When plain high-speed steel dulls halfway through a stainless plate, cobalt is the answer. The best cobalt drill bits carry a percentage of cobalt alloyed through the whole bit, so heat resistance holds even as the cutting edge wears back with sharpening. That makes them the sensible choice for hard, abrasive and heat-prone metals that punish ordinary drills.
What to Look For
Cobalt content matters. A five-percent alloy suits most shop steels, while an eight-percent alloy pushes further into stainless and tougher grades. Because the cobalt runs through the entire bit rather than sitting in a thin coating, you can resharpen a cobalt drill many times without losing its heat resistance. Look for a 135° split point, which self-centres on hard surfaces and cuts the walking that ruins a clean start. A shank that fits your chuck securely rounds out the choice, and for larger sizes a reduced shank lets a big bit run in a standard chuck.
Our Top Pick
For steady metalwork, choose a solid cobalt jobber bit with a split point. It bridges general drilling and demanding stainless jobs without needing a second set on the bench. If you run production or repeat work, a three-eighths reduced shank keeps larger sizes turning in a standard chuck without stepping up to an industrial machine. American-made cobalt bits are ground to consistent geometry, which shows in cleaner holes, less wandering and longer life between sharpenings.
- Alloy: five percent for shop steel, eight percent for stainless.
- Point: a 135° split point self-centres on hard metal.
- Speed: run slower on hard alloys and let the edge cut.
- Coolant: use cutting fluid to carry heat away from the tip.
How Cobalt Beats Plain HSS
The difference is heat. Drilling stainless or hardened steel builds temperature fast, and once an ordinary HSS edge softens it is finished for good. Cobalt keeps its hardness at higher temperatures, so the edge survives the job instead of burning away in the cut. That resistance also means you can hold a steadier feed without glazing the surface, which is important because a rubbing bit work-hardens stainless and makes the next hole harder still to start.
Technique That Protects the Edge
Let the bit do the work. Firm, constant pressure produces a continuous chip and keeps the edge cutting rather than rubbing. Rubbing generates heat and polishes the metal until it resists the drill entirely. Clear the flutes often, keep coolant on the tip and ease off as the bit breaks through the far side. Treated this way, a good cobalt bit will outlast a whole stack of cheaper drills and pay for itself many times over. It also helps to centre-punch the spot first, since a small dimple gives the point somewhere to sit and stops the tip skating across a polished surface before the flutes engage and pull the bit off its mark.