In response to a forum question here, I made quick video:
The longer focal length is often preferred, even though it means you need a little bit more depth in the build of your case/cage, and the reason is that the vertical area of the beam being more concentrated, is longer, with a longer focal distance, which is desirable for reducing the amount of wedge in your cut — the area of tight concentration is taller. Hopefully this video will help illustrate it.
I haven’t watched the video yet, but something I always do with my laser is to adjust the focus dependent on the material. If my material is 3mm, I lower the focal point 1.5mm. This puts it at the middle of the material and puts any cone shape in both directions. Makes square corners easier to come by.
This is something I haven’t been doing with the diode laser. Until watching Dougs video i never realized that was a thing lol. I do have my Z drop .8mm each pass as that was suggested to me in my newbie laser questions post a while back. With a moveable Z should i still be setting it down lower? or maybe i should be splitting the thickness by the number of passes? We seem to be mostly cutting 1/4" stuff and at 3 passes, so dropping .8mm isn’t getting us near as much as each cut should be making. Man every time i think im doing something right i realize just how much more there is to learn LOL
For me, the only person to answer those questions are you or your wife. If you’re happy with the cut, or the edge…. If not……
The hidden secret is, when cutting with a single pass with light/heat the thicker the material, the less likely the cut edge will be straight and 90. This is not applicable to only lasers, and additional considerations might need to be made based on cutting methods.
I say this because with respect to lasers, there are jobs where I want focal at the center. Some I want at the top, few, very few at the bottom.
There are materials I cut making multiple passes, some dropping on each pass, some never dropping. IMO, the answers are dependent on the machine, the material, the operator and the desired finished result, which means there are no wrong answers. When I use my cutting wire, well similar factors, and different lessons and methods to get close to straight and 90.
I feel with respect to x,y,z machines this is the biggest injustice marketing teams and YouTube have done. Everyone presents a 3D printer as buy or build it, and print objects. Or purchase this laser, and engrave things or cut things. Or buy my CNC, and make cutouts or sculptures. The reality is, purchase my machine, learn my machine, get it dialed in for one material, make some stuff, get brave, change materials, start dialing in again and learning, lather, rinse, repeat until you break/consume the machine, are ready to upgrade your machine or want to buy the next “better” machine.
Or do I have it wrong?