How to Design Holes, Threads, and Inserts for 3D Printing
1. Holes
Minimum hole diameter:
FDM: 2.0mm (smaller holes tend to close up)
SLA: 2.0mm (high detail capability)
Industrial SLA: 2.0mm (high detail capability)
MJF: 1.5mm (powder must be able to escape)
Compensation: Holes in 3D printed parts tend to print slightly smaller than designed due to material shrinkage and over-extrusion. Add 0.1–0.2mm to the radius for FDM, or 0.05–0.1mm for SLA.
Orientation matters: Holes printed vertically (along Z-axis) are more accurate and round. Holes printed horizontally may be slightly oval and need support material inside.
2. Threads
Printed threads are possible but have limitations:
Minimum thread size: M6 for FDM, M3 for SLA
Coarse threads (e.g., M6x1.0) work better than fine threads
Threads printed vertically are more accurate
Printed threads wear out faster than metal threads
Better alternatives to printed threads:
Heat-set threaded inserts — Brass inserts pressed into the plastic with a soldering iron. Strong, reusable, professional. Design a hole slightly smaller than the insert’s outer diameter.
Self-tapping screws — Design a pilot hole and let the screw cut its own thread. Works well in Nylon and ABS.
Tap after printing — Print an undersized hole and tap it with a standard thread tap. Best for larger threads (M5+).
3. Heat-Set Insert Design Guidelines
Hole depth: insert length + 1mm
Hole diameter: insert outer diameter minus 0.2–0.8mm (material melts around it)
Wall thickness around insert: minimum 2x the insert diameter
Leave 2mm+ clearance from the bottom of the hole to the opposite wall
4. Assembly Clearances
For parts that need to fit together:
Press fit: 0.0–0.1mm clearance
Snug fit: 0.1–0.2mm clearance
Sliding fit: 0.2–0.4mm clearance
Loose fit: 0.4mm+ clearance
These values are for FDM. SLA can use tighter clearances due to higher accuracy.
For critical assemblies, order a test print first to validate fit before committing to a full order.