Functional 3D-printed assemblies live or die by clearance. Too tight and parts won’t go together (or crack when forced); too loose and they wobble or fall apart. This tool gives you a calibrated starting gap for each kind of fit, scaled to your part size and print process.
How it works
Fits are defined by diametral clearance — the difference between the hole diameter and the shaft diameter:
- Negative (interference): the inserted part is larger than the hole, so it must be pressed or glued in for a permanent bond.
- Zero to small positive: press and snug fits that hold by friction.
- Larger positive: slide, free-running and loose fits that move freely.
The recommended gap starts from a base value per fit type, then:
clearance = base_fit_gap × process_scale + (nominal × 0.005)- process_scale is 1.0 for FDM and about 0.5 for SLA, because resin resolves finer detail.
- The size term adds a little clearance for bigger parts, where process error accumulates.
The result is split into the diametral gap, the per-side offset, and the exact hole and shaft sizes to type into your CAD.
Tips and notes
- Calibrate once: print a pin against a row of holes stepped in 0.05 mm increments and pick the one that feels right. Use that to offset all your fits.
- Over-extrusion is the usual culprit when holes come out too tight — check your flow/extrusion multiplier before adding more clearance.
- Holes printed vertically (along Z) come out rounder and more accurate than holes printed horizontally (along the bed plane), which sag slightly at the top.
- For threaded or load-bearing joints, consider heat-set inserts instead of printed threads — see the heat-set insert hole size tool.