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HSS and cobalt drill bits are both intended for metal, but “cobalt” bits are still high-speed steel bits: cobalt is added to the alloy to help the cutting edge withstand heat. The practical choice depends less on the label than on the steel you are drilling, how often you will use the bit, and whether you can keep it cutting without overheating.
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What we cover
Quick comparison
| Feature | HSS bit | Cobalt bit |
|---|---|---|
| Best suited to | Thin sheet, mild steel, aluminum, wood, and general workshop use | Stainless steel, harder steel, and repeated metal drilling |
| Heat tolerance | Good when used at suitable speed with lubrication | Better; cobalt alloy helps the edge retain hardness at higher temperatures |
| Cost | Usually lower | Usually higher |
| Sharpening | Can be sharpened with a suitable drill-bit sharpener | Can also be sharpened; cobalt is throughout the bit, not just a surface coating |
| Common failure | Edge softens or wears quickly after overheating | Can chip or break if forced, misaligned, or used with a loose setup |
What the labels mean
Standard HSS bits are a sensible starting point for ordinary mild steel and occasional holes. They are widely available, relatively inexpensive, and suitable for a cordless drill or drill press. Their limitation is heat: a bit that spins too fast, rubs instead of cutting, or is pushed hard without cutting fluid can lose its temper. Once the cutting edge is damaged, extra pressure usually makes the hole worse rather than restoring the edge.
Cobalt bits are made from HSS alloyed with cobalt, commonly in grades labeled M35 or M42. The cobalt improves hot hardness, which helps the edge keep cutting in materials that generate heat, such as stainless steel. It does not make a bit immune to overheating, and it does not mean you should run it faster. Cobalt bits are not simply HSS bits with a coating, so sharpening does not remove the cobalt benefit.
Which bit should you buy?
For a few holes in thin mild-steel brackets, sheet metal, or angle stock, a basic HSS drill-bit set for metal is usually enough. Buy a set with clearly marked sizes and replace a damaged bit rather than leaning harder on it. For repeated work in stainless steel or harder steel, a M35 cobalt bit set is worth considering. It costs more, but can last longer when the job and technique suit it.
Do not buy cobalt just because you expect it to drill any metal easily. A dull cobalt bit, a spinning workpiece, or a high-speed drill can still produce heat, chatter, and a ruined hole. For occasional household repairs, a small HSS set may offer better value. For a project with many holes in stainless, cobalt is the more forgiving choice if you use it slowly and keep it lubricated.
Drilling steel with a cordless drill
Secure the work before drilling. Clamp flat stock to a bench or use a vise; never hold a small piece in your hand. Mark the hole and use a center punch to make a small dimple, which helps prevent the bit from skating across the surface. If the hole is larger than about 6 mm (1/4 inch), drill a smaller pilot hole first, often around 3–4 mm (1/8 inch), then step up. A pilot hole helps the larger bit start cleanly, though an oversized pilot can leave the larger bit with too little material to guide it.
Use the drill’s low-speed range when available, especially for larger bits or stainless steel. As a starting point, a 6 mm (1/4-inch) HSS bit in mild steel may run around 1,000–1,500 rpm; in stainless, start closer to 500–800 rpm. A 10 mm (3/8-inch) bit generally needs a lower speed. These are working ranges, not fixed rules: reduce speed if the bit squeals, smokes, or turns the work blue. Apply steady pressure so the bit makes chips. A shiny surface with no chips usually means the bit is rubbing; stop, let it cool, and correct the speed, edge, or feed.
Use cutting oil on steel, particularly stainless. Add a little at the cutting point and reapply when it dries or the bit heats up. If the drill has a clutch, set it to drill mode rather than a low clutch setting, and keep the drill square to the surface. Breakthrough can catch suddenly, so ease pressure as the bit exits and keep both hands on the drill. A side handle is helpful with larger holes.
Failure signs and fixes
A squealing bit, blue or straw-colored chips, smoke, or a hot workpiece are signs of excessive heat. Stop instead of forcing the bit. Let the bit cool, add lubricant, and restart at a lower speed. If the cutting lips are rounded, chipped, or no longer produce chips under reasonable pressure, sharpening or replacement is due. A cobalt bit can chip if it is twisted in a crooked hole or grabbed at breakthrough; harder alloy does not equal greater toughness.
Thin sheet metal can snag even with a good bit. Clamp it firmly against a wood backing board, use a center punch, and reduce pressure as the bit exits. For a clean, larger hole in sheet, a step bit may be easier to control than a large twist drill. For thick steel, a drill press gives better alignment and control than a handheld cordless drill, but it does not remove the need for appropriate speed, sharp bits, and secure clamping.
Choose for the job, not the label
Choose HSS for occasional holes in mild steel and general-purpose use; it is the cheaper, practical option when you control heat and replace worn bits. Choose cobalt for frequent drilling in stainless or harder steel, where its better hot hardness can extend cutting life. In either case, a sharp bit, a clamped workpiece, low enough speed, and steady feed matter more than buying the most expensive set.