The Smell of Molten Projects in the Morning
Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Mechanical widgetry
Given the geometry of the Mariner’s Compass block:
Applying increasing insets to that top level produces a beveled result:
Seen from a distance, you need oblique light to make any sense of it:
It’s made from random sketch paper, rather than cardstock, and we all know how much the paper matters.
Time spent making a fixture is never wasted time:
The general idea: securely hold a piece of paper flat while cutting it, so that it cannot move or warp, while letting the cut pieces fall out without snagging on anything underneath. The sheet holder I made a while ago worked reasonably well, but those thin metal blades tend to warp while cutting small patterns in restricted areas and the pieces definitely don’t fall free.
The simple fixture I use while assembling the paper layers consists of four rivnuts poking through a chipboard upper layer, with a craft paper layer around the rivnut washers on the bottom:
The cutting fixture uses a similar layout around a hole for freely falling chips:
Next time, I’ll remove those three bars across the hole, because the MDF doesn’t need any support. Nearly all the chips fell out, so the fixture worked as intended.
I trimmed the flange off the rivnuts so they would sit flat on the MDF:
That’s the kind of job chuck stops really simplify.
The cutting fixture requires pre-cutting paper into 200 mm squares with four 5 mm corner holes, which can be done three-abreast on the platform bars, then putting each sheet in the fixture to cut the shapes. That’s not much of a disadvantage compared to messing up an unsupported sheet.
The cutting fixture has crosshair targets to align a LightBurn template using Print-and-Cut, thus eliminating the need to precisely locate the fixture on the platform. The finger-crushingly strong neodymium bar magnets do a fine job of holding the MDF in place on the steel platform.
The small cutout rectangle in the lower right corner frames the sheet number, done in binary code with 0 = 1 mm circle and 1 = 2 mm circle:
That’s the underside view of a completed stack with the 5 mm lower-right fixture hole on the left and the code for layer 11 = 0b1011 reading backwards. The small 0 holes have two lobes showing the Print-and-Cut alignment was off by maybe 0.3 mm; the off-center hole was in the blank sheet.
Obviously, cutting tiny circles with a big laser at 300 mm/s doesn’t produce perfect results. You can see small wiggles in larger shapes:
Unless you’re trying hard to find a problem, you’ll never notice them.
The Chimney Swallows block from page 128 of Beyer’s book:
The tool (blue & orange) and top cut (red) layers:
The long radial blue tool lines simplified selecting them when mirroring / duplicating the cut polygons around their symmetries. The orange tool circles aligned various midpoints / vertices / features during construction.
The inward curve along the outer edge started as a triangle with a node at about the middle of the curve. Deleting that node left the remaining two sides overlapped, but dragging one of them to match the curve worked OK. There’s probably a better way.
That curve defines the outer edges of the shapes along it, so I drew polygons from the corner intersections and dragged the outer edge to match the curve at high zoom.
The shape remains selected after dragging the side, which meant I could immediately apply a 1 mm inset to create the cut lines.
To my surprise, the swallow bodies are straight-sided polygons!
After taking advantage of all the symmetries, knock out the shapes defining each layer:
The swallows look like F-117 Nighthawks to me:
Maybe I have the colors wrong:
Fly away!
Starting from the Pyrotechnics quilt block on page 132 of Beyer’s book:
It looks more like flowers than fireworks to me, but there’s no accounting for taste.
Deploy enough 2 mm circles to catch the flower’s radial symmetry:
During the process of building the layout, a big circle positioned the cups at the base of the flowers, another delineated the joint between the cups and the petals, and more little circles caught the intersection of those circles with the petals. All that was for visualization and positioning, as you only draw one flower shape, then duplicate it around the pattern.
Although the cups and petals are surely circular arcs, it’s easier to draw a closed line triangle around the intersections, then pull the midpoint of a line into an arc (Bezier curve!) matching the pattern Closely Enough™ at high zoom. Because the arcs end at the intersection points based on circular arrays of points, they’ll all match up when they’re duplicated around the pattern; in fact, you need only one side of one petal, mirror it around the midline, and away you go.
Then the magic happens:
Which is easier to see without the original shapes:
Pick one of the closed shapes, apply the Offset tool to shrink it by 1 mm, duplicate as needed, and you get the outlines of the regions to cut with 2 mm between them. Plunk those shapes on a cutting layer, add the outer frame with locating holes for the fixture, and it’s ready to cut the top layer from black paper:
Knock out the cuts for each sheet of paper in the stack:
Then Fire The Laser™:
That was a nearly random selection of colors, but it’s hard to go wrong.
IMO, a frame makes it look even better:
This could be Art.
The Mariner’s Compass pattern on page 133 of Jinny Beyer’s The Quilter’s Album of Blocks and Borders:
Becomes a laser-cut layered paper design:
A face-on view with different colors:
This seemed like an appropriate use for the stack (well, several stacks) of colored paper I’ve accumulated over the years.
The illustration in the book is apparently a photograph of a quilt block Beyer put together, so I had to reverse-engineer the Platonic Ideal Block from the image:
Fortunately, after a bit of fiddling around, I could take advantage of the radial symmetry to duplicate most of the fundamental shapes, so producing the layout really wasn’t all that difficult:
The blue tooling lines (upper left) run along the centers of what would be seams in a fabric block, with 2 mm circles defining the endpoints for ease in snapping the lines.
This being the first block I attempted, I did it all wrong. LightBurn can form the convex hull over a group of shapes, so I just selected pairs of circles, created the hull, and iterated for the minimal shapes required to generate the whole design. That produces the basic layout, but what you really want is the collection of shapes between those hulls that define the actual cutouts, which appears in the lower left image.
Don’t do it that way, as explained tomorrow with a different block.
With all the shapes in hand, you duplicate them for all the paper layers you need, removing the shapes corresponding to the color of each sheet. Sheets lower in the stack have fewer cutouts, with the pattern in the lower right being second from the bottom.
The four holes in the corners fit over rivnuts in a fixture aligning the sheets in a tidy stack:
Yes, that’s a blooper sheet.
All in all, it’s easier than I expected to get nice results.
What else would you call it?
It’s a test piece with adhesive sheets between acrylic layers:
From top to bottom:
The pair of 2.4 mm layers add up to just an itsy more than the 4.8 mm thickness of the shattered glass atop the mirror. Unlike previous epoxy sealed coasters, the glass sits on a sheet of 3M LSE adhesive film to keep the pieces together, with the top 1.5 mm acrylic layer containing any slivers. Because there’s no epoxy involved, the project is finished with no muss, no fuss, no curing time, and no drippy edges.
The geometry comes from a scan of the glass piece:
That’s the GIMP Quick Mask result of manually drawing around the perimeter with the center of a 25 pixel diameter pencil, thus creating a 12 pixel gap to ensure the glass will fit inside the cut shape: at 300 dpi, the 12 pixel gap is about 40 mil = 1 mm wide. Slightly less would work as well, although I’ve discovered some of the glass cuboids have non-vertical walls sticking out to the side above the scanner’s depth of field.
Scribbling over the interior with a bigger pencil clears it out and a few fill operations produce a binary mask perfectly suited for LightBurn’s Image Trace tool:
Trace that outline into vectors, throw away the mask, and use the outline for a conformal cut.
The rings of acrylic and adhesive are 3 mm wide, generated from the outline by offsetting it 3 mm outward:
The tooling circle around the perimeter simplifies drag-and-drop alignment, because the geometric center of the perimeter shape isn’t quite in the middle of where you’d (well, I’d) want to align it. Grouping the outline with the circle keeps the center snap point where it should be.
Narrow rings of adhesive sheet turned out to be even more unmanageable than I expected. Perhaps a better way:
The “adhesive tape sheets for craft” are paper-based, rather than a plastic film, and are neither transparent nor durable. I used it mostly to get an idea of how well it sticks to acrylic, as it’s primarily intended for paper crafts.
The 3M LSE backing layer is plastic and the sheet becomes nearly transparent as the glass squishes down, although you wouldn’t want it on a mirror where you cared about the optical quality of the reflection. Underneath a chunk of smashed glass, it’s just fine.
All in all, it turned out well.
Next: how long does that craft adhesive last in abnormal conditions?
This is definitely not recommended practice:
It’s the sheet of 3/8 inch = 9 mm vintage acrylic that Came With The House™ and what you see is the relatively unwarped result of several months of pressure. The wood sheet in front puts the left edge mostly flat on the platform, with the steel disk weighting the rear end down; the front end sticks out through the laser’s pass-through door.
The general idea was to cut a rectangle that would be flat enough for the shapes I actually needed:
They’re grips for a set of Blockface printing shapes. The alert reader will note the rectangle sits atop three spikes, not the usual four; it was flat enough for the purpose, not actually flat.
The OMTech laser claims to be 60 W, but has an over-amped 50 W tube that’s barely adequate for the task. The cuts required two passes:
Cranking the tube current that high isn’t recommended practice, either, but sometimes ya gotta do what ya gotta do.
All’s well that ends well, though:
I also cut the Blockface shapes from double-faced adhesive foam and stamp pad rubber:
To my utter astonishment, the rubber cuts did not smell like a tire fire and the neighbors did not rise up in arms.
I cut several sets of grips / foam / rubber and sent them off for final assembly. The recipient seems delighted with the results and has been hand-printing up a storm.
The Smell of Molten Projects in the Morning 