What Is Tolerance? How to Design Parts That Fit Together?

For two parts to fit together, a gap (tolerance) must be left between them; a shaft and hole designed to exact dimensions in CAD will never fit together after printing. As plastic cools, it slightly expands and contracts, and nozzle traces shift measurements by a few hundred microns.

Practical values for FDM: leave 0.3–0.4 mm for a loose fit, 0.15–0.2 mm for a tight fit, and 0.4–0.5 mm for threaded or rotating joints. Holes always come out smaller than designed; model critical holes 0.2 mm larger, or bring them to the desired diameter with a drill after printing.

The best approach is to print a small tolerance test piece before the actual part. For orders requiring precise fits, we run this test for you.