Tolerance Test

A tolerance test print tells you the real clearances your current printer + filament + profile can produce without fusing (clearance fits) and without wobble (sliding/rotating fits). Run it with the exact settings you use for functional parts, record the smallest free-moving gap and the best pin/slot fit, and store that number per material/nozzle/layer-height so your designs assemble predictably.

TL;DR

Print a labeled tolerance test using your real functional-part profile, then record the smallest gap that moves freely and the tightest pin that still inserts smoothly. Use those numbers as your default design clearance/interference for that specific filament + nozzle + layer height.

Tolerance TestTopic-specific diagram for the concept, checks, and tradeoffs in this lesson.One variableKnown modelMeasureSave profile
A quick visual map of the main decisions behind a tolerance test and how to apply the result.

What a Tolerance Test Actually Measures

Tolerance tests are small calibration parts with multiple known gaps (for sliding/rotating clearance) and/or multiple pins and holes (for slip, snug, and press fits). They capture the combined effect of extrusion width, flow, temperature, cooling, seam blobs, first-layer bulge, and XY motion accuracy. The result is not a universal ‘printer tolerance’; it is a practical number for one material and one print profile.

Set Up the Test (So the Number Is Meaningful)

  • Use the same filament type, nozzle diameter, layer height, line widths, and cooling you plan to use on real parts.
  • If the filament is moisture-sensitive (PETG, Nylon, TPU), dry it first; wet filament can add blobs and strings that make fits falsely tight.
  • Confirm first layer is correct: not overly squished and not flared at the edges (elephant’s foot), because that can make holes/gaps look worse than they are.
  • Keep settings stable: turn off experimental features that change walls/flow dynamically (you want repeatability).
  • Pick a test model with embossed/engraved labels for each gap or pin size so you can record results quickly.

How to Run the Test and Record Results

  1. Slice and print with your normal functional-part settings (temps, speeds, cooling, wall count). Avoid “pretty” profiles that you wouldn’t use for assemblies.
  2. Let the print cool to room temperature before judging fit; plastics can shrink and stiffen as they cool.
  3. Clearance section: starting from the smallest labeled gap, test movement without forcing. Record the smallest gap that moves freely through the full travel.
  4. Pin/hole section: test insertion straight and without twisting tools. Record the smallest pin that inserts smoothly (slip fit) and the largest pin that still feels snug without cracking the part (snug/press boundary).
  5. Write down the context with the numbers: material, nozzle size, layer height, temperatures, and any special notes (enclosed vs open air, fan %, print orientation).

Common Problems and First Fix

Gaps fuse together until large sizes

Likely cause: Too much plastic (flow too high), printing too hot, or insufficient cooling on small features

Fix: Reduce flow slightly and/or lower nozzle temperature 5–10 C; increase part cooling for small features

Pins won’t insert even at larger sizes

Likely cause: Holes print undersized from line width/over-extrusion, and/or elephant’s foot on the first layers

Fix: Add a small chamfer to hole edges; reduce first-layer squish; calibrate flow

Parts move but feel gritty or seize after some motion

Likely cause: Stringing/ooze, seam blobs, or rough walls creating high spots on contact faces

Fix: Tune retraction and wipe/coast; move seam to a non-contact face; slow outer walls

Results vary a lot between runs

Likely cause: Wet filament, temperature swings/drafts, or inconsistent first layer/Z offset

Fix: Dry filament; verify bed leveling/Z offset; keep enclosure/room conditions consistent

Using the Numbers in Real Designs

  • Sliding or rotating fits: start with a designed clearance at or above your smallest free-moving gap; add extra margin for long contact lengths, dirty environments, or parts that will be painted.
  • Pin/hole and screw clearance: use the smallest smooth-insertion size as your default slip-fit; add a chamfer/lead-in on holes to make assembly tolerant of small edge bulges.
  • Press fits: use only small interference, and prefer tougher materials; PLA can crack. When in doubt, design a light push fit plus a retention method (clip feature, screw, or heat-set insert).
  • Store a separate baseline per material and per nozzle diameter (and often per layer height). PETG and TPU commonly need more clearance than PLA due to stringing/elasticity.
  • Re-test after any major change: new nozzle, different filament brand/color, different layer height/line width, enclosure changes, or a flow/temperature recalibration.