Cutting 1/4" acrylic - perfectly perpendicular cuts possible?

I'm getting close to purchasing a machine and have noticed when cutting 1/4" clear acrylic (or thicker) that the cuts are not perfectly perpendicular to the surface. I assume this has something to do with the laser being refracted by the material as it cuts through. The amount of error will vary from cut to cut (on the same machine) but has been present on all machines I have looked at. This would be problem if, for example, you plan to construct a perfectly square box out of clear acrylic. The amount of error I'm talking about is perhaps being off by as much as 1/4" from square when placing a 6" square on a perfectly flat surface.

Although this will not affect most of what I plan to do I'd like to know if this is a problem for laser systems which cost more than $30K. My budget is under 30K so I have only been getting demos of machines within this range.

Perhaps, is there a practical way to "flatten" these edges?

As things stand, I would be forced to use my CNC router for outside edges and then doing etching or inside cuts with the laser... Keeping everything perfectly aligned going from the router to the laser machine may be tedious for some shapes..

Unlike a CNC end mill, the laser beam is not parallel so you are correct the cut edge is not perfectly perpendicular to the top surface. It depends greatly on the thickness cut and focal length lens used.

There could also be some additional error from the laser head angle to the table, it may not be perfectly 90 degrees. On a CNC mill/router the table is usually machined flat with a fly cutter so it is parallel to the axis.

Having said that, 1/4" acrylic should have minimal error and unless you were doing 1/2" or greater and planning to screw pieces together it should not be a problem. Most of the time thin Acrylic is bonded with either an adhesive or using a solvent, the slight gap can actually be beneficial in aiding strength (capillary action will pull the epoxy or solvent in to the bond). It would need to be held at the correct angle during assembly.

If this is an issue for you, talk to your vendor and get the focal length that produces the lowest beam divergence and you should be ok. The cost may be slightly higher and you may lose some power, but the result will be what you want without a second process.

If you've done much cutting of Acrylic with an end mill you'll know it has problems too. I personally do all Acrylic on the laser as it eliminates the edge cleanup and flame polishing processes.

Zax.

Thanks for the quick reply. I did not realize that the laser beam is not perfectly perpendicular to the top surface. What surprised me about the demo pieces that were cut for me was the inconsistency among the 4 sides of the same rectangle. One side will be reasonably "square" to the surface while one or more of the other sides will be off by a varying amount. The amount I am talking about is not small. A 2" square I have when rested on its' edge will be out of square by 1/8" - this is the worst edge but even the others are off by an average of about 1/16". (Note: I'm doing this testing on a perfectly level, granite surface plate)

I do like the near flame polished quality of the cut surface. I too have not been able to eliminate the need for edge cleanup when using my router which is why I was hoping to switch this type of work off to a laser. I also do not like the tedious nature of getting small pieces held down properly for CNC router work.

I'll ask about focal length options next time I visit... thanks.

The kerf should be the same regardless of where it is on the item being cut... if not, the beam needs an alignment. The kerf for 1/4" material should also be very minimal... the beam has an hourglass shape, so that's what is creating the kerf in the first place. A 4" FL lens will mitigate this kerf, but again, it is usually only a problem for the most demanding of applications for such thin material. Anything cut with a 2" FL lens should stand on edge without falling over (unless it's like a foot tall).