Focusing depth

Discussions on optics for laser cutter/engravers

Focusing depth

Postby buildsomething » Sun Jun 20, 2010 12:47 pm

One question that I was wondering about is when cutting thick material...not engraving...but cutting, where would one focus the beam.....at the top of the material, bottom, middle or somewhere in between?

Is the location of the focal point of the beam a function of thickness of the material being cut?

Does speed and power have any significant effect on beam focus location?

How about the focal point with respect to material...ie plywood, solid wood, clear plastics, coloured plastics...etc etc.

Although I can start a series of experiments to determine whether this is a black art or not, I thought I should ask the group for their comments and knowledge in cutting thick materials....say 1/4" and up.

Maybe there is just a simple guideline to follow and I'm making more work than necessary. Inquisitive minds want to know. :D

I look forward to any replies.

Richard
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Re: Focusing depth

Postby lasersafe1 » Tue Jun 22, 2010 2:19 am

Thickest item I have cut was acryilic 1/4" with my 35W laser and a 2" focal length lens. I find that it is best to put the focus in the center of the material so it converges on the way in and diverges on the way out. This gives the narrowest kerf possible. I have also cut through popscicle sticks with this laser, but they are thin enough to not care about focal point.

If you are going to do a ton of cutting, I would invest in a 4" FL lens. This has a much longer Rayleigh length (length of narrow focus), so it very forgiving of small focus errors.
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Re: Focusing depth

Postby bdring » Tue Jun 22, 2010 2:49 am

Unfortunately, there are no magic bullets. Increasing your focal length will lower the angle, but will increase the spot size (see my optics page).

The best solution depends your thickness and what you want to optimize for. Cut speed, small kerf, best edge, etc.
Bart
"If you didn't build it, you will never own it."
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