Bosch Cobalt Metal Drill Bit Set

Overview

I’ve been steadily adding more metalwork to my projects, and that’s pushed me to be picky about drill bits. The Bosch cobalt set has been living in my shop for the last few months, and I’ve reached for it whenever I’m drilling steel that would glaze a standard high-speed steel bit. It’s a straightforward, no-nonsense set: cobalt alloy twist bits, two practical kit sizes, and coverage from the small pilot diameters up to a full 1/2 inch. There’s also an “M42” variant on some sizes (identified with a B suffix on the bit), which I’ll touch on, since it matters for tougher alloys.

What’s in the set

You can buy the set in a 14-piece or 21-piece configuration. Both include duplicates of common small sizes, which I appreciate because those are the ones most likely to snap when you’re working by hand. The range tops out at 1/2 inch, which is a real cutoff for many DIY and jobsite drills (if your drill has a 3/8-inch chuck, you won’t be able to chuck the 1/2-inch bit). The 21-piece adds some in-between sizes and extra duplicates in the sizes most of us abuse—1/16, 5/64, 1/8, 3/16, and 1/4.

Some variants in the line are labeled as M42. That alloy has a higher cobalt content than standard cobalt bits and holds hardness at elevated temperatures better. If you frequently drill stainless or hardened fasteners, it’s worth keeping an eye out for the M42-marked pieces.

Build and metallurgy

Cobalt bits aren’t coated; the cobalt is in the steel alloy itself. That matters for two reasons. One, you can resharpen them repeatedly without losing a surface treatment. Two, they resist softening when you inevitably build heat in tougher metals. The trade-off is that cobalt bits are typically a bit more brittle than plain HSS. Treat them as cutting tools, not prybars, and they’ll reward you with long life.

The grinds on my set were consistent out of the box—no burrs or lopsided lips. Flute finish is clean, and chip evacuation has been reliable in mild steel and conduit with light oil. I didn’t see any premature blueing or temper loss as long as I kept the speed sensible and took the time to break chips on deeper holes.

Performance in real materials

I put this set to work in the same places most users will:

• Hardened bolts and automotive fasteners: Starting with a center punch and a small pilot, I moved to 1/8 inch and then to 3/16 inch. With cutting fluid and a firm feed, the bit kept biting instead of skating. Even in stubborn fasteners, the bit didn’t polish the hole—progress was slow and steady, which is what you want on hardened material.
• EMT and rigid conduit: The bits cut cleanly without grabby breakouts. On a handheld drill, moderate speed with pecking passes cleared chips and kept heat down. I didn’t need to baby them.
• Mild steel angle and plate: No drama here. The cobalt alloy shrugs off the heat you build punching multiple holes in a bracket. Compared to standard HSS, I could feel the bit stay sharper across a full afternoon of drilling.
• Wood and plastics: Overkill? Probably. But if your drill is already set up, the bits produce clean holes in wood with a crisp entry. I wouldn’t buy cobalt for carpentry, but it’s nice not to swap kits for a quick hole.

I didn’t notice excessive walking provided I used a center punch or a starter hole. That’s good news if you’re working overhead or on rounded surfaces like pipe.

Technique that makes a difference

Cobalt rewards the right approach. A few habits that paid off for me:

• Mark and start: A center punch or a small pilot saves the larger bit from skating and premature edge wear.
• Slow the drill down: Handheld drills love to spin fast. Resist that. Lower RPM with firm, steady pressure keeps the bit cutting instead of rubbing. For hardened steel and stainless, slower still.
• Use oil: Even a dab of cutting fluid keeps the edge cooler and extends life. WD-40 works in a pinch on aluminum; a sulfurized cutting oil is better for steel.
• Peck drill on deeper holes: Back the bit out periodically to clear chips. Clogged flutes make heat, and heat kills edges.
• Don’t side load: Cobalt is tough but not flexible. Let the bit cut straight.

Follow those, and you’ll see why cobalt bits earn their keep.

Case and organization

The case is compact and packs a lot of steel into a small footprint. That’s the upside. The downside is that the bits sit close together, and it takes a careful pinch to pull just one without nudging its neighbor. The size markings in the holder are on the small side and can be awkward to read in low light or after they pick up some shop grime.

Two fixes helped me:
- I hit the size embossing with a silver paint marker, then wiped the surface clean so the numbers stand out.
- I store the most-used duplicates (1/8 and 1/4) in the front row for quicker access and fewer mix-ups.

I’d love to see bolder marking from the factory and a little more finger clearance in the trays, but neither issue stopped me from using the set daily.

Durability and sharpening

These bits hold an edge well in steel, which is the whole point of cobalt. After several sessions in mild and hardened materials, the 1/8-inch bit still cut cleanly without burnishing the hole. The bigger diameters (3/8 and 1/2 inch) will reward you if you bring the speed down and really lean on chip clearing; they’ll stay sharp longer and run cooler.

Because cobalt is through-hardened alloy, you can sharpen them on a bench grinder or a jig without worrying about grinding through a coating. If you don’t sharpen your own bits, the duplicates in the 21-piece set soften the blow of snapping a small diameter or dulling a frequently used size.

Picking between the 14- and 21-piece kits

The 14-piece kit covers the basics from 1/16 to 1/2 inch, with a sensible duplicate in 1/8 inch. If you mostly drill brackets, sheet, and the occasional bolt, that’s enough. The 21-piece adds more intermediate sizes and duplicates in the most abused diameters. I prefer the 21-piece because it reduces the urge to “make it work” with the wrong size when you’re between steps, and the extra 3/16 and 1/4 duplicates are practical breakage insurance.

If you regularly see stainless or tool steel, look for the bits identified as M42 (often marked with a B). They stand up better to heat and will outlast standard cobalt in tough alloys, especially if you don’t have a drill press to control speed perfectly.

Limitations

No drill bit is perfect. A few realities to keep in mind:
- Cobalt edges can chip if you wobble the drill or pry the hole open. Use a step bit or reamer if you need to enlarge.
- The 1/2-inch bit requires a 1/2-inch chuck. Many homeowner drills are 3/8-inch; plan accordingly.
- The case markings are hard to read in poor light, and the tight trays make bit retrieval fussy if your hands are gloved.

None of these are deal-breakers, but they’re worth knowing.

Value

Cobalt bits usually sit at a premium over HSS. In exchange, you get longer edge life in steel, especially if your technique isn’t perfect. This set hits a good balance: real cobalt alloy, sizes that cover common tasks, and useful duplicates without fluff. If you drill mostly aluminum, wood, or plastic, you won’t use their full advantage. If you drill steel weekly—or even monthly—you’ll save time and frustration.

Who it’s for

• Fabricators, mechanics, and serious DIYers who drill steel regularly.
• Anyone who’s tired of blue-smoking HSS bits on stainless or hardened fasteners.
• Users willing to slow down, use oil, and adopt good technique.

Who might look elsewhere:
- Woodworkers who need brad points and clean exit holes in hardwoods.
- Casual users who rarely drill steel and want the cheapest set possible.

Recommendation

I recommend the Bosch cobalt set. It has the right metallurgy for metalwork, holds an edge in difficult materials, and includes practical duplicates of the sizes we break most. In use, it cut cleanly through hardened bolts and conduit without drama, and with basic cutting technique it stayed sharp far longer than standard HSS. The case isn’t perfect—the size markings are hard to read and the trays are tight—but those are manageable annoyances, not functional flaws. If you’re drilling steel with any regularity, this set earns its keep and then some.