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Tolerance stacking 101

Tolerances indicate how much a part’s measurements can vary from its ideal dimensions. They are a core aspect of Geometric Dimension and Tolerancing (GD&T), which deals with communicating design intent for manufacturing purposes. The point of creating tolerances isn’t to prevent unavoidable variations part to part but to account for them from the beginning of the manufacturing process and control them as best we can. Proper tolerancing can help you save money, time, and resources during production runs while ensuring correctly manufactured components.

One key part of effective tolerancing is accounting for tolerance stack-up. By performing tolerance stack analysis, you can make sure a component’s tolerances are mathematically correct, physically possible, and truly beneficial to part production and performance. This article will give a broad overview of what tolerance stacking is, why it’s important, and how to utilize it in your part designs.

What is tolerance stacking or stack-up?

Tolerance stack-up is the process of adding tolerances together before manufacturing in order to understand their cumulative effect on part production. Final results from a tolerance stack are compared to tolerancing standards, regulations, and other limits in order to ensure the part design will produce high-quality components. This tells you the total amount a part can differ from specified dimensions.

Proper tolerance stack analysis also enables you to predict how your final component will look, function, and interact with other components — this is particularly important when it comes to manufacturing mating parts.

Accounting for tolerance stack-up helps ensure tolerances can be manufactured before manufacturing even begins. This prevents you from having to go back to the design process after moving to the prototyping or production stages, which can save time, money, and resources. Calculating tolerance stack-up can also save you money by helping you understand your tolerances in context, so you can optimize for cost and manufacturability.

There are two main kinds of tolerance stack analysis:

Worst-case tolerance analysis

This involves adding up all of the individual tolerances of a part or assembly to find the total sum. When performing worst-case tolerance analysis, you should set each tolerance to either the largest or smallest value in its range. Both the upper and lower limits should be evaluated to provide a full picture of the allowable tolerance range. Then, compare the total tolerance to the part’s performance limits in order to ensure proper design. Worst-case tolerance analysis should be used when mating parts are absolutely critical and there is little chance for rework or design modification once production has started.

Statistical tolerance analysis

This combines all the probabilities of the different dimensions — meaning the likelihood of each dimension being above or below its ideal value by a given amount— to determine the part’s chances of failure or success. There are a number of different statistical tolerance analysis methods, such as the Monte Carlo method and root sum square (RSS). Statistical tolerance analysis is useful for high-volume production where a small percentage of scrap is acceptable, as long as the majority of parts fall within the allowable tolerance range. This allows for larger tolerances upfront that make manufacturing easier and lower costs.

Regardless of which method you use to find your part’s tolerance stack-up, having an accurate understanding of your tolerance stack-up will help you create manufacturable products.

Tolerance stacking best practices

Hands tolerance stacking in blue gloves
Tolerance stacking involves finding the cumulation of all individual tolerances of a component or assembly in order to understand the potential range of final fits.

Here are some best practices for product teams to keep in mind during the design process, which can help ensure they’re properly accounting for tolerance stack-up.

Avoid over-dimensioning your part

When each part feature is labeled with upper and lower tolerances, a design drawing can become overcrowded and unclear. Not only does this cause confusion and make your part design harder to understand, but conflicting dimensions can also bring errors into your tolerance stack analysis. One way to counteract over-dimensioning is to only explicitly define tolerances for part aspects that truly need them. Undimensioned features would then be controlled by a general tolerance that is applied to the entire part unless otherwise specified.

Evaluate your tolerance stack’s sensitivity

Make sure you understand the consequences of tolerance stacking before you calculate your stack-up. Will it be a total disaster if tolerance design conditions aren’t met or will the part still be able to function properly? By contextualizing your final tolerance stack-up with manufacturing and performance, you can understand how large or small your tolerance stack can be without compromising success. Keep in mind that tighter tolerances will require more expensive manufacturing methods, so it’s important to balance cost considerations with tolerance sensitivity.

Consider post-manufacturing changes

Post-assembly changes, such as deflection and normal wear-and-tear, can affect the precision of a part after it’s been produced. Since this information impacts the creation of tolerances, it’s important to keep any post-production changes in mind as you determine your tolerance stack.

Follow general tolerance best practices

When it comes to tolerance stack-up, all the usual GD&T standards still apply. This includes being conscious about how an individual part feature will interact with other component elements, making sure a part is machinable and within reasonable limits of manufacturing capabilities, and keeping track of important part characteristics like material selection. Most importantly, you should make sure your tolerance stack-up is allowable and within any relevant GD&T requirements for a specific component.

Tolerance stacking with Fast Radius

Effective tolerancing facilitates high-quality builds with acceptable dimensions so the part can function in the way it was designed. Tolerance stacking involves finding the cumulation of all individual tolerances of a component or assembly in order to understand the potential range of final fits. Calculating your tolerance stack-up enables you to make sure parts can be properly manufactured as effectively as possible for the least cost.

Efficiently and accurately accounting for tolerance stacking is far easier with the help of a trusted manufacturer. Fast Radius’ GD&T experts are here to help ensure your tolerances are correct before manufacturing. We’ll also guide you throughout the entire manufacturing process, working with you each step of the way to ensure the best possible results. Contact us today to get started.

For more tolerancing how-tos and guides, visit the Fast Radius resource center.

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