Tolerances and Fit in 3D Printed Parts

The model is not the part

In CAD a hole is exactly 5 millimetres across and two parts meet with no gap. On the printer, the plastic has to be melted, squeezed out through a nozzle of finite size, laid down in a line with a rounded edge, and cooled, and each of those steps introduces small errors. The result is a part that is close to the model but not identical, and the difference is often just large enough to ruin a tight fit.

Tolerance is the amount of variation you accept in a dimension. A clearance is the deliberate gap you leave between two parts so they fit together. Planning both is the heart of designing parts that assemble properly. Because the numbers depend on your machine, material and settings, this article avoids fixed figures: the right values come from testing your own printer.

Why prints differ from the model

Several effects combine, and they vary between printers.

• Extrusion width and over-extrusion. Each line of plastic has a width set by the nozzle and settings. If the printer extrudes slightly too much, walls come out thicker and holes smaller. Under-extrusion does the opposite.
• Rounded edges and layers. Lines are rounded, so outer surfaces show layer texture and tiny steps, and a fit between two such surfaces is effectively slightly looser or tighter than the model suggests.
• Elephant’s foot. The first layers often squash outward, making the bottom edge slightly larger than the rest.
• Shrinkage. Plastics contract as they cool. Some shrink more than others, so the part may end up slightly smaller than designed, and that varies with the material, as seen in the filament materials comparison.
• Small holes and circles. Holes tend to print undersize, and tiny circles are approximated by short straight segments.
• Slicer path choices. Things like seam position, perimeter order and how the slicer treats thin features can shift dimensions.
• Machine calibration. Belt tension, step settings, a slightly warped frame or a loose pulley can all cause dimensions to drift, particularly over a larger part.
• Warping. Corners may lift as the part cools, changing flatness and fit.
• Moisture and temperature. Some filaments absorb moisture, which affects printing quality and final dimensions.

Because of this, “my printer prints within a tenth of a millimetre” is a rule of thumb that depends strongly on the part, the printer and how it was tuned, so test rather than trust a number.

Types of fit

Think about what you want the two parts to do.

• Loose or free fit. Parts move easily, like a lid sitting loosely or an axle turning freely. This needs a generous clearance.
• Sliding fit. The parts move relative to each other with little play, such as a drawer or a hinge pin. This needs a modest, consistent clearance.
• Snug or location fit. The parts go together by hand and stay put, such as a board sitting in a pocket or a lid that locates on a lip.
• Press fit. The parts are forced together and held by friction. A press fit in printed plastic is sensitive to small errors and can crack parts or loosen over time, so it should be used with care, and it is often better to use screws, as discussed in the notes on mounting hardware.

The more precise the fit, the more you need to test.

Planning clearances

A reliable approach follows a few steps.

1. Decide what each interface needs. Decide whether it should be free, sliding or snug.
2. Start with a clearance guess based on experience with your printer. If you have none, begin generous and tighten up after testing.
3. Print a test piece. A small tolerance gauge, a set of holes in several sizes, or a mating pair with graduated gaps shows what your printer and settings actually do. Print it in the same material, orientation and settings you will use for the real part, because these change the result.
4. Measure with calipers, and note the differences between model and print for outside dimensions, inside dimensions and holes.
5. Record the results as parameters in your CAD modelling file so you can apply them to later designs, and adjust them per material.
6. Iterate. If it is too tight, add clearance. If it is sloppy, reduce it. Change one thing at a time.

Remember that clearance is applied to both parts or just one, depending on the design, so keep it consistent. Designing the offset in a named parameter makes it easy to revise.

Design tricks that help

• Make holes slightly larger than the screw or pin, and drill or ream if you need precision. A small drill bit run through a printed hole often cleans it to a more accurate size.
• Use chamfers and lead-ins. A small chamfer on the edge of a hole or a pin makes assembly easier and hides the elephant’s foot.
• Avoid long, thin features that bend as they cool. Breaking a long part into sections often improves accuracy.
• Orient parts for the fit. A cylinder printed on its side comes out slightly oval compared with one printed upright, and holes printed vertically tend to be rounder than horizontal holes, which may sag at the top.
• Add relief. Small slots, ribs or crush features allow a snug fit to tolerate variation.
• Print lids and bases with small gaps and use fasteners instead of relying on friction alone. For electronics, this ties into project enclosures, where lids need to open repeatedly.
• Use inserts or captive nuts for threads rather than threading plastic directly.
• Think about assembly direction. Make sure parts can go together without fighting supports or first-layer flare.

Material and wear

Different plastics behave differently under a snug fit. Brittle ones can crack under press forces, while flexible ones may creep and loosen over time. Heat can relax a tight fit, which matters near warm electronics. If a moving joint will rub, consider materials with lower friction and wear resistance, and keep surfaces from binding with a bit more clearance. Lubricants can help, but first check that they are compatible with the plastic, since some attack certain materials.

Keep a calibration record

Write down what you learn: settings, materials, clearances that worked, and the offsets for holes and walls. When you change filament, nozzle or settings, repeat a quick test. Over time you build a personal table of clearances you can trust, which saves many reprints.

The goal is not a perfect part but a predictable one. Once you know how your printer behaves, you can design with the right allowances and have parts that fit the first time.