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.
The six fit types, and when to use each
| Fit type | Behaviour | Typical use |
|---|---|---|
| Interference | Parts must be forced together; permanent without disassembly | Axle-in-hub, permanent press inserts |
| Press | Requires a firm push; holds without adhesive | Bearing housings, snap-together enclosures |
| Snug | Goes together by hand with slight resistance | Covers, lids that should stay put |
| Slide | Slides freely but without visible slop | Rails, drawers, moving carriages |
| Free-running | Moves easily; slight play | Spinner mechanisms, loose pivots |
| Loose | Maximum clearance; never binds | Guides where the shaft moves constantly or gets dirty |
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.
Worked example
For example, a 10 mm nominal diameter slide fit in FDM: the tool applies a base gap for slide fits plus the size term (10 × 0.005 = 0.05 mm). Say the total diametral clearance comes to 0.4 mm — the hole would be modelled at 10.4 mm and the shaft at 10.0 mm, giving 0.2 mm of gap per side. That part will slide smoothly without wiggle on a well-tuned FDM printer.
The same 10 mm slide fit in SLA might call for a diametral clearance of about 0.2 mm (process_scale ≈ 0.5), giving a 10.2 mm hole against a 10.0 mm shaft — half the gap because resin printers reproduce fine detail far more faithfully.
Tips and notes
- Calibrate once per printer. Print a pin against a row of holes stepped in 0.05 mm increments and pick the size that gives the feel you want, then use that offset to adjust these values for your specific machine. Every printer differs.
- Over-extrusion shrinks holes. The most common reason for holes coming out too tight is an extrusion multiplier above 1.0. Fix the flow rate before adding clearance — you get better-looking parts that way.
- Orientation matters. Holes printed vertically (axis along Z) come out rounder and more accurate than holes printed horizontally (axis in the bed plane), which sag slightly at the top and can be 0.1–0.2 mm undersize on that side.
- For load-bearing joints, consider heat-set inserts instead of printed threads — they grip more reliably and tolerate repeated fastening. See the related heat-set insert hole size tool.
- Adjust the hole, not the shaft, whenever the mating part is a purchased component (a bolt, shaft, or bearing) with a fixed dimension. Resize only the printed part.