How to add hidden magnets to 3D prints
I was designing a honeycomb grid light modifier for my Amaran Ace that required magnets to get it attached to the light. I figured I would document how I designed the part in Fusion 360 and how I embedded the magnets to the final print.
The 3D digital files, including the full Fusion 360 design is available from Printables and MakerWorld.
If you prefer this content in a video format. I have just the video for you.
Measure the Magnets
Don’t trust the manufacturers size. Double-check everything and measure a few different units because there might be some variation from magnet to magnet. The magnet below is supposed to be a 2×5 magnet. Although the 5mm diameter is spot on, the thickness of 2mm is way off at just 1.7mm . If you don’t have a set of calipers, do get one if you will be doing any 3D modeling. You don’t want to waist print time and materials only to find out that your magnets don’t fit.
Add Clearance
Always add clearance when designing a cavity for the magnet. There is some chance a 5mm magnet will fit in a hole designed with a 5mm diameter. Either due to variances in magnet size and the fact that your printer will most likely not be able to print a perfect 5mm diameter circle. There are other factors like the extruded plastic expanding an contracting when it cools down after it’s deposited on the model.
I like to start by adding a .1mm clearance and doing a test print to verify that the combination of filament type, my 3D printer, and the magnet will all fit together nicely.
Different filaments will expand and contract differently. So, clearance for PETG might not fit the same as PLA on a given 3D printer. Or, it might, you’ll never know if you don’t test it first.
You want some resistance for the magnet to go in, but you don’t want it too lose because the magnet will jump out of the hole and stick to the hotend if it is too lose. Specially if you have a hardened steel nozzle since brass is not magnetic.
Account for Layer Height
If you are printing at .2mm layer height, the distance between the print bed and the bottom of the magnet in your design should be a multiple of .2mm.
On this design, the magnet will sit .4mm from the top surface of the print. This will give me 2 top surfaces when I slice the model with .2mm layer height, If I had placed the magnet .3mm from the top, the slicer software would have just approximated that to the nearest multiple of the layer height.
If the hole is too shallow, it might create artifacts on the print surface, or worse, the nozzle could collide with the magnet and cause the print to fail.
I like using parameters for things like this so I can change the model design based on the target layer height of the print.
Add a Pause to the Correct Layer
This is where I have seen a lot of YouTube videos and tutorials get it wrong. A lot of them tell you to find the layer that will cover the magnet, back up one layer, and add a pause.
That’s not a good approach.
PrusaSlicer and other slicers place the pause at the beginning of the layer. So, move the marker to the layer that will be covering the magnet and add a pause there. You will see this tiny little green marker indicating where the print will be paused and the nozzle will move out of the way.
Double-check magnet polarity
Double-check and triple-check magnet polarity. I put a mark on each magnet so I know which side goes facing the print bed. It goes without saying that adding the magnet with the polarity flipped will be extremely disappointing and might render your whole print useless.
Placing the Magnets
When the printer pauses, move as quickly as possible to place the magnets in the holes and resume printing.
I like using this very old flathead screwdriver – that hasn’t been used to fasten a screw in over a decade – to place the magnets. This way, I can get to the magnet hole from the side of the print.
I find this easier than using a stack of magnets. Getting the bottom magnet free from the whole stack of magnets can be kind of hard on these super strong magnets. And I don’t want to risk knocking the print off the print bed.
Sidenote
The print bed on your 3D printer is magnetized. If you position the model in a way that the magnet hole will be directly above the print bed magnet, you might have issues with the polarity and the magnet under the print bed pushing the magnet you want to place on the print away.
