The most common problem with functional parts is not strength, but fit: a lid that won't close, an axle that won't go in, a hinge that won't turn. The reason is almost always the same - in the model, the two parts are drawn with the same dimensions, and printing does not work like that.
Why the hole comes out narrower
A line of molten filament is not a mathematical line. It has width, slight elasticity and a little spread before it sets. Along an outer perimeter, these effects can be compensated for fairly well. With a hole, however, each small inaccuracy acts in the same direction: inwards. Printed holes therefore almost always come out narrower than drawn, while shafts and pins come out slightly thicker.
Add in the fact that the material shrinks when it cools (differently for different materials), and the picture is clear: two parts drawn "at zero" jam after printing.
The rule: clearance must be designed in
The working approach is simple — between the parts to be moved or assembled, set the clearance already in the model — everywhere below it's the TOTAL clearance, that is, the difference between the two diameters, not the gap on each side:
- Free motion (shaft in a hole, hinge, slider): start with about 0.3–0.4 mm total between the two surfaces.
- Close fit (lid that clicks; friction part): about 0.1–0.2 mm.
- Fixed joint with glue: 0.2–0.3 mm — glue also needs space.
The numbers are indicative, not a promise: the actual value depends on the material, the size of the part, the orientation on the print bed and the shape of the hole itself. A small hole "loses" proportionately more than a large one; a hole printed lying down (like a bridge) comes out more inaccurate than printed straight.
The cheapest test in the world
Don't order the whole assembly at once only to find out it doesn't fit. Make a test piece: a small plate with the hole and a bit of the axle, or just the snap-fit section. Such a piece weighs grams and costs the minimum order — and tells you exactly what clearance works for your geometry and material. Then you adjust the model and order the real thing.
This is not our ploy to sell two orders - this is how people who have been printing for years work. The first print of a new assembly is a sample almost by definition.
Round holes that must be exact
If the bore is critical (bearing, axle with small clearance), there are two practical options:
- Draw it smaller and expand it after printing — a drill or reamer gives a more accurate and smoother hole than any printer.
- Orient it vertically. A hole with its axis pointing up prints as a series of circles and is far more accurate than a hole lying on its side that builds like a bridge.
What to write in the note
When ordering parts that need to fit into each other or into an existing item, write it in the order note: what goes into what and how tight it needs to be. If we see geometry that we don't think will fit, we'll ask you before we print—better an email than an attractive part that does not work.
And if you don't want to deal with clearances and samples at all — describe the task and we'll make the model with working assemblies for you.