Designing Fits and Clearances

Three kinds of fit

A fit describes how two parts meet, such as a peg in a hole or a lid on a box.

• Clearance fit: the hole is bigger than the peg, so they slide or turn. Hinges, sliding lids and spinning wheels.
• Snug or friction fit: almost no gap. The parts go together by hand and stay put by friction. Prusa's design guide calls this a friction fit and contrasts it with parts that move like a hinge.
• Interference or press fit: the peg is bigger than the hole and has to be forced in. Plastic deforms to hold it. Test press fits on a sample first, and remember the part is weakest across its layers (see Orienting Parts for Strength).

Why the gap is never zero

Prusa's design guide is clear that parts meant to fit together need tolerance built in: a slot modeled at exactly the peg's size usually will not assemble. The reasons are physical. Every extruded line has some width and squish, the first layers spread (elephant's foot), materials shrink as they cool, and the printer itself has an accuracy limit. Prusa states its own printers are accurate to at least 0.2 mm Maker spec, and that warping or shrinking materials add to that.

Prusa gives no universal value, because the right gap depends on model size, orientation, how the parts interlock, calibration, settings and material. As a starting point for parts that move, it suggests at least 0.3 mm Starting point. Prusa does not say whether that is per side or in total; read it as the gap between the two surfaces, as in Print-in-Place Hinges and Joints, and let your tolerance gauge decide. Treat it as the first thing to test, not the answer.

Find your own numbers with a tolerance gauge

A tolerance gauge is a small test print with the same feature at several different gaps. You print it once and see which gap gives the fit you want. OrcaSlicer ships one in its calibration menu: a base with six hexagonal holes at 0.0, 0.05, 0.1, 0.2, 0.3 and 0.4 mm of tolerance Maker spec, checked with a 6 mm hex key or the printed hex tester that comes with it.

1. Calibrate first. Flow and first layer change every printed dimension. Do flow calibration and check Z offset before trusting a gauge.
2. Print the gauge in the real conditions. Same filament, same profile, and the same orientation the real part will print in.
3. Let it cool, then test. Try the key or tester in each hole. Note which gap slides freely, which is snug and which will not go in Measure it.
4. Measure too. Calipers on the holes and the tester tell you whether the error is in the holes, the outside shape, or both.
5. Write the result down with the filament and profile name. A different material or color can need a different gap.

You can make a gauge for your own design the same way: model one peg and a strip of holes stepping up in small increments, and print it before the real part.

Design gap or slicer compensation

There are two ways to fix a fit, and you should use one at a time.

• Change the model. Add the clearance you measured to the hole or take it off the peg. This is specific to the design and does not affect anything else you print.
• Change the slicer. OrcaSlicer has X-Y hole compensation (positive values make holes bigger, negative make them smaller) and X-Y contour compensation (positive makes outer contours bigger). Its tolerance test is meant for tuning exactly these. They affect every part printed with that profile.

If you change both the model and the compensation at once, you will not know which change fixed it. The full measuring and correction process is in Dimensional Accuracy, and the tolerance calculator on the Calculators page turns a measured size into a first corrected design size.

Details that make fits work

• Chamfer the entry. Prusa recommends chamfering the edges of parts that slot together so they are easier to assemble. A chamfer also clears the slightly flared first layers on the bottom of a part.
• Watch the first layers. If a part only binds at its base, the cause is usually elephant's foot, not the gap. OrcaSlicer and PrusaSlicer have elephant foot compensation for this; see Elephant's Foot.
• Round holes print as polygons. OrcaSlicer's polyhole option converts round holes to polygons with fewer sides, which it says print more reliably and match the intended diameter better.
• Test the real length. A gauge only a few millimeters deep tells you about a short fit. If the real part slides a long way, print a test at that length too Measure it.
• Save versions. Prusa suggests keeping design versions before you merge objects or cut holes, so you can go back and change a gap.

Where to go next

Moving joints printed already assembled have their own rules: see Print-in-Place Hinges and Joints. Threads and clips: Threads and Snap Fits. Metal threads in plastic: Heat-Set Inserts and Embedded Fasteners.