Getting flatter, straighter cuts

[Jake Vander Ende]

I'm currently using a 60W laser to cut 1/4" acrylic for board game components. I've noticed that the cut is always angled due to the focusing cone of the laser, so my fix so far has been to focus down below the surface of the material a bit to compensate for the cone. It's still not wonderful, however, and the edges are always a bit angled. With pieces that stand up on those cut edges, it means I can't make anything very tall since it's bound to fall over.

I tried a search, but I'm not really sure what key words to use and couldn't find anything useful.

What do you folks do to get straighter acrylic cuts? I'm currently using 20% speed, 100% power, 5,000 frequency with a single pass.

[Rich Harman]

You should get a straighter cut if you slow down. For example, if you cut so that it barely cuts through, the top of the kerf will be a few thousandths wide and the bottom nearly non-existent. As you cut slower the bottom will open up, making for straighter sides. I always focus at the surface and try to cut at a speed where the parts fall out on their own. This results in pieces with edges that appear straight. Some turn out perfect, some are a couple degrees from vertical - there is some variation.

However, if I were cutting parts that needed to stand on edge I would run that edge against a disk sander so that they were all consistent.

[Chris J Anderson]

Also if you use a longer lens it will straighten out the cut, so a 4" lens will be straighter than a 2" lens, and a 2" lens will be straighter than a 1.5"

[Keith Colson]

I typically get a degree or two on 1/4' acrylic. Some acrylic is worse than others due to its composition. If you characterize your laser by measuring the kerf you can compensate for it be shimming one edge of the material. e.g. lift one edge of the plastic 1 degree.

Cheers
Keith

[Rich Harman]

Also if you use a longer lens it will straighten out the cut, so a 4" lens will be straighter than a 2" lens, and a 2" lens will be straighter than a 1.5"

I'm not sure this is true. Yes, the depth of field is greater on a longer lens but there is a "wave guide" effect when cutting acrylic. Not sure "wave guide" is the right term... The shape of the cut does not match the shape of the laser beam. I can focus on the surface of 3/8" acrylic with my 65mm lens and the sides will be vertical - if I get the power/speed right. In my experience, the only time the shape of the beam has been evident is when cutting 1" foam. No matter what focal length of lens you use you need to get the speed correct in order to get straight sides.

[Jake Vander Ende]

Unfortunately, none of these are helpful so far.

I've found that slowing down has actually worsened the angle of the cut, despite all logic saying that it shouldn't. The slower beam seems to be heating up the inside of the vector grid unnecessarily.
I can't use a longer lens since I'm on a Zing24. I asked for one, but Epilog said my depth was too short for a longer focal length.
Finally, I can't just tilt my material since I need the cuts to be straighter all around. I have pieces rotated in many different directions to make the best use of space for a single sheet.

Are there any tricks to duplicating cutting paths and doing multiple passes with an auto-focus mid-job? That's the kind of advanced technique I'm not familiar with yet, if anyone has any insight on where/how to get started.

[Bill George]

What brand of laser are you using? With my ULS I discovered you can change the heat and PPI settings to reduce the melting affect.

[Kev Williams]

Right or wrong, I've come to (my own) conclusion that light refraction is why some plastic never cuts straight.

With my Triumph I can cut thru 1" of wood and get a dead-straight right-angle cut...

Cutting thru 3/8" square white acrylic gets me a near-straight cut.

But cutting thru some 3/8" square CLEAR acrylic results in a very sloped cut...