Rectangle doesn't fit: Saving a misaligned double-sided engraving

[Purplie]

The simple trick of cutting out a rectangle and flipping it, to align a double-sided engrave, is an essential technique.

Glowforge Interface - How to Align a Double Sided Engrave

I’m going to demo an alignment process below using my (ancient) version of Adobe Illustrator, but the process is the same for Inkscape or CorelDraw or Affinity Designer. Whichever one you like to use, you’ll do the same things, with the tools in that program. (I’m also going to give you the thought process that goes into creating a design with 2D Design software, to keep from having to write another one up, so if you can slog through it, you’ll pick up some stuff you need to know to make thing…

What if the rectangle doesn’t fit when you flip it over?

I ran into this problem. This rectangle wouldn’t fit, no matter what I tried. It was a parallelogram, not a rectangle! You can see a 2mm gap on the top left. (On the bottom left, it overlaps the frame by 2mm.)

The cause was that the Glowforge’s horizontal arm was slightly askew, not at perfect right angles to the side rails. I had just spent 5 hours engraving the first side, and a 2mm error wasn’t within my tolerance. A disaster!

What to do?

For my project, the slight distortion of shape was acceptable, but I needed precise alignment of cuts on the second side to the engraving on the first side.

First, I didn’t touch anything. I didn’t turn off the Glowforge, and I didn’t realign the horizontal arm (yet).

Next, I placed the rectangle as you see it above: The right-hand edge was set flush with the frame, and the left-hand edge was out of place. I used calipers to measure the error (2mm).

Then, I reopened the vector file in my editor (Affinity Designer), and applied a compensating skew to the design. I dragged the left-hand side down by 2mm, like this (exaggerated):

Consult your editor’s manual for how to do this.

I saved the change, opened it in the Glowforge interface, and aligned the right-hand edge to its original location. It worked out fine; the cuts on the second side were aligned with the engraving on the first side. (The slight distortion of shapes was within acceptable limits.)

[evansd2]

Do you think flipping the design in place in the GF ui and cutting the rectangle again would have worked?

You could cut the flipped outline out of the same surround material or you could insert something like cardboard and cut a new pocket jig out of that.

Seems like it would work?

[Purplie]

I don’t think it would have sufficed, because no matter how you reposition the physical piece, the geometry of the second cut still needs a compensatory skew in the direction opposite to the skew of the first cut.

[evansd2]

Yeah, good point.

[rbtdanforth]

@Purplie

the geometry of the second cut still needs a compensatory skew in the direction opposite to the skew of the first cut.

I was thinking about that and figured to take @evansd2 's thought one step further and put a bit of scrap underneath, and balancing the overlap, rerunning the cut with a manual height matching the new height, that would take one mm off each corner and it would now fit in the hole.

[rvogt]

Great save on the project!