How do they work?

A few important tidbits about this next sequence:

  • It’s not simply 1-2-3-done. Each step requires some evaluation and decision making, and must be spread out over time…it may take a couple days to complete!
  • Not all magnets are created equal. Even among the rare-earth neodymium type there’s considerable variation in pull among brands.
  • The glues we recommend using have a “geometric” cure…they may form a decent hold in the first few minutes, but may take hours (even overnight) to reach maximum strength. Given the magnets’ strength, patience is required to avoid pulling them out accidentally.

Read through all the steps before starting so you know what you’re up against.

Sourcing Magnets

The two halves of the D20 model are held together with 1/4" diameter neodymium disc magnets, 1/8" thick.

There are slots for up to four magnets on each half. Do not fill them all with magnets on the first go! The correct number will depend on the strength of the magnets you acquire. We learned this the hard way…

During development we used four magnets per half, with rare-earth magnets from a particular eBay seller. When that source dried up we needed a steady alternative. Ladyada recommended K&J Magnetics. Indeed, their “D42” magnet is just the right size for this project and can be ordered in any quantity (some other sources required batches of 20, 50, etc.).

Unknown to us at the time, it’s like this company’s neodymium magnets are laced with kryptonite or something. They’re the Altoids of magnets. With our usual total of eight in there (four per half), it’s nearly impossible to pry the D20 open!

Therefore: take this in stages. Start by installing just four magnets (two per half). If these came from K&J, that’s probably enough to hold the die closed. If from a lesser source, the die might fall apart when rolled, in which case you can add a third pair, test again, then possibly a fourth pair.

The magnets are inexpensive, so just order ten or a dozen in one go. You’ll have spares for any slip-ups, or they’ll be handy on the fridge or for other projects later.

Prep Work

Magnets have a specific polarity…they will only click together when facing the right way. If you install even a single magnet upside-down, it will now push the two halves of the D20 apart rather than holding it shut. This first step helps avoid mistakes…

Stack all your magnets together. Using a permanent marker, draw a dot on the top of the stack.

Allow the ink a moment to dry, then slide this magnet off the top of the stack and place it at the bottom. Draw a dot on the next magnet. Repeat until all the magnets are marked like this.

It’s crucial that the stack not get turned around during this process…all the dots must face the same way.

Now you have a polarity indicator when installing the magnets. A “dot” face will always attach to a “blank” face. Never dot-to-dot or blank-to-blank.

Gluing Magnets

This is a painstaking process. Do not rush through it.

I recommend a strong adhesive for this…either 5-minute epoxy or E6000 craft glue. Do not use hot glue. But especially…

Do not use JB Weld.

JB Weld contains metal particles…when magnets are involved, it will not stay where you put it. Great for other things, but not this project. DO NOT USE.

These photos all show E6000 glue, and that works fine…but having assembled this a few times now I think 5-minute epoxy holds a little better. It’s just time-consuming mixing a tiny fresh batch for each magnet…as you’ll see, it’s vital that they be done individually. E6000 still works well for some other steps later, so you may want both around anyway.

You might do okay with cyanoacrylate “super” glue. Personally, I seem to be cursed when it comes to that stuff…the only things I can get it to hold together are fingers.

Before gluing any magnets in place, do a test-fit in each of the eight magnet sockets. The 3D printing may leave little bumps or artifacts in the sockets that prevent the magnets from being fully seated. Make sure they sit flat and do not rock. Clean out each socket with a file until the magnet cooperates.

Pick one half of the D20…either half, doesn’t matter…and any one of the four magnet holes.

Mix and scoop a little epoxy or squeeze a daub of E6000 into this socket, then press a magnet into place there with the dot facing up.

Make sure the magnet is fully seated. Press it down with a toothpick or something. It will probably ooze some glue around the edges…that’s fine, scrape it up with the same toothpick.

When the glue or epoxy has gone tacky enough that the magnet won’t shift, walk away for at least 30 minutes. Make sure your glue is capped, go wash your hands, take a bike ride or watch some cat videos or something. Do not try to install the other magnets yet. Keep them far away (they have a habit of rolling across the table).

These glues have a “geometric” cure…they form a basic grip in a few minutes, but stronger holds take progressively longer. Even “5 minute” epoxy requires 24 hours to fully cure…five minutes is just the working time.

These magnets are so strong that they’ll tug on others in nearby sockets. We need to wait for the glue to set up a bit before adding additional magnets on the same side of the D20.

With 30 minutes passed, now add glue and a second magnet in the diagonally-opposite socket, also dot facing up.

Guide the magnet all the way into place, scrape away any excess glue and then hold the magnet there…do not just drop it and expect it to settle! Even at this distance the magnets have a very strong influence.

Keep an eye on it for a few minutes. Once the second magnet isn’t shifting, set this aside somewhere where the glue can fully dry overnight…at least several feet away from the other magnets on the table. Then we’ll work on the other half.

Oops! I got careless and set it too close, and now the stack of remaining magnets has snapped on.

You’re done for the day! Don’t attempt to pry the other magnets off until the glue is completely cured…otherwise you’ll just pull the magnet out of its socket.

Second Half…

In a moment we’ll add some magnets to the second half. But let’s take a look at something first…

Whatever two sockets you chose for magnets on the first half, we’ll be using the opposite two sockets on this other side. Things are mirrored when flipped over.

Remember, when we’re adding magnets to this second half, the first half should be nowhere in the vicinity. These magnets pull hard and will ruin your work! Put it way up on a shelf or something, far away.

Also, on the first half we installed magnets with the dots facing up. On the second half we want the blank faces showing. No dots!

Glue the first magnet into place as before, cleaning up any ooze and waiting for a tacky hold.

The dot on this magnet should be facing down. You should just see a blank magnet face, no dot.

After waiting the requisite 30 minutes, install the second magnet, also dot-side-down.

Hold this in place while the glue sets up, so the other magnet won’t pull it out or askew.


The glue on both halves must be completely dry before proceeding. Completely. That means you’ll resume tomorrow. Make sure the glue is capped, and go wash your hands thoroughly.

Do not allow the two halves anywhere near each other yet! Keep them well apart until tomorrow.

Test Fit

Once fully cured overnight, you can bring the two halves together. They should click together nicely.


To open the die (to access the electronics later), there’s a certain way you can grip it and pry the two halves apart. I find it helpful to put thumbs on the “9.” and “6.” faces (the only two faces with orientation dots) and hinge it open like a clam.

If any magnets pull out of their sockets: you’ll have to re-glue them and try again tomorrow.

Roll the die around the table a bit. Do the two halves stay securely together, or do they come apart?

Die holds together well: you’re done with this phase and can skip ahead to the next page!

Die halves come apart, or don’t feel quite secure: this calls for additional magnets.

Unless you’re using weak magnets, add these one pair at a time. Each half receives one magnet (follow the same dot/blank face orientation) and keep the two halves separated overnight until the glue is fully dry, then test again. If it’s now solid enough with three magnets, you’re done, otherwise you can add the fourth pair.

This guide was first published on Aug 07, 2015. It was last updated on Aug 30, 2023.

This page (Magnets) was last updated on Jul 21, 2015.

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