Optics recalculation.

Its good that the laser is cutting but I am still interested in tweaking the optics so it will cut thicker metal.

As I stated in this post that has a table which suggested that at a beam diameter of 10mm, an a focal length of 3 inches I could obtain a reasonable power density and still cut thicker metal.

Purchasing optics is an expensive enterprize so I thought it would be a good idea to reality check the diameter of my beam. The original guess I had from back in the day was that I had a beam diameter of 10.5mm. This was based on a table that someone gave me which used a formula to determine spot size based on the total distance between the laser and the focal point.

There are three problems with this plan. I had no idea where the chart for spot sizes came from, the distance the beam travels in my optics chain is difficult to determine, and my optics chain has a colimator which is unlabeled – I had no idea how much that thing expanded of the beam. I went with the calculated sizes on the chart, took a guess at how much the colimator expanded the beam, hooked up the optics and eventually it cut metal. That was fine but in order to maximize the potential of the laser I need more information about the real dimensions of beam size.

I performed some tests with the laser set at low power. I made 11 spots with thermal paper set at different distances from the laser. The spots at each distance are shown in the picture below.

Each row is labeled with the distance from the laser in centimeters. Also shown in the picture is a test shot of spots after it comes from my optics chain. The picture was loaded into a CAD program and the max width of each spot was measured. The totals for each row shown for the spot sizes made from distance of 400, 350 and 300 cm – the totals for the test shot is also shown. It looks like from this analysis that the length of my optics chain is very close to 350 cm.

I also used a micrometer and by eyeball took a couple measurements of each spot, and they correspond _reasonably_ well with what is predicted from the chart. You might wonder why I dont measure the spot size directly from the thermal paper and just leave it at that. If you inspect the spots closely you can see they really vary. I think this is because the laser control software does not do short pulses very well and the total amount of time the beam is on seems to vary. Another issue is that laser beams are a gaussian distribution where the center is a lot hotter than the outer portion and the beam’s “edge” is really not obvious.

Okay, but by look at the chart of expected sizes and by directly measuring the spots on the thermal paper, I think my beam size is somewhere around 3.6mm.

I also tried some crude experiments and I think my colimator expands the beam to around 10mm (exactly what i predicted in my first analysis. So if I were to use my original chart I would go with my current colimator, get a 10mm spot size and use a cutting head with 3 inch focal length.

Unfortunately there’s a problem. Laser mechanisms doesnt make a cutting head with 3 inch focal length. They have 4 inch and 2.5 inch. This is a chart of power densities and depths of field for 2.5 inches.

This is a new chart that reflects the stats if I went with 2.5 inch focal length. It looks like what’d be useful is to create a beam diameter of 8 mm.


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