As engineers, we can't forget that as the excitement of the creation within a conceptualization phase comes to a crescendo, a critical step in comparing design alternatives and detailing out a preferred approach involves completing tolerance stacks. A tolerance stack is simply an analysis of how the various components of a design will fit and/or perform together based on the anticipated components’ dimensional differences, which will be seen in production.
Depending on the criticality of what is being designed, a tolerance stack can be scaled as appropriate — think medical device versus a pencil box. In the case of a pencil box, a couple hand calculations and you may be well on your way to a final design with very low risk of issues. Now contrast that to a complicated disposable medical device that has dozens of interacting components made from several multi-cavity tools to keep up with the millions of devices needed each year — you better have a robust tolerance stack analysis method. It is the latter scenario that has driven the latest revisions in our tolerance stack methods.
While certain scenarios justify advanced tolerance analysis methods such as 3D Monte Carlo Simulations, we still see significant value in more traditional 2D tolerance stacks. Certainly, one of the most challenging aspects of a tolerance analysis is maintaining it as the program transitions through first-article approvals, pilot production, and eventually process validation.
The original tolerance stack is usually based on a compromise between what the development team is hoping for and the operations team and vendor base is willing to sign up for.
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