Introduction - How to design 3d printed joints?

The interface between parts can easily be one of the hardest problems to solve when designing something for 3d print. Many methods have been invented to join parts together such as: snap-fit, friction fit, captive nut, tapped-screw holes, bolts, and yes, even magnets! There are so many clever solutions for joining objects together, but how do you know which one to use?

In this post, I’ll share the process I went through when designing my latest multi-part creation and - specifically - how I was able to leverage magnets to creatively join the different components together.

Why use magnets?

For my project, Sigil Backpack, I decided to leverage magnets because:

  • They can be used in conjunction with a reed-switch (which is an electronic switch that can turn on or off depending on whether a magnet is nearby). This is awesome because it will allow the LEDs to turn off when you remove the hexagon piece which was a design goal for my project.
  • The parts will be able to smoothly and easily separate without putting too much strain on any of the components.
  • It’s satisfyingly tactile to remove and insert the hexagon sigil.
  • And of course, I’ve never used magnets in this way before so it seemed like a fun idea!

These design benefits are awesome, but keep in mind:

  • Magnets could interfere with electronics or other components.
  • They will raise the cost of production, since they’re not exactly cheap. (If this is something you plan to market ever).
  • And practically speaking, securing them in place is non-trivial.
  • Lastly, be careful when handling them!

Each project is going to have unique design considerations but in engineering, there is always another way to solve any given problem. Magnetic joinery is just one of many tools to keep in mind. So how did I incorporate the magic of magnetic fields into my design?

The Design Process

Sigil Backpack – the project I was working on – began with a very clear design requirement. There would be a hexagonally shaped light diffraction unit which is snapped into a receiver through magnets. Easy!

I am a very kinesthetic learner, so I opened up Fusion 360 and designed the first thing that came to mind: “Take a hexagon and then just extrude some pillars with a magnet-shaped hole”. It seemed reasonable at first. But this has an obvious problem which I should have realized sooner and that is: the geometry of such a shape is pretty much impossible without support structures (as you can see in these pictures). And even with support structures, the tolerances will probably never print consistently to my liking.

So I went back to the drawing board.

The next obvious design requirement became “the magnet holder should not require support”. Okay, the most direct solution for that requirement is fairly straightforward.

I ended up with a functional design comprised of two components.

The first component is a “magnet holder” which is essentially a block that can be printed horizontally and has a place for the magnet to sit.

The second component is just my hexagonally shaped part with a slot big enough for the magnet holder to slide into.

If you look closely, you’ll notice the magnet holder also sports a lip at one end, which effectively allows it to snap in to place and stay there. Provided tolerances are within +/- 0.2mm this will have a very snug fit. (maybe too snug).

At this point, here’s a picture of all that put together.

It actually works well! And now that I have a common interface between the magnet holder and another part, it’s just a matter of extending this design block. Back to Fusion 360 we go.

I created a box which had 4 slots. The outer most slots will house their own magnet holders while the innermost slots will receive the hexagonal unit. When the hexagonal unit is pushed all the way down, the magnetic force of the inner and outer magnets should snap in place and hold everything securely. So with that in mind, here is my first attempt:

And a quick video of the finished prototype:


I’m loving the tactile and audible snap of clicking the hexagon in place! That coupled with the reed sensor capabilities makes me very happy. If you found this useful and have designed your own 3d part which leverages magnets, don’t hesitate to let me know in the comments! There are so many more kinds of techniques you can employ when prototyping something, so check back soon for more posts about other joinery methods.

Until then, have fun and be safe!