What Tolerance Stack-Up means
A tolerance stack-up is the arithmetic of variation across a chain of dimensions. Each feature is made within its own tolerance, but an assembly dimension passes through several of them, and the extremes combine: three features each ±0.2 mm can leave a gap anywhere across ±0.6 mm in the worst case. Sheet metal adds its own flavour, because bend position, hole-to-bend and flat-pattern accuracy all enter the chain alongside machined features.
Why it matters on the shop floor
Managing stacks is a design activity, not an inspection one. The questions a DFM review asks are: which dimension actually carries function, and can the chain be shortened or interrupted? Datums that restart the chain, locating a hole pattern off one edge instead of two, and assigning tight tolerance only to the features that need it — these cut stack-ups at the drawing stage, where they cost nothing, instead of at incoming inspection, where they cost parts.
What to remember
- Worst-case stacking adds tolerances arithmetically; statistical stacking assumes they rarely all land at extreme.
- Sheet metal stacks include bend position and flat-pattern accuracy, not just machined features.
- Shortening the chain — better datums, fewer referenced features — beats tightening every tolerance.