Why castings need their own tolerances
The final size of an as-cast dimension is the result of wax contraction, shell behaviour, alloy contraction and any distortion in cooling and heat treatment. The variation grows with the nominal dimension and depends on whether the feature is free to contract or restrained by the shell. Casting tolerance standards therefore relate the tolerance to the size of the dimension and to a grade that reflects process and effort.
ISO 8062: the international framework
The ISO 8062 series covers dimensional and geometrical tolerances for moulded parts.
- ISO 8062-1 defines the vocabulary.
- ISO/TS 8062-2 gives rules for drawing indications.
- ISO 8062-3:2023 specifies general dimensional and geometrical tolerances and machining allowance grades for castings toleranced with plus-minus tolerances on indicated dimensions. It defines dimensional casting tolerance grades (DCTG), geometrical casting tolerance grades (GCTG) for straightness, flatness, roundness, parallelism, perpendicularity, symmetry and coaxiality, and required machining allowance grades (RMAG). The 2023 edition replaced the 2007 edition.
- ISO 8062-4 describes an alternative concept using general profile tolerances in a datum system, suited to parts defined by 3D models.
The grade obtainable depends on the casting process, alloy, size and production quantity. An informative annex of the standard gives typical grades by process; for investment casting, grades in the region of DCTG 4 to 6 are commonly quoted, with finer grades requiring agreement and additional effort. A general tolerance is invoked by a note on the drawing in the form prescribed by the standard.
| Nominal dimension (mm) | DCTG 4 | DCTG 5 | DCTG 6 | DCTG 7 | DCTG 8 |
|---|---|---|---|---|---|
| > 0 – 10 | 0.26 | 0.36 | 0.52 | 0.74 | 1.00 |
| > 10 – 16 | 0.28 | 0.38 | 0.54 | 0.78 | 1.10 |
| > 16 – 25 | 0.30 | 0.42 | 0.58 | 0.82 | 1.20 |
| > 25 – 40 | 0.32 | 0.46 | 0.64 | 0.90 | 1.30 |
| > 40 – 63 | 0.36 | 0.50 | 0.70 | 1.00 | 1.40 |
| > 63 – 100 | 0.40 | 0.56 | 0.78 | 1.10 | 1.60 |
| > 100 – 160 | 0.44 | 0.62 | 0.88 | 1.20 | 1.80 |
| > 160 – 250 | 0.50 | 0.70 | 1.00 | 1.40 | 2.00 |
| > 250 – 400 | 0.56 | 0.78 | 1.10 | 1.60 | 2.20 |
Total linear dimensional casting tolerance (DCT) in mm for grades DCTG 4 to 8 and nominal dimensions up to 400 mm, as tabulated in ISO 8062-3. The tolerance zone is normally placed symmetrically about the nominal dimension. Extract for orientation; the standard itself is authoritative and contains further grades, sizes, wall-thickness rules and geometrical tolerances.
The tolerance reference tool looks up a value from this table for a given dimension.
VDG P 690
The German guideline BDG/VDG P 690 (edition 2010-07) was written specifically for investment castings and is widely used on European drawings. It defines accuracy grades D1, D2 and D3 for linear dimensions, separately for material groups, and also covers form and position, surface condition and machining allowances. D1 is the general grade; D2 and D3 are tighter and are applied to selected dimensions, D3 typically requiring agreement with the foundry. The guideline is a separate document from ISO 8062 and the two systems should not be mixed on one drawing without stating which applies to which dimension.
Geometrical tolerances and datums
Individual geometrical requirements are specified with the symbols of ISO 1101: flatness, straightness, roundness, cylindricity, parallelism, perpendicularity, position, coaxiality, symmetry, run-out and profile. For castings the following points are important:
- Datum system (ISO 5459). An as-cast part should be located on datum targets, typically six points on three planes, placed on stable surfaces. The same targets serve the first machining operation.
- Independency (ISO 8015). Size and geometrical tolerances apply independently unless stated otherwise; a size tolerance does not by itself control form.
- Sizes and other dimensions (ISO 14405). Only features of size, such as diameters and widths, are unambiguously controlled by plus-minus tolerances. Step heights, centre distances and positions are better controlled by geometrical tolerances.
- Profile tolerances referenced to a datum system are an effective way to control complex cast surfaces defined by a 3D model.
- Cylindricity, run-out and tight position on functional features usually require machining.
ISO 2768 and ISO 22081
ISO 2768-1 gives general tolerances for linear and angular dimensions of parts made by material removal or sheet forming; it is appropriate for the machined features of a casting, not for its as-cast dimensions. ISO 2768-2 (general geometrical tolerances) was withdrawn in 2021 and replaced by ISO 22081, which sets out how general geometrical and size specifications are stated. Drawings that carry only an ISO 2768 note should be reviewed before a casting is quoted.
Angles, shrinkage and distortion
Angular tolerances are not covered by the general tolerances of ISO 8062-3 and are agreed individually. Pattern allowance for contraction is the foundry's responsibility and is refined on first samples. Distortion of long, thin or asymmetric parts is a main cause of out-of-tolerance geometry; straightening, fixtures in heat treatment or wider tolerances are the options.
Practical recommendations
- State one casting tolerance standard and grade as the general tolerance.
- Tolerance functional features individually and plan to machine those beyond casting capability.
- Define datum targets on the drawing.
- Agree gate witness, parting-line witness and the condition (as cast, heat treated, blasted) in which tolerances apply.
Standards cited here are listed with their status in the standards library.