{"id":8158,"date":"2026-10-28T04:01:37","date_gmt":"2026-10-28T04:01:37","guid":{"rendered":"https:\/\/chinametalfoundry.com\/?p=8158"},"modified":"2026-10-28T04:01:37","modified_gmt":"2026-10-28T04:01:37","slug":"casting-tolerance-stack","status":"publish","type":"post","link":"https:\/\/chinametalfoundry.com\/ru\/blog\/casting-tolerance-stack\/","title":{"rendered":"Casting Tolerance Stack: Datums, GD&T and Numbers That Hold in Production"},"content":{"rendered":"
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A casting tolerance stack is arithmetic with teeth. Your print calls \u00b10.5 mm on a flange face, the raw casting shows up with \u00b11.2 mm of as-cast variation, and nobody broke a rule \u2014 nobody ever added up the pieces. Build the casting tolerance stack before you quote the part, and you will know to the dollar which number is worth paying for.<\/p>\n<\/div>\n

This guide uses one real part \u2014 a 14.2 kg finished ductile iron pump housing running 6,000 pieces a year \u2014 and walks the same casting tolerance stack<\/strong> a buyer actually has to close. Every figure below is calculated from a stated assumption, not quoted from memory, so you can rebuild it with your own numbers. Where a rule carries legal weight we point at the standard itself: the datum, datum target and projected tolerance zone rules come from ASME Y14.5 dimensioning and tolerancing<\/a>, the die casting guidance comes from NADCA<\/a>, and measurement traceability practice follows NIST<\/a>.<\/p>\n

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A casting tolerance stack starts with the part, not the print: machined pads, cored bores and as-cast skin all behave differently.<\/figcaption><\/figure>\n

What a Casting Tolerance Stack Really Adds Up<\/h2>\n

Most buyers treat “casting tolerance” as one number \u2014 usually whatever sits in the foundry’s general note. It is not one number. It is four different sources<\/strong> that behave in four different ways, and only one of them is random. Our guide to casting tolerances and ISO 8062 classes<\/a> covers the classification system; this article covers what those classes do once the part meets an assembly.<\/p>\n\n\n\n\n\n\n\n\n\n
Layer<\/th>\nTypical on a 240 mm iron housing<\/th>\nBehaviour<\/th>\n<\/tr>\n<\/thead>\n
As-cast size and form variation<\/td>\n\u00b11.2 mm over 240 mm<\/td>\nRandom within the batch; grows with core shift and mold opening<\/td>\n<\/tr>\n
Pattern and tooling drift<\/td>\n+0.3 to +0.5 mm across a pattern life<\/td>\nOne-direction drift, not noise \u2014 it moves the mean<\/td>\n<\/tr>\n
Cleanup machining<\/td>\n\u00b10.10 mm per setup<\/td>\nReplaces the casting number on every face it touches<\/td>\n<\/tr>\n
Heat treatment movement<\/td>\n\u00b10.20 mm per 100 mm on steel grades<\/td>\nAppears only after stress relief or normalising<\/td>\n<\/tr>\n
Assembly mate<\/td>\n\u00b10.15 mm hole spacing<\/td>\nAdds at the joint, outside your supplier’s control<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

The classic failure is treating all five rows as interchangeable. They are not. Machining replaces a casting number; it does not add to it. Tooling drift moves the centre of the distribution, which is why a process can sit inside tolerance on Monday and fail on the far side of the same tolerance six months later. And the assembly mate belongs to somebody else’s drawing \u2014 but it still lands in your casting tolerance stack.<\/p>\n

The one-line version<\/h3>\n
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Pick the surfaces you will machine, let those carry the datums, and give every as-cast surface a tolerance it can actually hold without machining. Everything else in this article is just the arithmetic behind that sentence.<\/p>\n

Why Castings Need Different Datums Than Machined Parts<\/h2>\n

A machined block gives you three faces that are genuinely flat and square. An as-cast surface does not. Sand-cast skin carries ripple, parting-line step and core shift, so a full-face primary datum “rocks” on the fixture and every downstream measurement picks up that rocking as if it were part geometry.<\/p>\n