Drilling porcelain tiles: core bits and technique

Porcelain is very hard and absorbs almost no water – an ordinary tile drill slides on it and goes blunt in seconds. Porcelain needs a diamond core bit. And because a cracked tile next to a socket or tap means replacing it, the bit is chosen for the material, diameter and machine – and the hole is drilled in the right order, not by force.

How a diamond core bit works

A core bit does not drill like a twist drill – it grinds. Diamond grains on the rim abrade the material around the circumference, and the centre stays as a core. Hence two basic rules: no hammer action and no pressure. Hammer action knocks diamond out of the rim and can crack the tile; too much pressure overheats the rim and shortens its life.

Mounting: angle grinder or drill

• M14 – the bit screws directly onto the angle grinder spindle. Higher speed and better control from about a dozen millimetres upwards. Most vacuum-brazed WERTEC core bits have an M14 thread – see diamond core bits.
• HEX shank – for drills and drivers, typically small diameters for wall plugs: HEX core bits.
• Electroplated – for drills without hammer action, used with water. The diamond layer is thin, so they suit occasional holes rather than series work: electroplated core bits.

Which diameter

For an 8 mm wall plug take an 8 mm bit. For a pipe leave a few millimetres of clearance for the seal. A standard flush-mounted electrical box needs a 68 mm hole. A hole can be widened later – but never made smaller.

Which series

• Vacuum-brazed (WTA, MATRIX, BLUE) – diamond brazed to the body; fast cutting in porcelain, sintered stone, stone and wall tiles, for everyday work.
• TurboSpeed – turbo rim for faster drilling with acceptable edge quality; good where a rosette or cover hides the hole.
• Electroplated (WTB) – for occasional holes with a drill, with water.
• Drill-and-grind (WTAR) – a core bit you can also use to correct and widen a hole.

Step by step

• Mark the centre and check that the hole is not right at the tile edge – that is where tiles crack most easily. If you can, drill before fixing, with the tile lying flat on a firm base.
• Switch off hammer action – rotary drilling only.
• Start at an angle: hold the bit at about 30–45° until it cuts a crescent in the glaze, then gradually bring it upright. This way it will not wander on the smooth surface.
• Cool it: overheated diamond loses its edge within seconds. Wet the spot (sponge, spray bottle) or use bits made for dry drilling – and drill with breaks.
• Circular motion: with larger diameters rock the bit gently around the circumference so the slurry can escape and the bit does not jam.
• No force: light, even pressure. If the bit stops progressing, it is overheated or clogged – cool and clean it.
• Slow down at the end: go through the last millimetres without pressure so you do not break out the back of the tile.

Tile already on the wall: first the tile, then the wall

Drill only the tile with the porcelain core bit. Once you are through, switch to a masonry or concrete drill – only then may you use hammer action, and only if the drill does not touch the edge of the hole in the tile.

Common mistakes

• Drilling with hammer action – a cracked tile and lost diamond.
• A wet-only bit used dry – a burnt rim after one hole.
• Strong pressure and too high speed with large diameters – overheating.
• A hole at the edge without support – chipping.
• A porcelain core bit in concrete – the bit loses its diamond; for concrete use a masonry drill or a core drill.

The most used sizes in one case: diamond core bit sets.

Frequently asked questions