Why do we need to perform Tolerance Stack-up analysis?

Mechanical Design verification
Every designed gap, interference and functional features should have been calculated for tolerance stack up analysis in order to verify the design. The product or component design depends on stack up analysis. It’s a good engineering practice to design a product thinking of the number of the contributors to stack up for functional areas. Datum definition is also a vital part of the mechanical design and should be considered at the concept design stage.

Dimensions to show on your drawings
Dimensions that are used in a tolerance stack-up are necessarily shown on the drawing. All linear dimensions should be dimensioned from the datums. If the dimensions required for tolerance stack-ups are not on the drawing, problems might occur later in production. It is a must for all the functional dimensions to be present on the drawing.

Root Cause Analysis will be easy and smooth to prove your design’s fit & function:
During design validation and product validation, no failures related to interference, clearance, or travel should occur if your stack-ups are correct. If there are failures, it is easy to prove they are not related to design when the tolerance stack-up is ready. They will be more likely connected to the functional dimensions being out of the spec.

No need for repetitive tool changes between design validation and production (no trial and error)
Invariably when the product works for the first time , no dimension related tool changes are necessary. Injected molded tool changes are expensive and time consuming.
Your product will work without a glitch during the entire lifespan of the program:
No need for extremely costly tool changes after production started, or calls from customers for expensive warranty claims, especially in the automotive industry.

Our company annually undergoes certification audit to ensure compliance of our design procedure to specific standards. To be ready for audits, we have to maintain our documents in order, including tolerance stack-ups. This is frequently required in medical, automotive and consumer electronics industries.

Supplier’s communication
It is easy to answer requests from suppliers regarding tolerances and dimensions of their tools.
When the suppliers ask if they can open a tolerance or change a nominal dimension, all you need to do is to put their requested dimension or tolerance into your already prepared stack-up.

Performing a Stack up Analysis one of these two methods are to be used:

Worse-Case Tolerance Analysis (WCA).
Worse-Case Tolerance Analysis determines the absolute maximum possible variation for a selected distance, gap or other characteristic. This method assumes that all dimensions in the tolerance stack up may be at their worst-case maximum or minimum (but within specification), regardless of the improbability.

Statistical Tolerance Analysis (RSS).
Statistical Tolerance analysis determines the probable or likely maximum variation possible for a selected characteristic (dimension, gap or other). This method more realistically assumes that it is highly improbable that all dimensions in the tolerance stack up will be at their worst-case low limit or high limit at the same time.

Statistical Tolerance Analysis techniques usually predict less variation than the worst-case results for a tolerance stack up.

The decision on what method to use or a specific analysis is to be taken based on the number of variables (tolerances) in the Stack up Analysis and the design sensibility (critical features).

As a rule of thumb as the number of tolerances in a tolerance stack up increases, the benefits and validity of using the Statistical Analysis increases.

Generally a better concept design combined with a better dimensioning scheme implies Stack up Analysis with less dimensions/tolerances and subsequently resulting a minimum of variability of the characteristic analyzed.

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