Clearance, Press, and Interference Fits

Pick the fit by function (move freely, hold by friction, or lock), then set a starting clearance or oversize, print a small fit coupon in the same orientation/material as the real part, measure with calipers, and adjust one knob at a time. In FDM, holes usually come out smaller and pins often come out slightly larger, so your CAD offsets plus print orientation largely determine whether a joint slides, presses, or won’t assemble.

TL;DR

Pick the fit you want, print a small pin-and-hole coupon, then measure it. In FDM, holes usually print small and pins often print a bit large. Adjust your CAD clearance/oversize (or slicer XY compensation) until the joint slides, presses, or locks the way you intended.

Fit choices at a glance: clearance vs press vs interferenceTopic-specific diagram for the concept, checks, and tradeoffs in this lesson.measured clearanceToo tightSlip fitPress fitMeasure
This diagram links the fit type to the assembly force and motion you should feel.

Why FDM fit won’t match your CAD

FDM builds shapes from extruded beads, not perfect surfaces. Bead width, corner bulge, and layer steps push small features off-size. The common bias is internal features printing smaller (holes, slots) while external features print larger (pins, tabs). Orientation changes both size and feel: sliding along layer lines can feel smooth, while rubbing across layer steps can feel gritty even at the same measured clearance.

When to use each fit

Clearance fit
Parts that must assemble every time and still move: sliders, pivots/hinges, lids, and alignment features that can’t bind.
Press fit
Joints that should hold by friction: bushings, spacers, light-duty assemblies, and parts you may want to remove later.
Interference fit
Joints you want to lock: permanent assemblies, anti-vibration fits, and parts intended for heat/cold-assisted assembly or a press.

The knobs that really change fit

Designed gap/oversize
The CAD offset between mating features; your most predictable control because it travels with the model.
Slicer XY compensation
Horizontal expansion / hole compensation shifts many dimensions at once; fast, but it affects the whole part.
Orientation
Changes dimensional bias and surface contact; reorient if the joint feels inconsistent, rough, or sticky.
Material
Sets how much squeeze you can get away with: softer materials deform more; more brittle materials crack with less press.
Post-processing plan
If you’ll drill/ream/sand, leave stock and make sure you can reach the surfaces cleanly.

A repeatable workflow to dial in fit

  1. Choose the goal: free motion, friction hold, or permanent lock.
  2. Model a small fit coupon that matches the real joint (depth, lead-in chamfer, wall thickness, and mating length).
  3. Print the coupon in the same orientation and with the same nozzle, layer height, wall count, and material as the final part.
  4. Measure the printed pin OD and hole ID (or slot width) with calipers and record both the CAD sizes and the printed sizes.
  5. Decide where to correct: open the hole, shrink the pin, or split the change so either side can be reprinted and still assemble.
  6. Change one control at a time (CAD offset or slicer XY compensation), reprint the coupon, and re-measure.
  7. Apply the proven offsets to the full model and save your coupon notes with the settings you used.