Ceramic CNC Processing: What Engineers Should Know Before Quoting

The material decision comes first, not the toolpath. Alumina (Al2O3) at 96% to 99.5% purity is the workhorse. It is affordable, electrically insulating and wears slowly, which is why it shows up in pump liners, insulators and wear plates. Zirconia (ZrO2) is tougher and has a finer grain structure, so it takes a better polish and holds thin edges. Silicon nitride (Si3N4) and silicon carbide (SiC) are harder again and used where thermal shock or abrasion would destroy anything else.

The dividing line is hardness. Alumina sits around 1,400–1,600 HV and zirconia around 1,200–1,400 HV. That gap sounds small on paper and feels enormous at the spindle. Diamond tooling is not optional for either one. Cubic boron nitride works on some softer grades but wears quickly against alumina, so we keep polycrystalline diamond for nearly every ceramic job.

Machinable glass ceramic and boron nitride are the exceptions. Both cut closer to a hard plastic than to alumina, and both accept standard carbide tooling with sharp edges and positive rake. If your part is a prototype for a fixture or an insulating spacer, these two will save you real money.

Purity matters more than most drawings admit. A 99.5% alumina blank costs noticeably more than 96%, and the difference shows up in dielectric strength and in how cleanly the part polishes. For structural brackets and spacers, 96% is usually enough. For semiconductor and vacuum hardware, it is not.

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  Alumina 96%–99.5%Insulators, wear plates, pump liners. Cheapest hard ceramic we machine.
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  ZirconiaTougher, finer grain, better polish. Good for thin edges and cutting blades.
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  Silicon nitride / SiCThermal shock and abrasion resistance. Slowest to cut, highest tool cost.
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  Machinable glass ceramicCarbide tooling, tight features, low load. Prototype-friendly.

There are two ways to reach a finished ceramic part, and the choice changes your tolerance callouts. In green machining, the blank is pressed or cast, then cut while it is still in a soft, chalky state. Carbide tooling works here. You can mill pockets, drill cross holes and cut slots at speeds that feel normal. Then the part goes into the furnace and shrinks, typically 15% to 25% linearly depending on the grade.

Shrinkage is not perfectly uniform. A thick boss and a thin web cool at different rates, so a feature that was on size before firing can drift 0.3% to 0.8% after. That is the reason green machining is used for near-net shaping, not for final dimensions. We leave grinding stock on every critical surface and cut it after firing.

Fired ceramic is ground, not milled. Diamond wheels and diamond burrs remove material by brittle fracture at the edge, and the surface you get depends on grit size, feed rate and coolant. A 400-grit diamond wheel leaves roughly Ra 1.6–3.2 μm. Working down to 800 or 1,200 grit with a slower table feed gets you to Ra 0.8–1.6 μm, and lapping after that reaches Ra 0.2–0.8 μm on flat faces.

Tolerance follows the same logic. On a fired and ground alumina part we hold ±0.005 mm on dimensions that a diamond wheel can reach in one setup. Features that need a second setup, a thin wall, or a deep pocket are looser. Tell us which dimensions actually matter and we will hold those and open the rest. It keeps the price down.

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  Green stateCarbide tooling, complex pockets, then 15%–25% linear shrink in the furnace.
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  Fired stateDiamond grinding only. Final dimensions and finish are cut here.
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  Stock allowanceLeave 0.3–0.5 mm per ground face on critical dimensions.

Through holes are easy. Drill them with a diamond core drill, keep the diameter above 0.5 mm, and you get a clean bore. Blind holes are harder because the drill tip has nowhere to go and the corner generates a stress riser. If a blind hole is unavoidable, add a flat or a small relief at the bottom and keep the depth under 3× the diameter.

Sharp internal corners are the most common problem we see on ceramic drawings. A 90° inside corner concentrates stress and chips during grinding. Put a radius of at least 0.5 mm there, and 1 mm is better. The same applies to the root of a slot or the base of a thread relief.

Thin walls need support. Below about 1 mm wall thickness, alumina becomes fragile in the chuck and in the fixture. We can machine it, but the risk of a crack at clamping or in ultrasonic cleaning rises. If the design allows 1.5 mm, take it.

Threads are possible and worth a second look. Cutting an internal thread in fired alumina with a diamond thread mill works down to about M3. Below that, a threaded insert or a bonded sleeve is usually the better answer. Tapped holes in green ceramic distort during firing, so we do not rely on them for anything that must be precise.

Edge break matters. A 0.2 mm chamfer or a radius on every external edge removes the micro-chips left by grinding and drops the chance of a crack in service. It adds little to the cost.

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  Through holesDiamond core drill, Ø0.5 mm and up. Clean and repeatable.
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  Internal corners0.5 mm minimum radius, 1 mm preferred. Prevents grinding chips.
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  Wall thicknessKeep above 1 mm. 1.5 mm removes most handling risk.
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  ThreadsDiamond thread mill to about M3. Below that, use an insert.

Ceramic does not show a burr the way aluminum does, so visual inspection is a poor guide. We check dimensions on a CMM with a ruby or diamond-tipped stylus, because a standard tungsten stylus will wear against alumina and drift over a run. Flatness on a sealing face is measured on a granite plate with a dial indicator or with optical flat and monochromatic light when the callout is tight.

Cracks are the failure mode that matters most. A part can pass every dimensional check and still fail in service because of a subsurface crack from grinding. We monitor for that during the process rather than at the end: spark-out passes, coolant flow and wheel dressing are logged, and any part that shows a chip at the edge goes back for review.

Every order is fully inspected before shipment. Raw material certificates come from the ceramic supplier, in-process checks cover the ground dimensions, and a final report is issued on request. For medical and semiconductor customers we also provide the surface roughness trace and the flatness reading as separate documents.

Parts ship in 3–5 days once the ceramic blank is in hand. Blank lead time is the long pole on this material, and it varies by grade, so ask early if your schedule is tight. Our historical late-delivery probability is below 2%.

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  Diamond-tipped CMM stylusStandard tungsten wears against alumina and drifts during a run.
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  FlatnessGranite plate and dial indicator, or optical flat for tight sealing faces.
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  Crack controlMonitored in process: spark-out, coolant, wheel dressing.
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  ReportsMaterial certs, roughness trace and flatness data on request.

Pick ceramic when you need electrical insulation, wear life, low thermal expansion or chemical resistance in the same part. A pump liner in alumina outlasts a hardened steel one by a wide margin in abrasive slurry. An insulating spacer in a high-voltage stack has no metal equivalent that is not much larger. A semiconductor chamber component needs the low particle shedding that only a dense, polished ceramic gives.

Skip ceramic when the part takes impact or bending. Alumina has almost no plastic deformation before fracture, so a part that dents in service under a steel counterpart will simply crack in ceramic. If the load path is tensile or the part sees shock, metal is the right answer.

Skip it too when the geometry is dominated by deep pockets, sharp internal corners and thin walls all at once. Each of those is workable alone. Together they push grinding cost up sharply, and a redesign in 17-4PH stainless or titanium is often cheaper and just as functional.

Cost is driven by stock removal and setup count, not by part size. A small part with ten ground faces can cost more than a large simple ring. If you are comparing ceramic against metal, send both versions of the drawing and we will quote them side by side.

We machine ceramics on the same floor as our metal work: 127 high-precision CNC machines, 16 simultaneous 5-axis centers and a 4,000 mm maximum processing size. Ceramic jobs run slower and on dedicated diamond tooling, so plan the schedule around that rather than around the metal numbers.

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  Good fitInsulation plus wear in one part, low thermal expansion, chemical resistance.
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  Poor fitImpact loading, tensile load paths, anything that dents rather than cracks.
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  Cost driverNumber of ground faces and setups, not blank size.