I have broken and blunted dozens of expensive drill bits in metal, and for a long time I blamed the bits. I bought titanium-coated ones specifically to get through mild steel and still got nowhere. The bits were fine. I was running them far too fast and leaning on them, which is exactly the combination that turns a cutting edge soft.
Metal is drilled slowly, with weight rather than speed, and with something wet on the cut. Heat is the enemy of every single part of this job — it takes the temper out of the bit, it work-hardens the hole so the next attempt is worse, and it is what makes people push harder, which makes more heat.
What’s Inside!
What You Need
- The right bit – HSS for mild steel and aluminum, cobalt (M35 or M42) for stainless and anything hard
- A variable-speed drill with a real low-speed range
- Center punch and a hammer
- Cutting oil – or engine oil, or soapy water at a push
- Clamps or a vise. Not optional, see below
- Safety glasses, and a file or deburring tool for afterwards
Bit choice matters more here than technique, and the reason is not raw hardness — it is that cobalt is alloyed all the way through, so it keeps its edge at temperatures where plain HSS has already gone soft. A coating cannot do that, because the moment the coating wears at the cutting edge you are back to the base metal underneath. Which is precisely why my titanium-coated bits disappointed me. Which bit for which metal.
Clamp It. This Is the Safety Section.
Thin metal grabs the bit and becomes a spinning blade
When a bit breaks through sheet metal, the cutting edges catch the thin remaining material and the whole workpiece spins with the bit. A piece of sheet steel turning at drill speed with an edge on it will cut through a hand. This is the specific hazard of drilling metal and it happens at the exact moment the job appears to be finishing.
- Vise or clamps, every hole. Hand-holding thin material is the single most dangerous thing you can do with a drill.
- Back thin sheet with scrap wood. It supports the material at breakthrough and stops the bit grabbing.
- Ease off in the last fraction of a millimeter. That is when the grab happens.
- Safety glasses always. Metal swarf comes off hot, sharp and curled, and it goes upward.
- No gloves near the chuck. A glove that catches pulls your hand in. Bare hands and clamped work.
- Do not brush chips away with your fingers. Swarf is a razor ribbon and it is hot. Brush or hook.
The Method
1. Safety First
Safety glasses before anything else, and no gloves near the chuck – a glove that catches pulls your hand in with it. Metal swarf comes off hot, sharp and curled, and it travels upward.
2. Secure Your Workpiece
Vise or clamps, every hole, no exceptions. When the bit breaks through sheet metal it catches the thin remaining material and the whole workpiece spins with it – at drill speed, with an edge on it. Back thin sheet with scrap wood so it is supported at breakthrough.
3. Mark Your Drilling Spot
Center punch it. Not optional on metal: a twist bit has no point that bites, it has a chisel edge that skates, and the punch dimple is the only thing holding it for the first second. On round stock like pipe it is the difference between a hole and a scratch across the surface.
4. Choose the Right Drill Bit
HSS for mild steel and aluminum, cobalt (M35 or M42) for stainless and anything hard. Cobalt is alloyed all the way through, so it keeps its edge at temperatures where plain HSS has already gone soft – which is why a coating cannot do the same job, and why my titanium-coated bits disappointed me. Which bit for which metal.
5. Apply Lubrication
Cutting oil ideally, engine oil or soapy water otherwise. It cuts friction and carries heat out of the cut. Cast iron is the exception – drill it dry, because it is self-lubricating from its graphite content and oil turns the dust into a paste.
6. Start Drilling
Slow, and slower than feels productive. For anything above about 6 mm (1/4 in), drill a pilot hole first – the chisel edge at the center of a large bit does not cut, it crushes, and the pilot removes the material it would otherwise have to crush.
7. Keep Cool
Pause and re-oil every few seconds, and let the bit and the hole cool. On a run of holes, pause between them. Impatience here is what snapped most of the bits I have lost.
8. Clear Chips
Withdraw the bit to clear waste rather than letting it pack the flutes. Never brush swarf away with your fingers – it is a razor ribbon and it is hot. Brush or hook it clear.
9. Finish Drilling
Ease off in the last fraction of a millimeter. The bit grabs as it exits, and this is the moment the workpiece spins if it is going to.
10. Deburr and Clean Up
A countersink, or a larger bit spun by hand, knocks the burr off both faces. A drilled hole in metal leaves a raised lip on the exit side that will cut you later if it is left there.
Swarf tells you everything. Continuous curls means the bit is cutting properly at the right speed and pressure. Small broken flakes means too fast or not enough pressure. Powder means the bit is rubbing, not cutting — stop, because you are work-hardening the hole and every second makes it harder to finish. Blue or straw-colored swarf means it is already too hot.
Work Hardening, and Why the Second Attempt Is Worse
This is the thing nobody tells beginners, and it explains a lot of frustration. When a bit rubs instead of cutting — too fast, too little pressure, dull edge — the metal at the bottom of the hole is deformed and heated rather than removed. Stainless steel does this readily, mild steel does it too. The surface it leaves behind is harder than the material was to start with.
So the hole that would not progress becomes a hole that truly cannot progress, and swapping to a new bit does not help, because the new bit meets the hardened layer first. The fix is to prevent it: enough pressure that the bit always cuts, low enough speed that it never gets hot. Once a spot is work-hardened, you are into carbide or a different location.
Common Questions
Why does my drill bit keep breaking in metal?
Usually heat, occasionally a bit stuck in a hole that then twists. Running too fast takes the temper out of the cutting edge, which makes it dull, which makes you push harder, which breaks it. Slow down, add oil, and pause between holes. It took me a long time and a lot of bits to accept that the answer was patience rather than a better bit.
What speed should I drill steel at?
Slow, and slower as the bit gets bigger or the metal gets harder. There is no single number without knowing both, but the feedback is reliable: continuous curled swarf is right, powder or blue swarf is too fast.
Do I need cutting oil?
On steel and stainless, yes – it cuts friction and carries heat out of the cut. Engine oil or soapy water will do at a push. Aluminum is happier with a light oil or even nothing. Cast iron should be drilled dry.
Can I drill stainless steel with a hand drill?
Yes, with a cobalt bit, plenty of pressure and low speed. Stainless work-hardens faster than mild steel, so the one thing you must not do is let the bit rub. Commit to the cut and keep it cutting.
Why won’t the hole get any deeper?
Almost always a work-hardened spot from earlier rubbing, or a bit that has lost its edge. Neither is fixed by pushing harder. Try a sharp bit with more pressure and less speed; if it still will not bite, that spot has hardened and you need carbide or a new location.
What size pilot hole?
Roughly the width of the final bit’s chisel edge at its center – for most bits above 6 mm (1/4 in), something around 3 mm works. The point is to remove the material the big bit’s center would have to crush rather than cut.
If the swarf stops curling, stop drilling. Everything expensive that has ever happened to me in metal started in the ten seconds after I ignored that.
