M35 cobalt bits with 5% cobalt are the usual starting point for stainless steel, while M42 handles tougher alloys and drill-press work.
Stainless steel punishes the wrong drill bit faster than almost any other material. When you need cobalt drill bits for stainless steel, the grade choice matters more than brand—and that choice comes down to how much cobalt is in the steel alloy itself.
M35 vs. M42: Cobalt Grade Choices For Stainless Steel
For most 304 and 316 stainless work, M35 is the practical default, while M42 earns its place in tougher materials and drill-press work. Both are high-speed steel (HSS) with cobalt mixed into the alloy, not just painted on as a coating. Cobalt improves hot hardness, so the cutting edge keeps its temper at the temperatures stainless creates.
Bosch’s cobalt drill bit specs describe a fully ground HSS bit with 5% cobalt, two cutters, and two flutes for chip removal. Bosch lists stainless steel, cast iron, and heat-resistant steels among the intended materials.
| Property | M35 (5% cobalt) | M42 (8% cobalt) |
|---|---|---|
| Cobalt content | 5% in the alloy | 8% in the alloy |
| Best stainless match | 304, 316, general stainless | Harder, tougher stainless alloys |
| Heat resistance | Good for most jobs | Higher red hardness |
| Handheld drilling | Preferred—tougher edge | Works, but more brittle |
| Drill press use | Good, steady results | Excellent in a rigid setup |
| Typical targets | Sheet, plate, stainless hardware | Tool steel, hardened stainless |
| Starting point | Start here | Move up when M35 dulls fast |
That’s why M42 is not automatically “better.” In a hand drill, M35 is usually the better choice because M42’s harder edge can chip or snap if the bit catches. In a rigid drill press, M42’s extra heat resistance earns its keep, especially on hard or work-hardened stainless.
How To Drill Stainless Steel Without Ruining The Bit
Drill slow, push hard, and keep oil on the cut. That combination stops the stainless from work-hardening before the bit finishes the hole.
- Center punch the spot first, and use a smaller pilot bit for large holes, so the bit doesn’t wander.
- Run low speed: about 200–400 RPM for a 6 mm bit (roughly 1/4 inch) in 304 stainless, and 60–120 RPM for a 12 mm bit (roughly 1/2 inch).
- Apply firm, steady pressure so the bit keeps cutting. Too little pressure makes it rub, and rubbing hardens the hole.
- Flood the cut with cutting oil throughout, using thread-cutting oil, a dedicated stainless fluid, or soluble oil.
- Once the bit is engaged, avoid lifting or pecking. Repeated lift-off leaves a hardened ring that is miserable to cut through.
- Stop if the edge dulls. A sharp cobalt bit cuts clean; a dull one generates heat and worsens work-hardening.
Plain HSS and titanium-coated bits are the usual reason stainless holes go wrong. They overheat quickly and lose their edge, while cobalt alloy bits keep cutting at higher temperatures.
A Simple Buying Rule For Cobalt Drill Bits
Start with M35 unless the stainless is hardened. If M35 dulls fast or you’re working from a rigid drill press, step up to M42 for its extra red hardness. If you only drill stainless occasionally, a small M35 set covers most jobs; shops that live in stainless usually keep both grades around.
When you’re ready to buy, our tested drill bits for stainless steel guide compares the sets worth buying, so you can skip the guesswork.
FAQs
Can Cobalt Drill Bits Be Used In A Hand Drill?
Yes. For handheld work, M35 cobalt bits are usually the better choice because they hold an edge without being as brittle as M42. Center punch the spot, run a low spindle speed, and keep firm pressure so the bit cuts continuously rather than rubbing.
Do Cobalt Drill Bits Need Cutting Oil?
Yes, real cutting oil or a stainless-specific fluid should be used for every stainless hole. The oil cools the edge, flushes chips, and stops the stainless from work-hardening. Running dry is the fastest way to ruin both the bit and the workpiece.
References & Sources
- Bosch. “Cobalt Drill Bit Sets.” Official product page describing 5% cobalt alloy, fully ground HSS, and stainless steel applications.