How do I drill metal accurately without the bit wandering or grabbing?

Centre punching, step drilling and the breakthrough technique that stops a bit snatching a hole into a triangle.

Difficulty: beginner
Time: 20 min
Read: 3 min
Safety: caution

Short answer

Punch the centre, start with a small pilot, clamp the work and back thin sheet with scrap timber. Slow the speed and ease off pressure as the bit breaks through — that is the moment a bit grabs and tears a round hole into a three-cornered one.

Metal punishes two habits that timber tolerates: starting a bit on a smooth surface, and driving it straight through at full pressure. Fix those and metal drilling becomes predictable.

Safety

Thin sheet metal grabs at breakthrough and can spin, turning the workpiece into a blade. Always clamp it. Wear eye protection — metal swarf in an eye is a hospital visit — and never wear gloves that could catch on a rotating chuck.

What you'll need

centre punch
HSS drill bits
clamps or vice
cutting oil (optional)
step drill bit (optional)
eye protection

Step by step

Mark and punch the centre.A sharp centre punch and one firm hammer blow makes a dimple deep enough for the bit's chisel edge to sit in. Without it the bit will skate across the surface.
Clamp the workpiece.Non-negotiable on sheet. A snatching bit will spin an unclamped sheet at drill speed and it will cut you.
Back thin material with scrap timber.Sandwiching sheet against a block supports the metal at the exit and almost eliminates the grab and the burr.
Start with a small pilot.A 3mm pilot cuts the material that the larger bit's dead centre would otherwise have to push through. It also drills far more accurately.
Use a slow speed.Metal wants low surface speed. As a rule, the bigger the bit and the harder the metal, the slower the drill.
Apply cutting oil.Any oil beats none. It reduces heat and stops the swarf welding to the cutting edge.
Keep steady pressure so it always cuts.Continuous swarf is the sign it is cutting. A rubbing bit polishes and blunts, and in stainless it hardens the material.
Ease right off at breakthrough.The last fraction of a millimetre is where the bit tries to screw itself through. Reduce pressure and let it finish gently.
Use a step drill for sheet.A conical step bit cuts sheet metal in clean round holes with no grabbing at all, because only a short cutting edge is engaged. It is the right tool for anything thin.
Deburr both sides.A larger bit turned by hand, or a countersink, removes the sharp rim. Leaving it makes the part dangerous to handle.

Tips

A step drill bit is the single biggest improvement available for holes in sheet metal.
If the bit is squealing rather than producing swarf, it is rubbing. Slow down and push harder.
Backing thin sheet with a block of scrap wood solves grabbing, burring and distortion in one move.

Common mistakes

Not centre punching — The bit's chisel edge has nothing to locate in, so it skates and marks the surface, and by the time it bites it is off the mark.
Driving through the breakthrough at full pressure — The bit's flutes catch the thin remaining rim and screw the bit through, snatching the workpiece and tearing the hole into a triangle.

If it doesn't work

Hole came out triangular

Cause: Bit grabbed at breakthrough in thin sheet — Fix: Back the work with scrap next time, ease off at the end, or use a step drill.

Bit skated and scratched the surface

Cause: No centre punch mark — Fix: Punch first. Mark scratches can be polished out but the position is usually lost.

Bit blunt after one hole

Cause: Too fast, no oil, or hardened material — Fix: Slow the speed, apply cutting oil, and use cobalt bits in stainless.

Workpiece spun in your hand

Cause: Not clamped — Fix: Clamp everything, without exception. There is no safe way to hand-hold sheet metal against a drill.

Questions people ask

What speed should I use?

Slower than for wood, and slower again as the bit gets bigger or the metal harder. If the swarf is discolouring or the bit is smoking, you are too fast.

Do I need cobalt bits?

For mild steel and aluminium, good HSS is fine. For stainless, hardened steel or repeated work, cobalt pays for itself immediately.

Why does my bit squeal in aluminium and then jam?

Aluminium is soft and gummy; it welds to the cutting edge and then grabs. Use plenty of lubricant, clear the flutes often, and ease off at breakthrough.