Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Category: Software
General-purpose computers doing something specific
After the first two snowflake coasters, it finally penetrated my thick skill that putting a 1 mm hole in the flake cut from the center of the plywood would convert it into a decorative window hanging:
Snowflake Hanger – plywood
Admittedly, I may be using the word “decorative” in a manner you had not previously encountered, but work with me on this.
Cutting a similar flake from transparent acrylic looks better:
Snowflake Hanger – blue acrylic
Transparent acrylic turned out to be, well, too transparent, so I set up a LightBurn layout to “engrave” a light frosting on the flake before cutting it out:
Snowflake Hangers – engraving in situ
That worked for all subsequent flakes, but I had to do something about the first few flakes. After realizing that the time to engrave an object depends only on its width, I set up a rectangle with the proper parameters, snugged two forlorn flakes next to each other, and fired the laser:
Snowflake Hangers – retroactive engraving
I thought using cardboard was a Good Idea™ for a stable backing, but lightly vaporizing the top layer produced an unbelievable amount of filth:
Snowflake Hangers – frosted
I had to scrub those poor flakes with dish detergent and a toothbrush to get them even close to their former pristine state; the blue one may never recover.
Anyhow, frosted flakes look good if you don’t look closely:
Snowflake Hangers – frosted
The grid pattern comes from the window screen in direct sunlight; the vertical bars are DIY BirdSavers.
The LightBurn layout produces 120 mm coasters to fit my 20 ounce mugs:
Snowflake Coaster 120 mm – LB Layout
You get two hanging flakes: one plain plywood and one frosted acrylic!
The rattlecan chipboard coasters having passed their Best Used By dates, I figured a more durable seasonal version was in order:
Snowflake Coasters – overview
I laid out the design with the intent of cutting an acrylic snowflake with a bit of compensation to fit snugly into a plywood background:
Snowflake Coaster – LB layout
At the last moment I realized I could just cut two of the patterns on the left, swap the snowflakes, and get two coasters with very little scrap:
Snowflake Coasters – detail
Mary thinks the gap between the snowflake and the background looks OK. I’m not convinced, but studying the results suggests applying enough kerf compensation to close the largest gaps would results in the rest of the flake not fitting into its socket. Plus, of course, you’d have more scrap.
Embiggening the small dagger-shaped pieces around the center would be an improvement. Perhaps cutting those as a separate operation after arranging them in a corner would work.
Protip: Align the grain in those daggers with the rest of the plywood, because It Will Be Very Obvious if you don’t.
Applying a nice wood stain / finish to the plywood, perhaps before cutting it out, would certainly improve the result.
Invisible on the bottom: self-adhesive cork disks eliminating the need to glue the pieces to something else. I had thought of a blank plywood or MDF disk, but came to my senses just in time.
The original SVG fell with a blizzard from one of the many SVG snowflake generators out there. Because LightBurn uses only the stroke centerlines of SVG images and ignores the stroke width, it required some tweakage before becoming a coaster.
After saving an SVG flake from the blizzard, fire up Inkscape:
Import the SVG file
Center it in whatever page you’re using
Ungroup the flake from the frame (if it has one)
Delete the frame to leave only the flake
Select the flake
Invoke Path → Stroke to Path
Save as an SVG image under a new file name
Then fire up LightBurn:
Import the tweaked SVG file
Assign a layer with line (rather than fill) parameters
The rattlecan chipboard versions came out pretty well, because I’ve already explored much of the error space. The two-tone berries and leaves received Sharpie contrast touchup. They’re all in constant use on the kitchen table!
The wood veneer version over on the right looks surprisingly good (in person, anyway) for being a complete faceplant. The narrow sections suffered severe burning and fell apart where the grain runs perpendicular to the cut. The highlight spots for the berries fell through the honeycomb table and vanished in the chaff. Sanding the whole mess flat scuffed off most of the smudges, although I couldn’t bring myself to slather it with polyurethane.
The bright holly on the left is mirror-back acrylic pressed into a 3 mm deep (!) recess engraved in more scrap paneling:
Holly Coaster – mirror
I hand-painted the colors by scribbling Sharpie onto areas where the mirror backing was engraved away. A sheet of aluminized Mylar below the acrylic reflects some light back through the colors to make them slightly brighter.
As I recently learned, applying alcohol to laser-cut acrylic produces almost instantaneous stress-cracking, which accounts for the decorative crackle finish around the perimeter:
Holly Coaster – mirror flaws
The surface flaw beyond the berry over on the right apparently came from an acrylic fume explosion in the honeycomb below it, strong enough to torch the protective plastic film. Given that I was starting with a scrap mirror fragment, I didn’t perch it up on spikes, which is pretty much required to prevent such events.
The wood coasters have mmmmm excellent upside potential, but it’s obvious I have not yet mastered my craft.
Mary made a neck cozy based on a Craftsy description using a pattern I’d extracted from the low-res photo and rescaled to the proper size. The perimeter came out grittier than I like, but the laser had no trouble chopping it from a sheet of paper, and she cut smoothly around the lumps:
Neck Cozy mask – right
It looks better on her, but you get the general idea:
Neck Cozy – rev 1 finished
After some experience, however, she wanted to lengthen the top and bottom seams to improve the way it draped, which meant I had to modify the extracted pattern. The original pattern in the Craftsy photo was hand-drawn (which is perfectly fine) and the two halves were of two different sizes; we decided to work with the left half and produce a symmetric result.
Rather than fiddle with the bitmap, I drew a rectangle around the outside of the left pattern mask, converted it to a path, moved the nodes to key locations, then fiddled with the control points to lay the splines along the perimeter:
Neck Cozy pattern – LB splines
Removing the bitmap makes the splines much more obvious and shows the much smoother perimeter:
Neck Cozy pattern – LB extended splines
Adding a few nodes to change the splined shape is much simpler than fiddling with the bitmap, particularly when tweaking their position as directed by someone who knows how the fabric will eventually take shape.
Add fiducial marks and a label, duplicate the shape, mirror it to get the other half, and fire the laser:
Neck Cozy pattern – as cut right
Fabric cutting isn’t in my wheelhouse, but I made the offer …
Mary is putting together a dogbone headrest pillow for a friend who will be spending a lot of time in a chair. It’ll be similar to this one from a while ago:
Dog Bone Pillow
She used Bonnie Browning’s pattern and I offered to laser-cut it as practice for other projects I have in mind.
It eventually worked out well enough:
Dog Bone Pillow – cut pattern
Fold a piece of fabric in half, align the pattern’s bottom edge with the fold, cut around the perimeter, make two more, and sew ’em together.
My first mistake was attempting to assemble the two halves of the pattern from the PDF document into a bitmap image using The GIMP:
D0G-BONE pattern – rejoined
That is both tedious and unnecessary, as I found out while trying to align the pieces.
The end goal is a simple and symmetric vector path defining the outline, including a line across the bottom, suitable for laser cutting. Rather than assembling an image, tracing it into a bunch of vectors, then cleaning up the mess, just lay a smooth spline vector path around half of it and invoke symmetry, much as happened with the Lip Balm Holder.
So import the slightly misaligned bitmap into LightBurn, draw a rectangle over just the left half, convert the rectangle to a path, then add a few nodes anchoring the splines to key points of the image:
Dog Bone Pillow – LB half pattern first splines
Although it’s not visible, the top and bottom spline nodes defining the vertical line down the middle are not quite vertically aligned, even though I dragged them to the middle of the pattern. Unsurprisingly, the bitmap image is not exactly aligned with the axes, even though the conversion from PDF to bitmap is entirely digital; the original design may be off by an itsy that would never matter for its intended application.
Tweak the splines / control points, add a few more nodes, and in short order the vector path runs pretty nearly along the middle of the bitmap image:
Dog Bone Pillow – LB half pattern overlay
Rather than trying to draw the second half just like the first half, duplicate the path and mirror the copy left-to-right to get the right half of the pattern. Grab the lower-left corner of the copy and snap it to the lower-right corner of the original, whereupon you will find the two points at the top of those lines don’t quite line up.
This is a grossly zoomed look at the top center, with the two red angles showing the two halves not quite meeting in the middle:
Click on the center point to define the Rotate center
Zoom way in on the top center
Click-n-drag the right corner to snap it onto the left corner
Done!
What just happened is that the right half now directly adjoins the left half, with the upper and lower center points overlapping.
Invoke the node editor and delete the center lines from both halves, leaving just the (overlaid) top and bottom nodes. Select both paths, then invoke Edit → Auto-join selected shapes to merge the two halves into one:
Dog Bone Pillow – LB splines
I missed the clip line in the middle of the top, but that’s why the first version is always a prototype.
This was easy, but it’s good to stay in practice …
Loading the bike batteries into the Rolltop Cupcake Box reminded me I hadn’t updated the Lip Balm Holder around the latest tube of sunscreen. My excuse was I didn’t quite know how to model the not-quite-elliptical shape of the Coppertone sunscreeen tube in OpenSCAD, but now I can bypass that whole problem:
Lip Balm Holder – installed
The trick is to scan the bottom of the cap to get a high-contrast image:
Coppertone Sport Tube – lid scan
Import the image into LightBurn, draw a circle tangent to the outside of the cap’s smaller diameter, turn the circle into a path, drag the nodes and twiddle the control points to create a symmetric shape just outside the cap, then outset the result by 1.5 mm for clearance around the tube:
Coppertone Sport Tube – LB splines
That 3 mm of wiggle room lets us drop the tube into its socket without careful alignment.
The lip balm tubes all fit into 18 mm circles requiring no special design skillz:
Lip Balm Holder – LB layout
The mid-left oval goes around the Coppertone tube.
The top-mid drawing shows the 3 mm outset around each of the pieces, with the smaller tubes arranged to put their midlines tangent to each other and the oval tube. LightBurn does not, as far as I can tell, have a direct way to align a shape tangent to two other shapes at the same time, but iterating at increasingly absurd zoom levels gets the job done fairly quickly.
Welding those shapes together produces the top-right drawing, which serves as the template for the lower set of layers.
Deleting the inner details produces the mid-right blob for the bottom layer.
Most of the layers come from 3 mm plywood, with edge-lit acrylic on the top and bottom surfaces:
Lip Balm Holder – side view
Mary pronounced it better looking than the 3D printed version, which I agree clears a rather low bar, but it suffices for the job.
Burn some holes and draw lines 10 mm in from the physical corners, like this:
LB Camera Cal – corner target
Burn holes and lay in a 10 mm grid at the center point:
LB Camera Cal – center grid
The center grid as seen through the camera:
LB Camera Cal – center grid overlay
That’s after adjusting the X and Y offset to align the center of the imaged grid with the center of the design grid. That’s using the non-faded image to make the target lines more visible.
The corner markers don’t quite line up with the grid, but they’re not off by much (using the faded image to make the grid more visible):
You could, of course, split the difference among all five sites, but I think having the middle of the platform be more accurate than the far corners makes more sense.
The Smell of Molten Projects in the Morning 
