Tolerance Stacking (Stack-Up): Methods & Practices
Tolerancing is common in the manufacturing industry. Tolerance stacking refers to the method of tolerancing applied to the stacked-up geometric features.
It ensures that the production process is easy, accurate, and within the budget. The article below takes a deep dive into the concept of tolerance stacking, explaining everything in detail.
What is Tolerance Stacking or Stack-Up?
Tolerance stacking refers to the combined tolerance of components that have connected features. It's understood as the tolerance of the individual part feature adding up to have the overall impact on the tolerance of the entire component.
It is a critical analysis that ensures that the components and their features are suitable for manufacturing. It helps avoid situations where the component has features that fall within the tolerance but the component on its whole is not within tolerance.
Example of Tolerance Stacking
An example of a plate can be considered, which has pin holes on its sides. The location of the holes on this plate is a tolerance feature. It is possible that all the holes are present within the tolerance but at the maximum limits.
In such a situation, the holes having the largest size may not leave enough material to support the base plate. Hence, in such a situation, despite the main features being within tolerance, the overall component isn't.
The aim of considering tolerance stack proves to be essential and also it helps to reduce wastage on the part of time and money.
Methods of Tolerance Stacking Analysis
The two methods of tolerance stacking analysis are as follows:
Worst Case Tolerance Analysis
Worst-case tolerance stack analysis is a very simple analysis. It works on the assumption that the features are at their maximum levels of deviation, which is the worst-case situation for any manufacturer.
However, in a real production environment, it is a very unlikely thing to happen but you consider it in cases where stakes are high.
For instance, in components that require high accuracy, like medical parts and aerospace components. It is calculated using the formula given below.
The worst-case scenario ensures that all the parts are within the tolerance and the required engineering fit is achieved.
However, this confidence is only possible if the tighter tolerances are looked into. It is suitable for low-volume components as it is manageable to have this tight tolerance achieved.
Statistical Tolerance Analysis / RSS – Root Sum – Squared
The statistical tolerance stack analysis is a realistic method for tolerance stacking. It uses a statistical method for estimating the probability of deviations in every tolerance dimension.
It has a primary hypothesis that every dimension is different; some will be the same, some more, and some less. It is applicable in production environments of high volume where the minor margin for errors is acceptable.
Once the probability distributions are calculated individually, they are added together to compute the entire tolerance of the component. It gives an estimate of the parts that are meant to fail the quality tests.
The statistical technique used by this analysis is the Root Sum Squared method, and it has the following formula:
Tolerance Stacking Best Practices
However, when you are conducting tolerance stacking for the components, some of the best practices you can follow are as follows:
Keep Your Drawings Clean
The drawings are a way of communicating the intent of the design to the engineers.
One mistake that is usually observed in the work of inexperienced engineers is to add tolerance in every feature of the part. It is not required, and tolerance for the critical features should be defined only.
Hence, do not overpopulate your drawings, as it leads to confusion.
Understand the Manufacturing Challenges
Sometimes, designers place very tight tolerance as needed on some features, which makes it challenging for the manufacturers to manufacture components. Hence, the tolerance should be decided based on the functionality of the component.
Adhere to General Tolerance Best Practices
The general standards of GD&T apply to tolerance stack-up as well. Hence, the best practices of these tolerances should be followed.
One should be mindful of how every feature of the component interacts with one another. It allows for keeping tolerance within the limits of the production capabilities.
Consider Post-Manufacturing Part Quality
When creating the drawings for the components, the engineers must consider the changes the components will go through when they are used.
Hence, the maintenance and wear and tear will impact the tolerance, which requires conditions to enhance the durability of the component.
Tools for Tolerance Stacking
The latest CAD software has tools that have a tolerance stackup calculator as the tool has transformed a lot in a few years.
The designers can select from various tolerance stack-up methods to identify the possible tolerance in the components and integrate them into their drawings.
Such tools help engineers to implement different strategies for attaining precision in components.
Заключение
Tolerance stacking is an essential aspect of producing components that can fit perfectly. Having the relevant skills and expertise on the subject of GD&T and tolerance stacking is very important.
Мы в DEK ensure to manufacture your component keeping all these aspects considered. So, connect with us for your next projects.
Вопросы и ответы
Why is tolerance stacking important?
Tolerance stacking ensures that every part is manufactured within the defined tolerance limit for the features of the component. It improves the assembly and functionality of the components.
What is 1st and 2nd order tolerance stacking?
1st and 2nd order of tolerance stacking are the terms representing the two methods, which are worst-case tolerance stacking and statistical tolerance stacking.
What are the common difficulties of tolerance stacking?
The common challenges associated with tolerance stacking consist of selecting the suitable tolerance chain variation. It also includes finding a balance between tolerances and affordability, and assessing production capability.
