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How Technical Ceramic Parts Are Machined
A ceramic part is either cut soft and then fired, or fired first and ground with diamond. Most precision parts use both. This page explains each process, what it holds and where it fails, so you can tell which route a quote is pricing.
The processes at a glance
Typical results across shops that work these grades, not a promise for a given part. Geometry limits for each feature are on the design guide.
| Process | When it is used | Typical result | Watch for |
|---|---|---|---|
| Green or bisque machining | Before sintering, on pressed or pre-fired blanks | As fired, about ±1% of the dimension | Shrinkage factor must be known for that grade and batch |
| Sintering | After green machining, or on near-net pressed parts | Linear shrinkage typically 15 to 25% | Warp on long or uneven sections, distortion on thin walls |
| Diamond grinding | On sintered (hard) parts: faces, diameters, slots, profiles | ±0.005 to ±0.013 mm, Ra 0.2 to 0.4 µm | Edge chipping, subsurface damage, cost per ground surface |
| Diamond core drilling and ultrasonic machining | Holes, slots and pockets in hard ceramic | Holes from about 0.3 mm, depth up to about 10 × D | Exit breakout; drill from both sides on deep holes |
| Lapping and polishing | Seal faces, chuck surfaces, optical or sliding faces | Flatness 0.005 mm and below, Ra 0.05 to 0.1 µm | Thin parts bow when stress is not balanced |
| Laser machining | Thin substrates, scribing, small holes, marking | Fast on sheet up to about 1 mm | Heat-affected zone and microcracks at the cut edge |
| EDM | Electrically conductive grades only, such as reaction-bonded SiC | Sharp internal shapes with no cutting force | Not possible on alumina, zirconia or nitride insulators |
Green and bisque machining
Machining before the part is fired, while the material is soft enough to cut with carbide or PCD tools.
A green blank is pressed powder held together by a binder. It cuts like chalk, so milling, turning and drilling are fast and cheap, but it is fragile and must be fixtured gently. A bisque blank has been pre-fired at a lower temperature: still machinable, stronger to handle, with less shrinkage left to come.
Green machining suits volume parts and shapes that would take hours to grind: deep pockets, cross holes, threads for low-load use and complex profiles. The price is accuracy. Because the part shrinks afterwards, every fired dimension carries about ±1% unless it is ground. The full comparison is in green machining against hard machining.
Sintering shrinkage, worked through
The green dimension is the fired dimension divided by (1 minus the linear shrinkage). Shrinkage varies with grade, powder lot, pressing density and kiln position, which is why as-fired parts carry a percentage tolerance rather than a fixed one.
| Fired dimension | Assumed linear shrinkage | Green dimension to machine | Typical as-fired tolerance (±1%) |
|---|---|---|---|
| 10.00 mm | 18% | 12.20 mm | ±0.10 mm |
| 25.00 mm | 18% | 30.49 mm | ±0.25 mm |
| 50.00 mm | 20% | 62.50 mm | ±0.50 mm |
| 100.00 mm | 20% | 125.00 mm | ±1.00 mm |
Illustrative arithmetic, not data for a specific grade. The shop measures its own factor. Long, thin or uneven sections also warp, so flatness on a fired plate is loose until it is ground. More in sintering shrinkage explained. Background: sintering.
Diamond grinding
The process behind every tight tolerance on a ceramic drawing.
Fully dense alumina, zirconia, silicon carbide and silicon nitride are harder than any metal cutting tool, so they are ground with diamond wheels: surface grinding for faces, cylindrical and internal grinding for diameters and bores, creep-feed grinding for slots and profiles, and jig or CNC contour grinding for complex outlines. Typical results are ±0.005 to ±0.013 mm and Ra 0.2 to 0.4 µm.
Grinding removes material slowly and every pass leaves a thin layer of microcracks below the surface. Good practice is to rough with a coarse wheel, finish with a finer one that removes the damage of the first, and keep coolant at the contact zone. Edges are chamfered rather than left sharp, because a sharp edge chips. Each ground surface carries setup, passes and inspection, which is why the count of tight tolerances drives the price. Detail in how diamond grinding works.
Holes are made with diamond core drills or ultrasonic machining, in which a vibrating tool drives abrasive slurry into the part. Both leave a slight cone at a blind hole bottom and can break out at the exit, so deep holes are often drilled halfway from each side.
Lapping and polishing
For the faces that seal, slide, clamp a wafer or carry light.
Lapping runs the part against a flat plate with loose diamond slurry; double-side lapping does both faces at once and controls parallelism. Polishing follows with finer abrasive on a softer pad. Typical results are flatness of 0.005 mm and below and Ra 0.05 to 0.1 µm. Seal faces are checked against an optical flat and specified in helium light bands, each band about 0.3 µm.
Thin discs bow if the stress in the two faces is not balanced, so a shop may lap both sides even when only one is specified. Name the faces that need it and leave the rest as ground. See lapping, polishing and flatness.
Laser, ultrasonic and EDM
The non-contact routes, each with a narrow job.
Laser. Fiber, CO2 and short-pulse lasers cut, drill and scribe thin ceramic, mostly alumina and aluminium nitride substrates up to about 1 mm. Scribed sheets are snapped into parts. The cut edge carries a heat-affected zone and can hold microcracks, so it is not used on load-bearing or sealing edges without grinding after.
Ultrasonic. Suited to holes, slots and pockets in hard, brittle grades where a rotating tool would chip the edge. Slow, but gentle on thin features.
EDM. Works only on electrically conductive ceramics, such as reaction-bonded silicon carbide and some conductive composites. Alumina, zirconia, silicon nitride and aluminium nitride are insulators and cannot be spark eroded. More in laser, ultrasonic and EDM machining.
Choosing a route
The route sets the price more than the grade does. Tell the shop what the part does and which features matter, and let it propose the route. Grade properties are on the materials page.
- Hundreds to thousands of parts, no tight fitsGreen machining, then fire. Grind nothing, or one datum face.
- Prototype or a few dozen parts with tight featuresGrind from sintered stock blanks. No firing risk, no shrinkage model.
- Volume part with two or three precision featuresGreen machine near net, fire, then grind only the critical features.
- Seal face, chuck surface or sliding faceGrind, then lap; polish if the finish spec calls for it.
- Thin substrate with holes and outlineLaser cut and drill, or green punch before firing at volume.
- Complex shape, low stress, short lead timeA machinable grade such as Macor or hot-pressed boron nitride, finished with no firing step.
Questions about ceramic processes
Why does a ceramic part shrink in the kiln?
A pressed blank is compacted powder with pore space between the grains. During sintering the grains bond and the pores close, so the part densifies and shrinks, typically 15 to 25% in each linear dimension. The shop scales every green dimension by a shrink factor it measures from its own batches of that grade.
Can a sintered ceramic be machined with carbide tools?
Not in practice. Fully dense alumina, zirconia and silicon carbide are harder than carbide and are ground with diamond. Carbide and PCD tools are used on green or bisque blanks, and on machinable grades such as Macor.
Is laser machining a substitute for grinding?
For thin parts and features where the edge condition is not critical, yes. The laser leaves a heat-affected edge with possible microcracks, so load-bearing edges, seal faces and precision fits are still ground. Short-pulse lasers reduce the damage but cost more per part.
What does subsurface damage mean on a ground part?
Grinding leaves a layer of microcracks below the visible surface. A part can measure correctly and then fail under load or thermal cycling. Staged grinding with finer wheels, and lapping where it matters, removes the damaged layer left by each earlier step.
Not sure which route your part needs?
Send the drawing. We read the geometry, decide the route and match you with up to 3 shops set up for it. Free for buyers, reply within 24 business hours.