As-cast tolerances depend on the process, the size of the dimension and the metal. ISO 8062-3 sets dimensional casting tolerance grades, DCTG 1 to 16, so you can write a clear, measurable tolerance on the drawing instead of a vague request for good accuracy.
Lower grade numbers are tighter. Investment casting reaches the tighter grades, shell moulding sits in the middle, and hand-moulded sand castings sit at the looser end.
How ISO 8062-3 works
ISO 8062-3 gives a table of total tolerance values. You find your grade, then read across by the nominal size of the dimension. Larger dimensions get wider tolerances within the same grade, because shrinkage and mould movement grow with size.
The standard also defines required machining allowance grades, lettered A to K. These tell the foundry how much extra metal to leave on surfaces you will machine. Agreeing both on the drawing avoids arguments later about whether a face could be cleaned up.
A general note such as "Casting tolerances to ISO 8062-3 DCTG 10, RMA grade F" covers all untoleranced cast dimensions. Critical features can carry their own tolerances.
Typical grades by process
The standard's informative annexes show typical grades by process and metal. As broad guidance:
- Investment casting in steel typically reaches the tightest commercial grades, around DCTG 4 to 6.
- Shell moulding and machine-moulded green sand typically sit around DCTG 8 to 12.
- Hand-moulded sand castings in iron and steel typically sit around DCTG 11 to 14.
- Very large floor-moulded castings may need looser grades still.
Ask, do not assume
Check the current edition of the standard for the exact tables, and ask each foundry which grade it can hold consistently on your part. Asking for a tighter grade than the process allows raises cost and rejection rates without improving the part. If the grade you need is out of reach, the answer may be a different process, as compared in sand casting versus shell moulding.
Geometry and machined faces
ISO 8062-3 also covers geometrical tolerances such as straightness, flatness and roundness of cast features, using geometrical casting tolerance grades. Long, thin castings bend as they cool, so flatness on a large base plate may need its own note or a machining operation.
Machined faces are a different matter. Once a surface is machined, its tolerance comes from the machine tool, not the casting process, and can be far tighter. The casting tolerance then only needs to guarantee enough stock for the cut to clean up.
Tolerances in practice
- Agree which surfaces are datum surfaces, so both sides measure from the same place.
- Mark dimensions that cross the parting line, which usually need extra allowance for mould shift.
- Specify draft angles or let the foundry propose them, and record the result.
- Ask for a full dimensional report on first samples, as described in first article inspection.
- Recheck dimensions when patterns age, since wear moves sizes over time.
Next step
We review drawings with the foundry, check that the tolerance grade is realistic for the process, and inspect first samples against it. To go through your part, book a free sourcing review.