Engineering guide
GD and T for Ceramic Parts: Datums That Can Actually Be Held
Geometric tolerancing on a brittle material only works if the datum can be established on the part and maintained through machining. This guide covers where a datum scheme succeeds and where it collapses.
Foundation
Why the datum scheme decides the outcome
A datum that cannot be physically established cannot be held for machining or verified at inspection.
- Choose a datum that exists as a real surface on the part
- Prefer a face that the fixture can seat on without distortion
- Avoid datums on thin or unsupported sections
- Keep the number of datum references small
- State the datum on the drawing rather than leaving it implied
Which controls are realistic on ceramic
| Control | Practicality |
|---|---|
| Flatness | Very practical on ground faces |
| Parallelism | Practical; limited by part thickness |
| Perpendicularity | Practical against a ground datum face |
| Concentricity | Practical on bores and shafts, measurement-intensive |
| Roundness | Practical on a dedicated instrument |
| Position | Practical with a clear datum reference frame |
| Profile | Practical but inspection is more involved |
Each setup adds error
A part machined in four setups accumulates position error that appears in the final measurement.
How GD and T relates to size tolerance
Material condition modifiers behave differently on a brittle material, because there is no yielding to redistribute stress.
The part and the process
Frequently asked
What flatness can be held on a ceramic part?
Parallelism to 0.002 mm is a confirmed figure for the site. Flatness on a specific part depends on its size and thickness, and it is confirmed from the drawing.
Should I use the same datum scheme as the metal part it replaces?
Not automatically. The scheme should be one the ceramic part can physically support and that your incoming inspection can reproduce.
Why does the number of setups matter so much?
Because each re-datuming step adds position error. Reducing setups is often more effective than tightening a tolerance.
Can concentricity be verified reliably?
Yes, but it is measurement-intensive. Where position is the real requirement, a position tolerance against a clear datum is usually both cheaper and more meaningful.
Have a drawing that this guide applies to?
Send it with the material and the quantities. The review confirms what is achievable before any price is issued.