How do I drill stainless steel without work-hardening it?

Why stainless goes glass-hard under a rubbing bit, and the speed, pressure and cutting fluid that stop it happening.

Difficulty: intermediate
Time: 20 min
Read: 4 min
Safety: caution

Short answer

Slow speed, heavy constant pressure and cutting fluid. Stainless work-hardens the instant the bit stops cutting and starts rubbing, so a fast bit under light pressure polishes a hardened glaze onto the hole that nothing will then cut. Once through that, the job is easy.

Almost everyone who fails at stainless does the same thing: high speed, light pressure, because that is what works in softwood. Stainless responds to that by hardening at the surface, and the harder it gets the more the bit rubs, which hardens it further.

Safety

Swarf from stainless comes off as needle-sharp spirals that will cut you and can flick into an eye. Wear eye protection and gloves, and never clear swarf with a bare hand while the drill is turning. Clamp the work — a sheet that grabs and spins is the commonest injury with metal drilling.

What you'll need

cobalt or HSS-Co drill bit
cutting fluid or oil
variable speed drill or pillar drill
clamps
centre punch (optional)

Step by step

Use a cobalt bit, not a standard HSS one.HSS-Co, usually labelled M35 or 5% cobalt, holds its edge at the temperature stainless generates. A plain HSS bit will do one hole and then be blunt.
Clamp the work down hard.Stainless grabs. A sheet spun off the bench by a snatching bit is dangerous and it ruins the hole.
Punch the centre.A sharp centre punch mark stops the bit skating across a hard smooth surface, which is how most stainless is finished.
Set the speed low.Slower than you think. Stainless is a low-surface-speed material; the larger the bit, the slower it must go. If the swarf is coming off blue, you are too fast.
Apply cutting fluid before you start.Proper cutting fluid, or failing that a general oil. It carries heat away and stops the chip welding itself to the cutting edge.
Push hard and keep pushing.The bit must be taking a cut at all times. Constant firm pressure and a continuous curl of swarf is the target; the moment it stops cutting and starts squealing, it is hardening the hole.
Do not stop and restart in the same hole.Pausing lets the surface at the bottom of the hole harden. If you must stop, resharpen or change bit before going back in.
Step up in stages for large holes.A pilot then one or two intermediate sizes. Trying to take a 10mm hole in one pass in stainless overloads the bit and glazes the material.
Ease off at breakthrough.As the tip breaks through, the bit tries to snatch and pull itself into the hole. Reduce pressure over the last fraction while keeping the rotation going.
Deburr both faces.The burr on a stainless hole is razor sharp. A larger bit spun by hand, or a countersink, takes it off in seconds.

Tips

Blue swarf means too fast. Long continuous curls of bright swarf means you have it right.
If a hole has already hardened, you need a solid carbide bit or a new start alongside it — a cobalt bit will not cut a glazed surface.
A pillar drill makes this dramatically easier because you can apply real pressure without the bit wandering.

Common mistakes

High speed with light pressure — The bit rubs rather than cuts. Rubbing generates heat and the stainless surface hardens under it, so the hole becomes progressively harder to drill and the bit progressively blunter.
Drilling dry — Without cutting fluid the chip welds to the cutting edge, the edge tears out, and the bit is finished in one hole.

If it doesn't work

Bit spinning with no swarf coming out

Cause: Surface has work-hardened, or bit is blunt — Fix: Change to a sharp cobalt or carbide bit, push harder, run slower. If the glaze is set, start a fresh hole nearby if the design allows.

Swarf coming off blue and the bit smoking

Cause: Speed too high — Fix: Halve the speed and add cutting fluid. Blue means the bit is being tempered soft as you use it.

Bit skating across the surface

Cause: No centre punch on a hard polished face — Fix: Punch a deep dimple first, or start with a small bit at very low speed.

Bit snatches and grabs at breakthrough

Cause: Full pressure carried through the exit — Fix: Ease pressure over the last part of the cut, and always clamp the work.

Questions people ask

Can I use a titanium-coated bit?

The coating is thin and is gone after the first hole or two. For a one-off in thin stainless it may get you through; for anything repeated, cobalt is the answer because the hardness runs all the way through the bit.

What can I use instead of cutting fluid?

Any oil is far better than nothing. Ordinary machine oil or even cooking oil will carry heat. Water alone cools but does not lubricate, so it is a poor second.

Why is my stainless sink harder than the stainless bolt?

Grade and work history differ. Thin pressed sheet has already been cold-worked in forming, so it starts closer to hardened than annealed bar does.