121: Anodized Aluminum Moai

When Jen and I were first researching buying a laser, we didn’t plan on buying the one we ended up with. We were looking to get a small, semi-portable solution. But my brother Johnny found a family out in Terre Haute, Indiana, who had owned their laser for about a year and realized they weren’t really using it as much as they’d like and they wanted to sell it. Buying the used laser meant I inherited some of the previous owner’s mistakes—like a damaged lens and rulers that had art engraved on them—but I had plenty of time to cover up those mistakes with my own.

One of the benefits of buying used is that the previous owner didn’t need their material stock anymore either. Bundled with the machine were dozens of small samples of various laser-ready materials, including a piece or two of black anodized aluminum. I dug up one of those old pieces while cleaning out the office, and realized that I hadn’t spent much time engraving on the medium, so I decided to give it a try!

I’ll probably never find out who these two are, but I’m sure they had fun engraving this picture!

This piece of anodized aluminum had a remnant from the previous owner: an engraving test of their own that they didn’t keep. It also had one corner clipped off, so maybe the leftover engraving I have was a test run and they kept a more successful engraving. Either way, the laser is way too low-power to actually cut through this metal, so I’d be limited to engraving the surface. I don’t have the tools necessary to cut this into a better shape, so our two buddies will, for the time being, remain a part of this experiment.

Full color, desaturated, and then inverted so that it would engrave properly.

I settled on a picture of the large moai head Jennifer took some years ago. This picture converted into black and white really well, and as you have to with all materials that engrave from dark to lighter colors, I had to invert the picture so that the laser would fire on white, not on black.

My first engraving test was too powerful: I typically use 35% power to lightly engrave surfaces, but the laser overpowered the black, creating a blown out image. I settled on 20% power—recommended in the laser driver’s settings for this medium—and the contrast was much better. That’s what I get for ignoring Universal’s built in material settings!

Because it only took one pass to get an acceptable result, I decided to use this project to compare a few of the settings available in the driver. When engraving art that has shades of grey, the machine has to dither to convert those to art it can engrave. The machinist can choose to have the laser convert those shades to a patterned halftone, use an error diffusion method, or use a threshold to convert the art to black and white. You can also select from seven different Image Density steps; the lowest step is the quickest and skips the most space on the material between horizontal lines, and the highest takes much longer and leaves very little space between each pass. The difference between those settings is pretty remarkable, but I didn’t want to do seven passes for each, so I decided upon odd numbered settings.

In the end, I made a matrix of engraving settings: each row is a different Dithering option, and each column is a different Image Density. Twelve engraving passes later, here is the result.

The full grid of engravings. Most of the difference in detail is lost in these photographs.

As expected, the image density settings took different amounts of time to finish. Density 1 took only 47 seconds to complete. Density 3 took 2:08, while the default density setting of 5 took 4:10. The maximum density setting took a whopping 15:13. Since the engraving speed isn’t really affected by the dithering option, the results were almost identical down the columns.

Lower density engravings make the dither pattern more obvious.

It’s clear in person (and less so in this photography) that error diffusion is the best option to get a good clean photograph engraved onto this medium. I’d be willing to bet that will be true for most materials, as the halftone pattern (while great for low color artwork) just doesn’t blend a photograph’s natural shades well. This is even true in the lower image density settings.

While the dither pattern all but disappears at image density 7, the time spent is too costly.

I think the default image density setting, 5, was well chosen. It’s an excellent middle ground between the time spent and the resulting image. You can’t really get a sense that the image is comprised of multiple horizontal lines like you can in setting 3, but you’re also not spending 15 minutes on a 2″ by 2.25″ photo.

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