Edinburgh Hacklab

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