Laser not cutting exactly vertically

[mikemccand]

I cut through 1/2 acrylic and the part (a snowflake) came out well, but I noticed that the laser is not cutting precisely vertically, but rather at a small angle.

I also pulsed on the edge of a 2 x 4 and saw the same thing.

Has anyone else hit this? It seems like I need to adjust the mirror on top of the laser head just a bit -- any suggestions?

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[fixtureman1]

On my 60 watt if I have the beam hit the center of the final mirror I get an angled cut. I had to lower the lens assembly just a little so the beam hits higher up and that did the trick. I checked by test firing at the focal length then lowered my table and fired again and the spot hit way off.

[idank]

Depending on the angle you're seeing, this might be "working as expected". When calibrated correctly, the laser beam produces an hourglass shape at the vertical center of your material so getting a precise 90 degree edge is going be impossible on 1/2". On thin materials this is barely noticeable. You can get a nicer edge with a lens that is suitable for cutting thick materials, e.g. a 4" focal length lens.

[GerryG]

You'll always get a slight vertical taper when viewing cut material in profile. It's a combination of beam focal length and the loss of energy as it passes through solid material.
1/2" acrylic will have quite a pronounced taper to it. One way og etting round it is to look at setting the focal point about a third of the way into the material, so if it's a 9mm sheet, set it 3mm deep, as opposed to the top surface of the material. I've had mixed results with this trick, but it has worked well in some instances, so might be worth a try

[offworld]

Depends
If both edges of a cut disk for example face the same slant direction then it would be lens astigmatism so the z axis optical path is not parallel. If cutting again the same disk the edge angles are opposite then its a case of playing with the focal point to get to the centre of the beam waist.

[mikemccand]

Thanks for the replies. I am using the 4" focal length already, which I thought should have less hourglass taper effect, but maybe in fact what I'm seeing is "by design". It isn't such a big deal in practice; I don't cut 1/2" thick material so often.