Guides / Design

What Tolerance and Finish You Can Actually Ask For on a Ceramic Part

June 22, 2026

Ceramic tolerances are not worse than metal tolerances. They are differently distributed. A ground ceramic surface can hold numbers that would be respectable on hardened steel. The surfaces nobody ground are ten to a hundred times looser, and that is where drawings go wrong.

The numbers

Feature	As fired	As ground	Lapped or polished
Linear dimension	Around ±1% of the dimension	±0.005 to ±0.013 mm	±0.002 mm achievable
Flatness	0.1 mm and up	0.01 to 0.02 mm	0.005 mm and below
Parallelism	Not controlled	0.005 to 0.02 mm	Below 0.005 mm
Surface finish Ra	0.8 µm and up	0.2 to 0.4 µm	0.05 to 0.1 µm, lower on request
Perpendicularity	Not controlled	0.01 to 0.02 mm	Tighter with fixturing
Roundness	Not controlled	0.005 mm typical	Below 0.002 mm

Read these as what a competent shop holds in normal production on a part of ordinary size. Very large parts loosen, very small parts tighten, and every shop has its own sweet spot.

Why as-fired is so much looser

A pressed and sintered part shrinks 15 to 25% in the kiln, and that shrinkage is never perfectly uniform. A shop measures its shrinkage factor from its own batches and scales the green part accordingly, but powder lot, green density, part geometry and kiln position all move the result slightly.

That is why as-fired tolerance is quoted as a percentage rather than an absolute number, and why you should never inherit a ±0.05 mm general tolerance note from a metal drawing. See sintering shrinkage explained.

The cost curve is not linear

Going from as-fired to ground on a face is a large step. Going from ±0.013 mm to ±0.005 mm on the same ground face is a smaller step. Going from ±0.005 mm to ±0.002 mm is a large step again, because you have left ordinary grinding and entered lapping, temperature control and slow inspection.

A rough rule for budgeting:

1. Which faces must be ground at all? Each one adds setup and time.
2. Of those, which need to be tighter than ±0.013 mm? Each one adds passes and inspection.
3. Of those, which need a finish callout? Each one adds a separate operation.

Answer those three questions honestly and most drawings lose half their tight callouts without losing any function.

Surface finish is a function, not a decoration

Ra on a ceramic part is worth specifying when:

• The face seals, statically or dynamically
• The face slides against something
• The part is bonded, brazed or optically contacted
• The part is plasma exposed and particles matter
• The part is highly stressed and subsurface damage must be minimised

Outside those cases, as ground is fine, and a finish note simply adds cost. Note also that Ra alone does not describe a ceramic surface well. If the function depends on it, consider adding a bearing ratio or a maximum peak requirement.

How to mark it up

Use a general note that is honest about the route:

“Unless otherwise specified, as-fired surfaces to ±1%. Ground features to ±0.013 mm. Face A and face B lapped flat within 0.005 mm, Ra 0.1 µm max. All edges chamfered 0.2 x 45 degrees.”

That single block tells a shop everything it needs, keeps the cost where the function is and removes most of the back and forth. For the rest of the drawing detail, see 12 design rules and sharp corners and edge chipping.

Send your drawing and we match it with up to 3 shops, with the tolerances read the way a grinder reads them. Reply within 24 business hours.

Frequently asked questions

Keep reading

• Joining Ceramic to Metal Without Cracking It on the First Cycle
• What Is Possible and What Is Painful: Features in a Ceramic Part