Manufacturing & engineering
Tolerance stack-up
Definition
A tolerance stack-up is an analysis of how permitted dimensional and geometric variation combines to affect a functional assembly result, such as a gap, fit, alignment, or preload. It follows the dimensions and locating relationships that control the result rather than adding every tolerance on the drawing.
Also known as: tolerance stack, dimensional stack-up, tolerance chain
Updated
A functional dimension chain#
A stack-up begins with one output that must remain within limits. The analyst then follows a signed loop through the dimensions, datums, contacts, and locating features that increase or decrease that output.
ASME Y14.5 defines the US system for communicating dimensional and geometric tolerances. The stack applies those requirements to a specific assembly function. Dimensions outside the controlling loop do not belong merely because they appear on the same drawing.
Worst-case analysis#
Worst-case analysis adds the absolute effect of every allowed contributor in the harmful direction. It answers whether every combination that satisfies the released tolerances also satisfies the assembly requirement.
This method is conservative, but its result is easy to audit. If worst case fails, the design team can change the nominal geometry, locating strategy, or selected tolerances rather than assuming production will avoid the unfavourable combination.
When a linear stack is incomplete#
A one-dimensional chain can miss component rotation, clearance shift, assembly sequence, geometric tolerances, thermal growth, coatings, and part flexibility. Metrologic DCS's 2026 guidance specifically warns against oversimplifying the assembly, choosing an inaccurate positioning strategy, and treating flexible parts as rigid.
Three-dimensional software can model more of that behaviour. The SOLIDWORKS partner description of 3DCS lists Monte Carlo, sensitivity, gap-and-flush, and geometric relationship analyses. Those product features do not choose correct datums, inputs, or acceptance criteria for the analyst.
Sources
Related terms
Tolerance
A tolerance is the allowed variation from a nominal dimension, such as 10.00 ± 0.05 mm. Tolerances drive fit, cost, and inspection, and they have to be specified deliberately because a CAD model on its own stores only nominal sizes.
Statistical tolerance analysis
Statistical tolerance analysis estimates the distribution of an assembly result from distributions assigned to its contributing dimensions and geometric controls. Unlike worst-case stack-up, it predicts a probability of meeting a requirement and depends on valid assumptions about process spread, centring, correlation, and assembly behaviour.
GD&T
GD&T is a symbolic language for specifying the allowed variation in a part's form, orientation, location, and runout relative to datum features. In the US it is defined by ASME Y14.5, and it tells the manufacturer and inspector which geometry actually matters for fit and function.
Clearance
Clearance is the positive space between components, often measured in CAD as their minimum nominal separation. Required clearance must account for function and manufacturing variation; the absence of geometric interference does not by itself provide enough clearance.
Assembly
An assembly is a CAD model made of multiple parts or subassemblies positioned relative to each other with mates or joints. Assemblies are where fit, clearance, interference, and motion get checked.