Bosch Cobalt Metal Drill Bit Set

Why I reached for this cobalt set

Metal work exposes weaknesses in drill bits fast. Mild steel is one thing, but once you get into thick angle iron, stainless, or a frozen bolt, lesser bits glaze, squeal, and leave you with a hot, dulled tip. I picked up the Bosch cobalt set to see if a reasonably priced, mainstream cobalt option could hold up across common shop tasks without babying the bits. After several weeks on the press and in a cordless drill, I’ve got a good sense of where this set shines and where it could be better.

The lineup and what you actually get

The set comes in 14- and 21-piece configurations, each available in a standard cobalt alloy and a higher-heat M42 variant (labeled with a “B” in the model). Coverage is squarely aimed at common fractional sizes, with a hard stop at 1/2 inch:

• 14-piece: 1/16 through 1/2 with sensible steps; you get two 1/8-inch bits, which is a thoughtful inclusion since that size sees a lot of abuse.
• 21-piece: adds in-between sizes like 11/64, 13/64, and 15/64, plus duplicates of break-prone sizes (1/16, 5/64, 1/8, 3/16, 1/4). Those extra steps matter when you’re chasing hole fit or sneaking up on a tap size.

If you often work with stainless or need to drill hardened fasteners, the M42 versions (CO14B, CO21B) are the ones to consider. M42 cobalt steel carries more heat without softening, so it tolerates slower chip evacuation and less-than-ideal cooling better than standard cobalt. For general steel work, the non-M42 sets are still a meaningful upgrade from black-oxide or titanium-coated HSS.

Build quality and grind

Out of the box, the flutes and tips on my set were cleanly ground with no visible burrs or chips. Shanks were straight, and the larger diameters had a nice, consistent finish that didn’t snag in the chuck. I didn’t see any coating flaking because there is no coating to flake—these are cobalt-alloy bits through and through, which also means they can be brought back to life on a grinder when they eventually dull.

The cutting edges came uniform enough that I could start holes on flat steel plate with only a light center punch. On round stock or curved surfaces like conduit, I still recommend a punch and a short pilot, but the bits didn’t walk excessively once I established a dimple.

Performance in metal

This is where the set earns its keep. In mild steel plate and angle up to 1/2 inch, the bits cut predictably without shrieking, provided I kept the speed in check and used cutting fluid. Chips form as little curls rather than dust, and you can feel the bit pulling itself into the work when the feed is right. I was able to run repeated 1/4- and 3/8-inch holes without edge blueing, and after deburring the holes looked clean and true.

On stainless sheet and tube, the M42 variant is the safer bet, but even the standard cobalt set handled modest stainless tasks if I was disciplined about speed and lubrication. I tested on 304 sheet and a short section of 1-inch tube; keeping RPM low and pecking to clear chips kept temperatures reasonable and edges sharp.

The most telling test was drilling out a hardened fastener. With a good center punch and a small pilot, the 1/8-inch bit in the M42 set bit in and kept cutting where a titanium-coated HSS had earlier skated and dulled. It still took patience and coolant, but the bit stayed intact and usable afterward—a result I’d expect from a quality cobalt bit.

Versatility beyond metal

These are metal-first bits, but they’re not one-trick. In wood, they behave like a standard twist bit—fast and reasonably clean on exit if you back the work. They’re not a replacement for brad-point or forstner bits when you care about tear-out or flat-bottomed holes, but if you’re bouncing between a metal fabrication and some supporting carpentry, you won’t feel hamstrung.

Aluminum and brass cut easily with these, though I prefer a slightly higher speed in aluminum and a lighter hand in brass to avoid grabbing. The flutes clear chips well, and finish quality is good enough for clearance holes without reaming.

Heat management and technique tips

Cobalt helps, but it isn’t a license to go full trigger. A few habits extend the life of the set dramatically:

• Use cutting fluid or oil, especially in steel and stainless.
• Keep RPMs low for larger diameters and tougher alloys. A simple rule: the bigger the bit, the slower the speed.
• Peck drill to break chips and let the bit cool.
• Pilot drill for holes 3/8 and above; stepping up improves accuracy and reduces heat.
• Always center punch on hard or curved surfaces.

Follow that, and you’ll go a long time between sharpenings. Ignore it, and even cobalt will glaze and dull.

Case and organization

The case is compact and holds the bits securely, but it’s not perfect. The size markings in the case wells are small and low-contrast, so in a dim shop or with dusty lenses you’ll squint and double-check. The smaller sizes sit close together, which keeps the footprint tidy but makes retrieval fiddly with gloved hands. I appreciate the duplicates of common sizes in the 21-piece set, and I ended up labeling the high-traffic sizes on the lid with a paint pen to speed things up.

On the plus side, the bits seat firmly and don’t rattle out of place in transit. The hinge and latch have held up fine so far; I’ve tossed the case in a tool bag and into the truck without finding a nest of loose bits later.

Accuracy and hole quality

In steel plate and tube, holes came out round and within an expected tolerance for a hand drill and a basic drill press. The bits track straight if you establish a good start, and I didn’t see the kind of tip wander that leaves crescent-shaped openings. For tapped holes, the added in-between sizes in the 21-piece set made it easy to hit recommended tap drills without hunting for single loose bits.

Burrs are reasonable on exit and clean up quickly with a countersink or a pass of emery. If you care about pristine exits in thin stock, back the work or sandwich with scrap to avoid mushrooming.

Durability and sharpening

After a mix of steel, aluminum, and stainless, the cutting edges on the workhorse sizes still felt keen. The 1/8-inch bit has seen the worst abuse and remains usable—helped by the fact that there’s a spare in both set sizes. Because these are cobalt-alloy bits rather than coated HSS, you can resharpen them as needed without grinding through a performance layer. A quick touch-up on a sharpening jig brought a 3/16 back from a slight dulling with no drama.

Which version to buy

• Choose the 14-piece cobalt set if you mainly work in mild steel and want a compact range that covers the sizes you’ll actually use, including a 1/2-inch max.
• Step up to the 21-piece if you do tapping and appreciate those in-between sizes and duplicates of the common diameters.
• Go M42 if stainless, hardened fasteners, or extended drilling sessions are part of your weekly routine. The higher hot-hardness buys you margin when heat is unavoidable.

The trade-offs

No bit set is perfect. The case markings could be larger and higher contrast. The tight spacing makes plucking the smallest bits a bit of a chore. And if you routinely need letter and number sizes or metric, this fractional-only lineup won’t cover everything—expect to supplement.

But the fundamentals are strong: consistent grinding, true running, predictable cutting behavior, and durability that lives up to what cobalt is supposed to provide.

Recommendation

I recommend this Bosch cobalt set. It drills steel cleanly, tolerates heat better than standard HSS, and includes the sizes you actually reach for, with smart duplicates in the larger kit. The M42 variants in particular are worth it if you work with stainless or have to drill out hardened hardware. The case could be easier to read and a little more glove-friendly, but those are small trade-offs for the cutting performance and longevity on offer. If you’re outfitting a shop for reliable metal drilling without paying boutique prices, this set belongs in your rotation.