CNC engraving LEGO bricks using Fengrave

I build a lot of LEGO stuff, and would like to be able to mark them, with names and pictures.
I’ve been trying to use my glowforge cnc laser to do this. ABS doesn’t laser well (and the fumes are horrible) and after a bunch of tries I’ve given up.
I’d been messing about with using 1/16" bits to carve stuff into LEGO bricks, and that kinda worked.
But then I ran across fengrave and man is it the right way to do this. It relies on using a V-shaped bit and moving it in three dimensions, which provides for forming acute angles. (I’ve also used it for doing inlay, which is even more amazing. See the page for examples.)

Workflow: type what you want. In this case I found some kanji I needed. Import that into Inkscape. I had to inset it by 0.6mm in Inkscape to get narrower strokes. (It’s possible to do that in Fengrave as well, but it was harder to get results looking the way I wanted there.) Then resize it so it’s about 80% of the height of a LEGO (9.6mm), save as a dxf, and open that in Fengrave. The default values work well once you’ve changed the diameter to match the bit you’re using, and reduced the depth of cut to a max of 1.5mm (just less than the wall thickness of a LEGO.)

What I’m doing is sticking a piece of scotch tape over the brick, clamping it in a vise with soft jaws and a parallel beneath it, scribing the center point by marking diagonals with a fine tipped pen, and using that as the effective 0,0,0 point. LinuxCNC shows me the extents of the cut path, so I can touch off half the difference, which is to say, if it shows the path extents in X as 0.22 to 0.88, I take the difference (0.66) and divide it in two (0.33) and add that to the X to get 0.55, and touch off X to 0.55. Then I have the engraving as centered as I can manage based on the accuracy of the diagonal scribes.

Engrave it at fairly high spindle speed (but not really high: you don’t want to melt the ABS through rubbing, just set up a decent chip load) then spray with black spray paint, then peel off the tape and scrub the brick on a new sheet of 400 grit sandpaper briefly to clean up the bleed under the tape or anywhere the tape got pulled off during engraving.

The tape minimizes how much paint you have to remove. The paint gums up the sandpaper, which then rubs it into the brick and makes the surrounding area look muddy. Hence, tape and new sandpaper.

I’m using a 3/8" bit with a 90 degree angle, about 1500 RPM, with a feed of about 12 mm/second.

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Very nice @John_Bump! Thanks for sharing the whole technique as well as the picture. :slight_smile:

@Scorch, see nice work with Fengrave here! :slight_smile:

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Final result looks really good. Persistence and experimentation paid off.

(But where can I learn more about your cool looking geared ring?)

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It’s my wedding ring: kinectdesign.com. They also have an equally cool necklace. I’ve explained how differentials and slide rules work using it.

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