Stainless steel is tough, work-hardens the instant it overheats, and dulls ordinary bits in seconds. The best drill bits for stainless steel are cobalt bits run slow with firm, steady pressure and plenty of cutting oil. This guide explains exactly what to look for, the right technique, and how to shop the matching category.
What to Look For
Stainless resists heat and hardens the moment you let a bit rub instead of cut, so bit material matters more than anything else. High-speed steel (HSS) can start a hole but loses its edge quickly and glazes the metal. Cobalt bits contain roughly 5 to 8 percent cobalt blended right through the steel, so they keep their hardness at high temperatures and shrug off the abrasion stainless creates. Look for a strong 135° split point that starts without a centre punch and resists walking on the slick surface. Grades M35 and M42 are the ones you want — M42 has more cobalt and lasts longer in the hardest 300-series and duplex stainless.
The Best Type of Bit for Stainless Steel
Choose a solid cobalt (M35 or M42) jobber-length bit with a 135° split point. Cobalt handles the heat and work-hardening that defeat plain HSS, and the split point cuts cleanly at the low speeds stainless demands, without wandering off your mark.
- Material: Cobalt (M35 / M42) for heat and abrasion resistance.
- Point angle: 135° split point to prevent walking on hard, slick surfaces.
- Speed: Run slow — high RPM burns the edge and hardens the metal.
- Coolant: Flood with cutting oil to carry away heat and extend bit life.
Technique and Mistakes to Avoid
Let the bit cut — never let it spin without biting, because friction is exactly what work-hardens stainless into a glassy skin no bit can penetrate. Use firm, constant pressure and a steady low speed, and keep the hole flooded with cutting fluid. On larger holes, start with a smaller pilot and step up in stages so the full-size bit has less material to remove. Clear the chips often and re-oil as you go. If the surface ever glazes over, you have overheated it — move to a fresh spot or grind past the hardened layer with a fresh, sharp edge. Our USA-made cobalt bits are ground for precisely this kind of demanding, heat-sensitive work.
Getting the Size Right
Sizing matters as much as material when you plan to tap the hole for a thread. Undersize the hole and the tap jams and snaps in the hard stainless; oversize it and the threads are weak. Use a tap drill chart to pick the correct pilot diameter for your thread, and step up to it in stages on larger sizes so each cobalt bit removes only a little metal. A cobalt drill set covers the common fractional sizes so you always have the right diameter on hand.