Hi, ive an odd problem, I needed to re-align the mirrors in my laser today and all are in check and in the centre but when I put the nozzle and the lens back on it’s not pulsing. I take the nozzle off and I’m getting a pulse in the centre. I took the lens out of the nozzle and fired a pulse that went through.
But with the lens back in the nozzle I’m not getting scorch marks.
the lens is in the right way, with the bump side facing upwards. and it’s clean so now I’m at a loss on what to do.
Are you sure the lens is in right. The bump side meaning a mark or lens is raised on one side?
Sounds odd indeed. If the whole setup works without the lens, and the beam does not go through when the lens is installed and nothing else is changed… Maybe the lens exactly perpendicular to the laser beam, as in the mount is warped a bit? How often did you fire a pulse and get no result? Did the beam get blocked by something else that might have been in the way?
I was looking it this.
This is actually opposite of what’s generally advised for CO2 lasers. Convex side is expected to be closest to tube beam.
This is a recurring contention point for some reason.
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Thanks for all the help on this.
Ive had a fresh look at it today, I aligned all mirrors again and put the lens back in, and the same problem. so I’m opting to get a new lens now. It is about 3 years old so think it’s time for a new one. I have got the laser working but god knows what way the mirror is pointing into the lens.
I put the lens back in and then moved the mirror until I got a beam through the lens. It worked but it is not as powerful as I needed it to be I had to slow the cut down to 5-speed at 45% to cut through 3mm acrylics.
So now I need a good recommendation on where to get lenses for RD4040
I suspect it is due to the lens not being 100% transparent to the co2 frequency. If so, the flat surface would reflect back into the laser tube.
That’s true even for convex side up at least for the very center of the lens. However, that’s not usually the argument I hear. It’s something about the intuition of how the light bends with the flat vs convex surfaces… specifically, I think the idea that the flat portion will capture all the light without first distorting it. Personally, I find little about light intuitive so don’t try to rely on it.
Can you take a photo of the lens? I’d expect it to be visibly tired to cause the symptoms you’re describing.
If you can also take so photos of your alignment validation steps that may be useful.
I can see where the convex side up lengthens the depth of focus.
I suspect this is some of the counter-intuitiveness at play.
Take a look at this:
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The image also shows the lengthening of the field of focal depth, which is true for longer foal lengths.
You machine should have a meniscus lens in it, not a plano convex lens.
Either way both @Eddie1981, in his first graphic and @berainlb are correct, the convex side goes up…
Why was it needed?
Hi thanks for the replies, the lens I use or that it came with is
CO2 Laser Meniscus Focus Lens
Diameter: 20mm
Focal Length: 50.88
Edge thickness: 2mm
ive had to adjust the mirrors with the lens in and now it’s cutting but not as it used to so I have ordered a new lens just to see if that makes a difference, if not ill go back to the drawing board. one of the mirrors has a black spot on it so that may also be the problem.
Probably an issue 
One of the requirements is clean optics…
Thanks for the help
Questions can i use a normal mirror to replace the faulty one as I’m a glass artist so I have the tools to cut a mirror or does it have to be coated?
No glass anywhere in the optical path, unless it’s the material you wish to engrave…
Co2 IR will engrave or shatter glass…
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Front surface glass mirror, like in a telescope?
Regular glass is opaque to IR,… else you couldn’t engrave on it.
Focus lenses are designed to allow for IR wavelength.