HSS, cobalt or SDS: the right drill bit for steel, stainless and concrete

Use a high-speed steel (HSS) bit for mild steel, aluminium, wood and plastic; a cobalt HSS bit for stainless and hardened steel; and an SDS Plus or SDS Max carbide bit in a rotary hammer for brick, block and concrete. A blunt hole, a smoking bit or a chewed chuck nearly always means the wrong bit, the wrong speed, or hammer action switched on where it should not be.

The three families are not interchangeable: an HSS bit will not survive hammer action, and an SDS bit will not grip in a keyed or keyless chuck without an adaptor.

The everyday twist drill. DIN 338 fixes the length for each diameter (a 10 mm bit is 133 mm overall with an 87 mm flute), so a replacement from any maker fits the same job. A 118-degree point needs a centre punch to start; a split point, such as the Bosch PointTeQ range, starts on the mark without walking. Run them at the speed the diameter asks for: around 1,500 rpm for a 4 mm bit in mild steel, falling to about 600 rpm at 10 mm and 300 rpm at 20 mm, with firm pressure and cutting oil. Use a pilot hole of about a third of the final size for anything above 10 mm.

Cobalt bits are HSS with 5 to 8 percent cobalt alloyed through the steel, not a coating, so they hold their edge at the heat stainless generates. Stainless work-hardens when a bit rubs instead of cuts, so drill at half the speed you would use for mild steel, keep the pressure on so the bit is always cutting, and use cutting fluid. The same bits suit spring steel, cast iron and welds. A titanium-coated bit is cheaper but the coating goes with the first sharpening; cobalt is the same alloy all the way through.

The SDS Plus shank has two grooves that lock into a rotary hammer and let the bit move back and forth while the hammer pounds it. The carbide tip chips the masonry while the flutes clear the dust. Sizes run from 4 mm for plugs and anchors to roughly 26 mm, and the bit is named by three numbers, for example 10/100/160: a 10 mm diameter, 100 mm working length and 160 mm overall, so check the working length covers your anchor depth plus the fixture. Use the hammer setting without drilling into reinforcement; if the bit stops on steel, move the hole.

Bigger hammers use the SDS Max shank, which has three grooves and a larger diameter to take the blows of a 5 to 12 kg machine. It is the tool for through-holes from about 20 to 50 mm, core bits, chisels and point breakers. An SDS Max to SDS Plus adaptor lets a large hammer run smaller bits, but a small hammer cannot run SDS Max.

• Hammer action on an HSS bit: the edge shatters. Hammer is for carbide bits in masonry only.
• Too fast in steel: the tip blues and softens. Slow down as the diameter grows.
• Rubbing on stainless: the surface work-hardens and the bit skates. Keep cutting, use cobalt and fluid.
• Wrong working length: the anchor needs more depth than the flute allows. Read the three numbers on the SDS bit.
• Dull bits kept in service: HSS bits can be resharpened; carbide tips cannot be, so replace them.