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

For a pin-in-hole style joint, start by choosing the fit type, then prototype a small test coupon and measure it: holes tend to print undersized and pins oversized. Adjust the designed gap/oversize (or slicer horizontal expansion) until the assembly force and motion match your goal.

Clearance, Press, and Interference FitsTopic-specific diagram for the concept, checks, and tradeoffs in this lesson.measured clearanceToo tightSlip fitPress fitMeasure
A quick visual map of the main decisions behind clearance, press, and interference fits.

Why FDM fit is different than CAD

FDM prints have bead width, corner bulging, and layer stair-steps, so small features rarely match CAD exactly. A common pattern is internal features printing smaller (holes, slots) and external features printing larger (pins, tabs). Surface direction matters too: faces that slide along layer lines can feel smoother than faces that rub across layer steps, even if the dimensions match.

When to use each fit

Clearance fit
Sliding parts, hinges, lids, alignment features that must assemble reliably across printer variation.
Press fit
Friction joints, bushings, snap-in spacers, light-duty assemblies where you can tolerate some assembly force.
Interference fit
Permanent assemblies, anti-vibration locking, parts designed for heat/cold assisted assembly or use of a press.

The knobs that actually change fit

Designed gap/oversize
The CAD offset between mating features; the most repeatable control (works across slicers).
Slicer XY compensation
Horizontal expansion / hole compensation can shift many features at once; powerful but global.
Orientation
Changes both dimension bias and friction feel; rotate if the fit is inconsistent or gritty.
Material
Stiffness and creep set how much squeeze works: soft materials tolerate more press; brittle materials crack sooner.
Post-processing plan
If you might drill/ream/sand, include enough stock or access to do it cleanly.

Repeatable workflow to dial in a fit

  1. Define the function: must move freely, hold by friction, or lock permanently.
  2. Make a small test coupon that matches the real joint geometry (same hole depth, lead-in chamfer, wall thickness) and print it in the same orientation you’ll use later.
  3. Print the coupon with the same nozzle size, layer height, wall count, and material as the final part (changing these can change effective size and surface texture).
  4. Measure both sides of the joint (pin OD and hole ID/slot width) with calipers; write down the CAD size and the printed size.
  5. Decide what to change based on the measurements: enlarge holes, shrink pins, or split the difference to make the joint more tolerant of variation.
  6. Change one knob at a time (CAD offset or slicer XY compensation), reprint the coupon, and repeat until assembly force/motion matches the goal.
  7. Apply the same offsets to the full model and keep the coupon/settings notes for future projects.