Tolerances become important as soon as you design an assembly of parts. Of course each dimension must be provided with a tolerance. But in an assembly the tolerances become important because all parts must fit. In this article I tacitly will assume that we are dealing with interchangeable parts.
Tolerance Analysis and Tolerance Allocation
We often don’t speak of tolerance stack-up analysis when just two parts are assembled together. Dimensions and tolerances must be such that the parts will always fit. With more than two parts performing a tolerance stack-up analysis can be necessary. This is not a fixed rule though. For instance in mass production even for the assembly of two parts a tolerance stack-up analysis can yield interesting results. A tolerance stack-up analysis can be used for:
- the analysis of actual dimensions of produced parts;
- tolerance allocation, deriving tolerance specifications on technical drawings.
In this article I want to speak about tolerance allocation.
Optimal Tolerances
As a mechanical designer you want to put optimal tolerances on your drawings. Tolerances that are big enough for a low price and small enough to ensure a good fit of the assembly. A tolerance stack-up analysis is an excellent tool to derive these optimal tolerances.
Step Plan Tolerance Stack-up Analysis
A tolerance stack-up analysis consists of the following basic steps:
- First you determine which dimension in the assembly you want to analyze, the so called critical dimension.
- Then you determine the specification for this critical dimension.
- Thereafter, you build the chain of tolerances that influences the critical dimension.
- Next you add up all tolerances in the chain.
- Finally you compare the derived sum with the specification in step 2. Take action when the specification is not met.
Statistical or Worst-Case
In each step there can be difficult to answer questions. Like: must the specification in step 2 always be met under all circumstances? And how big must the margin be? In step 3 there could be a mechanical adjustment or there could be moving parts in the assembly. In step 4 there is the question of performing a worst-case analysis or a statistical analysis. What is the distribution of the dimensional variation and how do you statistically add up these tolerances? What first seemed like a relative simple question can turn into a complex analysis. In a next post I will dive into more detail of these steps.
Tolerance Stack-up Analysis Expertise
Jaap Vink of Vink System Design & Analysis has great expertise in tolerance budgeting and stack-up analysis and has conducted several projects in this field. Besides, I give the Mikrocentrum three-day training course Tolerance Analysis several times a year.
Please feel free to contact me for more information or if you want to do a tolerance stack-up analysis.
