Inspired by my Prusa MK4S printer, I’ve been using hexagon socket head cap screws and captive nuts to assemble FDM 3D printed parts that might have to be disassembled.

Instead of square nuts I’ll be using standard hex nuts.

To determine the size of the features for M3 screws and nuts I made a test piece. A CAD drawing of the test piece with relevant dimensions is shown below.

The dimensions are based on the relevant standards. ISO 4762 for the screws and ISO 4032 for the nuts. The screw has a head diameter of at most 5.8 mm. The nut has a maximum width between the flats of 5.5 mm. Both the bolt head and the nut are at most 3 mm high.

The features are slightly oversized to allow for squeeze-out and shrinkage. The hardware fits pretty well in the test pice as shown below. Both the bolt head and the nut lie below the surface of the printed part.

The test piece was printed in PETG on my Prusa MK4, using the SPEED parameters for “generic” PETG.

The nut is best pulled into its cavity by the screw. It then stays in place. The screw can be removed.