There has been a lot of discussion on here about lens orientation…after installing the 50 watt 1000mm puri tube I noticed that I had lost serious power…after a week of different test and playing with cutting power and speeds I decided to turn my lens over…mind you that I have a lens that is curved on one side and flat on the other…I’ve been running it with the flat side down…I could barely cut 1/4 inch acrylic at 8mm/s at 50% power that way…after turning the lens over with the curved side down…it Cuts 1/4 acrylic at 14mm/s at 45% power…I was entirely baffled by this…this goes against everything that I’ve learned…so what did I do…I put the old reci tube back in and noticed something strange…the beam size is way bigger on the new tube…6mm…the beam on the reci tube is under 4 mm…
I guess that has an impact on lens…any thoughts on this?
This is a company that does the big lasers, and the best optics info I’ve seen anywhere.
http://www.ophiropt.com/co2-lasers-optics/focusing-lens/knowledge-center/tutorial/cutting-head-optics
@Scott_Marshall …great info…thanks man…I still don’t know why it’s cutting better with it in the wrong way…I’ve changed the lens with different focal length lenses and the same effect…that’s with this tube only though…work the reci tube back in it Cuts better with the flat side down.
From what I’ve seen when reading up about the lasers, the larger the power (or higher you are running it) the thicker the beam will become. Even with my own tests, I notice a difference between kerf-width of 2 passes @ 5mA & 1 pass @ 10mA. That’s interesting that your 2 different tubes using the same lens prefer different orientations. You mention that 1 side is curved & the other is flat. Is it a convex or concave curve? It would be really nice to be able to see the beam. I know the beam has a specific wavelength (10600nm or something?) so I wonder if we can get equipment to “see” the beam (for a reasonable price)? That would make visualising what is happening really simple.
Whatever works, you’re certainly not doing any harm.
Maybe a out of spec lens?
I DO know what you mean, you gotta know, because…
I’m betting you figure it out eventually
I’m curious what happens if we focus the output of one lens through a smaller lens. Will we end up with a super powered nanometre width beam?
@Scott_Marshall …I’m leaving it in upside down…it can cut through 10mm acrylic at 5 mm/ at 50% power…I’ve never been able to do that before.
@Scott_Thorne That’s pretty decent to get through that thickness with 50% power. I’d be fairly happy about that. I just noticed when reading that article @Scott_Marshall linked, that the temperature absorption of the lens can cause additional surface curvature of the lens. This may be related to your lens not working so well in the normal orientation. Possibly the heat from the larger/thicker beam is causing it to curve more, hence changing your focal point (up or down from the normal focal point).
I just thought of something, Polarization.
I’ve been doing some reading on the physics of the Co2 laser, and the practices of large Co2 machines, and there’s a couple things we don’t worry about with the diminutive K40, but they are still in effect.
Polarization comes in directional and circular, and they use polarizing filters on large cutters for a couple of reasons, one being a linearly polarized beam cuts better and narrower along it’s long axis, and that makes for different behavior on the X axis than the Y axis etc. Not good.
Circularly polarized beams move through the mirrors more efficiently too, a big consideration when you’re using $3000 water cooled copper mirrors… 1% loss of 5 KW is 50watts, more than my laser puts out… burning up your expensive mirrors.
Our lenses and mirrors cause polarization via the internal stresses in the substrate and just plain poor quality, but it’s unintentional and ignored. If you get a “lucky stack up” of random polarization, you may well end up with a beneficial result – a high percentage of circular polarization maybe?
The other overlooked subject is beam diameter. It the large lasers, they actually strive for a large beam diameter out of the tube, then INCREASE it with a beam expander (this is where they circular polarize it too). The reason is to keep the watt density lower on the mirrors to reduce the damage from heating. They often will run a ¾” beam through the optics, then reduce it at the head. Your new tube may be throwing a narrow beam, and flipping the lens widens it out to where it cuts better?
Those are my deep thoughts for the day. Now it’s your turn, you get to see if there’s something there…
Scott
@Yuusuf_Sallahuddin_Y …that very well could be the case…I’m on vacation this week and the wife and I are just relaxing at home so I think more test are needed.
@Scott_Thorne I’d consider attaching a thermal probe to the side of the lens somehow to monitor it’s temperature & see if there is any difference/distortion of the cut width at different temperatures. Another idea to add to the list of things I will do to my machine as time/$ allows.
@Scott_Marshall …another thing about this puri tube that I noticed…at low power when I test fire the laser at tape…like I’m going to do some alignment the spot on the tape looks like a donut…burned on the outside but not in the middle…it does this at the tube also so I know that nothing is in the way of the beam.
@Scott_Thorne That sounds exactly like when I deliberately moved my workpiece closer to the lens to defocus it (so it wouldn’t engrave all the way through the thin leather at lowest power/highest speed). So it makes me think that it definitely is a focus issue.
edit: I just realised you said it does this at the tube also. So that is different, because there is no lens focusing it at that point. So it must have some kind of dead/weak spot in the centre of the beam. Like the “eye of the storm” sort of thing.
@Scott_Thorne
That sure could make it act weird. Optics isn’t as simple as most people think. I’d have to do some research to figure out what a ring shaped beam will do, but that’s sure sounds like at least part of it.
Edit-
Yuusuf may have answered it. If moving in past the focal length will create a ring (which makes sense), then flipping the lens may move the energy back in to the center, thus filling out the spot again with even energy density all the way across.
@Scott_Marshall …look at the photo that I’ve posted about the beam characteristics of my tube…notice the hollow spot?
@Scott_Thorne Yeah is very odd. A serious dead spot in the middle of the beam. I wonder if it is a tube fault or if this to be expected with higher powered tubes.
Talk to the makers of your laser tube about it.
Also you could have a faulty/damaged/worn out focal lens
If you have a spare try that,what size is the focal lens in mm and what is the focal length?
I too have noticed thaat my beam is donut shaped beam on my k40.on my old 12mm focal lens i did try flat side up and could not notice any diffrance,however latter found damage to the curved side-so maybe you have a damaged focal lens however the damage is not visable yet…my new18mm focal lens is cutting very well with a beam width of 0.1mm . on the back side of the work peaces .
I 2 have been thinking about how to make the laser beam visable/photographable ,from what i have read i would need a infrared camera to do this.
Also do you have air assist that blows air the focal le lens?maybe the beam is hitting the side of the air assist nozzel.on mine ths nozzel was compltly bolocking the bean untill i aligned the laser head .
Me too Mr Conroy, Been in search of that for years.
I have a credit card sort of tool that has a coating that fluoresces under IR. It’s designed for testing IR opto-interuppters, retro-reflectors and remote controls. Been a few years since I’ve seen it, but it’s probably in the shop somewhere unless one of my employees needed it more then I did. They must still be available. I got it from an Effector salesman if I remember correctly.
That would show the beam shape clearly, but only at very low power levels.
FLIR cameras have come way down, you can get one for your phone for about $250. Then you could shoot a target and film it.
Edit
Not easy to find, but I found one on ebay after much digging.
I’m NOT paying 40 bucks for a strip of treated paper.