Guides / Process

How a Ceramic Face Gets Flat, and What Flat Actually Costs

July 20, 2026

Grinding gets a face close. Lapping makes it flat. Polishing makes it smooth. They are three operations with three fixtures and three prices, and drawings that ask for all of them everywhere are the reason ceramic quotes surprise people.

What each step does

Grinding removes bulk material and gets the face within a few hundredths of a millimetre, with a finish around Ra 0.2 to 0.4 µm depending on grit and grade.

Lapping runs the part against a cast iron or composite plate with loose abrasive slurry between them. Because the abrasive is loose and the plate is flat, it corrects form. This is the operation that produces real flatness, and it also removes subsurface damage left by grinding.

Polishing uses finer abrasive on a softer carrier. It improves roughness and appearance and removes almost nothing dimensionally. Do not expect polishing to fix a form error.

Operation	Typical flatness	Typical Ra	What it corrects
Ground	0.01 to 0.02 mm	0.2 to 0.4 µm	Size and basic form
Lapped	0.005 mm and below	0.05 to 0.1 µm	Flatness, parallelism, damage layer
Polished	As lapped	Below 0.05 µm	Roughness and appearance

How flatness is measured

On small faces, the shop places an optical flat on the part under monochromatic light. Interference bands appear, and each band represents roughly a quarter of the light’s wavelength in height, which is on the order of 0.15 µm for the usual helium light source. Counting and reading the curvature of the bands gives flatness quickly and cheaply.

For larger parts, complex geometry or tighter requirements, the work moves to an interferometer or to a CMM with a suitable probe. Both are slower and both cost more, which is why a flatness callout on a big face is not the same purchase as the same callout on a small one.

When the finish is functional

Specify lapping or polishing when:

• The face seals, statically or dynamically
• The face slides, as in a mechanical seal or a valve plate
• The part is optically contacted or bonded
• The part is plasma exposed and particle generation matters
• The part is highly stressed and subsurface damage must be minimised, as covered in how diamond grinding works
• The part is zirconia in hot wet service, where surface condition affects ageing

Outside those cases, as ground is the right answer and any finish note is money you will not get back.

Practical notes

Lapping needs a stable part. Thin parts flex against the plate and come off flat only while they are clamped. Tell the shop if the part is thin or has an unsupported section.

Parallelism is a two face job. If you need two faces parallel and flat, both get lapped, usually together in a carrier. Specify the pair, not each face in isolation.

Corner and edge treatment comes first. A sharp edge chips during lapping and contaminates the plate. Chamfers are part of the finishing plan, not an afterthought. See sharp corners and edge chipping.

Cleanliness after lapping matters. Slurry residue in a pore or a corner is a contamination problem in semiconductor and medical work. Specify the cleaning and packing requirement, not just the finish.

The drawing line that works

“Face A lapped flat within 0.005 mm, Ra 0.1 µm max. Face B ground, parallel to face A within 0.01 mm. Remaining surfaces as fired.”

One sentence, three different costs, each one where the function is. The wider tolerance picture is in the ceramic tolerance and surface finish guide.

Send your drawing and we match it with up to 3 shops with lapping and inspection in house. Reply within 24 business hours.

Frequently asked questions

Keep reading

• HIP for Ceramics: What the Extra Furnace Cycle Buys You
• Glazing and Sealing: Making a Porous Ceramic Part Hold Up in Service