CAD Tolerances for 3D Printing: A Practical Guide
One of the most frustrating experiences in 3D printing is designing a part, printing it, and finding the holes are too small for the bolt, the pin doesn't fit the socket, or the lid won't close. This is a tolerance problem. Here's how to design around it.
Why FDM Has Dimensional Error
FDM printers aren't perfectly accurate. Typical dimensional accuracy for a well-tuned consumer printer is ±0.1–0.2mm per axis. Holes tend to print smaller than designed (due to material expanding as it's laid down). External dimensions tend to be close to nominal, sometimes slightly larger.
Fit Types and Recommended Tolerances
Press Fit (tight: requires force to assemble)
Use when parts should not come apart without tools. For a pin into a hole:
- Design the hole 0.1–0.2mm smaller than the pin diameter
- Example: 5mm pin → 4.8–4.9mm hole diameter
Transition Fit (snug: easy assembly, stays put)
- Design the hole equal to the pin diameter
- Example: 5mm pin → 5.0mm hole
- In practice, this produces a snug fit with some friction
Clearance Fit (loose: moves freely)
For axles, sliding parts, or ease of assembly:
- Design the hole 0.3–0.5mm larger than the pin
- Example: 5mm pin → 5.3–5.5mm hole
Standard Hardware (bolts, screws, inserts)
- M3 clearance hole: 3.4mm diameter
- M4 clearance hole: 4.5mm diameter
- M5 clearance hole: 5.5mm diameter
- M3 heat-set insert hole: 3.8–4.0mm
- M4 heat-set insert hole: 4.8–5.0mm
Writing Tolerances Into Your Prompt
Be explicit when you need a clearance hole vs a tight hole:
"A 60×40×5mm plate with four M3 clearance holes (3.4mm diameter) in the corners, 8mm from each edge."
"A 20mm diameter socket, 15mm deep, with 0.3mm clearance: it should accept a 20mm pin that slides freely."
Test Prints
Always print a small test piece before committing to the full part when fit matters. A simple 20×20mm block with a few different hole sizes lets you dial in the correct tolerance for your specific printer, filament, and print settings.
Layer Height and Horizontal Accuracy
Horizontal (XY) accuracy is generally better than vertical (Z) accuracy in FDM. If you have critical mating features, try to orient parts so the fit is along the XY plane rather than the Z axis.