Darkly Labs Community

Theory of how the laser cuts

I have a question to try and get my head around the action of the laser when making a cut.

I believe that the laser beam is conical in shape. (Other post) 

If so it would suggest that the perfect height of the laser above the workpiece would be at the convergence of the beam.

(pin point).as the beam passes through the material I assume the cut gets wider. (See attached image) What is the depth of field (Lense technology) for the lense. This would suggest the maximum thickness the laser can cut in one pass 

Am I on the right or wrong path to working this out image.jpgJohn

I’m a member of a forum for CO2 lasers in which a similar discussion has taken place. Such lasers cut easily acrylic plastic and thick pieces are sliced at various focus settings to determine optimum distance for cutting. There is divergence in a CO2 laser, which results in the recommendation of setting the focus at one-half the convergence point for thick material. The divergence in a piece of 10mm acrylic is a fraction of a millimeter and partially caused by the vaporization of the material beyond the beam perimeter.

The angle of the beam in your drawing is somewhat exaggerated, based on my recent experiment with the 1/8 turn focus change and measurement with a recticle. The cutting power of our LED lasers is not so powerful as to be able to manage any appreciable depth, hence the need for multiple passes on material as thin as 3mm. Having made one pass, then returning for additional cuts, I would expect the change in width of cut to be less than 0.01mm per pass. I had taken photographs with an iPhone microscope, but the application did not include the scale reference with the photo. The application does include on the screen a scale reference which allowed me to measure the width of each of the burns made for the test. The difference in width across a full turn of the lens was less than 0.10 mm. The difference in quality of the burn was substantial, however.

Lasers are devices of coherent light with a known aspect of collimated beams, which fits in with the above measurements and observations.


My diagram was just that, a representation of what I think how the beam generated.

So to expand on your reply , can I assume that if I was to find the exact focus of the laser on top of the workpiece and then lower the head half the thickness of the material I would be able to achieve the minimum number of passes to cut through the material. 

Given that the number of passes is still affected by the colour , density and glue type in plywood, of the workpiece

I understand that the burn will be wider than optium but within limits of what I would accept and the burn charring as least as possible

Would it be possible then to set the laser guard at the (Darkly) height and then bt placing a piece of material half the thickness of the job, under the guard and then lowering the head to the new level.




I realize I’m sometime obtuse in my posts. I am suggesting that the practical depth of cut of our laser is so shallow that your suggestion has some merit, I believe. I lack the engineering skills, but I’d like to have a vernier focus adjustment for the laser module, rather than the friction design currently in use. I’ve found that there’s enough flex in the gantry to have as much as 1-2 mm variation, depending on friction based on temperature and humidity or maybe just the mechanical design in general. The shallow depth of cut of this module laser is such that one could turn the bolt/crank/handle and get one mm per turn or similar. I think the toughest part of such a design is to get it to fit in the compact space available.

Hi Fred,

Just read your post after doing some experiments. We seem to agree on the same outcome.

Maybe Darkly could think more about you suggestion.

See my attached tests and feel free to comment (Anyone)

I’m a bit confused about the line beginning with "remove the laser guard and set the focus on top surface with the aid of Darkly’s focus piece.

My focus piece is a piece of flat aluminum of one mm thickness. The focus guard is to remain in place and the focus tool is placed flat on the surface of the work piece. The module is lowered to contact the focus tool but not too firmly. If one applies sufficient force, the focus tool will stick a bit and change the optimum focus point.

If you’ve been using your focus tool on edge, as is common in the CO2 laser world, I can now understand the difficulty you’ve had with getting a good focus.

I’ve found that when the laser is in good focus, the bottom of the extrusion is about 2 inches from the work, which then makes it very difficult to re-attach the laser guard.