Laser cutter questions. How to set focus?

[Adrian Page]

I attended a demo of a Chinese machine.
Trying out a variety of materials, focusing the laser by moving the lens up and down (with the help of a small acrylic spacer) looked pretty slick - UNTIL the whole lens assembly fell apart in the guy's hand!

That must have been embarrassing for the poor fellow! Was he able to repair it?

Adrian

[Kev Williams]

I haven't used or seen a Chinese machine without a motorized table, so I'm only speculating on the reasoning here, but my guess is that, in general, you don't want to move/mess with the optics. Once they are perfectly aligned, you want to leave it that way, which is why I'd think the tables move up and down, not touching the optics path. Moving the head up and down might be moving the optics (not sure, haven't seen how they work, but not sure how you'd move the head down or up without moving the optics path, would be something less than desirable as a general rule. It's not the "end of the world" type stuff, but my guess is that it just introduces one more thing into the equation that causes the possibly for something to go wrong in the path of the optics, alignment wise.

That's purely a guess.

When I bought my Triumph, it was largely an experiment, so I went as cheap as possible, which meant no motorized table. I have run into focus issues, strictly based on the height of the material. Like, the max thickness material I can engrave with the supplied lens barrel is about 3/4". If I'm doing something thicker I remove the table and put in a wood table that settles in about 4" lower. As it is I'm shimming up what I'm engraving to get it within focus range. The supplied lens barrel is about 3" long, and the barrel has about 3/4" of useable travel, and I've found no significant difference in where the beam hits the lens or ends up on the work regardless of where the barrel is clamped. At this moment Triumph is making me some longer barrels to about 8" long, and I'll be shortening an original length barrel. With these, I should have 5" of useable focus travel overall, and be able to accommodate 1-1/2" thick material on the main table.

I don't forsee any issues. I assume there will be some beam offset when using the longer tubes, but every job has to be aligned properly anyway so it shouldn't matter much.

[Adrian Page]

Thanks for posting that Kev. I find it interesting that you're not ordering a movable table after living without one.

Adrian

[Kev Williams]

Thanks for posting that Kev. I find it interesting that you're not ordering a movable table after living without one.

Adrian

Not that big a deal for me. My run-of-the-mill stuff is flat SS and aluminum panels, and a little wood. I also do a little acrylic cutting. For anything else, I just McGuiver something up as needed!

But my next machine will have a moving table!

[David Somers]

Kev,

Thinking ahead for Adrian. Do you think the lack of a moving table will affect the resale value of your machine?

Dave

[Adrian Page]

Dunno. Resale value of a Chinese laser appears to be roughly twice whatever it costs FOB China.

Adrian

[Mike Null]

You will be very lucky to get half your purchase price if you use it for a year. Less if you use it for more than a year. Remember, David is talking about a "used" machine price.

That fixed table will be a show stopper for most people.

[Adrian Page]

I'm going to order the machine with a moving table.

Adrian

[Adrian Page]

Well... I ordered the machine with a fixed table. It has arrived and I'm learning to use it. I have been cutting 1/8" to 1" material and focusing it with the plastic spacer provided and the lens tube. It takes 2 or 3 seconds to focus. I don't think it will cut much more than an inch thick material but I'm good for 1.25" with the tube that came with the machine.

Adrian

[Julian Harrison]

I'm still shopping for a laser. How do you set the focus on a Chinese laser cutter if you don't have an up/down motorized table? The laser I like wants $500 more for the up/down table and I think I can just block up the rotary fixture with a simple jig I could put in place of the knife bars. I just don't know how the focus is set. Does my plan have any merit? I would pony up the 500 bucks but I don't even know if I will ever engrave a cylinder. I need the laser to cut balsa and ply R/C airplane parts. I thought it might be nice to be able to engrave on a cylinder at some point but I have no projects in mind at this point.

Adrian

Saw this as opposed to banking material. Thought was a nice tool. Rather than buy one you can lay one out online just search create angle online. I will try and make a PDF after I get the mm set up with the angle. I am thinking 0-30mm might be ideal.

focus_lens.jpg

[David Somers]

Julian,

Not trying to nit pick you, but unless I am misreading your graphic insert, that ruler is actually measuring the distance from the bottom of the lens tube, the cone", to the material, not the distance from the lens to the material. The lens is much higher up in the tube. It would be misleading to refer to that as the "focal length" if I am viewing that correctly? It is still quite useful, just not measuring the actual focal length as the graphic states.

Dave

[Adrian Page]

Saw this as opposed to banking material. Thought was a nice tool. Rather than buy one you can lay one out online just search create angle online. I will try and make a PDF after I get the mm set up with the angle. I am thinking 0-30mm might be ideal.

focus_lens.jpg

I think I would be more inclined(see what I did there?) to make one on my laser cutter. BTW after living with the laser for a while I am glad I bought the fixed table. Setting the focus is extremely easy and quick on the Bodor. You place a gauge under the head, loosen the collar, the head drops down onto the gauge and you tighten the collar. Easy. I have had zero issues with it throwing off the focus.

The fixed table has a nice cone shaped sheet metal intake duct the size of the table so It sucks out fumes very well. With the moving table you don't get that.

Adrian

[Keith Winter]

David the lens is at a fixed point just above the cone in my machine. So yes you can vary how far the lens insert goes into the lens head, but the focal distance is always the same because the lens is not changing in it's placement relative to the object you are cutting. The only distance that is changing is in relation to the distance to the mirror above directing the beam down onto the lens. So if 6 mm is your focal distance that would never change no matter what since the lens remains the same distance from the object you are cutting. So the tool would be nice to have for sure

[David Somers]

Morning Keith!

I think we may be saying the same thing? Like you my lens is higher up in the tube, but it is always a fixed distance from the tip of the cone to the lens. And like you, my tube slides in and out of the tube that is attached to the mirror block. The distance from my lens to the mirror may change, but the distance from the lens to the tip of the cone is fixed. I was just pointing out that the tool is measuring the distance from the material to the bottom of the tube's cone. It isnt measuring the distance from the material to the lens itself which would be the focal length. If I took my lens with a 2 inch focal length and took that literally and made a spacer 2 inches high and used that to set the distance between the material and the tip of my cone I would find my focus was way off. I was just trying to point out a term that might be confusing to newbies not clear yet on that whole lens structure. Does that make more sense?

Dave

[Keith Winter]

I see now what you're saying. You're correct sir!