Beam expander

[Dave Vigness]

Do any of you with large beds use beam expanders to keep the beam nice and tight? is it worth in on a 1200mm and larger bed?

[sebastien laforet]

we are talking AT MOST 2 meters of laser path (from source to material). if your laser is so wrong that you need a beam expander, the problem is in the laser source.

also remember that adding anything in the path will decrease your final power (probably from 2 to 5% for a good expander)

i think you probably should try to use larger mirrors and lens if possible (25mm or even 30mm) to capture all of the corolla, and also make sure the "holes" in the mirror holders are large enough and do not block a part of the laser beam.

[Dave Vigness]

Guess I should have expanded on it a bit. I don't have one yet, just doing the planning. I had not thought I needed one as long as I got a decent laser tube, but Russ' RF laser series had me wondering if I had over looked it.

I'm planning on a machine that will hold a full sheet. Probably 99% of the work will be done on 1/4" of the bed, but sure as hell some where along the way she's going to get an idea that will need the whole sheet. I'm doing it for two reasons. First, to cut down on waste and eliminate my need to cut strips to feed my 500 x 700 cutting bed, and second... because I can.

But that size puts me at near 4 meters from tube to focal point, hence the question for those of you with large machines. Do you have any beam size problems? My other option would be to make it like the BuilYourCNC vertical machine and put the tube on the gantry. Takes the beam length out of the equation very nicely, but adds a lot of mass to the gantry. The machine would be almost exclusively cutting for Y movement, so in that regard the mass wouldn't be a problem, and engraving is all on the X so that doesn't care about the mass of the gantry.

The only other stumbling block I see is the rails for the Y axis. I see three options. Worst case, I find some V slot extrusion and set it up lie Further Fabrication did his smaller machine. Next, I get several pieces of shorter rail and but them up. Mounting on aluminum extrusion will greatly assist the alignment, but could still have problems. Third and most expensive wold be to find a pair of one piece 15mm or wider rails nearly 9' long.

One question on the tube on gantry option. BuildYourCNC also put the laser PSU on the gantry, but I could swear I've seen some flat full sheet machines that put long HV leads on and left the laser PSU off to the side. My current laser is my first dealing with HV wire. Can it take the constant flexing that would occur in a cable chain, or is that a non starter?

[sebastien laforet]

But that size puts me at near 4 meters from tube to focal point, hence the question for those of you with large machines. Do you have any beam size problems?

whenever i had problem with "power" on the bottom right quadrant (my laser source is top left), it was ALWAYS because of the mirrors were badly aligned and/or the table was not horizontal (thus changing the focus)

with badly aligned mirrors, you may still "hit" the 3rd mirror then the lens if not too far away from the source, and obviously at longer distance even a small error can lead to miss the 3rd mirror (often hitting the borders of the "hole" in the head mount)

from what is saw, the beam is NOT AT ALL larger at the end. adding an expander would only add one source of misalignment.

[Dave Vigness]

That was my thought, but it's an I don't know what I don't know kind of thing. Worst case I leave room to mount one between the tube and the #1 mirror. If I don't need it, it's just an extra few inches of air.