Two tolerances on every casting drawing
A cast and machined part carries two kinds of tolerance. The as-cast surfaces are held to a casting tolerance, set by the process, the part size and whether a dimension crosses the parting line or a slide. The machined features are held to whatever the drawing states, because machining produces them. Our as-cast blank is quoted to ±0.2 mm nominal; tighter features are machined and agreed per feature. A drawing that gives one general tolerance for everything forces either expensive machining or an argument at inspection.
How ISO 8062-3 expresses a casting tolerance
ISO 8062-3 is the international standard for dimensional and geometrical tolerances of moulded parts, including castings. It does not give one number per process. It defines grades, and the drawing calls them up:
| Element | What it means | What you choose |
|---|---|---|
| DCTG | Dimensional casting tolerance grade, 1 (tightest) to 16. The total tolerance for a dimension depends on the grade and the nominal size. | The grade the process can hold on as-cast dimensions |
| GCTG | Geometrical casting tolerance grade for straightness, flatness, roundness, parallelism and similar | Where form matters on as-cast surfaces |
| RMA / RMAG | Required machining allowance, in grades A to K by size | Stock left on surfaces that will be machined |
| Individual tolerances | Values written on specific dimensions override the general grade | Every critical as-cast dimension |
General tolerances: ISO 8062-3 – DCTG [ ] – GCTG [ ] – RMA [ ] (RMAG [ ]). Dimensions crossing the parting line or formed by slides: [allowance]. Machined features: as dimensioned.
We do not print an ISO 8062 tolerance table here: copy values from the edition of the standard your drawing cites. The grade that fits your part is agreed at drawing review, for the process and the dimension.
Other frameworks you will see on drawings
- NADCA Product Specification Standards for Die Castings: North American die casting tolerances, with separate values for dimensions across the parting line and for moving die components
- DIN 1688: found on older German drawings; replaced by ISO 8062-3
- Customer standards: many OEMs publish their own casting tolerance classes; send them with the RFQ
What moves a casting dimension
- Parting line and slides. A dimension across the parting line or formed by a slide adds the die's closing and slide variation.
- Size. Larger dimensions shrink more in absolute terms.
- Wall uniformity. Uneven sections cool at different rates and distort.
- Heat treatment. T6 quenching can move dimensions; check after heat treatment.
- Tool wear. Dimensions drift over tool life; the tooling quote should state the life it is built for.
Datums and the first machining operation
The first machining operation locates on cast surfaces; those surfaces need tight enough casting tolerances and a stable datum scheme. Agree the cast datum targets with us during tooling, so the fixture datum and the CMM datum match. After the first operation, the machined datums take over.
What to send with the RFQ
- Which surfaces stay as cast and which are machined
- The general tolerance standard and grade, if any
- Critical as-cast dimensions individually toleranced
- Datum scheme and cast datum targets
- Condition in which dimensions are measured: after T6, machining, coating