Managing Tolerance Variations Among Individual Parts to Achieve Functional Requirements
In particular for aerospace and defense parts and assemblies, which are high-complexity and high-precision, designing for manufacturability is just as important as designing for performance.
Tight tolerances are aptly named. For aerospace and defense systems to perform safely and reliably, they must tolerate significant stress, and deviations in a part’s dimensions during the manufacturing process can have serious consequences on performance.
As some small deviations – tolerances – are inevitable, engineers and program managers are responsible for managing tolerance stackups; calculating the total impact of every variation on the final assembly. Overlooking tolerance accumulations can lead to serious performance and safety consequences, and is simply not an option for modern aircraft and defense systems that rely on precision-engineered parts cooperating seamlessly.
In this article, we explain how OEMs can manage tolerance stackups and reduce risks associated with dimensional variations of mission-critical parts and assemblies. Where fit and function are paramount, so is understanding the cumulative effect of deviations.
Why Managing Tolerance Stackups Matters
A single manufactured part will have some small dimensional variation, also known as its tolerance. That tolerance on its own is fine, but when you put many parts together into a full assembly – each with its own tolerance – you suddenly have an accumulation of tolerances, or tolerance stackup, that can affect the assembly in potentially catastrophic ways.
There can be hundreds of contact points within aircraft assemblies, and if the parts involved in those functions carry too many deviations, it can lead to friction, misalignment, gaps, or increased stress – all of which negatively impact predictability and performance.
Additionally, airspace and defense assemblies have specific functions that rely on dimensional accuracy of the parts involved, such as calculating precise clearances, performing automatic sealing, and placing sensors for accurate missile targets.
Tolerance Stackups Risk Quality and Repeatability
Outside of the serious performance, safety, and reliability risks associated with overlooked tolerance stackups, this phenomenon isn’t ideal for manufacturing operations either.
Supplier trust is built on the production of high quality, tight tolerance parts and components that can be repeatable across production lots regardless of volume. Diversions in assembly performance can lead to:
- Costly rework and/or scrapped materials, parts, and components
- Increased assembly time to account for errors or manual adjustments
- Variability in parts and assemblies that erode customer trust
- A heavier time and responsibility burden on QA and inspections
Organizations that manage tolerance stackups effectively are able to reduce unnecessary costs, production timelines, and supply chain risks while upholding customer trust.
How to Avoid Tolerance Stackups
Aerospace and defense parts and assemblies need to withstand extreme conditions like warp speeds, crushing atmospheric pressure, and corrosion and wear, all while performing flawlessly. Nothing is 100% perfect, but the mission-critical nature of these aerospace and defense systems requires engineers to get as close as possible.
Tolerance stackups occur for valid reasons – to withstand those extreme conditions, parts and assemblies often combine complex geometries and multiple parts that each need to perform different functions.
With tight regulatory requirements from the FAA, DoD, and others, thorough analysis of the cumulative effect of every part’s manufacturing tolerance is crucial to achieving certification.
Avoiding tolerance stackups keeps projects on track and ensures swift regulatory approvals. Here are four ways to avoid them:
1. Use Best-Case Tolerance Calculations
Every feature is likely not at the extreme of its tolerance range all at once. Avoid calculating tolerance under this assumption and instead statistically combine tolerances – which assumes most parts fall near their average dimensions. Identifying which parts can have looser tolerances prevents over-constrained designs or excessive tightening of tolerances, which drives up costs and machining time.
2. Engage in Early Collaborations with Manufacturing
Getting on the same page early in the design stage ensures alignment between tolerance specifications that are technically achievable but may not be feasible at scale. Designing for manufacturability is an important consideration to keep tolerances from accumulating unintentionally and causing surprise issues with fit downstream.
3. Specify Your GD&T Framework
If your organization’s General Dimensioning and Tolerancing (GD&T) contains too many complex datums and insufficient controls, manufacturing and inspection can become more challenging and tolerance stackups more likely. Instead, use a single datum for feature patterns, use surface features as datums, create local coordinate systems, and add features for datums like flatness.
4. Establish Supplier Alignment on GD&T
A shared methodology for and alignment on General Dimensioning and Tolerancing (GD&T) among suppliers reduces the likelihood of variation despite suppliers using different machines and having different inspection methods and process capabilities.
What to Do When Tolerance Stackups Occur
Management of tolerance stackups should begin at the concepting and layout stages and continue through production and QA.
At Primus, we practice strong tolerance management by employing these best practices:
- Using statistical tolerance stackup analysis whenever possible. As previously mentioned, a statistical approach assumes most parts fall near their average dimensions, helping cut back on costs and machining time. This approach is ideal for assemblies with many interdependent features. Do use a worst-case stackup for safety-critical components where no variation can be accepted.
- Establish alignment between design engineers and manufacturers. Whether you’re working with a trusted partner like Primus Aerospace or an internal team, coordination between engineering and manufacturing should be a priority of your DFM strategy. Alignment helps identify over-tight tolerances and opportunities for process improvements.
- Reduce ambiguity and complexity in your GD&T framework. Define relationships among features, functional datum schemas, and fit and performance requirements to eliminate unknowns and control tolerance variations.
- Use precision inspection and metrology for validation. CMMs, digital inspection technologies, and laser scanning are a few methods for verifying that parts meet tolerance requirements. Help connect engineering intent and manufacturing output with high-quality measurement tactics.
Your Manufacturing Partner Should Specialize in Tight Tolerances
Suppliers who are unable to maintain consistent accuracy across parts only increase tolerance stackup risks and can keep OEMs from producing consistent dimensional accuracy.
Your supplier should have stable, controlled processes and apply advanced machining, tooling, and fixturing strategies to ensure you get the strategic advantage – the better tolerance stackups are managed, the less costly and more productive your operation can be.
Deliver reliable assemblies that support mission-critical performance by minimizing tolerance stackups. Dimensional variation is inevitable but can be well-managed to achieve a predictable assembly fit, functional performance requirements, high quality, and lower production costs.
