Category: Machine Shop

With all the layers laid out, duplicating and plunking one in the middle of the LightBurn workspace looks like this:

In Absolute Coordinates mode the laser will cut it at the center of the platform, so put a sheet right there:

The Position Laser tool puts the laser head at a selected location in the design, with the picture showing it at the center of the upper right target. The trick is to slide the paper on the platform to put that target directly under the red-dot pointer:

I occasionally fire a test pulse to verify the CO₂ laser hits the red dot:

Do the same for the target in the lower left corner, rotating the paper around the upper right target, and iterating until both targets match.

Because the printer does not produce accurately sized output, this can happen:

You’ll get a hint of the problem when it’s impossible to put the targets under the the laser head at the lower targets, although it may look close enough to work.

That offset was on plain paper with the output scaled by 100.3% vertically. Applying the same scaling (to the Happy Bird picture, but with the same page template) and printing on Art Cover paper produces this result:

Eventually, I can find a suitable scale factor to make the printed image come out right and the cuts align properly:

Those tenuous paper strands show why I couldn’t use the Letter page fixture and had to cut the sheets directly on the honeycomb: the weight of the uncut sections distorted the remaining sheet beyond belief. The squat silver cylinders in the pictures are neodymium magnets holding the paper flat on the honeycomb.

All the other layers have trivial positioning, because the cuts need not line up with anything on the sheet:

Just frame the first layout, center the sheet around it, set the MDF stops to kiss the edges of the paper, and all the other sheets will come out just fine.

One side effect of all this cutting is the happiest scrap bin ever:

With all the layers cut out, assemble them face-down in the gluing fixture, align the black mask on the front by hand, put the stack into a suitable frame, and it’s ready to show off:

That wasn’t exactly easy, but next time I can make different mistakes.

Processing the original PDF of a Cornell Painted Warbler produces a PNG image:

And an SVG file with shapes matching the regions to be cut out:

Both of those derive from the same PDF page, cropped to emphasize the bird, then scaled to match the 172×220 mm rectangle in the middle of the LightBurn template corresponding to the alignment fixture:

A key point: the blue outline around the image must be centered within the red outline marking the sheet cutout, because that dramatically simplifies the overall alignment process.

Protip: Crop the image in GIMP with a fixed aspect ratio of 172:220 before scaling the result to match the 172×220 frame, then derive both the PNG and SVG from the result.

The alert reader will recall the SVG shapes exclude the 6 pixel = 0.5 mm border closing the outlines, so the overall SVG size is 1 mm smaller than the PNG image. This will not be a problem, for reasons to become obvious.

Because Inkscape can handle both SVG shapes and color PNG images (LightBurn turns them into grayscale):

• Export the LightBurn template as an SVG
• Open with Inkscape
• Make the Tool layer elements visible
• Import the PNG

Without further ado, the PNG is automagically centered in both the blue and red rectangles:

Print it, measure the vertical and horizontal distances between the corner target centers, scale the overall image to make the printed result come out exactly right, then iterate until satisfied. Scaling it 100.3% vertically worked for what’s called Art Cover paper printed with Stationery High Quality in an Epson ET-3830 printer. Your mileage will be different.

Protip: Save the unscaled page as an Inkscape SVG, because the scale factor(s) depend on the paper / print quality setting / moon phase and you will print it several times under different conditions.

In LightBurn:

• Tweak a copy of the sheet template to eliminate the corner holes & suchlike
• Import the Black Mask SVG into LightBurn
• Drag-n-drop it at the center of the template
• Set the speed / power to cut black Art Cover paper

Which becomes the Black Mask layer:

The alert reader will note the slightly different template layout: it’s been undergoing continuous process improvement and I’m not re-capturing all the details.

The Black Mask SVG contains all the shapes-to-be-cut, so it forms the basis for all the sheets under the top mask. Drag-n-drop a copy of the Black Mask SVG at the center of a copy of the template, ungroup it, delete the outlines of all the shapes on the printed layer, and set the speed / power for ordinary Art Cover paper:

The shapes on each layer will be the same as the one above it, minus whatever shapes have the color of that layer. After you finish each layer, select all its shapes, duplicate them, drop them at the center of the next sheet template, and iterate.

Or just set up a line of templates spaced (say) 240 mm apart using the Grid Array tool:

If you’re so inclined, set up their binary layer number holes:

Then:

• Select the finished shapes in one layer
• Duplicate them (Ctrl-D)
• Use the Numeric Edits toolbar to subtract 240 mm from their current location
• Which causes them to appear exactly centered in the next template to the right without any drag-n-drop action

Because my CO₂ laser has a Ruida controller and homes in the rear-right corner:

• X coordinates increase to the left
• Y coordinates increase to the front

You can get used to anything if you do it enough and your coordinates will likely be different.

Protip: Adding a description of each layer’s contents / color will be of great assistance when you select the paper.

The end result of all that will be a set of sheet layouts ready to cut:

• Select one entire sheet template + bird shapes
• Duplicate it with Ctrl-D
• Center the dupe on the LightBurn workspace with P (not Ctrl-P, which will send it to your printer)

The next piece of the puzzle: correctly aligning a Letter piece of paper with those shapes …

After getting better at aligning printed sheets for laser cuttery and learning how to use GIMP’s Path Edit tool, a pair of Cornell Painted Warblers emerged!

We call this one “Happy Bird”:

And his companion “Pensive Bird”:

The page numbers refer to the PDF file containing the outlines of all six birds.

The wing feathers and butterfly require finer resolution than the black mask layer allows, so the first layer is printed on white paper. I put the berries and nut-like objects on that layer, too, mostly to reduce the number of layers.

After considerable fumbling, it turns out GIMP provides a straightforward way to create the outlines required for the top Black Mask, from which all the other layers descend.

A closeup of Happy Bird’s head as imported into GIMP from the PDF at 300 DPI = 11.8 pixel/mm:

Because the Cornell Warblers were intended as a paint-by-number project, the PDF has (gray) color numbers in (most of the) regions. Selecting the numbers with GIMP’s Select by Color tool and hitting Delete clears the region for Bucket Fill colors copied from the key sheet with the Color Picker tool:

I cropped the images to emphasize the birds and scaled the result to fit the 8×10 inch frame size. The picture is 172×220 mm within the red 200×250 mm frame cut from Letter size sheets of Art Cover paper stock. The corner targets come into play while aligning the paper with the laser pattern.

Creating the Black Mask layer also starts from the cropped / scaled image. Select the black lines by color, then Grow the selection by 7 pixels = 0.6 mm to make the selection about 1.5 mm wide:

Use Quick Mask to edit the selection. For example, I reduced the eye to a cutout through the mask revealing the white spot on the printed layer:

Eventually the selection will neatly bracket the borders between the colors:

The black legs and feet must be edited down to their original size, as became obvious on the first iteration:

The fixed-up Quick Mask looks like this:

The mask includes a 6 pixel = 0.5 mm border line around the perimeter of the image which serves as the outer edge of the cut-out areas and helps match the selections to the image.

The process of growing & tweaking the selection will create orphan pixels:

Each orphan pixel becomes a tiny triangle when imported into LightBurn:

You’ll first notice those when the laser hesitates while cutting the outlines, which may overcook the adjacent paper. I cannot reliably detect these other than by watching the laser, because a single pixel is simply too small to see on-screen at normal zoom sizes. You can delete them in the LightBurn file or, if you prefer a single source of truth, in GIMP to regenerate the whole thing.

The end result of all this is a single, rather complex, Selection that you must convert into a Path, which you then export to an SVG file and import into LightBurn for laser cuttery.

Next: Aligning and layering