NADCA publishes two tolerance sets for die castings. Standard tolerances are what good manufacturing practice achieves with normal inspection, die life and die maintenance. Precision tolerances are tighter, roughly plus or minus 0.002 in on the first inch of length against roughly plus or minus 0.010 in for Standard, and they are achieved through precise tooling development and sustained process control rather than by request.
Seven Categories, Two Sets
The standards present both Standard and Precision values across seven tolerancing categories:
- Linear dimensions
- Dimensions across the parting line
- Dimensions formed by moving die components
- Angularity
- Draft
- Flatness
- Cored holes for threads
Dimensions across the parting line and dimensions formed by moving die components behave differently from ordinary linear dimensions, because they depend on die closure and slide position rather than on cavity geometry alone. Buyers who apply one linear tolerance across a whole drawing usually create a problem in exactly those two categories.
Precision Is a Purchase
The important point for a buyer is that Precision is not a stricter request, it is a different cost structure. It is bought with tooling investment, tighter process control, more inspection and shorter maintenance intervals. When a drawing carries precision-class tolerances by default, someone has made a purchasing decision without pricing it.
The disciplined approach is Standard everywhere, with Precision applied only to the features where function requires it, and stated explicitly on the drawing so the quote reflects it.
What to Put on the RFQ
State which set applies and to which dimensions. "NADCA Standard, with Precision on the two bore diameters" is a quotable instruction. "Tight tolerances required" is not.
How Buyer24 Helps
Buyer24 keeps the specification consistent across every supplier on a request, so quotes differ on price and lead time rather than on which tolerance set each shop assumed. Get started →
FAQ
Does NADCA apply to sand or permanent mould castings?
No. The NADCA product specification standards cover die castings. Sand and permanent mould work is normally specified under ISO 8062-3 tolerance grades, with the Aluminum Association standards governing the process itself.
Can a foundry hold Precision on every feature?
Technically often yes, commercially rarely sensibly. Holding Precision across a whole part raises tooling cost, inspection cost and scrap rate simultaneously, and most of the tightened features contribute nothing to function.
Where does draft fit into this?
Draft has its own tolerance category in both sets. It is a castability requirement rather than a nuisance, and removing draft to suit a designer's preference is one of the more expensive changes a buyer can approve without noticing.
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