How to Drill Stainless Steel Without Work-Hardening the Hole

Stainless steel has a trick no other common metal plays: it gets harder while you work it. Let a drill spin against 304 without cutting — two seconds is enough — and the surface under the bit strain-hardens into a glaze that the same bit cannot re-enter. The hole is now harder than it was, the bit is hotter than it was, and every further second makes both worse.

Once you understand that one mechanism, every rule for drilling stainless follows from it. The bit must be cutting — always, from first contact to breakthrough. Everything below is just how to arrange that.

Why the hole fights back

Annealed 304 sits around 200 HB — soft enough. But austenitic stainless strain-hardens ferociously: cold-work the surface and its hardness can double. A sharp bit under firm feed removes metal faster than it hardens it. A dull bit, a timid feed or a spinning start does the opposite — it creates hardened skin faster than it cuts, and loses the race it started.

That is why stainless problems snowball. The first hesitant contact glazes the surface; the glaze blunts the edge; the blunt edge rubs harder; the rubbing makes more glaze. The failure was decided in the first two seconds.

The numbers: slow — much slower than feels right

Stainless wants around a third of the cutting speed of mild steel. With an M35 cobalt bit, aim near these:

With a plain HSS bit, drop another 30% and accept a short life. Those numbers feel wrong the first time — 380 RPM is barely turning compared with the shriek most cordless drills make in metal. The shriek is the sound of a bit being ruined.

Feed is the other half, and in stainless it outranks speed: firm, continuous, committed. You should see an unbroken chip curling off both flutes from bite to breakthrough. If the chips stop while the bit is still turning, lift off now — spinning in the hole is how the glaze forms. Chip colour is the gauge: silver is right, straw is the limit, blue means the edge has already been somewhere it should not go. The full diagnosis logic is in why drill bits break.

The bit: cobalt is not a luxury here

Ordinary M2 HSS starts losing its hardness around 550 °C, and a rubbing edge in stainless passes that in moments. M35 (5% cobalt) holds hardness roughly 50–70 °C deeper into the red zone and resists softening in exactly the conditions stainless creates; M42 (8% cobalt) extends it again for production work. The HSS / cobalt / carbide comparison covers the metallurgy; the workshop summary is one line: in stainless, cobalt is the difference between one hole and forty.

Geometry matters as much as grade: a 135° split point cuts right to the centre, so the bit starts where it lands without a punch mark and without the skating that begins the hardening cycle. A standard 118° chisel point pushes before it cuts — in stainless that push is the enemy.

From our range

Industrial 13-Piece M35 Cobalt Twist Drill Bit Set for Stainless Steel

M35 cobalt set with 135° split point — grade and geometry that survive 304/316.

Oil: the cheapest tool on the bench

• Any oil beats no oil. Proper sulphurised cutting oil beats WD-40, but WD-40 beats dry by a distance.
• Keep the tip wet, not damp — re-oil at every peck. The oil is doing lubrication as much as cooling: it stops the chip welding to the edge.
• If the oil smokes on contact, the edge is already too hot. Let everything cool before the next attempt.

Rescuing a hole that has glazed

You started someone else's abandoned hole, or the drill slipped its speed, and now nothing bites. Three ways out, in order of preference:

1. 01Switch to a carbide bit for a few seconds — carbide does not care about the glaze. Low RPM, heavy feed, just enough to get under the hardened layer, then back to cobalt.
2. 02Grind past it: a small mounted point or burr through the glaze, then drill normally.
3. 03If it is a through hole in accessible sheet, come from the other side — the far surface is still soft.

What does not work is patience with the same bit. The glaze is harder than HSS; time only polishes it.

Sheet and tube: change the tool, not just the speed

Thin stainless sheet adds the grabbing problem to the hardening problem — a twist drill's corners hook the sheet at breakthrough and tear a triangular hole. Above about 10 mm hole size, stop using twist drills in sheet entirely: a step drill keeps a full cylinder in contact and cannot triangulate, and an M35 or bi-metal hole saw covers the big sizes. Clamp the sheet either way; stainless offcuts spin like blades.

From our range

HSS M35 5% Cobalt Step Drill Bit 3-Piece Set (4-12/20/32mm) for Stainless Steel

M35 cobalt step drills 4–32 mm — the sheet-metal answer in stainless.

From our range

Industrial M35 Cobalt HSS Hole Saw for Stainless Steel – Titanium Coated, OEM Sizes

M35 cobalt hole saw — for the sizes past step-drill territory.