Which ISO 8062-3 casting tolerance grade should a sand or die casting drawing use?

What causes a casting tolerance dispute, from most to least common?

The most common cause is a machined-part title block reused on a casting drawing. Next come a casting grade chosen without reference to the process, drafted or parting-line features held to general geometry, wall thicknesses graded like other sizes, and a missing datum system.

1. The wrong standard in the title block. The drawing was started from a machined-part template and nobody changed the general note, so ISO 2768 is the only rule on the sheet.
2. A grade picked by wish rather than by process. ISO 8062-3 says in its introduction that the tolerances specified may determine the casting method, and it recommends that the customer discuss the accuracy required, the casting method, the parting surfaces and draft, the number of castings and the alloy with the foundry before the design or order is final. Its Annex A grades are informative: they are what can normally be held once the production factors have been fully developed, and for complex castings Annex A recommends one grade coarser. The 2007 edition also says the system applies when the manufacturer provides the pattern or die, or accepts responsibility for proving it.
3. Draft and parting-line features. In ISO 8062-3:2007, general tolerances on form and orientation do not apply to features with draft; those need individual tolerances. Dimensions across the parting line also carry die or mold mismatch, which the standard treats separately as surface mismatch (SMI).
4. Wall thickness held to the same grade as everything else. In ISO 8062-3:2007, wall thicknesses in DCTG 1 to 15 take one grade coarser than the grade given for other dimensions, so a DCTG 10 drawing means DCTG 11 on walls.
5. No datum system for general geometry. General orientation tolerances under ISO 8062-3:2007 need a datum system marked “ISO 8062-3 DS” in or near the title block. Without it, the caster and the receiver can fixture the part differently and both be consistent with the drawing.
6. A model with no dimensions. The standard states that it does not apply to 3D CAD models used without indicated dimensions, so a “cast per model” note plus an ISO 8062-3 reference does not on its own create a checkable requirement.

How do you check whether a rejected casting really fails?

Re-judge each rejected characteristic against the rule that actually governs it: as-cast or machined, which general note applies, and whether it is a wall, a drafted feature or a parting-line size. Only a feature that fails its correct requirement, measured from an agreed datum setup, is a real nonconformance.

Work through the reject report line by line with this checklist before any parts are scrapped or returned:

• As-cast or machined? Mark every rejected characteristic. If the drawing set has a separate casting drawing, use it; if it is one combined drawing, look for the machining symbols.
• Which note governs it? If the title block cites only ISO 2768, record that the as-cast requirement is disputed rather than failed, and take it to engineering.
• What is the casting limit? Look up the total tolerance for the agreed DCTG and the nominal size band, and split it equally unless the drawing gives asymmetric limits.
• Is it a wall? Use one grade coarser for wall thickness, per ISO 8062-3:2007.
• Is it drafted? General form and orientation tolerances do not apply to drafted features; check the draft angle and any individual tolerance instead.
• Does it cross the parting line or come from a core or slide? Measure mismatch separately and check whether the drawing limits it.
• Was the right nominal used? Measure against the dimension shown on the drawing, not a value read off the model; the standard does not apply to a 3D model without indicated dimensions.
• Was the part held the same way? Compare the datum setup and fixture used by the caster and by your inspection. A different setup is a common reason two reports disagree.
• Does it matter? For each true overrun, check fit, clearance and whether machined faces still clean up. A boss that is high but still assembles is a deviation decision, not a scrap decision.

Machined faces that do not clean up are a different problem from an as-cast size overrun: the casting tolerance and the machining stock were not sized together. The machining allowance guide covers how to size stock against the casting grade.

What general note should the casting drawing and RFQ carry?

Name the standard and its edition, the DCTG for as-cast sizes, the GCTG with its datum system if general geometry is wanted, and the machining allowance, then say in words that ISO 2768-1 governs machined features only. One clear note in the title block removes most of the room for a dispute.

A note in this form, adapted to your grades, works on most casting and combined drawings:

• AS-CAST FEATURES: GENERAL TOLERANCES ISO 8062-3:2023 – DCTG 8 – GCTG 5
• DATUM SYSTEM FOR GENERAL ORIENTATION: ISO 8062-3 DS | A | B | C
• MACHINING ALLOWANCE: RMA [value] (RMAG [grade]) UNLESS SHOWN
• MACHINED FEATURES: ISO 2768-1 CLASS m UNLESS OTHERWISE SPECIFIED

Check the exact indication against the drawing-indication clause of the edition you cite (Clause 9 in the 2007 edition, Clause 11 in the 2023 edition), and make sure the caster quotes from the same edition.

Choose the DCTG from the process band and the function, not from the tightest number the drawing can carry. Give individual tolerances for the as-cast features that locate, seal or mate, and for drafted features where shape matters. On the RFQ, add what the caster needs to confirm the grade:

• the casting process and alloy you expect, or a request for the caster to propose one;
• the expected annual quantity, because long and short runs sit in different bands;
• which features are critical, and which surfaces are machined;
• a request to confirm the DCTG and GCTG in the quote, and to propose the parting line and draft before tooling;
• a first article report that covers the casting in its as-cast state and the finished part after machining.

If an older drawing carries an ISO 8062 “CT” grade from the 1994 edition, update the note to the current edition before quoting rather than leaving the caster to translate it. The ISO 2768 and NADCA guide covers how the machined-feature note should read.