Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
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.
Setting up a piece of MDF and hitting the Frame button produced a lightly scorched line around the part perimeter, plus a slightly diagonal track leading from / to the Home position in the far right corner:
Fire while framing tracks
Doing another pass with LightBurn’s rubber-band frame produced the faint dotted circle.
Huh. Didn’t useda do that.
The laser should not fire while framing and, having just installed LightBurn’s 1.2.01 update, suspicion instantly fell on the most recently changed thing.
Which turned out not to be the case, as LightBurn’s tech support pointed out:
This is generally an indication of a failed high-voltage power supply, not a software issue.
OMTech’s support requested a video of the equipment bay, which didn’t seem like a useful way to convey the situation. Instead, I sent pix.
This picture shows the status of the 60 W laser power supply while the laser is incorrectly firing:
OMTech 60W Laser – uncommanded framing fire
The power supply has two LEDs on what looks like, but is not, an Ethernet jack near the bottom:
Orange P LED: good water flow
Green L LED: controller’s PWM signal
The LASER orange LED near the top turns on when the HV output is active and the laser should be firing.
In this case, L LED is off and the LCD shows “Laser signal OFF”, but the LASER LED is on and the LCD shows 2 mA beam current: the laser beam is ON, even though the controller has not activated the PWM signal.
Not only that, but I discovered the laser would fire while framing even with the lid up and the “safety interlock” sensor active.
Totally did not expect that.
For comparison, the power supply status during a manual pulse at 49% power:
OMTech 60W Laser – manual pulse 49%
In that case, the L LED shows the PWM signal is active, the LASER LED is on, and the LCD shows 14 mA of current to the tube. That’s how it should work.
Although the function of the TEST button seems very lightly documented, pressing it did not turn on the output (the LASER LED is off), despite lighting the L LED:
OMTech 60W Laser – Test button pressed
OMTech confirmed my suspicion:
We are afraid that the laser power supply is defective
Making a coaster with petals from the NBC peacock turned out to be trickier than I expected:
Chipboard coaster – rounded petals
Protracted doodling showed that I cannot math hard enough to get a closed-form solution gluing a circular section onto the end of those diverging lines:
Chipboard coaster – rounded petal geometry doodle
However, I can write code to recognize a solution when it comes around on the guitar.
Point P3 at the center of the end cap circle will be one radius away from both P2 at the sash between the petals and P4 at the sash around the perimeter, because the circle will be tangent at those points. The solution starts by sticking an absurdly small circle around P3 out at P4, then expanding its radius and relocating its center until the circle just kisses the sash, thus revealing the location of P2:
The dist variable is the perpendicular distance from the sash line to P3, which will be different than the test radius r between P3 and P4 until it’s equal at the kissing point. The radius update is (pretty close to) the X-axis difference between the two, which is (pretty close to) how wrong the radius is.
As far as I can tell, this will eventually converge on the right answer:
Having found the center point of the end cap, all the other points fall out easily enough and generating the paths follows the same process as with the simple petals. The program performs no error checking and fails in amusing ways.
As before, laser cutting the chipboard deposits some soot along both sides of the kerf. It’s noticeable on brown chipboard and painfully obvious on white-surface chipboard, particularly where all those cuts converge toward the middle. I applied low-tack blue masking tape as a (wait for it) mask:
Chipboard coaster – tape shield
Whereupon I discovered the white surface has the consistency of tissue paper and removing the tape pretty much peels it right off:
Chipboard coaster – white surface vs tape
Putting the chipboard up on spikes and cutting it from the back side, with tabs holding the pieces in place (so they don’t fall out and get torched while cutting the next piece), should solve that problem.
In the meantime, a black frame conceals many issues:
Chipboard coaster – rounded petals – front vs back cut
I must up my coloring game; those fat-tip markers just ain’t getting it done.
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
First you mix the epoxy, then you blend in the dye, then you dispense it into the thing you are making. If you’re using many colors, this is obviously not the right way to go about it:
The bar magnet holds the backplate against a bench block to keep it at right angles to the base while the adhesive cures. The base is three layers of MDF with no, small, and large holes fitting the cups. I expect many epoxy spills; scrap MDF reduces deep emotional bonding to the result.
The LightBurn project has the sign outline as a tool layer to simplify aligning the victims with the laser path, plus one layer defining the cuts for the three plates. I exported it as an SVG image with the same information as colored vectors for use in whatever laser control program you might use.
I eased a thin bead of clear epoxy along the frame sash, using less than I thought necessary, and aligned it atop the base plate:
Acrylic Coaster – frame epoxy
The excess epoxy formed fillets along the petals, a little oozed out the perimeter, and even less smeared on the top surface. The scrap acrylic didn’t have a surface mask, but that’s definitely a Good Idea for the next attempt.
Two drops of transparent red tinted the remainder of the epoxy well enough:
Acrylic Coaster – first color pour
The clear epoxy was still liquid (which is why the red epoxy was still pourable!), but the red tint stayed atop the fillet around the spot.
The next day:
Acrylic Coaster – epoxy coloring
Obviously, coloring epoxy for a single coaster makes absolutely no sense whatsoever, but ya gotta start somewhere.
You (well, I) can suck most of the inevitable bubbles out of the epoxy back into the dispensing pipette, but those last few bubbles will remain forever. Popping bubbles by waving a propane torch flame over the surface seems better-suited to tabletop-scale projects not involving an acrylic frame.
The epoxy puddles are about 1 mm deep inside the 2.5 mm thick frame, so (if this were a real coaster) the sashes between the petals would support the chilled mug and the petals would collect all the condensation.
Thicker epoxy would have more saturated colors and a white base plate might be in order.
With the risk of squishing excess glue through the kerf:
Chipboard coaster – excess glue
That’s the same coaster as in the first picture, carefully arranged with light reflecting off the flat glue surface. In real life, the nearly transparent glue doesn’t look nearly so awful, but smoothing much less glue than seems necessary across the bottom disk suffices.
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
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.
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
This file contains hidden or bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
The Smell of Molten Projects in the Morning 
