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Thread: Laser cutting issue

  1. #1

    Laser cutting issue

    OK, so I recently dialed in my Shenhui which included lowering the tube slightly to get better alignment, mirror replacement on 2 of 3 mirrors (new Mo mirrors) and a fresh lense (which I've since toasted due to soot - but that was an air cleaning issue). Cuts finally were as good as I had hoped for 2 years ago (I had sort of powered through or worked around issues until then). I have a 150w Reci and I typically cut acrylic at 95% power as fast as I can get away with. Right after dialing it in, I got 6mm at 20mm/s in the far bottom corner of the 1200x900 table which is what I often cut 3mm at - so I was pretty happy that day!

    And so far still fairly happy with the alignment.

    But I am still getting an issue where sheet 1 of a pattern, cuts great but sheet 2 maybe great, sheet 3-4 stops cutting on far side and so on.

    It seems the mirrors and their holders are getting hot to the touch - not burning but very warm - maybe 50degC. I think that either the tube is getting too warm (and tomorrow I will try and lower the limit on the chiller to see if that helps) or the expansion of the mirror and or the holders of the mirror is causing the alignment to go out of whack.

    If I let things sit/cool down for a while, they go back to OK (or better at least).

    Anyone with any experience or suggestions on this? How hot do the mirror holders get on a similar set up? My brother is running an 80w tube on the identical machine and does not seem to have an issue so maybe the mirrors are absorbing too much heat at 130w?

    Its a PITA as my sample cuts seem great but my production goes to pot.

    Thanks, GF
    Last edited by Graham Facer; 02-23-2015 at 2:48 AM.
    Graham Facer
    1530 Omni CNC router (run with Vcarve), Shenhui 1200 x 900 150W reci laser cutter (now with EFR F6 hopefully)
    48" Generic Vinyl cutter, Roland engraver, and a dalhgren and vanguard on the project table.

    ...My pet peeve is a good thread with no conclusion because the OP solved the issue and disappeared. Either that or bus fatalities are much higher than reported.

  2. #2
    I had exactly this problem on my Universal and it occurred right in the peak of summer which is December here. My issue form what I deduced was the laser tube was getting too hot, The too hot part was obvious on this machine as the cooling fans normally modulate via a PID controller. In December the fans would go to full speed and stay there after about 30 to 40 minutes of cutting. I redid all my cutting settings at 80% power and things have been perfect since.

    Your issue could still be the mirrors moving tiny amounts when they get hot. Mine have air blowing on the mirrors and I have not noticed them getting warm. Trying 80% power or less could be a good starting point to see if the laser temperature is the issue.

    Cheers
    Keith
    Universal Laser VLS6.60, Tantillus 3D printer, Electronic design
    edns Group, Mairangi Bay, Auckland, New Zealand

  3. #3
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    If the holders are anything other than room temp, you have an alignment issue and part of the beam is hitting the holder. Or your optics are so far gone they are losing a lot of energy to radiant heating. What type of optics (material) are you using?
    Hi-Tec Designs, LLC -- Owner (and self-proclaimed LED guru )

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  4. #4
    Quote Originally Posted by Keith Colson View Post
    I had exactly this problem on my Universal and it occurred right in the peak of summer which is December here. My issue form what I deduced was the laser tube was getting too hot, The too hot part was obvious on this machine as the cooling fans normally modulate via a PID controller. In December the fans would go to full speed and stay there after about 30 to 40 minutes of cutting. I redid all my cutting settings at 80% power and things have been perfect since.
    Is your laser in a conditioned space? If so, you've got something going on that needs to be fixed. You should not have to dial that machine down to 80% to keep it cool. And it's fairly normal for the fans to run for a while after it's finishing cutting, but you really should never have to dial that machine back to make it work. That would indicate there is an issue. Maybe it's a sensor that needs replacing.
    Lasers : Trotec Speedy 300 75W, Trotec Speedy 300 80W, Galvo Fiber Laser 20W
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  5. #5
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    It also sounds like your cutting bed is not the same height on all four corners, as well as tube cooling issues and mirror alignment.
    Stay at it.
    Scott

  6. #6
    I think its a cooling issue. Cuts this cool Monday morning am (cool for Vancouver in winter at least) have been good so far. I've lots to play with today but started with setting the cw-5000 to -6 from stock -2 to ambient and its better. I've had a hint of the issue so I'm not there yet but the holders seem to be cooler. My guess is that if the tube is warm the beam expands, hence more absorbtion/heat? Anyone know the physics on why cut quality goes done with a warm tube?

    GF
    Graham Facer
    1530 Omni CNC router (run with Vcarve), Shenhui 1200 x 900 150W reci laser cutter (now with EFR F6 hopefully)
    48" Generic Vinyl cutter, Roland engraver, and a dalhgren and vanguard on the project table.

    ...My pet peeve is a good thread with no conclusion because the OP solved the issue and disappeared. Either that or bus fatalities are much higher than reported.

  7. #7
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    Power drops, lasing action can get flaky at higher levels than usual.
    Hi-Tec Designs, LLC -- Owner (and self-proclaimed LED guru )

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  8. #8
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    My experience with cutting degrading the longer the laser ran ended up being the cartridge going bad. In the end it would cut fully out, then within 30 seconds of running the cuts started to degrade until it was more like vector marking instead of cutting. I ran a series of 1/2" squares and when flipped over you could see the cut completely thru then start fading even within a single square. The cooling fans ran full speed all the time too. I didn't realize how loud they were until the new cartridge was installed and the machine turned on. I first didn't think it was running at all until I checked the display.
    I have dual cartridges in mine so it was a bit harder to tell what was happening until I ran some test cuts with each cartridge separately and in fact I could cut with the single 75 watt cartridge as fast as using both together (60 & 75 watts).
    Universal Laser ILS 12.150D (48"x24") 135 watts total, with 60 watt and 75 watt laser cartridges. Class 4 Module (pass thru ability). Photograv 3.0, Corel X6, Adobe Design Standard CS4 Suite, Engrave Lab laser Version 8, Melco Single Head Comercial Embroidery Machine, The Magic Touch System with Oki C711WT printer, and Graphtec CE6000-60 plotter.

  9. #9
    Hi Mike,

    Well the Shenhui is not cartridge based but hopefully that helps someone. Cutting for 2 hours so far this AM the chiller temp seems key so far. Still have my abs job to finish which will be the test.

    GF
    Last edited by Graham Facer; 02-23-2015 at 7:00 PM.
    Graham Facer
    1530 Omni CNC router (run with Vcarve), Shenhui 1200 x 900 150W reci laser cutter (now with EFR F6 hopefully)
    48" Generic Vinyl cutter, Roland engraver, and a dalhgren and vanguard on the project table.

    ...My pet peeve is a good thread with no conclusion because the OP solved the issue and disappeared. Either that or bus fatalities are much higher than reported.

  10. #10
    So I have an update, I now suspect that the cooling system of the reci tube itself is not up to the task at the 95% power I was running it at.

    Today I carefully checked and flattened the knife table, moved and levelled the machine, levelled the knife table relative to the laser head, cleaned out the corrosion in the water pass throughs, blew out the chiller vents and rechecked the laser aim.

    Then I cut test after test. I repeatedly could cut in the far corner (bottom right on my machine) and would cut right through, but with the same settings, I couldn't cut through if running a series of test squares along the x axis. The chiller seemed to be keeping it's cool so I wondered it it might be that the cooling system in the tube itself could not transfer enough heat to the cooling water.

    To test this I lowered the power to 50% and tried the same speed as before and bingo it cut all the way across.

    So my tentative conclusion is the tube design itself is flawed. It doesn't have enough surface area to cool it fast enough.

    I ran out of time to determine the max power I can run and keep it at the optimal level but any comments from other owners of Chinese glass tubes are welcome.

  11. #11
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    It could be that the machine is not level. I'll assume you have the machine resting on it's feet and not the wheels. Even so, if more weight is supported on one corner it can introduce twist into the machine. On larger machines this can make the alignment shift as the head moves across the table.

    On my machine I actually need to have all the weight on one corner in order for it to be "level". More accurately I need to pull down slightly on one corner. I think they do not take the greatest care to make sure that the rails lie on the same plane when they build these. They just do 'close enough'.
    Shenhui 1440x850, 130 Watt Reci Z6
    Gerber Sabre 408

  12. #12
    Hi Rich, I moved the table and levelled it first, it is on its feet not the wheels. I also rechecked the aim at all corners on the 3rd mirror.

    I'll actually be fine if it's consistent, it's the variation that is killing me.

    Just odd that the chiller stays cool but the results show over heating.

  13. #13
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    Quote Originally Posted by Graham Facer View Post
    Hi Rich, I moved the table and levelled it first, it is on its feet not the wheels. I also rechecked the aim at all corners on the 3rd mirror.

    I'll actually be fine if it's consistent, it's the variation that is killing me.

    Just odd that the chiller stays cool but the results show over heating.
    The machine and bed may be level... but have you checked distance between bed and lens for multiple points on the table? The gantry could easily be wonky, leading to out of focus conditions at various spots.

    You mention you can always cut in one area of the table but not another... that has nothing to do with tube cooling.
    Hi-Tec Designs, LLC -- Owner (and self-proclaimed LED guru )

    Trotec 80W Speedy 300 laser w/everything
    CAMaster Stinger CNC (25" x 36" x 5")
    USCutter 24" LaserPoint Vinyl Cutter
    Jet JWBS-18QT-3 18", 3HP bandsaw
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    Delta 18-900L 18" drill press

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  14. #14
    Hi Dan, that head (lens really) to table level was also done yesterday. I have not checked at every spot on the table, mainly corners. But as I also made sure the front and back of the knife table was flat side to side, so it should be OK everywhere. That is what I referred to when I said levelling the bed.

    I was definitely off on lens to table height before yesterday though, so the problems described at the start of the thread were certainly worse due to that.

    Today or tomorrow I'll try and check power levels. I don't usually have that system (added by Greg) hooked up.

    Graham

  15. #15
    Anyone know the physics on why cut quality goes done with a warm tube?
    How long do you have

    The problem is the reflective index of the Mo mirrors, they are quite well below both copper and gold PVD coated so will get a lot hotter when running than a gold mirror does. They also corrode quite quickly and lose more of that index in a short period of time.(think 95% after a week or two) that 5% of the ERP adds up to a lot of power in a very small area so will raise the temperature of both the optics and the holder quite quickly.

    Set the CW5000 to come on at 19C and go off at 17C, anything over 22C on a fatbody tube will cause problems even though the makers say 30C is safe (it is but it's unwise to say the least)

    cheers

    Dave
    You did what !

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