CNC machining using alumina processing: an accurate process
This guide is for design engineers and buyers who need dense, wear-resistant ceramic parts and want to know where the process actually holds tolerance. It covers grade selection, tooling, cutting parameters, green versus fired machining, and how to inspect what comes off the machine.
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Key takeaways
Why alumina behaves differently from metal on a CNC machine
Alumina (Al₂O₃) is an oxide ceramic, not a metal. It is roughly three times harder than hardened tool steel and does not conduct heat well. That combination changes everything about how you cut it. Heat stays at the cutting edge instead of flowing into the chip, so tool life is measured in minutes unless you use diamond.
The material is also brittle. There is no plastic deformation to absorb a heavy cut, no chip that curls away and carries heat with it. A cutter that bites too deep does not bend the part, it chips it. Edge chipping is the most common reject on ceramic work, and it usually starts at a corner or a thin wall.
Purity drives hardness and cost. You will see 96%, 99%, 99.5% and 99.9% grades. Higher purity means better wear resistance and better electrical insulation, but also a higher firing temperature and more shrinkage. Pick the lowest purity that survives your application, not the highest.
Alumina is also porous after sintering unless it is hot isostatically pressed. Pores open during machining and can trap coolant. If the part goes into a vacuum chamber or a medical fluid path, say so before quoting so the shop can plan a cleaning step.
- 1HardnessAround 1,400–1,600 HV for 99% alumina, well above carbide.
- 2Thermal conductivityLow. Heat concentrates at the edge, so coolant delivery matters.
- 3Fracture behaviorBrittle. Chipping, not bending, is the failure mode.
Choosing the alumina grade before you cut metal or ceramic
Grade selection is the first accuracy decision, because it fixes shrinkage, hardness and the achievable surface finish. A 96% grade with a silica and magnesia binder is easier to sinter and cheaper, but it wears faster in a sliding application. A 99.5% grade holds a lapped surface longer but costs more per part.
Ask what the part actually does. A wear plate running against steel needs hardness and a low-friction finish. An insulating spacer needs dielectric strength and dimensional stability. A seal face needs flatness more than it needs a sharp corner. Each of those pulls the grade in a different direction.
If the drawing calls out a tolerance tighter than ±0.02 mm on an as-fired feature, plan on a diamond grinding operation after sintering. There is no way around it. Sintering distortion is not repeatable enough to hold that band without a finishing pass.
Send the application with the RFQ, not just the drawing. We quote alumina work with a free DFM review inside 12 hours, and the review usually catches a corner radius or a wall thickness that would crack during firing.
Green machining versus fired machining: picking the route
Green machining means cutting the part while the alumina is still a pressed or cast powder compact held together by binder. It cuts like a soft chalk. You can mill pockets, drill cross holes and turn profiles with standard geometry tools, and cycle times are short.
The catch is shrinkage. The compact grows denser and smaller during sintering, typically 15–20% linear for a pressed grade. You scale the CAD model up by the measured shrinkage factor for that specific batch, then grind critical features back to nominal after firing. If the shrinkage factor drifts batch to batch, the as-fired dimensions drift with it.
Fired machining means cutting fully dense alumina, usually above 1,500 °C sinter. Nothing moves after the cut, so dimensions are stable and inspectable. But now you are cutting a material near diamond hardness. Material removal rates drop hard, and every operation needs diamond tooling.
The usual production route is hybrid: green machine the bulk of the geometry, fire, then diamond grind the sealing faces, bores and any feature with a tolerance tighter than ±0.02 mm. That keeps cost down and puts the precision only where the drawing needs it.
- 1Green onlyComplex internal geometry, low volume, tolerances looser than ±0.05 mm.
- 2Fired onlySimple profiles, tight flatness, no green pressing capability required.
- 3HybridMost common. Green roughing plus fired diamond grinding on critical faces.
Diamond tooling and cutting parameters for alumina
For fired alumina, polycrystalline diamond (PCD) inserts and diamond-coated end mills are the only practical cutters. Carbide will cut for a short while at low speed, then the edge dulls and the cutting force rises until the part chips. Do not let a job run on a dull diamond tool hoping to finish the last few parts.
Speeds sit far below metal practice. Peripheral speeds in the 100–200 m/min range on a PCD face mill are typical for finishing, with light depths of cut. Feed per tooth stays small, often 0.02–0.05 mm. The goal is a continuous, light shaving action, not a heavy bite.
Coolant is a debate. Flood coolant controls dust and keeps the edge cool, but it can load pores. Many shops run dry with high-pressure air extraction, especially for green machining where dust is the main hazard. Either way, extraction at the cut is not optional.
Rigidity matters more than spindle speed. Alumina does not tolerate chatter, because chatter is a repeated impact load and the material fails by fracture. Use the shortest tool holder you can, support thin sections, and reduce feed before you reduce speed when the cut starts to sing.
Inspection and the accuracy limits you should plan around
Alumina does not forgive a bad inspection setup. Ruby styli are the practical choice for contact measurement, and the probe force has to stay low. A worn stylus on fired ceramic reads small and the error grows quietly across a batch.
Optical measurement works well for edge profiles and hole positions, but it struggles with deep bores and internal features. For those, use a CMM with a small ruby tip and accept a slower cycle. The measurement plan should be agreed before the first part is cut, not after.
The realistic tolerance floor on a fired, ground feature is ±0.005 mm. That applies to a specific dimension the shop can grind and measure. A whole drawing at ±0.005 mm is not the same request, and it will not hold across every feature.
Surface finish has its own ladder. As-machined fired alumina lands around Ra 1.6–3.2 μm, a good grind gets to Ra 0.8–1.6 μm, and lapping reaches Ra 0.2–0.8 μm. Every step down costs time and tool wear, so only call out the finish the function needs.
- 1Edge chippingThe dominant reject. Catch it at the machine with a 10× loupe.
- 2PorosityShows up after grinding. Flag fluid or vacuum exposure early.
- 3Flatness driftCan move during lapping if heat builds up. Control the lap temperature.
Step-by-step: running an accurate alumina job
Follow in order. Skipping step 2 is the most common cause of a scrapped batch.
- 1Lock the grade and the process routeConfirm purity, density and whether the part is green machined, fired machined or hybrid. Get the shrinkage factor in writing from the material supplier before any CAD scaling.
- 2Scale the model for shrinkageApply the supplier's linear shrinkage factor, typically 15–20%, to the green model. Keep the nominal model separate as the finished target. Do not scale the drawing itself.
- 3Review geometry for firing riskCheck wall thickness, sharp internal corners and large unsupported spans. Add radii of at least 0.5 mm at internal corners. Walls under 1 mm and thick-to-thin transitions are crack starters.
- 4Green machine the bulk geometryCut pockets, slots and holes with sharp carbide or PCD tooling. Leave 0.3–0.5 mm stock on every fired feature. Green dust is a respiratory hazard, so extract at the cut.
- 5Sinter and measure the shrinkageFire to the grade's schedule, then measure a known feature to back-calculate actual shrinkage. Log it per batch. If it drifts more than 0.3% from the planning value, re-scale before the next run.
- 6Diamond grind fired featuresGrind bores, faces and any feature tighter than ±0.02 mm. Use light passes, 0.01–0.02 mm depth, with plenty of coolant. Dress the wheel often to keep the edge sharp.
- 7Lap or polish sealing facesFor seal faces and wear surfaces, lap toward Ra 0.2–0.8 μm. Flatness is the specification here, not roughness alone. Check with an optical flat before polishing further.
- 8Inspect and documentUse a CMM with a ruby stylus or an optical system. A standard touch probe will wear and drift on fired alumina. Record dimensions, flatness and any edge chipping on the report.
Alumina grade and process route by application
Pick the row that matches the part's job, then confirm with DFM.
| Application | Typical grade | Route | Tolerance band |
|---|---|---|---|
| Wear plate against steel | 99% Al₂O₃ | Fired + grind | ±0.01 mm on thickness |
| Electrical insulator spacer | 96% Al₂O₃ | Green + fire | ±0.05 mm as-fired |
| Pump seal face | 99.5% Al₂O₃ | Fired + lap | Flatness under 1 μm |
| Threaded ceramic insert | 99% Al₂O₃ | Fired + diamond thread mill | ±0.02 mm on pitch diameter |
| Prototype bracket, low load | 96% Al₂O₃ | Green only | ±0.05 mm as-fired |
| High-purity chamber part | 99.9% Al₂O₃ | Fired + grind + clean | ±0.01 mm, porosity controlled |
Decide the route before you cut
Alumina accuracy comes from the process route, not the machine. Lock the grade, scale for shrinkage, and put diamond grinding only where the drawing needs it. Send the application with the drawing and we will return a DFM review with the quote.
Frequently asked questions
Can you hold ±0.005 mm on alumina?
Yes, on a specific fired and diamond-ground feature. That is our standard tolerance capability and it applies to a dimension the shop can measure with a ruby stylus CMM.
It is not a blanket tolerance for the whole drawing. Features that stay as-fired are looser, and thin walls or long bores are harder to hold at that band.
Should I design for green machining or fired machining?
If the geometry is complex and tolerances are looser than ±0.05 mm, green machining is cheaper and faster. The part shrinks during firing, so as-fired dimensions move with the batch.
If flatness or a tight bore matters, plan on a fired diamond grinding pass. Most production parts use both: green for the bulk, fired grinding for the critical features.
How much does alumina shrink during sintering?
Linear shrinkage is typically 15–20% for a pressed alumina grade, and it depends on the specific grade and the firing schedule. The exact factor comes from the material supplier.
We log measured shrinkage per batch. If it drifts more than 0.3% from the planning value, we re-scale the green model before the next run.
What tooling do you use on fired alumina?
PCD inserts and diamond-coated end mills. Carbide is not viable on 99% alumina at production rates because the edge dulls fast and cutting force climbs.
Peripheral speeds run around 100–200 m/min for finishing, with small depths of cut and light feed per tooth.
Can alumina parts be threaded?
Yes, with diamond thread milling on fired material or thread cutting in the green state. Fired threads are usually ground or milled, not tapped, because tapping generates too much torque on brittle material.
Expect thread pitch diameter tolerances around ±0.02 mm. Sharp crests chip easily, so a small root radius helps.
What surface finish is realistic?
As-machined fired alumina is around Ra 1.6–3.2 μm. Diamond grinding reaches Ra 0.8–1.6 μm. Lapping gets to Ra 0.2–0.8 μm on sealing faces.
Call out only the finish the part needs. Every step down adds cycle time and diamond wear.
Send your alumina drawing for a DFM review
Upload the model and the application notes. We reply with a quotation and free DFM analysis within 12 hours, and your files stay confidential.
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