ESWithout water to carry heat away, a dry core bit lives or dies by two settings you control: how fast it spins and how hard you push. Get the balance right and the crown stays sharp and cuts steadily. Get it wrong in either direction and you hit the two classic failures - a glazed, polished rim that stops cutting, or a rim that overheats and sheds its diamond.
Why speed and pressure have to work together
Diamond does not slice; it grinds. Each grain has to be pressed into the material hard enough to scratch it, then allowed to clear before it rubs again. Spin too fast on a dry bit and friction builds faster than the air can cool it. Push too hard and the bit digs in, loads the tool and heats the rim. The right combination keeps each grain biting and clearing in turn, which is what keeps the bond wearing back at the rate that exposes fresh diamond.
Setting the speed
Dry bits run slower than wet ones, and larger diameters run slower than small. A bit's maximum RPM rating is a ceiling to stay under, not a target to reach. On a variable-speed grinder or drill, start slowly and only come up to a speed where the bit cuts without squealing or smoking. If you have to guess, err slower - an under-speed bit cuts more slowly, but an over-speed bit is destroyed.
Reading the feed pressure
Feed pressure is best judged by what the bit is doing rather than by a number. The bit should sound like it is steadily grinding, with visible progress and a steady stream of dust leaving the hole.
Signs of too much pressure
- Rim turns blue or straw-coloured from heat.
- The machine labours and the bit squeals.
- Dust stops flowing - the flutes are packed.
- Chipping or a blown-out exit edge.
Signs of too little pressure
- The bit polishes instead of cutting; progress stalls.
- A bright, shiny rim - the bond has glazed over.
- Rubbing sound rather than a grinding one.
When the bit glazes from too little pressure, do not simply lean harder on a hot bit. Ease off, let it cool, then feed it firmly into an abrasive surface for a moment to strip the glazed layer. Our guide to glazing and dressing explains the recovery in more detail.
Peck drilling
If there is one technique that separates a bit that lasts from one that burns out, it is peck drilling. Instead of feeding continuously, push the bit in a few millimetres, lift it clear to let the flutes throw the dust out, then feed again. On dry cuts this is not optional - it is the only way the dust leaves the kerf and the only chance the rim has to cool. Keep the pecks short and rhythmic.
Entry and exit
Start at a shallow angle to cut a groove before standing the bit upright, and reduce pressure in the last few millimetres so the exit edge does not blow out. Support the back of the workpiece with an offcut so the crown meets something solid as it breaks through.
When to stop and switch to wet
If the material is very hard porcelain, the hole is deep, or the diameter is large, dry drilling fights the physics. A wet bit with a water feed will cut faster, cooler and longer. Switching is not a failure; forcing a dry bit through a job it was never meant for is. For the materials where dry works well, see our notes on dry core bits for granite and marble.
Safety
Dry drilling throws the most dust, so a dust mask or respirator is essential, along with eye and hearing protection. Clamp the workpiece, use the tool's side handle, respect the bit's RPM rating and follow the manufacturer's instructions. Keep the working area dust-controlled and never remove a guard to reach a tight spot.
Speed and pressure in short
- Slower than wet, slower still as diameter grows.
- Enough pressure to cut, not enough to heat or glaze.
- Peck to clear dust and let the rim cool.
- Light pressure on entry and exit; steady pressure through the middle.
- Switch to a wet bit for hard, deep or large holes - see the diamond drill bit range.
