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What cobalt bits do well
Cobalt twist bits are a sensible choice for drilling steel, stainless steel, and other hard metals, especially when ordinary high-speed steel bits dull too quickly. The cobalt is alloyed throughout the bit, not applied as a coating, so sharpening the cutting edges does not remove the cobalt. Most sets use M35 cobalt steel; M42 has a higher cobalt content and can tolerate more heat, but it is harder and can be more brittle if abused.
These bits still need the right technique. Cobalt does not make a bit immune to overheating, wandering, or breaking. For clean, controlled holes in metal, use a drill press when possible, clamp the work, start with a center punch, and apply steady pressure at a modest speed. A cordless drill works for portable jobs, but use its regular drill setting—not hammer mode—and keep the workpiece secure.
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Choose a set for the holes you actually drill
For general repair work, a set spanning roughly 1/16 to 1/2 inch covers many common fasteners and brackets. A fractional set is easy to shop for in North America. A metric set is more useful for metric hardware, while a combined set costs more and takes up more space. Check the size increments: a set that jumps from 1/4 to 5/16 inch may not cover a job that needs a close-fitting hole.
Before buying, inspect the case and size markings, and check whether the set includes replacement-sized bits for the small diameters you use often. Thin bits are easy to snap and may wear faster. A good case helps keep them sorted; it does not make a low-quality bit cut better.
For a first metalworking set, compare fractional M35 cobalt drill bit sets by size range, flute finish, and case. Do not pay extra for a large assortment if most of the bits will sit unused.
Cobalt, coated HSS, or carbide?
| Bit type | Best use | Trade-off |
|---|---|---|
| Black oxide or titanium-coated HSS | Wood, plastic, and occasional holes in mild steel | Usually cheaper; coating wears off, and the bit may dull sooner in repeated metalwork. |
| M35 cobalt | Regular drilling in steel and stainless steel | Costs more than basic HSS; still needs lubrication, controlled speed, and a sharp edge. |
| M42 cobalt | Frequent work in tougher alloys or hotter cutting conditions | Often more expensive and potentially more brittle; not automatically better for every job. |
| Carbide | Specialized, rigid setups and very hard materials | Very hard but brittle; handheld drilling and poor alignment can chip or break it. |
For a few holes in mild steel, a sharp, inexpensive HSS bit is often enough. Cobalt is a better investment when you repeatedly drill stainless steel, work with thicker stock, or are tired of replacing dulled bits. For ordinary construction fastening, cobalt bits are not a substitute for masonry bits or a hammer drill.
How to drill metal without ruining the bit
Choose a slower speed as the bit diameter and metal hardness increase. As a rough starting point, a 1/8-inch bit in mild steel can often run around 1,000 to 1,500 RPM, while a 1/2-inch bit may need roughly 250 to 400 RPM. Stainless steel generally calls for slower cutting and steady feed. These are starting ranges, not guarantees: the alloy, bit geometry, and drill rigidity all matter.
Use cutting fluid or an appropriate metal-cutting lubricant, particularly for stainless steel and deeper holes. Keep the bit cutting with steady pressure; if it squeals, smokes, turns blue, or stops producing chips, pause and let it cool. Rubbing without cutting can work-harden stainless steel, leaving a harder surface that is more difficult to drill. Do not force a stalled bit with a high-torque cordless drill: the bit can snap, the work can spin, and the drill can twist in your hand.
For holes larger than about 1/4 inch, consider drilling a smaller pilot hole first, especially with a handheld drill. Clamp the metal close to the drilling point, and use a sharp center punch to keep the bit from skating. For thin sheet metal, a backing block and light pressure help prevent grabbing as the bit breaks through.
Features worth checking before you buy
Look for clearly stated M35 or M42 material, a useful size range, and a case that holds each bit securely. Split-point tips can start more easily on metal and reduce walking, but they do not eliminate the need to clamp and mark the work. Standard round shanks fit most drill chucks; verify the shank size if you plan to use a small drill or a quick-change holder.
Most twist bits are meant for a keyed or keyless drill chuck. Avoid using them in an impact driver with a loose hex-shank adapter for precision metal drilling: impact action and extra play can break small bits or leave rough holes. Use a drill or drill press with the workpiece secured. If you already own a basic set and only need occasional holes in mild steel, buy a few quality replacement bits in the sizes you use rather than replacing the whole set. If stainless work is frequent, a M42 cobalt set may be worth comparing, but careful speed and feed still matter more than the label.