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Old Aug 28, 2013, 04:32 AM
SB7 is offline
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Laser Question

Might not be the correct forum for this, but thought I'd ask anyway.

Most of us probably know that the beam width coming out of a Co2 laser is much larger then what is necessary for it to cut nicely through materials. The focusing lens takes care of this and intensifies the beam to a small point. (At least that's how I understand it) Trouble is it transforms the beam into a cone or hourglass shape, the apex of this cone shaped beam is the 'ideal' focal point for cutting.

The thing I don't like about lasers is because the beam is transformed into a cone shape it usually gives you slightly angled cuts, instead of a nice right angled cut. Yes you can minimise the size of these angles buy going to a larger focusing lens, but because the beam is wider, you lose cutting power. Granted this is a much larger problem for thicker materials then it is for thinner materials.

But it got me wondering, why aren't optics available that adjust the beam to a constant width. We have beam expanders that increase the diameter of the beam. Why not a beam narrower? Something that straightens and deceases the size of the beam so that the cutting beam is of constant diameter, eliminating the angled edges issue... Or are things like this available?
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Old Aug 28, 2013, 10:02 AM
c131frdave is offline
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Yes. What you describe isn't entirely accurate. Depending on the laser cutting device, a lens can focus the beam to a near point on a secondary lens which then "straightens" it out past that. But you are talking about a very expensive system. Every time light passes through glass, there is light loss regardless of how good the glass is, and/or the lens can actually change the wavelength of the light. This is why camera lenses make poor telescopes- tons of glass = massive loss of light.

So when you put lasers through multiple lenses, your losses may not outweigh your gains of having a straight beam.
Current "inexpensive" laser CNCs use a long focal length lens. The beam width coming from the CO2 tube is quite small, and the lens is only narrowing the beam marginally. The cauterization of the wood as the beam passes through it will cause more distortion in the piece than the shape of the beam. I can't imagine what kind of precision you would need to not be able to get by with current "inexpensive" technology.
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Old Sep 18, 2013, 09:23 AM
portablevcb is offline
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You are correct that the beam is a cone shape. You can see it quite well on thicker material, such as 1/4" balsa and foam. As stated, it is not really an issue for most model building so we live with it.

Why not fix it? Most laser cutters are designed for cutting very thin material so there is no reason for the mfg's to design a laser with a longer lens. The big industrial ones are cutting metals that are less than 1/8" thick so a bit of a taper to the cut is negligible.

We never cut any material thicker than 1/4" so it has not been a problem for us.

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