Best Metal Drill Bits for Door Installations: Why the BOSCH CO21B Outperforms Cheaper Brands

We tested several options for metal doors, stainless steel frames, and lock hardware. After our technicians in NYC used them in the real world, one clear winner stood out: the BOSCH CO21B Cobalt Drill Bit Set. Here’s our hands-on review and comparison.

What Didn’t Hold Up in the Field

We began with several affordable options to keep costs low, but most wore out quickly, even after just a few jobs.

COMOWARE Cobalt Drill Bit Set (15-Piece)

• Marketed as M35 high-speed steel with a wide size range
• Dulls quickly, especially on stainless steel
• Only suitable for light-duty or one-time use

IRWIN Cobalt Alloy Steel Drill Bit Set (15-Piece)

• Dulls under repeated use, especially with harder metals
• Acceptable for softer materials, but not reliable long-term

Cobalt Drill Bits for Metal and Steel – 13-Piece Set (1/16″ – 1/4″)

• Fully grounded 5% cobalt construction sounded promising
• Performance declined after limited use on door frames
• Lacked consistency and wore down faster than expected

M35 Twist Drill Bit Set (13-Piece, 1/16″ – 1/4″)

• Advertised for stainless steel and cast iron
• Performed poorly under real work demands
• Like others, dulled faster than we could justify

COMOWARE Hex Shank Cobalt Step Drill Bit (1/4” – 1-3/8”)

• Looked ideal for general metal and plastic use
• Usable for soft applications, but didn’t hold up to the door frame metal
• Better suited for hobby or one-time use than pro installation work

What Performed: BOSCH CO21B (21-Piece Set)

Once we switched to the BOSCH CO21B, the improvement was immediate.

• M42 cobalt construction cuts cleanly through stainless steel, cast iron, titanium, and other hard metals

• Split-point tips on 1/8-inch and larger bits allow for precise starts without drifting — no center punch needed

• Three-flat shank design holds securely in the drill chuck to prevent slipping

• Still sharp after multiple real-world installations

• Comes in a compact, durable case with a full range of bit sizes

We mostly use the thinner bits from this set during our installations, and they’ve far outlasted anything we’ve previously tested.

Ideal Use Cases

• Hinge preparation on metal doors
• Mortising for locksets and latches
• Drilling into frames for strikes and closers
• General metal fabrication tasks involving aluminum, stainless, and steel

Final Verdict

If you’re a DIYer working on one or two installs, cheaper sets like COMOWARE or IRWIN might get the job done, but expect them to dull quickly. If you’re a contractor, technician, or anyone handling door installs regularly, the BOSCH CO21B is an investment that pays off in time saved, cleaner holes, and longer tool life.

We use it on real jobs weekly, and it hasn’t let us down.

Drill Bit Comparison: What We Tested and What Held Up

Here’s a quick side-by-side breakdown of the drill bit sets we tested while installing and repairing metal doors in NYC. This table highlights the material, performance, cost, and our recommendation for each option.