How to Drill Through Metal Without Burning Your Bits

Metal punishes bad habits. The rule that fixes most of it is simple: go slow, push hard, and keep the cutting edge cool. Fast speed with light pressure is what turns a sharp bit blue and leaves you spinning uselessly in a shiny dimple.

Pick the right bit first

Most jobs around a house or garage are mild steel, aluminium or stainless, and the bit you grab decides how easy the next ten minutes will be.

• HSS (high speed steel): the standard grey bits. Fine for aluminium, brass, copper and thin mild steel.
• Titanium coated HSS: the gold coloured ones. The coating reduces friction and buys you a longer life, but only until the coating on the cutting edge wears through. It cannot be resharpened without losing the benefit.
• Cobalt (HSSCo, often M35 or M42): the dull bronze looking bits. Cobalt is alloyed through the whole bit, not just plated on, so it handles heat far better. This is what you want for stainless steel and anything hardened.
• Step bits: excellent in sheet metal up to about 3mm. They self centre, cut round holes rather than triangular ones, and deburr as they go.

Whatever you use, check the tip. A bit that has gone blue at the point is already dead and will only generate more heat. Sharp bits cut cool, blunt bits rub and burn.

Mark and centre punch the hole

This step is not optional on metal. A drill bit landing on a smooth flat surface will skate sideways and score your workpiece before it bites.

1. Mark the centre with a scribe or a permanent marker.
2. Set a centre punch on the mark and give it one firm hit with a hammer. You want a visible dimple, not a crater.
3. Sit the drill tip in the dimple before you pull the trigger.

An automatic centre punch, the spring loaded type you press down until it clicks, is worth the small cost if you do this often. It works one handed and leaves a consistent dimple.

Set the speed low

This is where most people go wrong. Metal wants a low surface speed and a big bit at high RPM overheats almost instantly. As a rough starting point on a cordless drill in low gear:

• Small bits under 4mm in aluminium or mild steel: moderate speed is fine.
• Bits from 5mm to 10mm in steel: slow, roughly 500 to 800 RPM.
• Anything above 10mm, or any stainless: as slow as your drill will hold steady, often under 400 RPM.

The bigger the bit and the harder the metal, the slower it goes. If you are using a drill with two gear settings, gear 1 is the one you want for almost all metal work. Hammer mode stays off. Hammer action does nothing useful in metal and just batters the cutting edge.

Pressure, lubrication and chips

Lean into it. Firm steady pressure keeps the cutting edge biting into fresh material rather than rubbing on the surface it just work hardened. You are aiming to produce continuous curling chips or spirals of swarf. Fine dust or sparks means the bit is skating and heating up.

Add cutting fluid for anything other than a quick shallow hole. Proper cutting oil or tapping fluid is best, but general purpose 3 in 1 oil works well enough for mild steel in a pinch. Add a drop every few seconds. Aluminium is the exception where oil is optional, though a little paraffin or WD40 stops the swarf welding itself to the flutes.

Back the bit out every ten seconds or so to clear the chips and let air get in. Packed flutes trap heat and jam the bit.

Stainless steel needs a different mindset

Stainless work hardens the moment you let a bit spin without cutting. Once the surface has hardened, a normal HSS bit will not touch it. So on stainless: cobalt bit, very low speed, heavy constant pressure, plenty of fluid, and never let the bit dwell in the hole while spinning. If you have already glazed the surface, you usually need to drill through the hardened layer with a fresh cobalt bit under real force, or grind that spot away and start again.

Breaking through and finishing

Ease off the pressure as the bit is about to break through the far side. A twist bit grabs at breakthrough, and on thin sheet that grab can snatch the workpiece out of the clamp and spin it into your hand. Clamp everything, always, and never hold sheet metal with your fingers.

Finish by knocking off the burr on both faces. A countersink bit spun by hand, a larger drill bit twisted a couple of turns, or a deburring tool all work. Burrs are sharp enough to cut you and they stop fixings sitting flat.

Safety points worth repeating

• Eye protection every time. Hot swarf flies further than you expect.
• No gloves near a rotating bit, and tie back long hair.
• Clamp the workpiece to a bench or use a vice. Do not rely on grip strength.
• Sweep up swarf with a brush or magnet rather than your hand.
• Hot metal and hot bits look exactly like cool metal and cool bits.

Common problems and what they mean

If the bit is spinning but not cutting, it is either blunt or the surface has work hardened. If the bit walks off the mark, you skipped the centre punch. If the hole comes out oval or triangular, the bit is unevenly sharpened or the drill was rocking. If smoke and blue tips are involved, the speed was too high and the pressure too light.