Tolerance stack-up issues in high-precision screw machining are a critical concern for ensuring that parts fit and function correctly when assembled. When multiple parts with specific tolerances are assembled, the cumulative effects of slight deviations can lead to significant functional issues, even if the parts have been manufactured to spec. When Micro Precision Components reviews customer drawings, we check for tolerance stack-up issues and can help our customers to resolve them. Below is a summary of the issue that can help address this basic but fundamental problem during the design process for your precision components.
What is Tolerance Stack-Up?
Tolerance stack-up refers to the cumulative effect of dimensional tolerances and geometric tolerances across multiple parts in an assembly, or across multiple dimensions in a single machined part. When tolerances are stacked, small deviations from the nominal dimensions in individual components can combine and cause a part to fall outside its functional requirements.
Suppose for example that a machined component is specified with three lengths with tolerances of +/- 0.0005 inches. The overall length of this part will therefore be subject to a tolerance of +/- 0.0015 inches, which may not be acceptable even if all the sections fall within their tolerance range. This is an example of linear stack-up, but the same concept can apply to angles and other geometric tolerances such as flatness, roundness, and parallelism.
Impact of Tolerance Stack-Up in High-Precision Machining
- Fit and Function: The most immediate consequence of tolerance stack-up is improper fit. For example, if the tolerances of mating parts are not accounted for, the components may not fit together correctly, resulting in difficulty during assembly, poor performance, or part failure.
- Manufacturing Concerns: In some cases, tolerance stack-up issues can cause confusion when it comes time to machine your parts. Projects can be delayed while the tolerancing problems are solved.
- Increased Wear and Tear: Even if the tolerance stack-up does not cause a part to fail its requirements, a poor fit can still cause undesirable wear in an assembly.
- Assembly Issues: In some cases, the parts may not even assemble correctly due to tolerance stack-up, causing delays in production or the need for rework.
How to Fix Tolerance Stack-Up
- Tolerance Analysis: One of the most important steps in managing tolerance stack-up is performing a tolerance analysis. This can be done manually using mathematical formulas or more efficiently using computer software that models the entire system of parts. This helps to predict how individual tolerance variations will combine and affect the final assembly.
- Worst-case analysis: This method assumes the worst-case scenario, where all tolerances stack in the most unfavorable way. While conservative, it ensures that the part still works even if every dimension is at the extreme of its tolerance.
- Statistical analysis: Using statistical methods (e.g., Monte Carlo simulation), this approach considers the probability distribution of each tolerance and predicts the likelihood that the part will function within the required limits.
- Precision Machining Techniques: Using high-precision machines, such as CNC (Computer Numerical Control) machines, and optimizing the setup can help minimize dimensional variations. MPC can hold tolerances of up to 0.0001”, which can help resolve tolerancing issues that may arise when using less precise machining sources.
MPC’s Tight-Tolerance Machining and Engineering Support
Micro Precision Components’ high-precision Swiss and CNC machining expertise can help prevent tolerance stack-up from impacting your project in two ways. Firstly, our ability to manufacture to the tightest tolerances allows us flexibility in accommodating your design and any changes it may need to resolve tolerance stack-up. But we can also offer direct engineering and design feedback from our production team to assist in resolving tolerance stack-up both on the parts we manufacture and in their final assemblies.