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If I understand it correctly in a CO2 laser engraver/cutter the laser itself is fixed in position and mirrors are used to redirect the laser to the material.

Is there some mechanism used to keep the focal point on the material or is the focal point set to the midpoint of the machine's range? Wouldn't the beam be wider at the edges of the machines travel?

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... the laser itself is fixed in position and mirrors are used to redirect the laser to the material.

Yes. This system is called "flying optics". The delicate laser is fixed in position and only the mirrors and lens move about. This simplifies the mechanical setup and reduces the inertia and required motor sizes.

Wouldn't the beam be wider at the edges of the machine's travel?

enter image description here

Figure 1. Divergence specification for Syntrad's ti60.

The beam divergence can be quite small. The Synrad unit above has < 7 mrad which would give a divergence of up to 7 mm/m of beam length. The optical path (mirrors, etc.) at maximum range would have to have large enough mirrors and the final collimation lens diameter would have to cope with the enlarged beam. The other course of action would be to correct for the divergence at source.

enter image description here

Figure 2. See Science ABC for example.

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  • $\begingroup$ correcting collimation at the source, effectively focusing the laser would result in more severe divergence at the point of contact with the work, as well as increase the energy per unit area on the mirrors and subsequent lenses, causing damage/reduction of life. $\endgroup$ – fred_dot_u Aug 6 '19 at 21:07
  • $\begingroup$ That sounds right, Fred, but I don't understand why there would be more divergence at the point of contact. Can you elaborate? $\endgroup$ – Transistor Aug 6 '19 at 21:21
  • $\begingroup$ I use a focusing tool at the nozzle of the laser, calibrated for the lens within. It is expected that this places the focus of the collimator lens at the material surface. Below that level, visible in thicker materials, one can see a diverging "burn." The divergence from mirror to mirror is apparent, of course, but is far less severe than that beyond the focus of the lens. For particularly thick cuts, I'll raise the bed slightly above focus, getting a wider out-of-focus on the surface, a narrow inside beam and a wider exit. 0.05 mm deviation on 6 mm thick = 1 mm over 120 mm $\endgroup$ – fred_dot_u Aug 6 '19 at 22:42
  • $\begingroup$ I use the same technique for my 30 W laser too but I always thought that a narrower incoming beam on the final lens would have a narrower cone angle at the point of focus giving a greater "depth of field" or less divergence on the cut. Am I mistaken? Thanks for fixing my spelling mistake. $\endgroup$ – Transistor Aug 6 '19 at 22:51
  • $\begingroup$ The distance from the (only) lens to the work is much less than the distance from the laser tube to the lens. Also note that the lens should have the flat side pointed at the work. $\endgroup$ – fred_dot_u Aug 6 '19 at 23:39

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