Bosch Cobalt M42 Drill Bit

First impressions

A thin curl of blue steel told me a lot before I stopped the press. The Bosch cobalt bit was tracking straight, breaking chips predictably, and coming out of 304 stainless without a hint of squeal. That was my first pass with the 1/8 inch size, and it set the tone for the rest of my testing. Over a few days, I drilled mild steel bar, 304 stainless plate, angle iron, cast iron, and some 6061 aluminum, using both a bench-top drill press and a 18V cordless drill. I also spent extra time with the 1/16 inch bit (model CO2131B) to see how the smallest diameter held up.

Build and materials

The bit is made from M42 high-speed steel with 8% cobalt. That matters for two reasons: hot hardness and wear resistance. Cobalt doesn’t make the bit “coated”; it changes the alloy itself, so the cutting edges keep their hardness when heat builds up in tougher steels. The tip is surface-hardened and the body uses a thick web-helix geometry. That thicker core adds stiffness (less flex, less chatter), and the helix favors material removal in ferrous metals. You pay for that stiffness with a bit more thrust requirement, which I’ll get to.

Everything else is what I’d expect from a jobber-length metal bit: straight shank, standard flute polish, and clean grind. There’s no split-point grind here, so you’ll still want a center punch for precise starts.

Setup and test conditions

• Tools: bench-top drill press (belt-drive, 250–3000 RPM) and 18V brushless drill/driver
• Lubrication: sulfurized cutting oil for steel and stainless; WD-40 for aluminum; dry for cast iron
• Workholding: vise and clamps; painters tape for sheet to reduce burring
• Techniques: center-punch for all metals, peck drilling in stainless and aluminum, consistent feed pressure

I kept speeds slow for stainless (e.g., around 600–800 RPM at 1/8 inch, slower as diameter increased), moderate for mild steel, and higher for aluminum.

Performance in steel and stainless

In mild steel, the bit cuts cleanly and confidently. The thicker web helps keep it from wandering when your punch mark isn’t perfect, and the flutes clear chips well. I could push feed pressure a touch higher than with a standard HSS bit without the edge glazing or squealing.

Stainless is the real exam. M42 shines here. Even in 304, I didn’t see the edge soften after multiple holes, provided I kept the speed low and used cutting oil. The bit stayed sharp enough to produce shiny, continuous chips with good color and minimal blueing. The surface-hardened tip bites better than a generic jobber bit, and the helix doesn’t load up as quickly as some M35 cobalt bits I’ve used.

The trade-off: because the core is thicker, you need to maintain steady pressure. Feather-light feed in stainless can rub and heat the edge; solid, controlled pressure keeps the bit cutting and cool.

Cast iron and aluminum behavior

Cast iron likes dry drilling, and the bit complied—crisp holes, powdery chips, and no chatter. In aluminum, chip evacuation is always the challenge. The bit’s helix is aggressive enough to pull chips, but aluminum’s tendency to stick means you’re better off with a touch of lubricant and short pecks. With that, holes were clean and oversize was minimal.

Small-diameter durability

The 1/16 inch size is a good stress test. Tiny bits are prone to snapping, particularly in stainless or when the drill isn’t perfectly perpendicular. The cobalt bit’s thicker web gives it more backbone than a standard HSS micro bit. I drilled pilot holes in mild steel angle and a couple in stainless sheet without breaking it. Technique matters—use a light touch and let the tool cut—but it’s more forgiving than most at this diameter.

Precision and starting

Without a split point, the bit will walk on smooth metal if you skip the center punch. With a good punch mark, starts are predictable. In thin sheet metal, I had better results stepping up in small increments rather than jumping straight to final size. For example, 1/8 inch final holes were easier when I started with the 1/16 inch and then went to 3/32 inch before the last pass, especially with a handheld drill.

Heat resistance and edge life

Cobalt’s advantage is most noticeable after a dozen or more holes in tougher steels. Where plain HSS begins to polish the cutting edges and lose bite, the Bosch cobalt bit kept cutting with only a slight increase in feed needed. I didn’t see chipping at the cutting lips, and the wear land progressed as expected for M42. If you’re used to black-oxide HSS bits, expect a meaningful jump in life as long as you keep speeds conservative and use oil in steel.

It also resharpens well. Because the alloy is through-hardened, not coated, you can restore the edge on a bench grinder or a drill doctor without burning through a surface treatment.

Power and machine considerations

The thick web means a bit more thrust is required, especially above 1/4 inch. On a small cordless drill, you’ll notice it—holing 3/8 inch in steel is doable, but a drill press or a high-torque drill makes it smoother. For tiny sizes (1/16–3/32 inch), the extra stiffness is a benefit; the bit is less whippy and less likely to flex and snap.

Sizes and coverage

The line spans common fractional sizes from 1/16 inch up to 1/2 inch, in single-bit packs. Lengths are jobber and appropriate for general metalwork: enough flute length for bracketry, plate, and angle, without the wobble penalty of extra-long bits. I like that they’re sold individually—it’s easy to replace the few sizes you burn through most often without buying a whole set.

Where it excels

• Drilling stainless and hardened steels at correct speeds and with lubrication
• Small diameter pilot holes where stiffness matters
• General fab work in mild steel, angle iron, and channel
• Jobsite use where bits get hot and you can’t always baby them

Limitations

• No split-point grind, so a center punch is non-negotiable for precise starts
• Requires a bit more thrust than a thin-web HSS bit; small, low-torque drills may struggle in larger diameters
• In aluminum, you’ll need to peck and lube to avoid chip welding in the flutes

Tips for best results

• Punch your hole locations and use a pilot bit on sheet metal
• Keep RPM low in steel and stainless; use cutting oil generously
• Apply steady, confident feed—don’t let the edge rub
• Peck drill in gummy materials and clear chips often
• Resharpen as needed; cobalt takes a clean new edge

How it stacks up

Compared with plain HSS and titanium-coated bits, the Bosch cobalt bit lasts longer in heat, holds an edge in stainless, and resists micro-chipping. Versus other cobalt options (M35), M42’s higher cobalt content does add a margin of hot hardness that you can feel on tougher metals. Carbide still wins for hardened steels and production work, but cobalt is far more forgiving and versatile for handheld and jobsite use.

Verdict

I reached for this bit repeatedly because it’s predictable: it starts cleanly with a punch, runs cool with reasonable technique, and holds its edge in steels that quickly humble ordinary HSS. The thick web-helix design adds stability in small diameters and helps with chip control, at the cost of slightly higher thrust. If you’re willing to use a center punch and modulate your feed, it rewards you with straight holes and long service life.

Recommendation: I recommend the Bosch cobalt bit for anyone drilling metal regularly—especially stainless, mild steel, and cast iron—who values durability over fast starts. It’s a sturdy, long-wearing option that performs best with proper technique and lubrication. If you need self-centering convenience above all else, look for a split-point alternative; otherwise, this is a reliable, hard-working bit that earns its spot in a metalworking kit.