Why Pieces Crack While Drilling — and How to Stop It
You’ve spent hours shaping a piece. The edges are smooth, the surface perfect — you’re drilling the final hole when you hear it: ping!
The piece cracks, and all that work is gone in a second.
Every artist and maker has felt that gut-drop moment. It’s not bad luck — it’s preventable. Once you understand why glass, ceramic, and porcelain pieces crack during drilling, you can stop it from ever happening again.
Why Pieces Really Break While Drilling
After years of working with artists and drillers — and seeing hundreds of real setups — here are the most common causes of cracking, binding, and blowout:
Debris buildup inside the hole
As you drill, the bit removes material from the cutting edge — and that debris needs to be ejected from the hole.
Fluted drill bits (used for metal or wood) are designed with spiral grooves to carry chips out.
Abrasive diamond bits, on the other hand, don’t have flutes because they grind instead of cut. They remove material as fine dust or slurry, not long chips.
That dust can quickly pack into the hole, creating resistance and friction. The trapped heat and drag cause the driller to push harder, which adds pressure on the material and leads to micro-fractures that turn into cracks.
Dull or cheap drill bits
Inexpensive electroplated bits have only a thin layer of diamond grit bonded to the metal surface. Once that coating wears off, the bit stops cutting — you’re effectively pressing bare metal against the piece.
Friction skyrockets, heat builds, and stress transfers directly into the material, often causing cracks or chips.
Sintered diamond bits are different. They contain a continuous layer of diamond embedded throughout the metal bond. As they wear, new diamond particles are exposed, keeping the bit sharp and the pressure even.
Electroplated bits are disposable; sintered bits are durable. Sharp, well-balanced sintered bits cut cleanly, run cooler, and protect your work from breakage.
Vibration and wobble
Excess vibration is one of the most overlooked causes of cracking. It often comes from low-quality drill presses with loose tolerances — worn bearings, unbalanced motors, or loose chucks.
Wobble can also develop in the quill, the chuck, or from a bent drill bit. When the bit doesn’t spin perfectly true, it oscillates with every rotation, transferring shock waves into the material.
If the workpiece isn’t firmly supported, those vibrations multiply. Thin edges and narrow sections act as stress amplifiers, so even minor wobble can trigger cracks.
A precision-built drill press with tight bearings, a true-running chuck, and proper clamping makes all the difference.
Blowout on the backside
When drilling completely through a piece, the exit side is where cracks or chips almost always occur. As the bit breaks through, the final thin layer of material is unsupported and fragile. The bit catches that layer and “pops” a chip or fracture on the backside — a blowout that can ruin hours of work.
The best solution is to drill halfway from each side so the holes meet in the middle. The blowout still happens, but inside the material where it’s invisible, not on the edge.
The challenge is alignment. Without precision, the two holes rarely meet perfectly.
The Gunther Alignment System solves this by using a counter post that matches the exact diameter of your drill bit. You align both sides before drilling, go halfway through, then flip the piece. The starter hole on the back slides over the post, perfectly centering the second hole so the two meet cleanly in the middle — no visible blowout, even in thin glass, porcelain, or stone.
Wrong drilling speed or slow RPM
Drilling speed isn’t just about how fast the spindle turns — it’s about surface speed, or how fast the cutting edge moves across the material.
With large-diameter bits, the outer edge travels farther per revolution. That’s why large bits can run at lower RPMs and still cut effectively. Spin them too fast and the cutting edge overheats, wearing down the diamonds prematurely.
With small bits, low RPMs are often the real problem. When the bit spins too slowly, it can grab or bind. That forces extra pressure, generating more heat and vibration — and that’s when cracks happen.
Running at moderate to high RPMs with light, steady pressure produces a cleaner cut and helps coolant flow through the bit. While higher RPMs do generate more heat, through-bit coolant easily manages it by continuously flushing debris and cooling the cutting edge.
Rule of thumb: use lower RPMs for large bits, higher RPMs for small bits, and always maintain proper coolant flow to control heat and friction.
Poor coolant or no coolant flow
Surface water helps a little, but the real problem happens inside the hole.
If coolant can’t reach the cutting edge, heat and debris build up quickly — dulling the bit, increasing friction, and stressing the material until it fractures.
A pressurized through-bit coolant system solves this by pushing water directly through the drill. The internal flow:
- Flushes out debris continuously
- Cools the bit right at the cutting edge
- Lubricates the drilling zone and extends tool life
- Stabilizes pressure and minimizes vibration
The result is smoother drilling, cleaner holes, and far fewer cracks — especially in thin, delicate, or high-value pieces.
Unstable or unsupported material
If the piece moves or flexes while drilling, stress instantly transfers through the material.
Always keep your work firmly clamped or supported, especially with curved or thin pieces. A stable work surface prevents chatter, wobble, and cracks.
Material flaws and internal stresses
Sometimes the problem is invisible.
Tiny cracks, uneven density, or residual tension from firing or annealing can release suddenly under heat or vibration — making some pieces far more fragile than they appear.
While this is sometimes unavoidable, material selection can reduce the risk. Choosing well-annealed glass, properly fired ceramics, or high-quality blanks helps minimize internal stress.
Even when flaws exist, good cooling, steady RPMs, and light, consistent pressure can be the difference between a piece that breaks and one that survives the drilling process.
The Role of Cooling and Debris Removal
Controlling heat is critical when drilling glass, ceramic, or porcelain — but surface cooling alone isn’t enough.
Dripping or spraying water on the surface only cools the outside. The real friction, heat, and dust buildup occur inside the hole, at the cutting edge — exactly where surface water can’t reach.
As the bit grinds deeper, debris and slurry collect in the hole. This traps heat, increases pressure, and dulls the bit prematurely. The bit then starts rubbing instead of cutting, generating even more heat and stress on the material.
A pressurized through-bit coolant system eliminates these issues by delivering coolant directly through the drill core. The flow:
- Flushes debris before it can pack or bind
- Cools the cutting edge where heat forms
- Lubricates the drilling zone for smoother operation
- Stabilizes pressure to keep holes round and minimize vibration
The result is precise, clean drilling — and a dramatic reduction in cracking and edge chipping.
Why So Many Artists Use the Gunther Multidrill
The Gunther Multidrill Water Swivel was built specifically for this kind of work.
It delivers a steady through-bit coolant flow, runs true with minimal vibration, and uses long-lasting German diamond bits that stay sharp and balanced.
It’s designed for glass, ceramic, and porcelain drilling, giving artists confidence to work faster, cleaner, and without the constant worry of heat or resistance.
How to Prevent Cracks in Your Next Drilling Session
Here’s a quick checklist you can follow right now:
✅ Use a sharp, high-quality diamond bit.
✅ Keep coolant flowing through the bit, not just over it.
✅ Use moderate to high RPMs with light pressure — let the bit cut, not grind.
✅ Support the workpiece firmly and back it with something soft.
✅ Slow your feed rate as you approach breakthrough.
✅ Check for wobble — make sure your bit and spindle run true.
Following these steps eliminates most cracking issues — and protects the hours you’ve invested in your work.
For more suggestions check out this guide:
HOW TO DRILL GLASS, STONE, AND CERAMIC WITH GUNTHER DIAMOND TOOLS
Conclusion: Stop the “Almost Done” Disaster
When your piece cracks mid-drill, it’s not bad luck — it’s physics.
Heat, debris, vibration, and dull bits all add up to that heartbreaking ping.
A stable setup, sharp bit, correct RPM, and through-bit coolant flow keep your work cool, clean, and crack-free.
That’s the same principle behind the Gunther Multidrill System, trusted by artists who are done losing their best pieces to the final drill.