Tag: Improvements

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:

The trick is to scan the bottom of the cap to get a high-contrast image:

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:

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:

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:

Mary pronounced it better looking than the 3D printed version, which I agree clears a rather low bar, but it suffices for the job.

The LightBurn layout as a GitHub Gist:

After going through the LightBurn camera alignment / calibration process, I thought it would be interesting to see how well the corrected image matches the design grid.

Burn some holes and draw lines 10 mm in from the physical corners, like this:

Burn holes and lay in a 10 mm grid at the center point:

The center grid as seen through the camera:

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.

In any event, a few millimeters works for most purposes, even if you’d want to verify the alignment for critical operations before firing the laser in earnest.

Going through the LightBurn lens calibration and camera alignment routine produces an orthographic view of the laser platform from a camera with an ordinary lens perched somewhere above it.

Early on, I stuck a camera to the lid of my OMTech 60 W laser:

The uncorrected view from the camera (through VLC):

After calibration and alignment, LightBurn underlays this view of the platform behind the workspace:

The correction depends critically on the camera maintaining its position / orientation / focus, which turns out to be a bad assumption for the camera I’ve been using, because the (metal) focus locking screw binds directly on the (metal) lens threads. This works, until vibrations slightly loosen the screw and the lens shifts ever so slightly.

After noticing the focus had shifted again, I tucked a snippet of silicone insulation from some 30 AWG hookup wire into the screw hole to compress against the lens thread, then re-did the entire sequence with some attention to detail.

Pulsing the laser in each corner produced pinholes exactly 700×500 mm apart. One diagonal is 859.0 mm and the other is 861.5 mm, pretty close to the ideal 860.2 mm.

Next, to measure the offsets from some known positions …