Never, Ever Run Your Laser Cutter Unattended

While running some finger-joint test pieces, this happened:

Detached laser lens holder
Detached laser lens holder

The knurled ring just below the Tight→ label worked its way loose and released the lens holder tube collet, whereupon the whole affair fell out and dangled on the air hose & wires as the gantry continued to zigzag along the finger pattern.

As is my custom, I was watching the proceedings and managed to poke the controller’s STOP button, which was a mistake. What I should have done was slap the EMERGENCY STOP mushroom switch, because the STOP button just tells the controller to cancel the current action and return to the home position, which resulted in dragging the lens holder across the plywood and platform.

No harm done, as far as I could tell, and it realigned easily enough.

The more typical laser cutter failure seems to be having the controller execute the Halt and Catch Fire instruction, resulting in at least a ruined workpiece, sometimes a ruined laser, and occasionally a serious conflagration.

Lesson learned: practice slapping the Big Red Switch every now and then.

Laser Cutter: Scan vs. Cut Alignment

Laser-cutting alignment pin holes in the most recent smashed-glass coaster raised the question of whether it’s feasible to engrave a deep recess around a hole with Good Enough accuracy for things like recessed screw heads.

The test image:

Scan vs cut offset
Scan vs cut offset

The top two rows create engraved recesses and cut holes from 1.0 to 1.5 mm and the next two rows run from 1.5 to 2.0 mm. The bottom row has 1.0 mm holes centered in engraved pits from 0.5 mm to 3.0 mm; obviously, the first hole will subsume its pit.

The first pass looked promising, although the edges of the engraved pits seemed ragged:

Scan vs cut alignment - first test
Scan vs cut alignment – first test

Perhaps the replacement power supply has different timing than the original one?

I’m still surprised that the core of a laser-cut hole falls right out of the sheet, right down to a sliver from a 1 mm hole:

Cut hole cores
Cut hole cores

Recalibrating the scan offset got the errors down to 0.1 mm in either direction:

Scan offset - 300 200 mm-s 0.15mm offset
Scan offset – 300 200 mm-s 0.15mm offset

The lines in the middle column are spaced 0.15 mm apart at scan speeds of 300 mm/s (top) and 200 mm/s (bottom).

Another test pattern puts an engraved rectangle inside a dot-mode cut line with 1 mm spacing on all sides:

Scan vs cut alignment - 300 mm-s 0.15mm
Scan vs cut alignment – 300 mm-s 0.15mm

That’s wonderfully accurate!

A few more test pieces later:

Scan vs cut alignment - test pieces
Scan vs cut alignment – test pieces

Returning to the pits-and-holes test, with one engraving pass:

Scan vs cut alignment - holes x1 engrave
Scan vs cut alignment – holes x1 engrave

That’s lined up to be looking directly down the 3 mm pit in the lower right, which looks fine.

Two engraving passes makes the pits deeper (nearly through the 2.5 mm arylic) and somewhat messier, but still nicely aligned with the holes:

Scan vs cut alignment - holes x2 engrave
Scan vs cut alignment – holes x2 engrave

Engraving the recess before cutting the hole seems to produce a better result, perhaps because both the engraving and the cutting encounter uniform surfaces.

All in all, this worked out better than I expected.

High Impact Art: Coaster 5

This came out all glittery:

Smashed Glass Coaster 5 - top view
Smashed Glass Coaster 5 – top view

Epoxy tinted with transparent black dye does a pretty good job of not obliterating the cracks between the cuboids. In person, the cracks seem less conspicuous around the borders of the glass pieces, but they’re visible enough for this ahem use case.

Under the proper lighting, a few bubbles appear along and above the black layer:

Smashed Glass Coaster 5 - oblique view
Smashed Glass Coaster 5 – oblique view

The new thing this time around were three pins holding the layers in alignment while the epoxy cured:

Smashed Glass Coaster 5 - alignment pin
Smashed Glass Coaster 5 – alignment pin

The conical end comes from grabbing an 8 mm snippet of 3/64 inch steel rod in a pin vise and twirling it against Mr Bench Grinder for a few seconds.

The pins pretty much dropped into 1.1 mm holes created while cutting the sheets. The tiny circles mark the laser path around the pin holes:

Coaster 5 - layers
Coaster 5 – layers

The “holes” in the top sheet (upper middle) are in the Tool 2 layer so they’re not cut, because it was easier to match-drill holes halfway into the top sheet with the drill press than to figure out how to convince the laser to not punch all the way through. Engraving (along the lines of the earring borders) might work, but I’m not sure how well a high-aspect-ratio hole will engrave.

The mirror sheet is reversed left-to-right in order to cut it from the back of the reflective layer. I’m not certain this is necessary, because acrylic is basically opaque to 10.6 µm IR light and any doubly attenuated reflected light will diverge strongly from the focus point at the top surface, but it’s the recommended procedure and easy enough to do.

The cork cuts with its adhesive layer up and blue tape on the bottom to prevent soot from accumulating in all the surface crevices.

The alignment pins worked surprisingly well:

Smashed Glass Coaster 5 - edge alignment A
Smashed Glass Coaster 5 – edge alignment A

The top sheet sticks out 0.3 mm on one side:

Smashed Glass Coaster 5 - edge alignment B
Smashed Glass Coaster 5 – edge alignment B

Oddly, there’s no place where the top sheet is indented by any noticeable amount, so there may be slight size differences depending on all the colors and ages in that stack of plastic sheets.

I’ll cure the next one top-side down, giving the bubbles an opportunity to rise toward the mirror layer and maybe become less conspicuous:

Smashed Glass Coaster 5 - curing
Smashed Glass Coaster 5 – curing

The tricky part: finding and arranging glass chunks within a 100 mm circle!

Avoiding narrow gaps and acute angles in the perimeter, as the notch on the left side, should simplify draining the epoxy.

Smashed Glass Coaster: Conformal Perimeter

Snugging the perimeter around the smashed glass fragments definitely improves the result:

Smashed glass coaster - top view A
Smashed glass coaster – top view A

It’s just under 100 mm = 4 inch across the longest dimension and surprisingly glittery:

Smashed glass coaster - top view B
Smashed glass coaster – top view B

The coaster is a five-layer sandwich half an inch thick:

Smashed glass coaster - edge view
Smashed glass coaster – edge view

From the top:

  • Clear acrylic: 1.5 mm = 1/16 inch
  • Black acrylic: 1.5 mm = 1/16 inch
  • Clear acrylic: 3.2 mm = 1/8 inch
  • Mirror acrylic: 2.7 mm
  • Cork: 2.7 mm cut from a standard round coaster base

The smashed glass pieces sit atop the mirror, so the trick is making the layers around it add up to the same thickness. This is not possible by adding the nominal dimensions, because cast acrylic sheet thickness isn’t well controlled; I’ve finally written the actual (metric!) thickness on the sheets so I can select which 1/8 Inch sheet has the proper thickness.

A chipboard template (seen atop the finished coaster) verified the glass pieces fit easily within their openings:

Smashed glass coaster - top view - fit template
Smashed glass coaster – top view – fit template

I laid the clear frame on the mirror, poured generous epoxy puddles along the middle of the fragment openings, eased the glass in place, and gently pressed the slabs down to get a uniform epoxy layer, with the excess oozing under the frame all around. Then lay the black frame around the glass atop the clear, squirt more epoxy along the gaps around the glass, pour more epoxy atop the fragments, ease the top sheet in place paying considerable attention to coaxing the bubbles along to the edge, align the sides, and wait.

The epoxy cured while stuck atop a styrofoam pillar to let it drain smoothly off the edges:

Smashed glass coaster - epoxy curing
Smashed glass coaster – epoxy curing

I encouraged the epoxy out of the acute corners, as shown by the larger puddles, over the next few hours until the epoxy stiffened up. Those puddles also show the transparent black tint, to the tune of four drops in 8 ml of epoxy, which turned out to just barely suffice for the job. The whole assembly sat level while curing, but the layers didn’t remain aligned even after gently shoving them around while the stack cured.

The black epoxy joins nicely with the black frame layer to conceal most of the remaining bubbles. A different color frame with matching epoxy might looks less ominous, but colors more transparent than dark gray would likely reveal the bubbles.

It Would Be Nice™ if the acrylic sheet on the top had a transparent plastic film cover, but it arrived with brown paper on both sides. Despite that, I spattered only a few tiny drops on the bare surface and managed to scrape most of them off without further damage.

Overall, I think the conformal perimeter looks much better than the polygonal outline smashed glass coasters.

Smashed Glass Earrings: Sample Set

The POC earrings had a pair of 1.5 mm acrylic disks epoxied around the glass fragment, with the “gold” ring captured in a rebate around the rims. That process was both tedious and messy, so I tried laser-engraving a deeper rebate into a 3 mm sheet, then epoxying the fragment and the ring in place:

Earrings - epoxy curing
Earrings – epoxy curing

They’re stuck to a strip of Kapton tape to keep the epoxy off the bottom surface of the glass, while aligning it with the surrounding disk.

Peeling the protective film / plastic off reveals the acrylic disk:

Earrings - 25 20 16 12 mm first look
Earrings – 25 20 16 12 mm first look

They all required more effort to remove the epoxy remaining around the ring, but it worked out better than I expected.

A lighter background shows off their internal structure:

Earrings - 25 20 16 12 mm set
Earrings – 25 20 16 12 mm set

A closer look at the pairs:

  • Earrings - 25 mm pair
  • Earrings - 20 mm pair
  • Earrings - 16 mm pair
  • Earrings - 12 mm pair

As always, glooping clear epoxy around the edges fills many of the internal cracks and reduces the glittery aspect of all those glass-to-air-to-glass interfaces, but I don’t see another good way to keep the fragments under control.

The results may not be up to a “fine jewelry” standard, whatever that may be, but a slipcase box definitely improves the presentation:

Earrings - presentation case
Earrings – presentation case

If I had the courage of my convictions, I’d go for the Mr Clean look myself, but …

Layered Paper Coaster: GCMC Test

A few more attempts at layered paper construction, done with plain white Art Paper of various vintages:

Layered paper coasters
Layered paper coasters

The middle one comes from a version of the original GCMC marquetry shape generator, tweaked to produce just the frame SVG, called by a Bash script to change the sash width, and imported into LightBurn for laser control:

LightBurn - Marq-6-0.6-0.0mm
LightBurn – Marq-6-0.6-0.0mm

I generated the plain disk for the bottom by deleting all the inner shapes.

The left and right coasters use LightBurn’s Offset tool to reduce the size of the interior holes on successive layers:

LightBurn - Marq-8-0.40-20.0mm-Layers
LightBurn – Marq-8-0.40-20.0mm-Layers

Although the GCMC version turned out OK, you’ll note it lacks the central disk, as I was unwilling to tweak the code enough to make the disk diameter vary with the kerf width.

Applying the LB Offset tool requires selecting only the inner shapes (it has an option to ignore the inner shapes) and applying the appropriate offset. Because the tool remembers its previous settings, it’s straightforward to step the offset from 1.0 mm to 7.0 mm on successive patterns.

Applying glue (from a glue stick!) to the bottom of each disk, aligning them atop each other, and pressing them together becomes tedious in short order. If I had to do a lot of these, I’d be tempted to add three wings (not at 120° angles!) around the perimeter with holes for pegs, then stacking the layers in a fixture to ensure good alignment. A polygonal perimeter would simplify trimming the tabs.

Spray adhesive might be faster, but each layer would have sticky edges and the finished coaster would become a dust collector par excellence.

I like the overall effect, but …

The OpenSCAD source code as a GitHub Gist:

#!/bin/bash
# Layering paper cutouts
# Ed Nisley KE4ZNU - 2022-08-21
Flags='-P 4 --pedantic' # quote to avoid leading hyphen gotcha
SVGFlags='--svg --svg-no-movelayer --svg-opacity=1.0 --svg-toolwidth=0.2'
# Set these to match your file layout
ProjPath='/mnt/bulkdata/Project Files/Laser Cutter/Coasters/Source Code'
LibPath='/opt/gcmc/library'
ScriptPath=$ProjPath
Script='Marquetry Layers.gcmc'
[ -z "$1" ] && leaves="6" || leaves="$1"
[ -z "$2" ] && aspect="0.50" || aspect="$2"
[ -z "$3" ] && center="0.0mm" || center="$3"
numlayers=8
sashmin=2.0
sashstep=2.0
sashmax=$(echo "$sashmin+$sashstep*($numlayers-1)" | bc)
echo min: $sashmin step: $sashstep max: $sashmax
for sash in $(seq $sashmin $sashstep $sashmax) ; do
fn=Marq-$leaves-$aspect-$center-S$sash.svg
echo Output: $fn
gcmc $Flags $SVGFlags --include "$LibPath" \
-D "NumLeaves=$leaves" -D "LeafAspect=$aspect" -D "CenterDia=$center" \
-D "Sash=${sash}mm" \
"$ScriptPath"/"$Script" > "$fn"
done
view raw layers.sh hosted with ❤ by GitHub
// Marquetry Layers
// Ed Nisley KE4ZNU
// 2022-08-21 layered paper test piece
layerstack("Frame","Leaves","Rim","Base","Center","Tool1"); // SVG layers map to LightBurn colors
//-----
// Library routines
include("tracepath.inc.gcmc");
include("varcs.inc.gcmc");
FALSE = 0;
TRUE = !FALSE;
//-----
// Command line parameters
// -D various useful tidbits
// add unit to speeds and depths: 2000mm / -3.00mm / etc
if (!isdefined("OuterDia")) {
OuterDia = 120.0mm;
}
if (!isdefined("CenterDia")) {
CenterDia = 20.0mm;
}
if (!isdefined("NumLeaves")) {
NumLeaves = 8;
}
if (!isdefined("Sash")) {
Sash = 4.0mm;
}
if (!isdefined("LeafAspect")) {
LeafAspect = 0.50;
}
// Leaf values
LeafStemAngle = 360.0deg/NumLeaves; // subtended by inner sides
LeafStemHA = LeafStemAngle/2;
LeafOAL = OuterDia/2 - Sash - (Sash/2)/sin(LeafStemHA);
LeafWidth = LeafAspect*LeafOAL;
L1 = (LeafWidth/2)/tan(LeafStemHA);
L2 = LeafOAL - L1;
// message("Len: ",LeafOAL," L1: ",L1," L2: ",L2);
LeafTipHA = to_deg(atan(LeafWidth/2,L2)); // subtended by outer sides
LeafTipAngle = 2*LeafTipHA;
// message("Width: ",LeafWidth);
// message("Tip HA: ",LeafTipHA);
LeafID = CenterDia + 2*Sash;
LeafOD = LeafID + LeafOAL;
// message("ID: ",LeafID," OD: ",LeafOD);
// Find leaf and rim vertices
P0 = [(Sash/2) / sin(LeafStemHA),0.0mm];
m = tan(LeafStemHA);
y0 = -(Sash/2) / cos(LeafStemHA);
if (CenterDia) { // one sash width around center spot
a = 1 + pow(m,2);
b = 2 * m * y0;
c = pow(y0,2) - 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];
if (FALSE) {
message("a: ",a);
message("b: ",b);
message("c: ",c);
message("p: ",xp," n: ",xn," y: ",y);
}
}
else { // force sharp point without center spot
P1 = P0;
}
P2 = P0 + [L1,LeafWidth/2];
P3 = P0 + [LeafOAL,0mm];
P4 = P3 + [Sash/sin(LeafTipHA),0.0mm];
P5r = P4.x * sin(LeafTipHA) / sin(180deg - LeafStemHA - LeafTipHA);
P5 = rotate_xy([P5r,0.0mm],LeafStemHA);
P6 = rotate_xy(P4,LeafStemAngle);
t2 = pow(tan(-LeafTipHA),2);
a = 1 + t2;
b = -2 * t2 * P4.x;
c = t2 * pow(P4.x,2) - pow(P3.x,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 - P4.x)*tan(-LeafTipHA);
// message("p: ",xp," n: ",xn," y: ",y);
P4a = [xp,y];
P6a = rotate_xy(P4a,LeafStemAngle - 2*atan(P4a.y,P4a.x));
if (FALSE) {
message("P0: ",P0);
message("P1: ",P1);
message("P2: ",P2);
message("P3: ",P3);
message("P4: ",P4);
message("P4a: ",P4a);
message("P5: ",P5);
message("P6: ",P6);
message("P6a: ",P6a);
}
// Construct paths
LeafPoints = {P1,P2,P3,[P2.x,-P2.y],[P1.x,-P1.y]};
if (P0 != P1) {
StemArc = varc_ccw(P1 - [P1.x,-P1.y],LeafID/2);
StemArc += [P1.x,-P1.y];
LeafPoints += StemArc;
}
RimChord = length(P4a - P6a);
RimThick = OuterDia/2 - Sash - length(P5);
RimPoints = {P4a,P5,P6a};
RimArc = varc_cw(P4a - P6a,P4a.x);
RimArc += P6a;
RimPoints += RimArc;
//--- Lay out the frame
linecolor(0xff0000);
layer("Frame");
if (CenterDia) {
goto([CenterDia/2,0mm]);
circle_cw([0mm,0mm]);
}
repeat(NumLeaves;i) {
a = (i-1)*LeafStemAngle;
tracepath(rotate_xy(LeafPoints,a));
}
repeat(NumLeaves;i) {
a = (i-1)*LeafStemAngle;
tracepath(rotate_xy(RimPoints,a));
}
linecolor(0xff0000);
goto([OuterDia/2,0]);
circle_cw([0mm,0mm]);

Acrylic Coasters: Edge Finishing, Round 4

Lacking a 4-jaw chuck for the lathe, this should suffice:

Coaster Epoxy Rim - chuck-in-chuck setup
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
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
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
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:

Coaster Epoxy Rim - turned 6 petal mirror
Coaster Epoxy Rim – turned 6 petal mirror

This one has a 3M double-sided tape with low-surface-energy adhesive layers between the mirror and the fluorescent blue frame + petal, with epoxy between the top layer and the frame.

Coaster Epoxy Rim - turned 4 petal
Coaster Epoxy Rim – turned 4 petal

If I never tell anybody, they’ll think the slightly granular look if 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
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.