laser cutter acting strange

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Matt Harrington

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Jan 12, 2018, 6:37:15 PM1/12/18
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I was doing some long cuts with Bodie and we experienced some odd behavior: the laser intensity seemed to be reducing through the duration of the cuts.  By the end, the cuts were much more shallow.  I was able to confirm this by repeating a cut I had done earlier which was now no longer able to cut through the material.

We were taking notes on all of our settings in each cut so I don't think we were making an obvious mistake.  Focus still appeared good.  Ventilation appeared to be working properly.  The water tank was getting warm but not hot.

His cuts: 1/4" ply, 5mm/s 99%
My cuts: 1/8" ply, 10mm/s 99%

Did we break it somehow?

Kenny

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Jan 12, 2018, 7:19:56 PM1/12/18
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Hi guys, most of the info below probably won't be applicable since it
sounds like you already checked several things on the list. Seemed like a
good time to start putting together documentation for our fancy new
machine repo on github. Feel free to format and edit as desired.

CO2 laser efficiency depends on a whole lot of factors. At the top of
that list after power level, focus and time on target:
* tube temperature
* optics alignment
* optics surface contamination
* beam path attenuation over path length

=== Tube Temperature ===
Running long jobs with high power output should be done with a chiller to
maintain any sort of consistency. Putting ice in the water bucket would
help considerably. A change from 30C (warm summer day) to 10C can result
in a considerable output change. It has been a while since I looked at
this stuff, but I'm remembering somewhere in the 20%-40% higher output
power.

=== Optics Alignment ===
If a portion of the emitted energy is spilling off the edge of a mirror or
lens, that power is lost. Worse than that, it is lost in the form of heat
where we don't want it. Possibly even damaging optics retainers, machine
enclosure, more sensitive machine bits or even escaping the machine and
becoming a significant operator hazard. I've seen unwanted heat build up
and produce flames, melt machine parts and do all sorts of other bad
stuff. If the laser ever acts in a particularly unexpected way, be
proactive and get help checking optics alignment.

=== Optics Surface Contamination ===
A laser cutter has kurf, just like a table saw spitting out sawdust.
Instead of visible chunks of material, ablated material is so small that
it floats around in the air. Most of this is drawn out by ventilation
and, ideally, captured by filters. But some solids will deposit on
surfaces inside the machine including lenses and mirrors. A significant
increase in deposits will be observed when cutting materials with higher
active resin content. This creates a sticky film which grabs more
non-sticky particles. Regular cleaning should be part of the standard
maintenance procedure.

=== Beam Path Attenuation Over Path Length ===
As noted above, ablation generates an eruption of particles, filling the
machine with a cloud of dirty air. Just like headlights in fog, this
molecularly loaded air will absorb and reflect light energy. Suspended
moisture will refract laser light, sending it off course. And this will
get progressively worse as the beam needs to travel farther to get to the
cutting head. This means cuts near the far left of the bed will have
higher power on target while the near right with longest beam path will
have much lower power on target. To resolve this, reduce the rate of
smoke generation such that the ventilation can keep up by turning down
output power and feed speed. Or pause your job every once in a while to
let the machine vent.

-Kenny
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Matt Harrington

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Jan 12, 2018, 7:34:15 PM1/12/18
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Thanks Kenny, this is great information.  The water felt warm but not hot, it's possible it could have been enough to have an impact.  Also, I believe someone was cutting particle board which would have that high resin content.  If you are comfortable with trying again before doing a cleaning, let me know and I'll re-run my earlier test.  Also, I would like to learn how to clean this machine when you have a chance.


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Matt Harrington

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Jan 12, 2018, 8:10:41 PM1/12/18
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Someone else is using it now and experiencing similar issues.  The water feels cool but not cold.

Matt Harrington

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Jan 15, 2018, 6:13:11 PM1/15/18
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I added ice to the reservoir then resumed testing on 1/8 in ply at 5mm and 99%. I am attaching an image that shows how low the power gets. It cannot even cut the tape.

I cleaned the mirrors with iso and a qtip and the mirror over the cut lense was dark. The pause and cleaning and lowering the power to 80% resulted in a very dark cut line. Shortly after, power dropped again

The apparent duty cycle is very low right now.  I am getting by regularly pausing the machine. 


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