Masonry Drill Bits Explained: SDS, TCT and Diamond for Stone, Brick and Concrete

Anyone who has spent an afternoon with a drill in hand knows the feeling: the bit spins, the dust trickles, and the hole goes nowhere. Or worse, the tile cracks, the bit glows blue and the drill smells hot. Nine times out of ten the problem is the bit, not the drill. Stone, brick, block and concrete each behave differently under a spinning edge, and the shank, tip and flute that suit one will ruin another. What follows is a plain explanation of the common types, what they are for, and how to choose before you start.

Match the shank to the drill first

The shank is the plain end that goes into the chuck, and it decides which drill the bit will fit. Two systems dominate masonry work.

SDS Plus has a 10 mm shank with slots and grooves. It locks into the chuck but slides back and forth, so the bit can hammer without the drill’s mechanism fighting it. Most corded hammer drills and smaller rotary hammers take SDS Plus, covering the great majority of holes from 5 mm to around 20 mm.

SDS Max uses an 18 mm shank and is found on heavier rotary hammers. It suits big holes, deep fixings and chiselling. If you are drilling 25 mm or more into concrete, this is the system you want.

There are also ordinary round-shank masonry bits for a three-jaw chuck, and hex-shank bits for impact drivers. An SDS bit will rattle uselessly in a plain chuck, and a round bit will slip in an SDS chuck without an adapter. Buy the shank your drill takes.

The three families of masonry bits

SDS bits

These are the workhorses for concrete and hard brick. A carbide tip is brazed onto a helical flute, and the shank drives it with a hammer action. Rotary hammers deliver that blow pneumatically, which is far more efficient than the ratcheting hammer of a combi drill. The flutes are deep and wide because concrete dust is bulky; clear it often, or the bit will jam and overheat.

Use them for concrete, engineering brick, hard stone, and any hole where a plug and screw is going. Sizes are nominal — a 6 mm bit makes a hole slightly larger than 6 mm, which is why plugs specify a drill size.

TCT (tungsten carbide tipped) bits

TCT describes the tip material rather than the shank. Many round-shank masonry bits are TCT, and so are some SDS bits. The tip is a small carbide plate, harder and more heat-resistant than steel. It cuts by scraping and crushing, so it needs hammer action in hard material, but can be run without hammer in soft brick, block and mortar.

These bits are cheap and widely available, and good for lighter work. They dull faster than a quality SDS bit and the tip can chip if you drop the drill. Replace them when the carbide edge looks rounded rather than sharp — a blunt bit polishes concrete instead of cutting it.

Diamond bits

Diamond bits have no cutting edge. Diamond grit is bonded to the rim, and they grind rather than cut. That makes them ideal for tiles, porcelain, glass and natural stone, where a hammer blow would chip or crack the surface. Diamond cores produce clean, accurate holes for pipes, cables and taps.

They need care. Run them slowly, keep the pressure light and cool them. Most tile cores are used wet, either with a water-fed drill or a sponge and spray bottle. A dry diamond bit will glaze and stop cutting. They are not the answer for deep holes in reinforced concrete; that is a job for a proper core drill and a wet diamond core.

Which bit for which material

• Brick: TCT or SDS with hammer action. Soft stock brick may need no hammer at all.
• Concrete: SDS Plus or SDS Max with full hammer action. Keep the bit cool and clear dust often.
• Blockwork: TCT or SDS, light hammer or none. Blocks are soft and crumbly; too much hammer makes a ragged hole.
• Natural stone: TCT at low speed with little or no hammer. Diamond core for a clean finish or a large diameter.
• Tiles and porcelain: Diamond only, no hammer, plenty of water.
• Mortar and render: Sharp TCT, no hammer, so the surrounding brick stays intact.

Speed, pressure and dust

The most common mistake is running a masonry bit too fast. Carbide and diamond both work better at lower speeds with steady pressure. As a rough guide, the larger the bit, the slower the drill. A 6 mm masonry bit can run at a decent speed; a 20 mm core should be barely turning.

Let the bit cut. Pushing hard bends the bit, overheats the tip and can crack the material around the hole. Use a pecking motion — drill for a second or two, pull back to clear the dust, then go again. Never force a bit that has stopped cutting.

Dust is not just a nuisance. It clogs the flutes, traps heat and hides the tip, so you cannot see whether you are cutting or burning. A vacuum hose close to the bit makes a real difference. Wear eye protection and a mask; silica dust from concrete and stone is not something to breathe in.

If you hit steel reinforcement, stop. A masonry bit will not cut rebar. Move the hole, or use a bit designed for metal. Do not enlarge a hole by wobbling the drill; use the next size up.

Building a sensible bit set

You do not need every bit on the shelf. Start with a small SDS Plus set in 5, 6, 8 and 10 mm, which covers most wall plugs and fixings, then add 12 mm and 16 mm for larger anchors. Keep a few TCT round-shank bits for lighter work and a diamond core or two for tiles. Buy decent carbide; cheap bits are false economy in hard concrete.

Store bits in a case so the tips are protected, and replace them when they stop cutting. A sharp bit is quicker, cooler and less likely to damage the surface around the hole. For fixings into structural concrete or load-bearing stone, take advice before drilling. Choose for the material in front of you, use the right speed, and the drill will do the work.

Photo: dokudokunomi / Pixabay