I just finished this coaster using cherry-clad MDF with red acrylic inlays; both materials from an assortment I got from my valentine (she knows me well!). A few new techniques here that I had a lot of fun with.
The design was inspired by tiles we saw a few years ago at the Alhambra, created in Inkscape by composing a bunch of variations on a sixteen-point star. I am still a rank amateur with this app but am blown away by its power. The second and third rings were each created by overlapping two 16-point stars with different “spoke ratios” (the radius of the outer points divided by the radius of the inner point) and then taking the difference between the two areas. For the third ring I also trimmed out a circle so you got the five-sided end result.
the round-bottom points in the outer ring were created by distributing triangles and circles around the same circular path, breaking all the paths apart, and unioning each touching triangle and circle. That union bit at the end was critical to get a single cut around the edge.
I cut the design from the cherry sheet along the red lines in the screenshot, and engraved the circle in blue. I also cut out a solid circle the same size to act as a base. The trick from there was to cut the acrylic pieces so that they’d inlay perfectly into the wood.
This is way more complicated that you might imagine! First of all, the laser cuts with a tiny but meaningful “kerf” (the width of the cut itself). The kerf is different from material to material, which is why I added that little stub of red cut line outside the circle in the Inkscape screenshot. The width of that test cut measured .014″ using my calipers, which meant that the cutouts were really .007″ bigger than my design. Inkscape came to my rescue again with an “outset” feature that expanded all the shapes for the acrylic cuts.
But wait, there’s more. It turns out that the laser doesn’t make a perfectly vertical cut. It’s actually a cone shape with the point focused on the top edge of the material. This means the bottom of the cut is just a little bit wider that the top, screwing up the inlay fit. BUT (and this is cool), if you flip and reverse your design, you cancel that out and get a perfect match. This was super-simple given my symmetric design; I just flipped over each piece of acrylic before pressing it into place. I have to say it was deeply satisfying having each inlay just click into place so beautifully.
Since this was going to be a coaster, I poured two coats of clear epoxy resin over the whole thing, sanded down the drips and then cut out (thanks again Glowforge) a circle of self-adhesive 2mm cork for the bottom.
I’m quite proud of the end result! A lot of work for a simple coaster, but now that I’ve figured out the quirks, new designs should come together pretty quickly. So many neat concepts packed into that Glowforge.