Common Assembly Design Mistakes in CNC Machining and Sheet Metal Fabrication
In both CNC machining services and sheet metal fabrication projects, engineers often design parts that look perfect in CAD but fail during assembly. The main issues come from overlooking assembly clearance, fastening methods, accessibility, and clear annotations.
This article summarizes four key mistakes with real cases, practical improvements, and an engineering checklist for battery enclosures, sheet metal housings, brackets, and precision machined parts.
1. Ignoring Assembly Clearance and Process Allowance
Problems
0-to-0 fit designs that make assembly impossible.
Rivet/screw/insert holes without clearance, forcing rework.
Coating thickness (anodizing, plating, powder coating) not considered.
Cases
Apple MacBook housings: anodized frames jammed keys because no allowance was given; later fixed by post-machining critical areas.
Automotive sheet metal panels: ignored paint thickness caused rework; later fixed with a 0.2 mm allowance.
Improvements
Key Point: In precision CNC machining and sheet metal fabrication, zero clearance is not precision — always leave “process space.”
2. Mismatched Fastening and Material Properties
Problems
Overuse of costly special rivets.
Self-tapping screws in thin walls or thin sheet metal → stripped threads.
Aluminum–steel direct contact → galvanic corrosion in humid/salt environments.
Cases
Tesla battery enclosures: aluminum housings + steel frames corroded; later insulated with washers and coatings.
Boeing 787 fuselage: composite–metal joints required insulating layers to prevent galvanic corrosion.
Improvements
Use ISO/DIN standard fasteners.
Hole spacing ≥ 2× diameter.
Thin sheet metal: use rivet nuts or weld nuts instead of self-tapping screws.
CNC aluminum: use threaded inserts instead of direct tapping.
Separate dissimilar metals with insulation or coatings.
Key Point: Fastening must match material and structure — never force a method that the material cannot support.
3. Overlooking Accessibility (Assembly & Maintenance)
Problems
Cases
BMW battery packs: early designs required complete teardown to replace a module; later versions reserved ~25 mm clearance for tools, enabling module-level service.
Home appliances: motor screws hidden in tight corners made repair slow; later redesigned for better access.
Improvements
Simulate assembly in CAD.
Apply DFMA analysis early.
Reserve ≥20–30 mm tool clearance.
Place wear parts on accessible sides.
Key Point: A design is only complete if it can be assembled and serviced — always check tool clearance and maintenance paths.
4. Missing Critical Design Annotations
Problems
Cases
Battery support frames: inconsistent welding due to missing notes → high rework; fixed with standardized weld symbols.
Medical devices: failed assemblies caused by skipped tolerance stack-up analysis; later became mandatory.
Improvements
Add weld symbols and lengths.
Provide sectional views when needed.
Perform tolerance stack-up on critical dimensions.
Add slots or adjustment features if cumulative tolerance is unavoidable.
Key Point: Clear drawings ensure consistent assembly — always define welds and check tolerance stack-up.
Assembly Design Checklist
Did you add assembly clearance and account for surface finishing thickness?
Is the fastening method compatible with material and structure?
Can tools reach during assembly and maintenance (accessibility)?
Are welds, GD&T, and tolerance stack-up clearly defined?
Why Considering Assembly Saves Cost
❌ Ignored assembly → rework, custom fasteners, delays → higher cost.
✅ Assembly considered → standard parts, fewer errors, stable delivery → lower cost.
Rule of thumb: In CNC machining and sheet metal fabrication, 1 extra hour of design review can save 1 week of rework.
Engineering Tip
Run a virtual assembly in CAD.
Request a DFM review from your supplier.
Apply the four-point checklist as a self-audit.
Learn from industry cases to avoid repeating mistakes.
Summary Table
| Mistake | Industry Case | Core Fix |
|---|
| Clearance & Allowance | Apple MacBook anodizing jam; Automotive panels paint issue | Add 0.1–0.5 mm clearance; consider coating thickness |
| Fastening & Materials | Tesla battery corrosion; Boeing 787 joints | Use ISO/DIN fasteners; insulate dissimilar metals |
| Accessibility | BMW early battery packs; Appliance motor screws | CAD simulation, DFMA, reserve ≥20–30 mm tool clearance |
| Missing Annotations | Battery support frames; Medical devices | Standard weld symbols; tolerance stack-up analysis |
Call to Action
Ready to optimize your design?
If you are working on battery enclosures, sheet metal housings, or CNC machined components, our engineering team can provide a free DFM review and practical feedback to reduce rework and costs.
👉 Contact Bergek CNC to discuss your project today.
