Date: Fri, 28 Oct 2011 12:27:05 +0100 From: Martin Ling <martin@earth.li> To: "members@edinburghhacklab.com" <members@edinburghhacklab.com> Subject: Laser engraving anodised aluminium (i.e. shiny Apple hardware) Hi folks, Last night I put 3.5 hours of laser time into a broken Macbook Pro case trying to figure out how to do this. After about 30 test cuts I finally zeroed in on some usable engraving settings. My final settings were speed 65, power 33, spacing 0.03mm, bidir off. This is painfully slow: an 80x60mm piece took two hours. But the result was sharp and even, the engraved areas smooth grey. Everything else sucked in one way or another. My test piece was the lower body of a 15" pre-unibody Macbook Pro. I used the bottom of the case and the wrist rest areas, which are all of the same material. I am told that the top case (i.e. back of the screen) on the unibody models is the same stuff too, but haven't confirmed this yet. The back of the iPad (v1, at least) looks similar as well at a glance. In any case, for any similar material the following notes may be of use: This material is a bastard to engrave consistently. The relationship between speed, power and result is neither linear nor straightforward. There are effectively three layers you can expose depending on settings, and only the middle one seems to offer a consistent finish. A very light cut will turn the material white. A design engraved in this way will look much as though it was printed or painted on with white paint. The white stands out well, but the sweet spot in the settings that produces it is extremely narrow. Too much and it will cut through to grey, too little and it will not penetrate and leave the surface silver. Unfortunately the starting surface is not perfectly consistent and nor is the laser. It seems impossible to get this right over anything more than a very small area. A slightly deeper cut will go through the white and expose a grey layer. The grey stands out well when the original silver is catching the light, appearing darker. It's not quite as prominent from as wide a range of angles as the white, but it's thick enough to expose consistently. It also looks more obviously engraved rather than painted on. At the edges of the engraved area the white layer shows through as a brighter hairline, which I think is a nice effect. Going deeper still through the grey will expose lighter material again, but this material is not consistent. I think it may be a filler layer applied to smooth the bare aluminium before further coatings. It is a mix of light and grey following what I assume are variations in the underlying metal. The laser doesn't seem to be able to clear its way through to the metal itself. The depths we are talking about here are in microns. You can run your finger over after cutting to the bottom layer and still not feel an edge. There is no risk of structurally compromising the material. It would be nice to go faster, but all the ways to do so introduce problems: - Increasing speed above about 65 makes the delay from laser turn-on to actually reaching full power visible, with some material not fully cut at the leading edges of the design. Thin lines perpendicular to the sweep may not be cut at all. - Bidirectional mode would double the speed but the cuts in the two directions do not line up accurately enough, giving a double image effect. It's possible this could be improved by tweaking some backlash parameter somewhere, I've not looked into this yet. - Since the metal dissipates the heat, the area affected by the laser is very narrow compared to other materials. A scan step of more than 0.03mm leaves visible lines between passes. It's possible we could get a wider cut by using higher power and deliberately defocusing slightly, but again I've not investigated this yet. Hope this is useful to someone! Martin
Date: Fri, 28 Oct 2011 16:47:53 +0100 From: Martin Ling <martin@earth.li> To: members@edinburghhacklab.com Subject: Re: [hacklab-members] Laser engraving anodised aluminium (i.e. shiny On Fri, Oct 28, 2011 at 12:30:40PM +0100, aaron wrote: > > Great work Martin, any pictures of your results? This stuff is tricky to photograph - have just put some not ideal but okay photos up here: http://void.printf.net/~martin/photos/Engraving/ Test cuts on the wrist rests, two copies of the intended final design on the bottom. The larger one is at speed 200, power 31, bidir on - this exposes just the white layer, it looked good on the tests and for the first (bottom) half of the pattern. However you can see the double image caused by the bidirectional mode, the loss of thin vertical lines, and the failure to engrave some areas. It was fast though, only took 40min or so to cut. The smaller one is speed 65, power 33, unidirectional and exposes the grey layer. It's sharp and solid, just unfortunately not quite as prominent from some angles as the other one. And took 2h to cut. > When are you going to cut the final design for Wren? When I've convinced myself the same settings will work on the top of the newer unibody model. And when said model is next near the cutter. Martin
