Drilling glass or ceramic is unforgiving. The material is hard, brittle and sensitive to heat, and a hole that starts badly stays bad. A hollow core diamond drill bit solves most of this — it grinds a clean ring instead of crushing a point, frees a core, and keeps the load light. But only if it is set up and fed correctly. The bit does the cutting; the setup decides whether the edge chips.
This guide covers the six steps to clean holes in glass and ceramic with a hollow core diamond drill bit, and it assumes the bit is already the right family for the job. For harder, thicker masonry and reinforced concrete the technique changes — that is the world of the core drill bit selection guide. Here we stay on thin-walled, brittle-material work.
Why a hollow core diamond drill bit beats a twist drill
A twist drill is designed to cut and clear chips from ductile metal. On glass and ceramic it acts as a wedge: it pushes the brittle material apart and cracks it. A diamond core bit instead grinds a thin ring of material away, leaving a cylindrical core inside that drops out or is pried free. Because the cutting load is spread across a ring and the diamond removes material by abrasion rather than wedging, the bit is far gentler — as long as it is allowed to grind, not forced to punch.
This is the same fixed-abrasive principle that governs all diamond tooling: diamond grinds, it does not cut like a knife. The diamond drill bit shares its working principle with grinding wheels and discs, which is why the same rules about speed and coolant apply here. Where a twist bit clears chips from a deep flute, a core drill bit carries swarf out in the coolant, so flow matters more than flute depth.
The practical upshot is that brittle material rewards process over power. A light, slow, well-cooled cut produces a clean hole; a fast, dry, heavy cut produces a crack. Almost every failure on glass and ceramic traces back to one of those three variables being wrong.
1. Set the core with a guide, always
The first contact decides the hole. A wandering start scratches the surface and often cracks the edge before the bit has even bitten. Fix it before you drill:
- Use a drill guide, a template, or a pilot hole.
- For hand work, start at a shallow angle to establish a crescent, then straighten the bit once it has a hold.
- Mark the centre and use a small amount of putty or tape as a dam for coolant where the surface is smooth.
Never start a hollow-core drill bit flat and fast on a glossy surface — that is how most chips happen. If the hole must be perfectly placed, a drill press with the work clamped is worth the setup time, because it removes the operator’s hand tremor from the equation.
2. Match the bit diameter and wall to the finish you want
core bits come in tight size steps for a reason: the hole is only as good as the bit’s runout. Use the closest nominal size rather than forcing an undersized bit to open up, and choose a thin-walled bit — it cuts a narrow kerf, removes less material and runs cooler. The hollow core bit set covers small, stepped sizes for exactly this kind of work, and for even finer detail there are diamond nail drill bits down to 2 mm.
Thin walls are not just about speed. A narrow kerf means less diamond in contact, less friction and less heat, so a thin-walled bit tolerates the low spindle speeds that brittle material demands. Where the hole is deep relative to its diameter, choose a bit with enough clearance to let swarf escape — a packed bit stops cutting and starts rubbing, and rubbing is what kills both the bit and the workpiece.
Also check the shank and the chuck. A bit that does not run true in the chuck will cut an oversized, out-of-round hole no matter how good the abrasive is. Clean the shank, seat it fully, and reject a bit whose body is bent — on a brittle material there is no second chance.
3. Slow the spindle down
The single most common cause of failure is speed. Diamond is a grinding abrasive; it needs time at the contact point to cut, not friction. Run the drill far slower than you would for metal — often a few hundred RPM — and let the diamond grind steadily. High speed on glass means heat, and heat means chips and a burnt-out bit.
The right speed also depends on bit diameter: a large bit has a higher rim speed at the same RPM, so it usually wants a lower setting than a small one. If you cannot measure RPM, judge by the cut — a steady grinding note with fine, milky swarf means you are close. A high squeal means too fast; a bit that smears rather than cuts means too slow or too dry.
| Material | Speed | Feed pressure | Coolant |
|---|---|---|---|
| Glass (thin) | Very low | Light, steady | Water or cutting fluid, flooded |
| Glass (thick) | Low | Light to moderate | Flooded, keep hole wet |
| Ceramic tile | Low to moderate | Moderate | Water |
| Porcelain | Low | Light | Flooded, porous material loads fast |
4. Flood the cut — heat is the enemy
Coolant does two jobs at once: it carries heat away from the cutting edge and it flushes the fine swarf that would otherwise grind the hole and load the diamond. A few drops are not enough. Build a small dam of putty around the hole, keep it full of water, and refresh it as you drill. Let the bit come up for air if you cannot keep the cut submerged. A hot bit glazes and stops cutting; a hot workpiece cracks.
Water is the usual coolant for glass and ceramic because it is effective, cheap and clean. Where a machine cannot run wet, a light cutting fluid can substitute, but never run a diamond core bit dry on glass — the heat has nowhere to go and the failure is sudden. On very fine work, a small pump or even a squeeze bottle held steady can keep the contact point wet for the whole cut.
5. Feed with light, steady pressure and let the bit work
The correct pressure for a core drill bit is just enough to keep it cutting, no more. Too little and the diamond rubs and glazes; too much and the brittle material cracks and the bit grabs. Apply steady, even pressure and let the sound tell you — a steady grinding note is right, a high squeal means too fast or too dry, a thud means too much pressure on a bit that has lost its diamond. Clear the core periodically on deeper holes so swarf can escape.
Hand drilling is where most breakage happens, because the feed is unsteady. If you must drill by hand, brace the workpiece, keep the bit square to the surface, and accept a slower rate than a drill press would give. Speed and pressure are the two dials; on brittle material both stay low.
6. Break through with care and finish the edge
The exit is as risky as the entry. As the bit breaks through, reduce pressure — the support under the bit is suddenly gone and a heavy hand will chip the back face. On the exit, ease off and let the core drop out before withdrawing. Where the hole must be clean, lightly chamfer or dress the edge with a fine abrasive rather than filing, which can flake brittle material.
If you are drilling a series of holes, check the bit between holes. A bit that has lost its diamond edge will rub and heat rather than cut, and the next hole is where it fails. Retire or dress the bit as soon as the cut rate drops, not after it has damaged a workpiece. With care, a good hollow-core drill bit will cut dozens of clean holes in glass and ceramic before it needs replacing.
Drilling parameters at a glance
| Material | Spindle speed | Feed | Coolant |
|---|---|---|---|
| Thin glass | Very low | Light, steady | Flooded water |
| Thick glass | Low | Light to moderate | Flooded, keep wet |
| Ceramic tile | Low to moderate | Moderate | Water |
| Porcelain | Low | Light | Flooded; loads fast |
| Mirror / coated glass | Very low | Very light | Flooded; back the exit |
Treat the table as a starting point and listen to a hollow core diamond drill bit as it cuts. The right setting is the one where the bit grinds steadily, stays cool to the touch, and the edge comes out clean.
Quick hollow core drilling summary
- Guide the start: with a hollow core diamond drill bit, a controlled first contact prevents most chips.
- Closest size, thin wall: a narrow kerf cuts cooler and cleaner.
- Slow the spindle: diamond grinds, it does not burn through.
- Flood the cut: coolant protects both the bit and the edge.
- Light steady feed: enough to cut, not enough to grab.
- Ease through the exit: reduce pressure as the bit breaks out.
For glass, ceramic and porcelain work, choose a thin-walled core bit sized to your hole and keep the cut flooded — CDFIT can match a bit and coolant setup to your material and hole depth. Browse the thin-walled core drills or the full diamond and CBN tools range to start.