Choosing and fitting screws in 3D printed parts

Clearance holes: print them oversize

Printers shrink holes. Add roughly 0.2 to 0.4 mm over the nominal clearance so you do not have to drill every part.

• M2 → print 2.4 mm
• M2.5 → print 2.9 mm
• M3 → print 3.4 mm
• M4 → print 4.5 mm
• M5 → print 5.5 mm
• M6 → print 6.6 mm

Heat-set inserts are the strongest option

A brass insert melted into the plastic gives you a real metal thread that can be undone hundreds of times. This is what production hardware does.

Print the boss hole slightly under the insert's outside diameter so the melted plastic has somewhere to flow.

• M2 insert → 3.2 mm hole
• M2.5 insert → 3.6 mm hole
• M3 insert → 4.0 mm hole
• M4 insert → 5.6 mm hole
• M5 insert → 6.4 mm hole

Self-tapping straight into plastic

Fine for parts you open once or twice. Use a hole about 0.8 times the screw's outside diameter, keep the boss wall at least as thick as the screw radius, and stop the moment the head touches down.

Thread-forming screws designed for plastic (Delta PT, Plastite and similar) hold far better than a wood or sheet-metal screw because they push plastic aside instead of cutting it away.

Trapped nuts and captive hardware

A hexagonal pocket for a nut, printed into the part, is free and strong. Size the pocket 0.2 mm over the nut's across-flats measurement and pause the print to drop the nut in if it needs to be fully enclosed.

Square nuts and T-nuts work the same way and are less likely to spin.

Stop parts splitting

Screws pulling along layer lines is the usual failure. Orient the part so the screw load presses layers together rather than peeling them apart.

Always use a washer under a pan head on printed plastic — the head sinks in otherwise and the joint goes loose.

Torque by feel and stop early. Printed plastic gives no warning before it cracks.

Common questions

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