Focus depth and gas pressure
Many materials require high pressure assist gas for effective laser cutting. These materials include stainless, mild steel with air, and aluminum with N2.
Usually these call for a focus set well below the surface of the material. The adjustment is generally a percentage of the thickness of the material – focus halfway into the material, or at the bottom of the material, etc.
All this is empirical, because we have no way to actually see what’s happening in there, really. Anyway – this is production – the result is all that matters.
I wonder, though – how much of the focus adjustment is due to deflection of the lens and/or lens mounting due to the pressure? And, what about gas lensing?
In certain circumstances, gas pressure gradients can act like a lens. I don’t see much info about this. There are a few papers and studies about “gas lensing”, but those are aimed at using lensing effects to replace standard optics. Nevertheless, gas flow into the area below the focus lens can affect the beam shape drastically.
A real-life occurrence
In one of our Optimo systems, we made some test burns below the nozzle in plastic. The system had a 5″ lens, and I positioned the target about 5″ away from the nozzle. The burn should have looked like it did at the lens – but it didn’t.
There was a shallow burn the size of the raw beam, then a very sharp spike perhaps half the diameter of the beam. As we tested with lower gas pressure, the spike became smaller. At 20psi or so, it did not show at all. See gas lensing illustrations.
Gas port configuration
The gas was ported to an annular slot, and a series of holes allowed gas from the slot into the area just below the lens. All the holes pointed at the center of the beam path just below the lens.
What cured the problem
We had some stainless steel mesh available, and we just rolled a cylinder of several layers and put it in the head to diffuse the flow of gas. It solved the problem nicely.
The manufacturer (Prima Industries) revised the design of that part of the head. I don’t recall what else was changed, but it fixed that issue.
If you have a cutting head / lens / gas system that is unproven, or if you are trying to operate at a much higher pressure than usual, watch out for strange cutting issues – you may encounter gas lensing.