Recently I made a suggestion for fastening magnets to acrylic: clear adhesive tape.
Glue is difficult because most glues don’t like the smooth surfaces of magnets and acrylic, and it’s all too easy to make a mess of your shiny project. Friction fitting is a no-go because acrylic doesn’t tolerate pressure: it crazes or cracks.
But tape is an inelegant solution.
The other day dan offered an interesting approach which he uses to insert bearings in fidgets: a 24-petal cutting pattern which absorbs pressure from friction fitting but still holds tight.
I call the design a sunflower.
In Inkscape I imported the image he posted, traced a petal with the Bezier tool, multiplied it using the Clone tool, cleaned up the nodes, combined the paths, then resized it to 8mm magnets and cut a sunflower in some scrap acrylic. After a few trials I had it nailed. The magnet resisted, then grudgingly entered the center of the sunflower. And it held there. I then remodeled some rulers which use magnets. They work great.
The Clone tool is kind of fun so I decided to make some variants while the process was fresh in my mind.
Here is Dan’s original 24-petal design along with two variations: 20 and 32 petals.
After some trial cuts, I’ve learned that the 32-petal version does not scale successfully to an 8mm magnet diameter in 1/8" acrylic The petals become too thin and melt and the center gets weird. I haven’t tried it at a larger size. The two low-petal-count versions are fine at 8mm.
The 32-petal version prints fine at full size (3" outer diameter). The version that didn’t print well was 0.486" outer diameter (0.307" inner diameter). I think that, resized, it will probably work fine for magnets that are 12mm or larger in diameter.
I’m now neck-deep in experiments trying to figure out when the sunflower works and when it doesn’t. Since I want to fasten 8mm magnets, the cuts are quite close to each other. The proximity heats adjacent cuts, which I think annoys the sunflower. What does a sunflower do when it’s annoyed? It sheds petals: they look like tiny teeth, glittering in a halo around the intruding magnet.
Petal-shedding doesn’t always happen. It depends on how tight the fit. And, I think, the age and quality of the acrylic. I had consistently frustrating results on some clear acrylic from TAP Plastic’s scrap bin (hey, it was cheap!). Better luck with fluorescent green acrylic from TAP’s regular inventory. All were cut using Proofgrade settings for green acrylic.
Today I’m going to try an 18-petal sunflower with petals in alternating colors, so I can cut them in two separate passes. I’ve also added a little loop where each petal joins its neighbor. This might help absorb the stresses, as suggested in this article.
Finally, I realized that the sunflower approach offers a way to glue magnets to acrylic. It disrupts the glossy surface sufficiently to disguise the glue and offers myriad crevices for an adhesive such as CA to leech ’n’ grab the hapless magnet.
The 18-Petal Two-Pass Sunflower with Loops worked quite well. To put it to practical use, I designed a compact reading stand for my Kindle ebook reader. It consists of two pieces which cling to each other magnetically when they’re not supporting the Kindle.
It’s 1/8-inch acrylic, cut using settings for Proofgrade acrylic. The sunflower magnet fit is quit snug. I insert the 8mm magnets by pressing down with a flat surface, generally the other piece of the Kindle support.
After making a few of the Kindle stands, I found that one of the sunflowers cracked when I inserted the magnet. The good news: it cracked on the bottom edge, which suggests that the loops are deflecting the pressure away from the sunflower petals. If you decide to cut that design, I suggest increasing the outer diameter of the sunflowers by about 0.004 inches.
I’ve alternated colors for the petals so they can be cut in two passes, minimizing heat build up. And the interior end of each petal ends in a small loop to minimize strain from pressure. I am a flawed draftsperson so you will notice some irregularities (petal length on the 30-petal design is shorter than the others, loops are not perfect little curves, some loops are wider than others) but still… At least the shapes are consistent within each petal variant, and I used Inkscape’s node snapping feature to nail the joints between every petal.
I invite your experimentation. The 18-petal version works quite well for me, resized to hold 8mm magnets. Bigger petal counts would match to wider magnets.