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Why printed parts that should fit often do not
Anyone who has printed a shaft and a hole from the same design file knows the disappointment: on screen they fit perfectly, in your hands the shaft will not enter the hole at all. The reason is that 3D printers are not perfectly accurate. Printed holes usually come out smaller than drawn, because plastic shrinks as it cools and because the nozzle traces the inside of a circle slightly inward. Printed shafts usually come out larger than drawn, for the mirror-image reasons. The gap between them closes from both sides at once, and a design with no allowance for this has no chance.
The fix is to design the gap deliberately. You need the clearance you actually want between the parts, plus an allowance for the printer’s error on each side. If you want 0.2 mm of clearance and your printer is accurate to 0.1 mm per side, the designed gap must be 0.2 plus twice 0.1, which is 0.4 mm. The calculator splits that gap symmetrically: the hole grows by half the gap and the shaft shrinks by half the gap. For a 10 mm nominal diameter, the hole becomes 10.2 mm and the shaft 9.8 mm, and after printing errors eat 0.1 mm from each side, the real clearance is close to the 0.2 mm you wanted.
How to find your printer’s tolerance
Printer tolerance is not a specification you can read off a datasheet. It is a property of your printer, material, and settings together, and the only honest way to find it is to print a test. Print a set of test holes and shafts at known sizes, for example holes from 9.8 to 10.4 mm in 0.1 mm steps around a 10 mm shaft, and see which combinations fit. The smallest gap that fits smoothly is a good estimate of twice your per-side tolerance plus your desired clearance, and from there the per-side tolerance follows.
Measure with calipers as well as by feel. A shaft that fits by force is not a good fit: it will wear, crack, or seize over time. A good sliding fit moves freely without wobble. A good press fit needs firm hand pressure but no tools. Decide which kind of fit you want before you calculate, because a press fit and a sliding fit need different clearances. As a starting point, sliding fits for small parts often use 0.2 to 0.3 mm of clearance, while press fits use 0 to 0.1 mm, relying on the plastic to compress slightly.
A worked example
Suppose you are printing a hinge pin and knuckle with a nominal diameter of 8 mm, and you want a smooth sliding fit with 0.3 mm of clearance. Your printer’s per-side tolerance, measured from test prints, is 0.15 mm. The designed gap is 0.3 plus twice 0.15, which is 0.6 mm. The hole becomes 8 plus 0.3, which is 8.3 mm, and the shaft becomes 8 minus 0.3, which is 7.7 mm. Printed, the hole shrinks toward 8.15 mm and the shaft grows toward 7.85 mm, leaving about 0.3 mm of real clearance, which is what you wanted.
Now suppose you ignore tolerance and design the hole at 8.3 mm with the shaft at 8.0 mm, a gap of only 0.3 mm. After printing, the hole shrinks to about 8.15 mm and the shaft grows to about 8.15 mm, and the parts do not fit at all. That is the classic failure, and it happens to experienced printers as well as beginners, because the error is systematic and invisible until assembly. The calculator exists to prevent exactly this one mistake.
Tolerances at different sizes
Printer tolerance does not scale neatly with part size. A 0.1 mm per-side error matters enormously on a 5 mm shaft, where it is four percent of the diameter, and barely matters on a 100 mm part, where it is a fraction of a percent. Small mating parts therefore need proportionally more attention than large ones, and sometimes need a different approach entirely, such as printing a test piece at the exact size before the full part. Large parts, by contrast, are usually limited by shrinkage and warping rather than by nozzle-level tolerance.
Frequently asked questions
Why are my holes always small even when shafts are right? Holes shrink more than shafts grow, because the slicer traces circles with a finite nozzle width and the plastic contracts inward. Some slicers offer a hole-size compensation setting for exactly this reason.
Should I just drill holes after printing? For critical holes, yes, drilling or reaming after printing gives the most accurate result. But designing the right printed size first means less material to remove and less risk of cracking.
What clearance do I need for a moving part? More than you think. Moving parts need room for the motion itself plus wear over time, so start at 0.3 mm and test, rather than starting tight and hoping.
These figures are estimates and general guidance only, not a guaranteed print result. Your material manufacturer’s datasheet and your printer’s own calibration govern every setting. Use the calculator above, print a small test fit first, and only then print the full assembly.