I was hoping to build a laser engraver out of a 1W diode I have. I’d like to be able to focus the laser into the smallest dot possible both to have a very fine kerf and to bring the maximum heat to the target spot. Could you talk about the theoretical and practical limitations to how small I can make that dot, and different strategies for focusing it? Thanks!
Interesting question! I did a bit a reading up on the lens metrics that govern the minimum diameter of a focused spot and it doesn’t look too hard to derive. Newport has an article specifically on focusing and collimating a laser beam that goes over lens specifications and their relation to spot diameter. You will also note that the minimum kerf is both determinate the focal point and the material that you are cutting. Typically materials with a lower melting point will require decreasing your laser’s dpi in order to lower the cutting temperature, resulting in a better cut with a smaller kerf.
As a numerical example, let’s look at the case of the output from a Newport R-31005 HeNe laser focused to a spot using a KPX043 Plano-Convex Lens. This Hene laser has a beam diameter of 0.63 mm and a divergence of 1.3 mrad. Note that these are beam diameter and full divergence, so in the notation of our figure, y1 = 0.315 mm and θ1 = 0.65 mrad. The KPX043 lens has a focal length of 25.4 mm. Thus, at the focused spot, we have a radius θ1f = 16.5 µm. So, the diameter of the spot will be 33 µm.
This is a fundamental limitation on the minimum size of the focused spot in this application.
Regarding how you focus your laser there are two main ways to do this, electronically and mechanically, and both require an initial determination of your laser’s focal point. If you do not know the specifics of the lenses in your assembly, I would recommend to achieve this distance through simple trial and error. If you set up your laser in a test jig, where the distance from the laser to the sample material can be adjusted, you can conduct a series of tests to determine the appropriate distance by analyzing the cut. I was thinking that using an enlarger for photo development would make a nice Z-axis.
Once you have determined your focal point, you will want to make a focus jig that will allow you to accurately adjust the focus depending on your materials thickness. This could either be a removable piece, or an extension off of the front of your laser module. My Epilog has a removable inverted V touch tool that I use to focus.
The following video shows an Epilog using an electro-mechanical touch tool that uses a spring loaded limit switch to determine the focus. The advantage of something like this is your repeatability and accuracy is much higher then doing it manually. You could easily replicate this by attaching a limit switch to a probe attached to your z-axis and send the feedback to your stepper controller.
Remember, lasers with the power to cut are exceedingly dangerous. Make sure you take every precaution necessary to protect you, those around you and your equipment. Laser beams can extend a bit further then light sabers.
I hope this has helped steer you in the right direction and best of luck with your project!
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