A fundi needs one 6 millimetre hole through the hinge plate of a steel gate. He fits a bit from the set, squeezes the trigger fully, and leans in. The bit squeals, a thin thread of smoke rises, the tip turns straw and then blue, and after a minute there is a shiny dimple in the plate and a bit that will never cut again.

Nothing about the drill was wrong. The bit was the wrong grade, it was turning three or four times too fast for its diameter, and nobody put a drop of oil on it. Each of those costs nothing to get right, and together they decide whether a set of bits lasts a month or several years.

This article is about the bits rather than the machine, which our guide to choosing a drill covers. Here: what each bit is made of, which drill and mode it needs, how fast it should turn, why bits burn, and how to keep a small set sharp enough to be worth owning.

Three Materials, Three Completely Different Ways of Making a Hole

A hole in wood, steel or concrete looks the same when finished. Each is made by a different physical action, and the bit is shaped for that action alone.

Wood is sliced. Timber is a bundle of fibres, and a clean hole comes from severing them at the rim before lifting the waste out, so wood bits carry a centre point against wandering and spurs or a knife edge at the rim. Brad point bits do this for small holes, spade bits for fast rough holes, augers for deep holes in posts, and Forstner bits for flat bottomed holes such as hinge cups.

Metal is sheared. A twist bit has two cutting lips that peel a chip off the bottom of the hole, exactly as a lathe tool does, and the chip curls up the flutes and out. That only happens when the lip is sharp, the bit is turning slowly enough, and the pressure is firm enough for the lip to bite. Fail any of the three and the lip rubs instead of cutting, which is the beginning of every burnt bit.

Masonry is crushed. Nothing slices concrete. A masonry bit has a blunt wedge of tungsten carbide brazed onto a steel body, and it works by being hammered forward thousands of times a minute while it turns. Each blow chips a little stone, the rotation moves the wedge to fresh stone, and the flutes carry the dust out. Without hammer action it simply polishes the concrete and heats up.

The bit families do not cross over. A masonry bit in steel rubs. An HSS bit in concrete loses its edge on the first piece of aggregate. The one exception worth knowing is that an HSS twist bit drills small holes in wood perfectly well, so a good metal set doubles as a small wood set.

HSS, Cobalt, Carbide Tipped and Coated: What the Words Mean

The terms on a bit set sound like marketing and are actually metallurgy. They tell you what heat the bit will survive.

Carbon steel is the cheapest grade, and what a very cheap combination set is usually made of. It loses its hardness at the temperature that turns steel blue, so one hot hole in steel finishes it. It is adequate for wood and nothing else.

High speed steel, or HSS, is alloyed with tungsten, molybdenum, chromium and vanadium so that it stays hard at temperatures that soften carbon steel. It is the default for mild steel, aluminium, brass and wood, and at 60 shillings for a 6 millimetre bit on our current price list there is no economy in buying carbon steel for metal work.

Cobalt bits are HSS with cobalt in the alloy, usually five or eight percent, which raises the temperature the edge tolerates again. That matters in stainless and hardened steel, which work harden under a rubbing edge and generate far more heat than mild steel. Cobalt is the whole bit, not a coating, so it can be resharpened indefinitely, though it is more brittle.

Carbide tipped bits carry a brazed tungsten carbide insert, far harder than any steel and still hard when hot, but brittle, so it is used as a crushing wedge rather than a slicing edge. Look at the braze on a cheap masonry bit: if the carbide sits off centre or the joint is porous, the tip will shear off in the first hard hole.

Coatings are the gold or black finish. Titanium nitride, the gold one, is a very thin hard layer that reduces friction and is gone from the lips the first time you sharpen the bit. Black oxide resists rust and adds nothing to cutting. Neither turns a soft bit into a hard one; a gold carbon steel bit is still a carbon steel bit.

Match the bit, the drill mode and the speed to the material Bit Drill and mode Speed Wood Brad point, spade, auger or Forstner Any drill, hammer OFF Fast for small, slow for spade and auger Steel HSS twist bit; cobalt for stainless Any drill, hammer OFF, oil on Slow, and slower as diameter grows Masonry and concrete Carbide tipped; SDS shank for rotary hammer Impact drill hammer ON, or SDS rotary hammer Moderate; let the hammer do the work
The three pairings that decide almost every drilling job. The most common failures are a masonry bit with the hammer switched off, an HSS bit with the hammer switched on, and any bit in steel at full trigger.

Which Drill, and Why the Shank Matters

A plain round shank fits the three jaw chuck on a rotary or impact drill. An SDS shank is different in kind. Grooves along its length engage keys in the holder of a rotary hammer, so it is locked against turning but free to slide a few millimetres. That sliding is the whole point: the hammer's piston hits the back of the bit, and the bit must move for the blow to reach the tip. SDS-plus is the common shank for bits to roughly 25 millimetres; SDS-max is the heavy shank for large core and breaker work.

An impact drill makes its hammer action with two ribbed discs clicking past each other as the chuck turns, giving many small taps. That handles blockwork, brick and occasional holes in concrete to about 10 or 12 millimetres. Ours run from 3,200 shillings for a 680 watt machine to 6,000 shillings for a Bosch GSB 570, and both drill wood and steel with the hammer switched off.

A rotary hammer makes far heavier blows and does not care how hard you push. For daily anchor holes, or anything above 12 millimetres in concrete, it is the tool, and at 7,125 to 7,900 shillings for an 800 watt machine it is not a big step up. It takes SDS bits only, and a chuck adapter for round shank bits must never be used with the hammer on.

The rule that follows: hammer action only ever goes with a carbide tipped bit. An HSS bit under hammer blows chips its lips within seconds. Set the mode before you fit the bit.

Speed: Bigger and Harder Means Slower

This is where most bits die. What matters to the cutting edge is not revolutions per minute but how fast the edge moves through the material, which machinists call cutting speed and measure in metres per minute. Doubling the diameter doubles the distance the edge travels per turn, so the same RPM gives twice the cutting speed. Handbook figures for HSS in mild steel sit around 25 to 30 metres per minute; aluminium and brass tolerate two to three times that, and stainless wants about a third.

Most hand held impact drills exceed 3,000 RPM at full trigger, which is about right for a 3 millimetre bit in steel and roughly four times too fast for a 13 millimetre one.

Sensible top speed for an HSS bit in mild steel Revolutions per minute, computed at a cutting speed of 30 metres per minute 3,000 2,000 1,000 0 3,180 1,590 955 735 3 mm 6 mm 10 mm 13 mm Halve these for stainless steel. Double or treble them for aluminium and brass.
Speed falls as diameter rises because the cutting edge travels further per turn. These figures come straight from the cutting speed formula, RPM = cutting speed times 1,000 divided by pi times diameter, and are a ceiling, not a target. Illustrative.

You control speed with the trigger and, on a two speed drill, the gear selector: low gear for anything above 8 millimetres in steel, spade and auger bits and hole saws; high gear for small twist bits and for masonry. Listen as much as look. A bit that is cutting makes a steady, low sound and a continuous chip. A squeal is the sound of rubbing.

A rough rule you can carry in your head. In steel, the RPM you want is about 10,000 divided by the bit diameter in millimetres. Ten thousand over 6 is roughly 1,600; over 13 is about 770. It is not exact, but it is always closer than full trigger.

Pilot Holes, Pressure and the Chisel Edge

The two lips of a twist bit do not meet at a point; they are joined by a short flat called the chisel edge, and that edge does not cut. It is pushed into the metal and extrudes it sideways. On a 12 millimetre bit it is 2 or 3 millimetres wide and takes most of the force you apply, which is why large holes feel so much harder than they should.

A pilot hole removes the metal the chisel edge would have to push through. Make it a little larger than the chisel edge, about a quarter to a third of the final diameter: a 4 millimetre pilot for a 12 millimetre hole. Larger pilots leave the big bit nothing to steady it, so it grabs at break through, which is how wrists get hurt and bits get snapped.

Before the pilot, mark the centre with a punch, since a twist bit walks across smooth steel until something stops it, and clamp the work: a plate held by hand spins when the bit breaks through, and a spinning plate is a blade. Then press firmly and steadily, so the lip bites deep enough to raise a chip on every turn, and ease off just as the bit breaks through. In deep holes, withdraw the bit every few millimetres to clear the flutes.

Thin sheet is the exception: a twist bit leaves a triangular, torn hole in it, so drill sheet on a scrap of timber or use a step drill, which has no chisel edge at all. In timber the pilot is for the screw rather than the bit: a hole about the diameter of the screw's core lets the threads grip without splitting the grain.

Why Bits Burn

A drill bit does not fail from use. It fails from heat, and the heat comes from an edge rubbing rather than cutting. When a sharp lip is cutting properly, most of the heat goes into the chip and leaves with it. The moment the lip stops cutting, because it is dull, turning too fast, or pressed too lightly to bite, the heat stays at the tip. Hotter steel is softer steel, so the edge dulls further, rubs harder and gets hotter still. From the first squeal to a ruined bit is often less than a minute.

The colours tell the story. Steel at a couple of hundred degrees oxidises to straw, and above roughly 300 degrees it turns blue, the temper colours a blacksmith reads. On carbon steel, blue means the edge has lost its hardness for good. HSS tolerates far more heat, but a blued HSS tip that has stopped producing chips has been rubbing, and its lips need grinding back past the damage.

Four habits keep bits cool: the right speed for the diameter, pressure firm enough to raise a chip, cutting oil on steel (even old engine oil is far better than nothing and roughly doubles bit life), and cleared flutes. Aluminium welds itself to a dry bit, so it needs lubricant even more than steel; paraffin is the traditional choice. Masonry bits burn differently: the carbide does not soften, but the braze holding it does if the bit is run without hammer or with the flutes packed with dust.

Keeping a Small Set Sharp and Usable

Sharpening separates a set that lasts from one bought again every few months, and it takes a bench grinder and ten minutes of practice. Standard twist bits have a point angle of 118 degrees, so each lip sits at 59 degrees to the axis. Behind each lip the tip must fall away, the lip relief, so only the edge touches the bottom of the hole; without relief the bit rubs on its heel. And both lips must be the same length and angle, because a bit with unequal lips cuts with one of them, drills oversize and heats twice as fast. Hold the bit to the wheel at that angle, lip horizontal, and roll it gently downward so the relief forms behind the edge. Dip it in water after every pass; the tip must never colour on the grinder.

What a correctly ground tip looks like 118 degree point 59 degrees each side of the axis Lip Lip Four checks before the bit goes back in the box Both lips the same length, meeting on the axis Each lip at 59 degrees, so the point is 118 degrees Relief behind each lip: the heel sits lower than the edge Chisel edge short and centred, no burr on the lips Test it in scrap: two equal curling chips means both lips are cutting. One chip means one lip is doing all the work and the hole will be oversize.
A twist bit ground to the standard 118 degree point with equal lips. The two chip test on a piece of scrap is the quickest way to know whether a freehand grind has worked. Illustrative, not to scale.

Do not sharpen carbide tipped masonry bits on a grey bench grinder wheel; carbide needs a green silicon carbide or diamond wheel, and at 100 to 150 shillings for a 6 or 8 millimetre Total masonry bit it is rarely worth the trouble below 12 millimetres. Do not resharpen a bit whose shank has spun in the chuck and galled. Our article on bench grinder sharpening covers wheel choice and stance.

The set itself should be small and honest. For metal, HSS in 2, 3, 4, 5, 6, 8, 10 and 13 millimetres, with doubles of 3 and 6 because those are the ones that break. For masonry, carbide tipped 6, 8, 10 and 12 to match your plugs and anchors, in SDS if you own a rotary hammer. For wood, brad points to 10 millimetres, spade bits at 16, 20 and 25, and a 35 millimetre Forstner if you fit hinges. Everything else is bought when a job needs it.

Storage decides how usable that set stays. Keep bits in an index case with sizes marked, so a dull bit gets noticed rather than hunted for, and wipe them with an oily rag before they go away, which at the Coast is not optional: a bit rusted in the flutes cuts badly and packs with swarf. The full range, with the drills and grinders that use them, is on our workshop and industrial tools page, and the wider kit is covered in our guide to essential workshop power tools.

Key Takeaways

  • Wood is sliced, steel is sheared and masonry is crushed. Each material has its own bit, and the families do not cross over.
  • HSS is the minimum for metal. Cobalt is for stainless and hardened steel. Carbide tipped is for masonry. Coatings are thin and disappear on the first sharpening.
  • Hammer action only ever goes with a carbide tipped bit. An SDS shank exists so the bit can slide and receive the rotary hammer's blow.
  • Bigger and harder means slower. In steel, aim for roughly 10,000 divided by the diameter in millimetres, and never full trigger above 6 millimetres.
  • Pilot a little wider than the chisel edge, about a quarter to a third of the final size. Centre punch, clamp, and ease off at break through.
  • Bits burn because they rub instead of cutting. Correct speed, firm pressure, cutting oil and cleared flutes keep the heat in the chip.
  • Sharpen HSS to a 118 degree point with equal lips and relief, cool it in water, test it for two chips, and store it oiled in a marked case.

Frequently Asked Questions

Can I use a masonry drill bit on metal or wood?

No. A masonry bit has a blunt carbide tip designed to crush and chip stone with the hammer action on. In steel it rubs and polishes without cutting, and in wood it tears a ragged hole and wanders. Use a high speed steel twist bit for metal and a brad point, spade or auger bit for wood. The one crossover that works is an HSS twist bit in wood for small holes.

Why does my drill bit turn blue and stop cutting?

Because it was rubbing instead of cutting. A bit that is turning too fast for its diameter, being pushed too lightly to take a chip, or already dull, generates heat at the tip faster than the chips can carry it away. The steel colours straw and then blue as the temperature rises, and once the edge softens it stops cutting altogether. Slow the drill down, press firmly enough to raise a continuous chip, use cutting oil on steel and withdraw the bit to clear deep holes.

Do I need an SDS rotary hammer or will an impact drill do?

An impact drill with a carbide tipped masonry bit handles blockwork, brick and occasional holes in concrete up to about 10 or 12 millimetres. If you are drilling structural concrete regularly, holes above 12 millimetres, or many anchor holes a day, an SDS rotary hammer does the job in a fraction of the time and stops the bit burning, because its piston delivers far heavier blows and the SDS shank lets the bit move freely in the holder.

What size pilot hole should I drill before a large hole in steel?

Slightly larger than the chisel edge of the final bit, which is roughly a quarter to a third of the final diameter. For a 12 millimetre hole a 4 millimetre pilot is about right. Larger pilots make the big bit grab and snatch, especially as it breaks through, and smaller ones leave the chisel edge still pushing instead of cutting.

Can HSS drill bits be sharpened at home?

Yes, on an ordinary bench grinder, and it is worth learning for anything from 5 millimetres upward. Hold the bit at the point angle of about 118 degrees, grind each lip to the same length with clearance falling away behind the cutting edge, and cool the bit in water between passes so the tip never discolours. Carbide tipped masonry bits need a green silicon carbide or diamond wheel and are rarely worth regrinding at the small sizes.