As you read this, fabrication teams across the country are struggling to fit components that looked perfect on paper. The parts were fabricated correctly, met specifications, and passed inspection-but now they won’t mate the way they’re supposed to. In most cases, the root cause isn’t a result of fabrication challenges, but of engineering decisions made months earlier during design.
When these fit-up challenges surface, the impact goes far beyond a few fabrication and assembly headaches. Fit-up problems often lead to scrapped parts, rework, and budget overruns that could have been avoided if they were addressed before a single piece of metal was cut.
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Where Assembly Problems Start
So what does a problematic design actually look like? Issues such as unclear datums (the reference surfaces or edges that everything is measured and aligned from), missing alignment features, and parts that rely on manual forcing or visual alignment are common early warning signs. There are also a few specific red flags that tend to show up again and again during assembly:
- Tabs that are too loose or too tight. Tabs are meant to guide parts into position and hold them steady during fastening or welding. When they’re too loose, parts shift and require manual adjustment. When they’re too tight, forcing them into place can distort the material or damage the finish.
- Holes that don’t self-locate. In a well-designed assembly, holes line up naturally when parts are positioned correctly. When they don’t, workers end up eyeballing alignment or enlarging holes on the fly just to make fasteners fit.
- Bends that stack tolerances across multiple parts. Every bend has a small acceptable variation. That’s not a problem on a single part, but when an assembly involves multiple components with multiple bends, those small variations add up. What may be a fraction of a millimeter off on one part can become a significant gap or interference once four or five parts come together.
If you were to look at each part individually, with no context for the full assembly, nothing may seem wrong. But when multiple components come together on the shop floor, the issues become apparent quickly.
Suddenly, the process requires too many hands, clamps, or fixtures just to hold parts in position before fastening or welding can begin. Assemblies that should be straightforward turn into a juggling act, leading to increased assembly time and inconsistent results from build to build.
Four Traits of Assembly-Friendly Designs
When designing components for an assembly, the goal is an end result that’s forgiving, with features that guide parts into position naturally. Here are a few key characteristics that set these designs apart:
Self-Locating Features
Self-locating features are design elements that guide parts into the correct position without manual adjustment-reducing the need for measuring and “eyeballing” during fit-up. Tabs and slots are a common example. When sized correctly, a tab slides into its corresponding slot and holds the part in place, removing guesswork from the equation. Formed flanges can serve a similar purpose, acting as built-in locators that establish alignment before fastening or welding begins.
The benchmark for a well-designed assembly is whether one person can put it together without excessive force or adjustment. When parts fall into place naturally, labor time is reduced, the risk of error is minimized, and the need for improvised fixtures or extra hands is eliminated.
Keeping bends oriented in the same direction throughout an assembly helps parts behave more consistently during fabrication and fit together more predictably during assembly. When bend directions vary from part to part, each component responds slightly differently to forming, which can affect overall dimensions, squareness, and alignment.
Those small differences may not matter on a single part, but they start to compound when multiple components are brought together in an assembly. Inconsistent bend direction increases the risk of tolerance stacking, makes parts harder to fixture, and often leads to extra adjustment during fit-up.
By standardizing bend direction where possible, designers can simplify the assembly sequence, reduce alignment issues, and minimize troubleshooting when parts don’t line up as expected.
Built-In Forgiveness
Sheet metal behaves differently from machined components, and there will always be some variation in material, bending, and flatness. Designs that account for this reality go together far more easily than those that assume perfect conditions.
Building in forgiveness means creating parts that can tolerate small variations without causing fit-up problems:
- Slotted holes, rather than tight round-to-round alignment, give fasteners room to find their position.
- Slightly oversized tabs allow parts to fit together without force.
- Clearances at mating surfaces help prevent interference when tolerances stack.
The alternative is a design that only works under ideal circumstances. When parts require perfect flatness, exact bend angles, or precise hole-to-hole alignment with no adjustability, even minor variations lead to prying, rework, or parts that simply won’t go together without modification.
Clear Assembly Sequence
How parts fit together is only one part of the equation. A well-designed assembly also needs to consider the order in which components come together. When the sequence is clear, each step builds logically on the last, and parts can support themselves during positioning.
Achieving this sequence starts with establishing primary datums. When these datums are clear and consistent, fabrication teams know where to start and how each subsequent part should relate to what’s already in place. Without them, small errors early in the process compound as assembly progresses, and parts that looked fine individually begin to drift out of alignment.
The goal is to design an assembly that your sheet metal shop can build the same way every time, by any trained worker, with predictable results. A clear sequence makes that possible.
Assembly Method and Finish: Two Often-Overlooked Factors
Good part design goes a long way, but the chosen assembly method and finishing requirements can also make or break the build. The choice between welding, mechanical fastening, or a mix of both has a direct impact on speed, repeatability, and labor cost. The same goes for finishing processes like powder coating or plating, which can quietly complicate fit-up if they’re not factored in from the start.
Choosing the Right Assembly Method
There’s no one-size-fits-all answer when it comes to selecting the right assembly method. Each approach has strengths and constraints, and the best option depends on what matters most for a given project-whether that’s speed, adjustability, appearance, or some combination of the three.
Mechanical Fastening
Mechanical fastening is fast, repeatable, and offers the most flexibility during fit-up. Parts can be adjusted before final tightening and disassembled later if needed. The tradeoffs are more hardware to manage and fasteners that remain visible unless concealed by design.
Welding
Welding creates a permanent connection with a clean look, but it’s slower and more skill-dependent. Once parts are tacked, there’s little room for adjustment. Heat distortion is also a factor, and most welded assemblies require finishing work afterward.
Mixed Approaches
Some assemblies benefit from a combination of mechanical fastening and welding. This can offer a good balance of strength, flexibility, and appearance, but it requires more planning upfront. Sequencing matters-whether you weld or fasten first can affect fit, access, and final alignment.
| Assembly Method | Assembly Speed | Flexibility During Fit-Up | Common Tradeoffs |
|---|---|---|---|
| Mechanical fastening | Fast and repeatable | High, allows adjustment | More hardware, visible fasteners |
| Welding | Slower, more skill-dependent | Low once tacked | Heat distortion, finishing required |
| Mixed (fastening + welding) | Moderate | Medium | More planning required, sequencing matters |
The Finishing Factor
Finishing processes such as powder coating, plating, and anodizing are sometimes specified without much thought for how they’ll affect assembly. But coatings add thickness, and that added thickness can change how parts fit together.
Fit changes after coating. Coatings affect hole sizes, slot fits, and mating surfaces. A design that assembles perfectly in raw metal may become a problem once parts are coated, especially in tighter assemblies where there’s little room for variation. When the original design doesn’t account for finish thickness, the result is often interference, forced fits, or last-minute rework.
Masking adds labor. When certain surfaces need to stay bare for welding or fastening, someone has to mask them before finishing and remove that masking afterward. If no one planned for this, it adds time and cost.
Post-finish rework erases efficiency gains. Assemblies that require grinding, welding repairs, or other touch-up after finishing often lose whatever efficiency good fabrication provided. The cleaner approach is to design assemblies that can be intentionally finished before or after assembly, rather than ones that force a messy sequence of finishing, reworking, and refinishing.
Case Study: How a Simple Redesign Transformed an Assembly
Smooth assembly is the result of intentional design choices that account for tolerances, forming behavior, finishing processes, and real-world fit-up. When those factors are considered early, assemblies go together faster, more consistently, and with far fewer surprises.
For example, our team at Approved Sheet Metal recently worked with a customer on a multi-part enclosure that was creating consistent challenges during assembly. The original design relied entirely on loose fasteners and manual alignment. As a result, each build required two people, took longer than necessary, and produced inconsistent results. The team even resorted to makeshift fixtures to hold parts in place long enough to fasten them.
To address the problem, ASM recommended a series of small design adjustments, including adding tabs and slots so parts could self-locate during fit-up. Bend directions were standardized across all components, and the number of unique fasteners was reduced to simplify hardware management and minimize opportunities for error.
These simple changes transformed the assembly process. A single person could now build the enclosure, parts fell into place naturally, assembly time dropped, and consistency improved from build to build. The makeshift fixtures that were once required were no longer needed.
While fabrication costs increased slightly, the total project cost dropped noticeably because labor time and rework were significantly reduced.
It’s a good reminder that small, intentional design changes made early in the process can dramatically improve how an assembly comes together, and often reduce total cost as well.
The Value of Involving Your Fabricator Early
Most of the issues covered in this article share a common thread: they’re far easier to address during design than after parts have been fabricated.
When ASM gets involved before designs are finalized, potential assembly risks can be identified while changes are still simple and inexpensive to make. Adjusting a flange length, adding an alignment feature, or redistributing tolerances across parts are small modifications that can dramatically improve how an assembly comes together without significantly increasing fabrication cost.
Early collaboration typically leads to faster builds, fewer surprises during assembly, and more predictable project costs overall. For prototype and low-volume production, this is especially important. At lower quantities, labor and rework often represent a larger share of total project cost than raw materials or fabrication. Getting the design right from the start matters more, not less, when there are fewer units to absorb mistakes.
There’s a limit to what precision fabrication can accomplish on its own. If an assembly is sensitive to small variations or lacks built-in adjustability, even the most accurate parts won’t go together easily or consistently. When design and fabrication teams work together from the beginning, many of these problems can be avoided entirely.
If any of the challenges in this article sound familiar, or if you’d like to avoid them on your next project, reach out to ASM. The earlier we’re involved, the smoother the build.