Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Lacking a 4-jaw chuck for the lathe, this should suffice:
Coaster Epoxy Rim – chuck-in-chuck setup
Which is just the Sherline 4-jaw chuck chucked in the lathe’s 3-jaw chuck, with both chuck Jaw 1 positions lined up and marked on the acrylic disk fixture. The picture is a recreation set up after the fact, because I lack a good picture of the overall scene.
Now it’s easy enough to center the fixture, stick the coaster in place with reasonable accuracy, then tweak the Sherline chuck to center the coaster:
Coaster Epoxy Rim – turning setup
Because the bottom layer is a laser-cut disk, eyeballometrically aligning its edge to a simple pointer worked surprisingly well:
Coaster Epoxy Rim – locating mirror edge
Turning the OD down to match the bottom disk meant I could finally get decent results with zero drama:
Coaster Epoxy Rim – turned samples
From the bottom, this one has a 3 mm mirror, the 3 mm fluorescent green frame + petals, and a 1.6 mm top sheet:
If I never tell anybody, they’ll think the slightly granular look of the tape was deliberate; it looks OK to me.
And, for completeness, the crash test dummy from the start of this adventure:
Coaster Epoxy Rim – turned 6 petal black
I don’t know how to avoid the bubbles, as the usual torch-the-top and pull-a-vacuum techniques pop bubbles at the epoxy-air interface. These bubbles are trapped under the top acrylic sheet, even though I was rather painstaking about easing the layer down from one side to the other while chasing bubbles along.
Maybe I can define bubbles as Part of the Art?
Definitely fancier than chipboard, although not nearly as absorbent.
A long-forgotten pad of Art Paper in assorted colors came to the surface:
Layered Coaster – tweaked
An angled view shows off the layering a little better:
Layered coaster – side view
Done manually with LightBurn’s Offset tool: shrink the frame’s interior openings (which lie outside the frame) by 1 mm per step, then cut each shape into a different color. The black layer is a complete disk, stuck atop a plain chipboard disk for stiffening.
In the cold light of day, I think I offset the green layer by 2 mm.
It’s not a particularly useful coaster, because you want a flat surface under your drink, but it does look pretty. Nowhere close to that good, but I like it.
The next time around, I’ll automate the process by stepping the sash width by 1 mm and saving each SVG image separately.
Yes, there really is a difference between 35 mil and 57 mil chipboard:
Chipboard coaster – 35 mil white vs 57 mil kraft
The thinner leaves (0.92 mm) have one delicate white surface that presents much better color when scribbled with fat-tip colored markers. The thicker frame (1.45 mm) is ordinary kraft chipboard which seems much more durable and looks terrible when colored.
Although it may be a case of gilding the dandelion, a durable kraft frame sets off the petal colors and, being slightly thicker, may also protect them from immediate destruction by sweaty drinks.
We’re talking artsy coasters here, not cheap disposable junk. Right?
So I assembled a coaster from shattered glass in a clear surround with black epoxy atop a mirror base:
Smashed Glass Coaster 2 – mid-layer glass pour
Each fragment sits on a blob of black epoxy that eventually oozed out to fill the gap between the mirror and the transparent layer. You can see the oozing start around the two fragments in the upper left.
A top layer of black acrylic sits flush with the upper surface of the glass, seen here with the protective paper in place before pouring black epoxy into the gap around the perimeter of each fragment:
Smashed Glass Coaster 2 – masked top
Peeling the paper away exposes an almost perfect surface, with the epoxy forming a slight curve between the black acrylic and the glass:
Smashed Glass Coaster 2 – overview
The mirror doubles the number of glass cuboids and their glittery gaps:
Smashed Glass Coaster 2 – fragment detail
All in all, it turned out well, but the epoxy pouring and leveling is tedious.
It might be possible to assemble a coaster upside-down, with the black layer stuck to something like Kapton tape and the fragments carefully aligned in their openings to make the entire top surface a plane. The tape should keep the epoxy from oozing out of the gaps, although a perfect seal may be impossible.
Then fill the gaps with black epoxy, lay the clear middle layer in place, run a dollop of epoxy on each fragment, lay the mirror in place, and hope there’s enough epoxy to fill all the gaps and not enough to make a mess around the perimeter.
With a bit of luck, that wouldn’t require so much hand finishing.
The next coaster must have a perimeter shrinkwrapped around the fragments, if only to break the low-vertex-count polygon tradition.
So I clamped it to the Sherline’s tooling plate and milled off the rim:
Smashed Glass Coaster – meniscus removal
Given the Sherline’s cramped work envelope, all the action took place along the rearmost edge, requiring eight reclampings indexed parallel to the table with a step clamp.
The cutter cleared off everything more than 0.3 mm above the surface of the glass chunks. I could probably have gone another 0.1 mm lower, but chopping the bit into the edge of a shattered glass fragment surely wouldn’t end well.
Polishing the dark gray milled surface might improve it slightly, at the risk of scuffing whatever poured epoxy stands slightly proud of the glass:
Smashed Glass Coaster – leveled edge
Perhaps if I define it to be a border, everybody will think it was intentional.
Just to see what happens, I laid some smashed glass in puddles of epoxy:
Smashed Glass vs epoxy – samples
Backlighting with the LED light pad reveals more detail:
Smashed Glass vs epoxy – backlit samples
The chunk on the left is the proof-of-concept shot glass coaster with a form-fit black acrylic mask atop a clear epoxy layer on a clear acrylic base. The chunk at the top is raw shattered glass fresh from the pile. The two chunks on teardrop acrylic scraps are bedded in transparent black and opaque black tinted epoxy.
A look through the microscope at all four, laid out in that order, with the contrast blown out to emphasize the grain boundaries:
Smashed Glass vs epoxy – magnified comparison
You may want to open the image in a new tab for more detail.
The raw chunk has air between all its cuboids, so it’s nicely glittery. All the others have much of their air replaced by epoxy.
Clear epoxy produces an essentially transparent layer where it fills the gaps, because its refractive index comes close enough to the glass. The stretched contrast makes the gaps visible again, but the backlit image shows the unassisted eyeball view.
Transparent black dye sounds like an oxymoron, but it fills the gaps with enough contrast to remain visible. The overall chunk is not particularly glittery, but it’s OK.
Opaque black dye produces a much darker tint; the slightly tapered thin layer between the glass and acrylic (the small white circles are air bubbles) cuts down on the transmitted light. The gaps remain nearly as prominent as in the air-filled chunk, although with very little glitter.
Bedding the glass in epoxy against an acrylic sheet should reduce its tendency to fall apart at the slightest provocation, although the proof-of-concept poured coaster showed the epoxy must cover the entire edge of the glass sheet to bond all the slivers in place.
Before trying to make decorative coasters from colorful acrylic, I figured a few practice sessions in chipboard would be in order:
Chipboard coasters
They’re colored with wide tip Sharpies of various ages and, as the yellow and uncolored sections show, chipboard never gets very bright. On the other paw, chipboard is also known as “beer mat”, so at least I have the right general idea.
The patterns come from a GCMC program producing SVG figures for LightBurn to apply kerf compensation:
Chipboard coasters – cut and color
It’s obviously too late to have me color within the lines.
The overall frame in the upper left and the base plate in the upper right get the kerf compensation, which (for chipboard) turns out to be +0.15 mm outward (thus making the holes smaller and the diameter larger). If I were doing marquetry, I’d want to arrange each piece on a separate wood veneer sheet with proper grain orientation and similar fussiness, but that’s not the point right now.
Without compensation, the pieces have a drop-in fit with an obvious gap:
Coaster – chipboard – no kerf comp
Adding a mere 0.15 mm on each side produces a very snug fit:
Coaster – chipboard – frame 0.15 out
In fact, the pieces go in from the back and require hammering gentle tapping to persuade all the corners into place.
Protip: putting a dark color on the frame and around the edges conceals many flaws.
Increasing the compensation to +0.20 mm means the pieces no longer fit and, when eventually battered into the frame, the surface becomes a concave-upward dish.
With the (colored) pieces in the frame, I covered the base plate with a thin layer of good old Elmer’s Yellow Wood Glue, dropped the top over it with some attention to good alignment on all sides, and clamped the assembly between two planks for a while. Obviously, you’d want to make more than one at a time, but they’re rather labor intensive.
The GCMC program produces the patterns from the coaster’s dimensions:
Outer diameter
Number of leaves around the center
Center spot diameter
Sash width (it’s really a muntin, but quilters say sash)
Leaf aspect ratio (max width / overall length)
Due to the relentless symmetry, finding the points describing half a leaf and half the sector between two leaves suffices to generate the entire coaster by various rotations around the center. The code performs no error checking whatsoever, so some dimensions emit a hard crash rather than a coaster.
A geometry doodle with some incorrect values:
Coaster Geometry doodle
Poinr P1 (where the leaf snugs against the circular sash around the center spot) sits at the intersection of a line and a circle, so the code solves a quadratic equation with grisly coefficients:
a = 1 + pow(tan(LeafStemHA),2);
b = -2 * tan(LeafStemHA) * (Sash/2) / cos(LeafStemHA);
c = pow((Sash/2) / cos(LeafStemHA),2) - pow(LeafID/2,2);
xp = (-b + sqrt(pow(b,2) - 4*a*c))/(2*a);
xn = (-b - sqrt(pow(b,2) - 4*a*c))/(2*a);
y = xp*tan(LeafStemHA) - (Sash/2) / cos(LeafStemHA);
P1 = [xp,y];
Given the geometry, the “plus” root is always the one to use.
A doodle working out that intersection, as well as for P5 out at the widest part of the leaf, carrying some errors from the geometry doodle:
Coaster Geometry equations
Both of those doodles have errors; the GCMC source code remains the final arbiter of coaster correctness.
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The Smell of Molten Projects in the Morning 