Short answer:
HSS for the everyday — mild steel, wood, aluminium, plastic.
Cobalt for the tough stuff — stainless, cast iron and hard, high-alloy steels.
Cobalt costs a bit more, but it holds its edge at much higher temperatures — which is exactly what tough metals throw at a drill bit.
Both are “HSS” really — the difference is what’s in the steel. Standard HSS (high-speed steel) is a great all-rounder. Cobalt bits are HSS with 5–8% cobalt alloyed in, which lets them keep cutting when the going gets hot. Here’s how they compare and when to reach for each.
HSS vs cobalt at a glance
| HSS | Cobalt (M35 / M42) | |
|---|---|---|
| Best for | Mild & structural steel, wood, aluminium, plastic, general fabrication | Stainless steel, cast iron, hardened and high-alloy steels, repeated drilling |
| Heat resistance | Good | Excellent — keeps cutting when HSS would soften |
| Edge life in tough metal | Blunts quickly on stainless / hard steel | Lasts far longer under heat and wear |
| Toughness | More flexible, forgiving of flex and wobble | Harder but slightly more brittle — avoid side-load and snatching |
| Re-sharpening | Yes | Yes — cobalt runs all the way through, so it sharpens back to full performance |
| Price | Lower | Higher — but cheaper per hole on the jobs that eat HSS |
When to choose HSS
For most everyday drilling, standard HSS drill bits are all you need — mild steel, timber, aluminium, plastic and general workshop or site work. They’re cheaper, they cut cleanly, and they’re more forgiving if the drill wanders. If you’re doing a lot of repeat holes in ordinary material, titanium-coated HSS lasts longer again without stepping up to cobalt.
When to choose cobalt
Reach for cobalt the moment the material gets hard or hot:
- Stainless steel — the big one. Stainless work-hardens as you drill it: too much speed or a bit that rubs instead of cutting and the surface goes glass-hard and blunts a standard HSS bit almost instantly. Cobalt shrugs off the heat. (More in our stainless guide.)
- Cast iron and tough alloy steels — abrasive and hard on edges; cobalt holds up.
- Production drilling — lots of holes back to back builds heat. Cobalt lasts, so you swap bits far less often.
The trade-off: cobalt is a touch more brittle, so keep it cutting with firm, steady pressure, don’t let it snatch on breakthrough, and use cutting fluid on metal.
Both, in every size, from stock
HSS in 0.1 mm increments, plus M35 and M42 cobalt for the tough jobs. Over a million bits in stock, same-day dispatch, free next-working-day UK delivery on orders over £50.
And the speed matters too
Whichever you choose, the right drilling speed is half the battle — the wrong RPM will blunt even a cobalt bit. As a rule: the bigger the bit and the harder the metal, the slower you go.
Frequently asked questions
Are cobalt drill bits better than HSS?
Not universally — they’re better for hard, hot work. Cobalt resists heat far better, so it wins on stainless steel, cast iron, hardened alloys and repeated drilling. For everyday mild steel, wood, aluminium and plastic, standard HSS is cheaper and does the job just as well.
Can I use HSS on stainless steel?
You can, but it won’t last. Stainless work-hardens and generates heat, which takes the edge off standard HSS quickly. Use cobalt, run it slow, keep firm pressure so it’s always cutting, and use cutting fluid.
What’s the difference between HSS and cobalt?
Cobalt bits are HSS with 5–8% cobalt alloyed into the steel. That cobalt lets them hold their hardness at much higher temperatures, so they keep cutting where plain HSS would soften and blunt.
Are cobalt bits worth the extra money?
On tough materials, yes — they last several times longer than HSS on stainless and hard steel, so they work out cheaper per hole. On soft, everyday material there’s no need to pay the premium; HSS is fine.
Can cobalt drill bits be sharpened?
Yes. The cobalt is alloyed all the way through the bit, not just a coating, so re-sharpening brings back full cutting performance.
UK Drills has supplied drill bits to UK trade and industry since 1985, from our trade counter in Hindley, Wigan. Not sure which to choose for your job? Email info@ukdrills.com — we’re happy to advise.
