Laser power dying out partway through job

[leeroybrown]

I have been having some problems with my 50w Chinese laser cutter recently. Having had the cutter for 14 months and not really had many issues with it (apart from the usual Moshi problems) I originally thought the laser tube had died, after all it had lasted 14 months and not been replaced. So, I ordered two more tubes, replacement mirrors and replacement focus lens. Unfortunately I ordered the wrong size focus lens so have not replaced it yet - am going to re-order.

Anyway, the issue is that the laser beam stops cutting about halfway through jobs. The head continues to move, the laser is firing from the tube, through the mirrors just not coming out of the nozzle. I have calibrated all the mirrors at each end of the axes so I know that the beam is entering the laser head. I have posted a link to a 10 minute video below to show everyone the problem and any thoughts or ideas will be most appreciated. I am scratching my head and banging it against the wall at the moment as I really don't know what the problem could be. This video shows the job running at 85% power. I ran the same job on 20% power and it is fine - completes the job but unfortunately 20% power doesn't cut through the material. When I ran it again on 20% power and manually increased the power whilst the job was running, it stopped working shortly after I got the power back up to around 50% or more. It's almost as though the laser power is too much for the laser/lens/nozzle to handle.

I don't know whether it is the focus lens as the beam seems to dissipate just before it stops working. What I cannot understand is that it is a solid beam for a while and cuts fine, then stops. The water running through the laser is cool enough, all the power seems to be fine - I just don't get it!

Here is the link to the video:https://drive.google.com/file/d/0BwF...ew?usp=sharing

Approx. 4:06 the laser stops firing but continues moving.
Approx. 4:41 I use another piece of wood to break the laser beam - proving that the beam is actually firing up until the laser head
Approx. 6:45 - 7:32 I reposition the laser head and do a manual test
Approx. 8.18 I do another manual test and the laser stops working near to the end of the cut and there is a close up of the manual cut

Thanks in advance!

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[LaserMikey]

hmm that is weird... so the laser is definately making it as far as the last mirror.

Have you tried doing a test laser at all four corners of the machine table? Ie go to top left, test laser, top right test laser, bottom right, bottom left?
That would be certain the laser mirrors are properly calibrated.

One thing i do notice tho (unrelated to your probelm) but the cuts seem very wide (maybe its just the charring), is it correctly focussed? The laser should have a preset focal point which would be a predtermiend distance from the lens head. If it doesnt have an autofocus youd need a measuring jig to set the table height. Ie if you engrave is the width of the engrave "fat"?

[leeroybrown]

hmm that is weird... so the laser is definately making it as far as the last mirror.

Have you tried doing a test laser at all four corners of the machine table? Ie go to top left, test laser, top right test laser, bottom right, bottom left?
That would be certain the laser mirrors are properly calibrated.

One thing i do notice tho (unrelated to your probelm) but the cuts seem very wide (maybe its just the charring), is it correctly focussed? The laser should have a preset focal point which would be a predtermiend distance from the lens head. If it doesnt have an autofocus youd need a measuring jig to set the table height. Ie if you engrave is the width of the engrave "fat"?

Hi LaserMikey,

Thanks for your response. Just to answer your questions:

Yes, the laser is definitely making it to the last mirror. I have done the "tape test" in all four corners of the machine table. The laser is passing through the near centre of the last mirror at all four corners so it is properly calibrated - it is so weird.

As far as the cut width, it has always been like that and has never caused any issues. I will try and manually focus it again though, just to quadruple check everything!

If you or anyone else has any other advice, please let me know as soon as you can as it is holding our business up! We want to get back into the swing of things being new year, but we can't!

[LaserMikey]

Hmm haha weird. ill have a ponder and see if i can think of anything

As far as cut width im almost certian it is defocussed. Sometimes i deliberately defocus so i can do engraving a thicker line.

Here is mine cutting right now, as you can see it should be much thinner. This has the advntage of cutting easier as the beam is well "more focused"



This diagram explains it pretty well.



If it doesnt have autofocus, find out from the manufacturer what the focal length is or how far it should be by trial and error. You can make a simple guide cut out of wood or soemthing u can use to make sure the material is always XXmm or whatever away from some spot on the laser head.

[buddydog]

WAGuess here but you have a cracked lens. Has some stuff in the crack and heats up and changes focus drastically. Second guess: cracked 45* down mirror on head. And last guess: A different mirror is moving and even though it appears to be entering the head it actually isn't. It's landing somewhere nearby.

[herby08]

hi there
did you do a ramp test? that's where you take a strip of wood lift the right side about 1/2 inch then cut a straight line. then leave it there and measure the distance from top of material to the head where the line is thinnest , that's you focal distance and should always be the same .
greetings
waltfl

[Pplug]

Cooling problem?

[LeeWay]

I don't know much about lasers, but is that material warped? It appears that after the part is cut out, it drops. That may be the issue with the width of cut because the FP isn't where you think it is.

[Pplug]

I don't know much about lasers, but is that material warped? It appears that after the part is cut out, it drops. That may be the issue with the width of cut because the FP isn't where you think it is.

The part would have to warp several inches to not get some type of burn on it if the laser was actually aligned/firing well.

[LeeWay]

I understand that. I was only commenting on the width of cut. I have a plasma cutter and it needs a precise height above the material to cut optimally. It uses THC. I take it that these lasers need manual height control, so once you set the FP, if it is warped in a corner, it may not be optimal and would make a wider kerf cut.

[buddydog]

I still think the beam is changing direction at some point. Try fastening a business card over the entrance hole on the head. Watch the entrance area while a test job is running. Just before the beam stops burning the job, does it move across the card in a line? If it does then the problem is in optics before the head. If not then remove the lens, run a test job on cardboard. If it continues to burn then it's likely the lens. If it still quits with no lens then the last 45* down mirror has some problem. Maybe the head has some very hardy bug crawling around in it? :-)

[Pplug]

I still think the beam is changing direction at some point. Try fastening a business card over the entrance hole on the head. Watch the entrance area while a test job is running. Just before the beam stops burning the job, does it move across the card in a line? If it does then the problem is in optics before the head. If not then remove the lens, run a test job on cardboard. If it continues to burn then it's likely the lens. If it still quits with no lens then the last 45* down mirror has some problem. Maybe the head has some very hardy bug crawling around in it? :-)

I agree 100%

[LaserMikey]

as most people have mentioned, i think we need to take a look at the lens up close. Do you cut alot of MDF? That gives off nasty residue meaning your lens is likely clouded.

At the very least give it a clean with some isopropyl and look for cracks or residue as people have mentioned. Maybe it is not even sitting flat?

When you said you did the four corner check and "near centre"? how exact do you mean? I use a calibration system like this that ensures each corner of the machine hits exactly in the centre of each mirror.


I still think the beam must be reflecting somewhere inside the laser head/mirror assembly. That or i would be very careful running it with the cover open... in case the beam is reflecting back out somewhere :/