Which drill bit should I use for metal?
Which drill bit should I use for metal?
An HSS twist drill, ideally with a 135° split point, run slowly with firm pressure and cutting fluid.
Two things destroy bits in metal: heat and rubbing. Heat comes from too much speed, and the correct surface speed for HSS in steel is far lower than most cordless drills default to — 500-1000 rpm for a small bit and a few hundred for anything above 10 mm. Rubbing comes from too little feed pressure: a bit that is not cutting is polishing, which work-hardens the surface and generates heat without producing swarf. The visible test is the chip. Continuous curls of swarf mean the bit is cutting properly; dust and squealing mean it is rubbing. Cutting fluid carries heat away and stops swarf welding to the flute, and even a squirt of general oil roughly doubles bit life.
The options, ranked
| Option | Good at | Bad at |
|---|---|---|
| HSS jobber bit | Cheap, resharpenable and adequate for mild steel, brass and aluminium. | Loses temper if it overheats, and a blued bit never cuts properly again. |
| HSS with 135° split point | Self-centring, so it starts without a punch mark and does not skate across the surface. | Slightly more fragile at the tip in a hand drill. |
| Cobalt (HSS-Co) bit | Holds hardness at higher temperature, so it survives stainless and tougher alloys. | More brittle and several times the price. |
| Titanium-coated HSS | The coating reduces friction and extends life in mild steel. | Coating is microns thick and gone at the first sharpening, so it is a consumable advantage only. |
| Step (cone) bit | Excellent in sheet metal: it will not grab and tear the way a twist drill does at breakthrough. | Limited to thin material and gives a slightly chamfered hole. |
Technique that changes the outcome
- Centre-punch the mark so the bit cannot wander
- Pilot-drill anything above about 6 mm, then open out
- Slow speed, heavy steady pressure, and stop when swarf stops coming
- Apply cutting fluid or oil continuously — not once at the start
- Ease pressure just before breakthrough so the bit does not snatch
What goes wrong
- High speed, which blues the tip and ruins the temper permanently
- Letting the bit spin without cutting, which hardens the surface for the next attempt
- Holding sheet metal by hand; it will spin on the bit and cut you
Compiled practical guidance rather than a published standard. Product formulations vary by brand and market, so read the label — and where the work is structural, electrical or gas, the regulations that apply where you are govern rather than anything here.
Other drill bit questions
| For | Use |
|---|---|
| Tile | A spear-point carbide or diamond bit in rotary mode only — hammer action shatters tile, every time. |
| Concrete | An SDS-plus carbide bit in a rotary hammer — a normal hammer drill will get there eventually and blunt several bits doing it. |
| Masonry | A carbide-tipped masonry bit in hammer mode for brick and block; step up to an SDS rotary hammer above about 10 mm. |
| Porcelain tile | A diamond core bit, cooled with water, at low speed — carbide spear points do not cut porcelain. |
| Stainless steel | A cobalt bit at low speed with heavy constant pressure and cutting fluid — and never let it stop cutting. |
| Wood | A brad point bit for clean holes up to about 13 mm; an auger for depth and a Forstner for a flat bottom. |
| Plasterboard (drywall) | Almost none — an ordinary HSS or brad point bit in rotary mode, and the real question is which fixing goes in the hole. |
| Glass | A spear-point carbide or a diamond bit, run slowly under water — and toughened glass cannot be drilled at all. |
| Plastic | An HSS bit ground to a shallower rake, or a plastic-specific bit, at low speed with a backing board — acrylic especially will crack. |
| Hardened steel | Solid carbide in a rigid setup, or soften the spot first — HSS and cobalt cannot cut anything much above 50 HRC. |
| Aluminium | A sharp HSS bit at high speed with a wax or paraffin lubricant — the problem is aluminium sticking to the bit, not hardness. |
| Large holes in wood | Spade for speed and roughness, auger for depth, Forstner for a clean flat bottom, hole saw for big diameters in thin stock. |
Common questions
- Which drill bit should I use for metal?
- An HSS twist drill, ideally with a 135° split point, run slowly with firm pressure and cutting fluid.
- What should I avoid?
- High speed, which blues the tip and ruins the temper permanently Letting the bit spin without cutting, which hardens the surface for the next attempt Holding sheet metal by hand; it will spin on the bit and cut you
Related pages
- Which drill bit should I use for tile? A spear-point carbide or diamond bit in rotary mode only — hammer action shatters tile, every time.
- Which drill bit should I use for concrete? An SDS-plus carbide bit in a rotary hammer — a normal hammer drill will get there eventually and blunt several bits doing it.
- Which drill bit should I use for masonry? A carbide-tipped masonry bit in hammer mode for brick and block; step up to an SDS rotary hammer above about 10 mm.
- Which drill bit should I use for porcelain tile? A diamond core bit, cooled with water, at low speed — carbide spear points do not cut porcelain.
- Which drill bit should I use for stainless steel? A cobalt bit at low speed with heavy constant pressure and cutting fluid — and never let it stop cutting.
- Which drill bit should I use for wood? A brad point bit for clean holes up to about 13 mm; an auger for depth and a Forstner for a flat bottom.
- Which drill bit should I use for plasterboard? Almost none — an ordinary HSS or brad point bit in rotary mode, and the real question is which fixing goes in the hole.
- Which drill bit should I use for glass? A spear-point carbide or a diamond bit, run slowly under water — and toughened glass cannot be drilled at all.
- Which drill bit should I use for plastic? An HSS bit ground to a shallower rake, or a plastic-specific bit, at low speed with a backing board — acrylic especially will crack.
- Which drill bit will go through hardened steel? Solid carbide in a rigid setup, or soften the spot first — HSS and cobalt cannot cut anything much above 50 HRC.
- Which glue works on metal? Glue and adhesive
- Which blade cuts metal? Blade
- Which paint should I use on metal? Paint
Sources
- Typical range compiled from manufacturer specifications — At One Place