How do you manage tolerance stacking in complex assemblies to ensure accurate fit and function in final production?
How to manage tolerance stacking in complex assemblies for precise fit and function in production.
Accepted answer
For over 10 years I have worked at Volkswagen. One of my roles is working on tolerance analysis for new model introduction.
Methods range from simplified 1D stack-up analysis to 100,000+ unit full Monte Carlo simulations using purpose built software. We use CATIA with plugins, but there are other software packages such as VisVSA or 3DCS.
For simple (think 1-D) stack-ups there is a excel based sheet called CATS 1-D.
You'll need to take into consideration not only single part tolerances, but also the process contributions such as how the parts datum to one another and clearances for the parts to their locating features.
For example the width of two parts with a ±0.5mm single part tolerance in the measurement area, with ±0.2mm tolerance at a locating pin hole, and ±0.01 clearance for the locating pin would have a ±1.42 worst case stack-up.
We normally use a RSS stack-up or a 100k unit simulation to determine the expected process output and expected defect rates, then drive design (typically locate scheme or nominal/tolerance changes) or process changes to improve the results.
