Designing for real-world fit
Parametric models are only useful if the output actually fits. Here is what you need to know about tolerances, clearances, and print-ready dimensions.
Tolerances and clearances
FDM printers are not CNC machines. Expect dimensional accuracy of +/- 0.2mm on a well-tuned printer. For parts that fit together:
| Fit type | Gap per side | Use case |
|---|---|---|
| Tight / press fit | 0.1–0.15 mm | Snap joints, bearing seats |
| Sliding fit | 0.2–0.3 mm | Lid on box, drawer in slot |
| Loose / clearance | 0.3–0.5 mm | Cables through holes, screw clearance |
Expose clearance as a parameter so users can tune it for their printer:
param clearance: float = 0.3 [0.1:0.5:0.05] "Fit clearance (mm)"Wall thickness
Minimum practical wall thickness depends on the nozzle:
| Nozzle | Min wall | Recommended |
|---|---|---|
| 0.4 mm | 0.8 mm (2 perimeters) | 1.2–1.6 mm |
| 0.6 mm | 1.2 mm | 1.8 mm |
Set your wall parameter minimum accordingly:
param wall: float = 2.0 [1.2:6:0.2] "Wall thickness (mm)"Screw holes
Printed holes come out smaller than designed because the plastic contracts inward. Add 0.2–0.3 mm to the radius:
| Screw | Nominal diameter | Printed hole diameter |
|---|---|---|
| M3 | 3.0 mm | 3.4 mm |
| M4 | 4.0 mm | 4.4 mm |
| M5 | 5.0 mm | 5.4 mm |
| #6-32 | 3.5 mm | 3.9 mm |
Overhangs and support
FDM printers can handle overhangs up to about 45 degrees without supports. Design with this in mind:
- Chamfer edges at 45 degrees instead of sharp overhangs
- Use teardrop-shaped holes for horizontal holes (avoids drooping at the top)
- Orient the model so large flat surfaces are on the build plate
Layer height considerations
Vertical dimensions snap to layer height. If your layer height is 0.2mm, a 5.1mm tall feature will be printed as either 5.0mm or 5.2mm. For critical vertical dimensions, use multiples of common layer heights (0.1, 0.15, 0.2 mm).