[MikeSinclair]

Hi,

I've been using a CO2 laser engraver/cutter since 2002 and have created a tips and techniques document on what I've learned (and wish I knew before). I'd like to share it. It's really a work-in-progress. I work at Microsoft Research Hardware Lab where our two lasers get more use than a screwdriver.

The document is attached.

It describes methods for precision bending thermal plastic, welding, creating smooth beveled edges and much more.

Hope you find it useful.

[Dean448]

Good document Mike. Lots of tips on cutting plastics. I wonder if laser cutting plastics is less popular these days since we now have 3D printing.
Bunch of good stuff here but light on techniques to focus and how to check and correct beam alignment. Important in a machine that large.
Different when cutting wood you focus on the top and the beam (kerf) widens below. Which I think is from beam geometry.


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[MikeSinclair]

Thanks for the reply!

Yes, I was light on the focus and measuring beam alignment as it's rather involved as you'll see. I'll try to add it in the future versions. The out-of-focus value was by experimentation. You extend the focus distance till "sufficiently out of focus" to get the size of beam you want. To test, cut an out of focus straight "groove" in the plastic (I usually do this by 1" to 2.5" out-of-focus), but don't cut all the way through. Then cut an in-focus marker line down the same path you cut the groove, but not all the way through. You then cut an in-focus path, all the way through but at 90 deg, across the groove axis. You then look at the edge under a magnifier/microscope and evaluate the quality of the groove and the location of the marker cut. The groove should appear gaussian in profile. Do this with a groove and marker line cut parallel to the X-axis and one parallel to the Y-axis and evaluate both this way. If the beam is elliptical (IE because you only have one laser feeding the beam), you'll notice a marked difference in the width of the two grooves. This can be somewhat mitigated by arranging two identical grooves cut with a fixed X,Y displacement from each other to partially accommodate the beam's ellipsoidal shape and the non-perpendicularity of the beam with respect to the bed. IE, the beam's center moves away from the expected center by a small X and Y when out of focus. You can evaluate this offset by measuring the distance between the bottom of the groove compared to the marker cut, in the X and Y examples. This will enable you to derive an X & Y offset per-inch-of-out-of-focus for your machine. None of this is usually a problem if cutting normally through (thin) materials. When the device is made to vector cut out of focus, the beam expands as expected but differently in the X vs Y directions. Also, if the optics aren't aligned to cause the beam to be perpendicular to the floor, you will experience an offset in X and Y when out-of-focus. Note, most laser machines do not care that the beam's center is exactly perpendicular to the laser's floor so you may have to account for this for out-of-focus cuts or rasters by shifting your vector or raster paths. Note, with a machine with two lasers producing the single output beam, they usually arrange one of the laser's beam rotated at 90 degrees to the other, making the combined beam more circular and doesn't form as high an aspect ratio ellipse in the combined beam. It's too bad you can't also adjust the out-of-perpendicular angle of the beam, (if it's not perpendicular).

For thick thermal plastic materials like acrylic, the exit kerf is usually smaller than the entrance kerf as the beam looses energy as it progresses through the material. There is also a countering light-piping optical effect, keeping the beam's diameter small as it cuts. Also, because of a single laser beam's elliptical profile, the exit geometry will be different than the entrance one. For example, try to cut a circle in a thick piece of thermal plastic (0.5 -> 1" thick). Measure the two diameters (at 0 and 90 degrees wrt its position in the machine) at the top and at the bottom of the slug produced. You'll notice that the two diameters on the top of the piece are equal to each other and different at the bottom, indicating a non-circular geometry at the beam's exit.

Re: Laser cut vs 3D print. I use both but usually prefer a laser job for a number of reasons. This is IMHO

1. The laser job is usually quicker
2. The intrinsic properties of the raw material usually result in a more robust part with better surface finish.
3. The post-laser processing usually results in less effort and time.
4. Part-to-part friction is less for a laser processed job. IE - gears made from Delrin operate smoother than 3D printed ones.
5. Part tolerances are usually better from a laser. IE - Axel and bearing fit is mechanically better.
6. You're not limited to the discrete layer thickness of 3D filament for part dimensions.
7. Laser processed parts can be larger than 3D parts usually.

That said, 3D printing can generate parts that can't be easily produced by a laser.

Hope that helps and sorry if I confused.