If you’ve partnered with a precision machining company to make assemblies or subassemblies, you might already know that mating parts can present tolerance stacking challenges.
For our purposes in this piece, let’s talk about tolerance stackups that occur with mating parts. Before manufacturing begins, the tolerances for each part must be calculated together and considered collectively—not just individually—to ensure the assembly will meet all critical specifications.
Here’s an example of why. . .
Let’s say we’re making an assembly that must measure 2.0″ tall to function correctly. That assembly is comprised of two mating parts: part A and part B. Per the print, both A and B must be 1.0″ tall, and each has a +/- 0.1″ tolerance.
We could manufacture parts A and B to measure 1.10,” 1.0,” or 0.90,” and each part—individually—would technically be “in print” and pass inspection. However, once mated, the assembled part’s measurement might be wrong.
If both parts, A and B, were made to measure 0.90″ tall, the assembly would measure 1.80″ tall instead of 2.0″ tall. In other words, the completed assembly would be out of print and would not pass inspection.
How to Prevent Tolerance Stacking Problems in Precision Machining
While many precision machine shops have the engineering and manufacturing expertise to prevent tolerance stacking issues from impacting the functionality of an assembly or subassembly, there are best practices customers can follow when designing mating parts.
1. Run a stackup study
A stackup study allows you to look at tolerances at their extreme dimensions to identify potential problems. You can see how the assembly will turn out if the mating parts’ tolerances are set to the smallest and largest variances or if each part is at one extreme.
Not every modeling program has this capability, so it’s not a guaranteed solution, but it is an excellent first step in mitigating potential problems.
2. Avoid over-tolerancing
When designing mating parts, engineers have a tendency to make tolerances tighter than necessary to avoid tolerance stacking problems, but this “solution” often does more harm than good.
Over-tolerancing adds complexity to the design, directly contributing to increased costs and lead times. This approach isn’t always successful at preventing tolerance stacking issues in the first place—and in some cases, it can negatively affect the functionality of the assembly or subassembly.
Instead of making every tolerance extremely tight, we recommend determining how the tolerances stack up against each other.
3. Identify critical surfaces
When manufacturing an assembly or subassembly, your precision machining partner needs to know which surfaces are considered critical and which can have some variance and still be functional.
For example, picture a traveling coffee cup with a friction-fit, “pop-on” plastic lid. The diameter at the top of the lid can have some variance, and the lid will still be functional. The diameter at the bottom of the lid is the critical surface, as it contains a groove for a rubber o-ring to seal the coffee cup and prevent it from leaking.
If we were manufacturing these lids, we would need sufficient information about the critical surface (e.g., surface finish Ra, tolerances, application) to ensure functionality.
4. Choose an appropriate surface finish
Surface finish refers to the roughness or smoothness of a given surface.
When mating parts rub together or make firm contact against each other, it’s best to have a smooth surface with a smaller Ra. If a part requires wet paint or powder coating, a rough surface is preferred because it provides more surface area for the finish to adhere to.
Some precision machining shops, including Reata, have a standard “as machined” finish that might be sufficient for a customer’s assembly or subassembly. Still, understanding the part’s required surface finish is critical.
It’s best to call out if mating parts will slide against one another, make firm contact, or will have wet paint or powder coating to help us understand what surface finish is required.
5. Communicate with your precision machining partner
Be sure to tell your precision machining partner that you’re requesting a quote for a mating part(s). Here’s everything you should plan to communicate:
- Specify that the RFQ is for mating parts
- Articulate where the part mates and at which features
- Call out which features and surfaces are critical and why
- Note GD&T or dimensions that indicate the type of fit for mating parts (e.g., indicate slip fit vs. press fit)
- Clarify whether the end user will see or hold the part, so we can eliminate sharp edges if necessary and achieve the desired aesthetic requirements
Work with a One-Stop Contract Manufacturing Company
The best way to ensure mating parts are functional for end users is to rely on the same contract manufacturing company to machine both mating parts and complete the assembly. This approach streamlines the entire manufacturing process by giving the manufacturer full visibility into the project.
As a result, you’re assured a fully functioning assembly and a much more convenient experience: managing one supplier relationship and inspecting one completed assembly. Request a quote from Reata, and let us be your one-stop contract manufacturing partner.