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Design8 min read

Designing Parts for 3D Printing

Key design principles to keep in mind when creating CAD models intended for FDM 3D printing — wall thickness, overhangs, and tolerances.

Design Rules for FDM Printing

If you're designing a part that'll be 3D printed, there are some guidelines that'll help you get the best result. These are specific to FDM (the most common desktop 3D printing method).

Wall Thickness

  • Minimum: 1.2 mm (2 perimeters with a 0.4 mm nozzle)
  • Recommended: 1.6–2.0 mm for structural parts
  • For thin cosmetic walls: 0.8 mm is possible but fragile

Thinner walls can print but are prone to breaking. When in doubt, go thicker.

Overhangs and Bridges

FDM prints layer by layer, so any surface that overhangs more than ~45° from vertical needs support material.

Tips:

  • Keep overhangs under 45° where possible
  • Use chamfers instead of fillets on bottom edges
  • Bridges (horizontal spans between two supports) can work up to ~60 mm
  • If supports are needed, design flat surfaces where they'll attach for easy removal

Tolerances and Fit

3D prints aren't as precise as CNC machining, but you can still get good results:

  • General tolerance: ±0.2 mm
  • Tight fit (press-fit): Design with 0.1 mm interference
  • Sliding fit: Add 0.2–0.3 mm clearance
  • Loose fit: Add 0.4–0.5 mm clearance

Always print a test piece for critical fits before committing to a full batch.

Orientation Matters

How a part is oriented on the print bed affects:

flowchart LR subgraph Good["✓ Good orientation"] A[Load] -->|across layers| B[Strong] end subgraph Bad["✗ Weak orientation"] C[Load] -->|between layers| D[Weak] end
  • Strength — Parts are weakest between layers. Orient so the load goes across layers, not between them.
  • Surface finish — Vertical surfaces are smoother than angled ones.
  • Support needs — Rotating a part 45° can sometimes eliminate supports entirely.

Holes and Threads

  • Horizontal holes tend to come out slightly oval. Design them 0.2 mm oversize or plan to drill them out.
  • Threaded holes: Use heat-set brass inserts for reliable threads. Printed threads work for low-stress applications but wear quickly.
  • Screw holes: Design for the screw's core diameter and let the screw self-tap into the plastic.

File Formats

Export your model as:

  • STL — most common, universally accepted
  • 3MF — better than STL (preserves units, colour, metadata)
  • STEP — best if you want me to modify the design

Need Help?

Not sure if your design is printable? Send it over and I'll review it for free. I'll flag any issues and suggest fixes before we print.