloy): These nickle-based superalloys work-harden aggressively and generate extreme cutting heat. M35 is the minimum grade recommendation; M42 is often preferred.
M35 is also notably tougher than M42. The higher cobalt content in M42 slightly increases brittleness. For interrupted cuts — drilled holes that enter at an angle, cross a slot, or hit a second material — M35's toughness advantage over M42 matters.
M42: The 8% Cobalt Grade
M42 is the highest-cobalt standard HSS grade at 8% cobalt, with a modified alloy formula that also increases the molybdenum content relative to tungsten. The result is the highest hardness in the standard HSS family — 67–70 HRC versus M2's 63–65 — with exceptional hot hardness.
M42 is the correct choice for the most demanding applications:
- High-hardness steel above 45 HRC
- Nickel-base superalloys and heat-resistant alloys
- Hardened tool steels where HSS is used instead of carbide
- High-speed production in stainless and alloy steel where maximizing tool life per resharpen is the goal
The trade-off for M42's hardness is reduced toughness. At 67–70 HRC, M42 is more susceptible to chipping under shock loads than M2 or M35. In interrupted cuts or applications with vibration, M42 bits are more likely to chip than to simply dull. For those applications, M35 is the better choice even though M42 is technically the harder material.
Resharpening and Cobalt Content
A common question: does resharpening remove the cobalt? The answer is no. Cobalt in M35 and M42 is an alloying element distributed throughout the entire steel matrix — it's not a surface coating. When you grind a drill tip, you're removing steel from the very tip, but the steel that's exposed after grinding has the same cobalt content as the original. The resharpen inherits the same material properties.
This is one of the strongest economic arguments for resharpening cobalt-grade bits. A quality M42 bit costs $20–$50 depending on size. A resharpen costs $2–$5. The cobalt content — the thing you're paying for — is still there after grinding. Running a cobalt bit to failure and replacing it with a new one throws away the most expensive part of the tool.
Which Grade Do You Actually Need?
The honest answer for most shops: M2 for most drilling, M35 for stainless and alloy steel, M42 only for the hardest materials where M35 isn't sufficient. The cobalt premium is worth paying when the application demands it — and not worth paying when M2 will do the job equally well.
If you're drilling mild steel, aluminum, or general structural shapes, you're paying a premium for cobalt content you'll never use. If you're drilling 316 stainless or Inconel all day, M42 pays for itself in extended tool life and fewer resharpen cycles.
We Resharpen All HSS Grades
MachinistPost resharpens M2, M35, and M42 HSS drill bits by mail. Same WinsloMatic geometry, same flat-rate pricing — regardless of cobalt content. Don't throw away expensive tooling.
Resharpen Your Bits →M2 is the workingman's HSS. M35 is the right step up for difficult materials. M42 is the specialist grade for extreme applications. Know which one you're using and why, and you'll get the most out of every dollar spent on tooling.