Tolerance and Clearance: Printed Holes Come Out Small, Every Time

A circular hole is printed as a many-sided polygon whose corners touch the circle, so the flats fall INSIDE it. The smaller the hole, the fewer segments and the worse the effect. On top of that the nozzle pushes a line of plastic that spreads slightly, and the plastic shrinks as it cools. Both add material where the hole should be and remove it from outside surfaces — so holes shrink and pegs grow. That means an interference fit and a clearance fit are not symmetrical on a printer, unlike on a lathe. A peg and hole modelled the same size will always be an interference fit, and usually a very tight one.

Measure across several diameters and at several heights. A printed hole is not round and not the same size top to bottom — the first layers are squashed wider and the seam where each layer starts leaves a small bump. Take the smallest reading for a hole and the largest for a peg. Those are what actually decide whether the parts go together, and averaging hides exactly the feature that will stop them. Deburr before measuring. The small ridge left where each layer starts, and the elephant-foot bulge on the bottom layers, both read as material that is not really in the way — and both come off in seconds with a deburring tool.

The best answer to a tight tolerance is usually to remove the need for it. A slot instead of a hole, a chamfer at the entry, a split collar with a screw, a deliberate flexure — all of them turn a precise dimension into an adjustable one. A CHAMFER on every hole and peg entry is nearly free and makes an imperfect fit assemble anyway. It also prints better, because the chamfer at the bottom removes the elephant-foot problem from the part that has to enter. And where a real fit is needed, use a real part: a brass insert for a thread, a steel pin in a printed housing, a bearing pressed into a printed seat. Printing everything is a constraint nobody imposed.