Tolerance stack-up

Stack-up is the argument behind the argument when a print looks over-toleranced. Each dimension on its own seems needlessly tight, then the assembly view shows 4 of them chaining into one clearance, and the designer has divided a functional budget across the chain. The estimating question is never whether a single tolerance is achievable. It is what the chain costs, because the tightest link usually sets the process, and the process sets the price.

Reading stack-up at quote time changes real numbers. A bore and a face that stack into a seal fit may force grinding where turning would have held either one alone. A datum scheme per the print’s GD and T that chains through 2 setups can add a fixture, or force operations into 1 setup on a more expensive machine, which is a routing decision with the part’s whole margin inside it. The estimator who spots the chain quotes the expensive truth once. The one who prices each dimension in isolation finds the truth on the floor, after award, at the floor’s prices.

Stack-up is also where scrap hides. Parts can pass every individual check and still fail the function at the extremes if the print’s budget was optimistic, and on a program the cost of that discovery lands in sorting, in concessions, and in first article rounds that were priced as one. A quote that flags a suspicious chain before award, with the money attached, reads as expertise rather than hedging, and it is exactly the kind of print-reading that separates the slow careful quote from the fast wrong one in how to cut RFQ response time from 10 days to 1.