Tolerances: how to design parts that fit together

What tolerances and clearance are

Tolerance is the amount by which a real dimension may deviate from the one in the model. For parts that fit together, clearance belongs to it too, that is, the gap you deliberately leave between them. Depending on the size of the clearance, a joint is either loose or tight, which engineers call a fit.

In FDM printing the material shrinks a little after printing, and each layer is a squashed strip of plastic, not a perfect edge. That is why two parts drawn exactly against each other, with no gap, usually will not fit.

What clearance to leave between parts

There is no single universal number. It depends on the size of the part, whether the joint lies horizontally or vertically, the shape and the material. As a good start for moving parts such as hinges, Prusa recommends a clearance of at least 0.3 mm, and writes more in its guide Modeling with 3D printing in mind.

For tight joints and snap-fits use a smaller gap, and for freely rotating parts a larger one. If an exact joint matters, first print a small test piece with just the joint, and only then the whole part.

Why holes come out smaller

Holes and openings are usually a little smaller after printing than in the model. Slicers can compensate for this partly, for example with the X-Y hole compensation function in OrcaSlicer. The surest way, though, is to allow for it in the design.

• Screw holes should be drawn slightly larger, or drilled out to the exact size after printing.
• Chamfer the edges of joints. A small chamfer on the edge of both the pin and the hole makes assembly easier.
• The bottom edge is often slightly flared, because the first layer is pressed against the bed. It is called elephant foot, and a small chamfer helps on a mating surface.

What to send us

Draw the clearance directly into the model, as the print is made exactly from it. Save the model in millimetres, ideally as STEP or 3MF, which remember the units. If a part has to fit something specific exactly, tell us which dimensions are critical and what they go with.