A part that looks perfect on screen can still fail on the bench. The lid that will not close, the shaft that binds in its bore, the bracket whose holes miss by a fraction of a millimeter: nearly all of these trace back to tolerance stack-up that was never checked in the CAD assembly. At eCadCam, our Los Angeles product design team has spent more than 22 years modeling assemblies that have to go from screen to real hardware, and tolerance planning is one of the most valuable habits we can share.
What Is Tolerance Stack-Up?
Every manufactured feature varies a little from its nominal dimension. When several parts are assembled, those small variations add together. A gap between two parts might be controlled by four or five different dimensions, and if each one lands at the edge of its allowed range, the combined error can be many times larger than any single tolerance.
Stack-up analysis asks a simple question: in the worst realistic case, will the assembly still work? Answering it early, while the design is still in CAD, costs almost nothing. Answering it after tooling or a production run is expensive.
Choosing the Right Fit Type
Before assigning numbers, decide what the mating features need to do. Most fits fall into three families:
- Clearance fits leave a gap so parts slide or rotate freely, such as hinges, sliding covers, and pins in loose holes.
- Transition fits may have a small gap or a small interference, useful for locating parts accurately without much force.
- Interference (press) fits make the shaft slightly larger than the hole so friction holds the parts together, common for bearings and bushings.
Choosing the fit first keeps tolerance decisions tied to function instead of guesswork. It also tells us where tight tolerances are worth paying for and where loose ones are perfectly fine.
How to Set Up Tolerances in a CAD Assembly
Start with the functional requirement
Identify the critical gaps and alignments: how much clearance a moving part needs, where a seal must compress, or how closely two holes must line up. These are the dimensions that deserve attention.
Map the dimension chain
Trace every dimension that contributes to that gap, from one side of the assembly to the other. In SolidWorks, Fusion 360, and other CAD platforms, we use assembly mates, measurement tools, and tolerance analysis add-ons to see which features drive the result.
Run worst-case and statistical checks
A worst-case check adds every tolerance at its extreme and is the safest approach for low-volume or safety-critical parts. A statistical (root-sum-square) check assumes variations rarely all hit their limits at once, which can allow looser, cheaper tolerances on higher-volume products. The right method depends on quantity and consequences of failure.
Dimension for how parts are made and inspected
Datums and dimension schemes should match how the part will be machined, molded, or printed and how it will be measured. Our post on engineering drawings and GD&T covers how those callouts are communicated to a shop.
Tolerances Depend on the Manufacturing Process
A tolerance is only meaningful if the chosen process can hold it. CNC machining can hold much tighter tolerances than injection molding, which in turn is typically tighter than most 3D printing. Plastic parts can also shrink, warp, or absorb moisture, and 3D printed holes often print slightly undersized. As a general starting point for printed prototypes, designers add a clearance of roughly a few tenths of a millimeter to mating features, then adjust based on the technology and material.
That is why we often print a fit-check prototype before committing to production tooling. If you are weighing options, see how 3D printing can validate an assembly quickly, and how tolerances differ by technology in our guide to 3D printing accuracy.
Common Tolerance Mistakes to Avoid
- Applying one tight tolerance to every feature, which raises cost without improving function.
- Ignoring thermal expansion, finishing, paint, or plating thickness that changes final dimensions.
- Forgetting that snap fits, living hinges, and press fits need very different allowances.
- Never checking the assembly in its worst-case state, only at nominal dimensions.
- Skipping a physical prototype for assemblies with multiple mating parts.
Get Assemblies That Fit the First Time
Good tolerance planning turns CAD models into products that assemble smoothly, perform reliably, and cost less to make. Our product design and CAD services include assembly modeling, fit analysis, and manufacturing-ready documentation, backed by in-house prototyping when you need to test a fit in your hands.
Have an assembly that needs to go together right? Request a quote from eCadCam in Los Angeles at 213.489.1173 or visit us at 4212 San Fernando Rd, Glendale, CA 91204, and tell us about your project.