How to insert nuts (or other fixtures) into your prints

In this post we show you how to insert nuts or other fixtures firmly into your parts. This can be useful for joining multiple parts together, attaching other objects to your prints or just increasing the strength of fixtures in your prints. We will use our recent project of a 3D printed remote control stealth boat as an example of the technique shown here.


1. Measure the nut or fixture

While designing the stealth boat it was clear that the hull and top sections were too big to be printed in one piece. Therefore I decided to use nuts and bolts to hold the sections together, this way I could make it strong but also ensuring it could be taken apart and reassembled easily (unlike glue for example). I measured the nuts I wanted to use and drew up a CAD model of them.


2. Design the positioning hole for your fixture but make it 0.5mm smaller than your fixture

For example if the nut outer diameter was 7mm, use 6.5mm as the diameter for your hole. Use the below design guidelines as general rules of thumb

  • Radius the edges so that the fixture is guided into the hole, the exact radius depends on the size but a good starting point for nuts under 10mm is a 1mm radius
  • Make sure there is a shaft below the nut for the corresponding bolt to go through
  • When printing ensure there is at least 3 perimeters around these holes as the first perimeter will melt away when the fixture is inserted, this will ensure a tight fit so that the fixture is strongly attached
  • Ensure there is a lip or similar to hold the fixture in place
  • Ensure there is enough room so that the fixture would sit 1mm below the top plane of the opening/hole
  • See the cross section below for a visual representation

3. Heat your fixture

After the part has been printed, heat up your fixture until it is hotter than the melting point of the filament you are using. Be careful when heating up the fixture and use all necessary safety precautions.


4. Place heated fixture into hole/slot

Push the fixture in the holes until you feel it hit the end/lip (if you keep pushing it will melt the bottom plastic too and go in too far).


5. Wait until cool and then remove excess

There may be some excess plastic that has melted and is covering some part of the fixture, if this has happened carefully cut away with a knife (it is usually very easy to cut away). Make sure the fixture isn’t protruding past the top plane of your surface as this would result in a non-flush connection between the two parts to be joined.

Once this is done you can attach the other part and screw the bolts in. The picture below shows the purple top section has been screwed into the nuts we inserted into the grey hull. Now it is all done and you have strong, replaceable parts!