What Drill Bit to Use for Metal: HSS, Cobalt and Titanium Explained

For most metal drilling around the house, a standard HSS twist bit is all you need. Step up to cobalt when you hit stainless steel or hardened fixings, and use a step bit for thin sheet metal where a twist bit would grab and tear. The bit material matters less than most people think; the coating and the point angle usually decide whether the hole goes well or the bit dies halfway through.

HSS: the default choice for mild steel and aluminium

High speed steel is the workhorse. It holds an edge at the temperatures generated by hand drilling and it is cheap enough that snapping one is annoying rather than expensive. Plain HSS bits handle mild steel, aluminium, brass, copper and most galvanised sheet without complaint.

The main limit is heat. HSS starts to soften somewhere around 550C, and the tip of a bit spinning too fast in steel gets there faster than you would expect. If your bits keep going blue at the tip, the bit material is not the problem, the speed is.

Titanium coated HSS: better wear life, same core

Titanium nitride bits are HSS underneath with a thin gold coloured surface layer. That layer is harder than the steel below it and reduces friction, so the bit stays sharp longer and clears chips more easily. In practice you get a noticeably longer working life in mild steel, which is why most multi packs sold for general use are coated.

The catch is that the coating is microns thick and only exists on the outside. Once you sharpen a titanium bit, you have sharpened the coating off the cutting edges and you are back to plain HSS performance. That is not a disaster, but it does mean coated bits are best treated as consumables rather than as tools you maintain.

Cobalt: for stainless steel and anything hardened

Cobalt bits, usually labelled M35 or M42, are not coated. The cobalt is alloyed through the whole bit, typically 5 percent for M35 and 8 percent for M42. That raises the temperature the steel can tolerate before it loses hardness, which is exactly what you need in stainless.

Stainless is the material that catches people out. It work hardens, meaning that if you let the bit rub instead of cut, the surface gets harder and the bit then cannot bite at all. Once that happens, no amount of extra pressure will help; you either need a fresh sharp bit and a lot more feed pressure, or you drill through the hardened skin from the other side. Cobalt bits, low speed and firm continuous pressure are the combination that avoids the whole problem.

Cobalt is also more brittle than plain HSS. Use it in a drill press or a well braced hand drill, and avoid twisting the bit sideways as you break through.

Step bits for thin sheet metal

A twist bit in sheet metal thinner than about 2mm tends to grab as it breaks through, snatching the workpiece around and leaving a triangular hole. A step bit, sometimes called a unibit or cone bit, cuts a clean round hole because only one cutting edge is engaged at a time and there is nothing for the bit to hook on.

Step bits also deburr as you go and cover a whole range of sizes in one tool, which makes them the obvious pick for electrical enclosures, ducting, car body panels and stainless sinks. They are not suitable for anything thick; past around 4mm the steps run out of depth.

Hole saws and specials

• Bi metal hole saws for holes above roughly 16mm in sheet or thin plate. Run them slowly and use plenty of cutting fluid.
• Sheet metal or Zip bits with a split point and a shallower flute, sold specifically for thin gauge work.
• Carbide tipped bits for hardened plate, tool steel or old cast iron. Expensive and brittle, but they will go where HSS simply will not.
• Left hand drill bits for extracting snapped bolts, since they often back the broken stud out as they cut.

Point angle and split points

Most bits sold in the UK come ground at 118 degrees, which is a general purpose angle suited to mild steel and aluminium. A 135 degree point is flatter, spreads the load over more of the edge and lasts longer in stainless and other tough alloys. Where 135 degree bits really help is that they are almost always split point, meaning the centre has been relieved so the bit starts cutting immediately instead of skating across the surface.

If you are buying one set to cover everything, a 135 degree split point cobalt set is the most versatile option. If you are drilling wood and plastic as well, keep a separate 118 degree HSS set for that, because the split point tends to grab in softer materials.

Speed and pressure matter more than the bit

The single most common reason a metal bit fails is running it too fast. As a rough guide for HSS in mild steel: a 3mm bit is happy around 1500 to 2000rpm, a 6mm bit around 900rpm, a 10mm bit around 500rpm, and anything above 12mm wants to be under 400rpm. Stainless needs roughly half those numbers. On a cordless drill, that means using low gear and feathering the trigger rather than pinning it.

Pressure should be firm and constant. A bit that is allowed to spin without biting generates heat instead of chips, and heat is what kills the cutting edge. Use cutting fluid on steel and stainless, and use paraffin or WD40 on aluminium to stop the swarf welding itself to the flutes.

Finally, always centre punch before you start. Even a split point bit will drift on a smooth machined surface, and a punch mark gives the tip something to sit in while the hole establishes itself.