Next, Donner shines a flashlight up inside the CO2
laser’s beam-delivery system. If the gaskets were
dirty and the bellows are full of dirt and debris, the
bellows themselves might have holes, and the mirrors in the beam-delivery path may be damaged (see
Figure 3). “Often, unless you replace the bellows
and optics, including the cutting lenses and mirrors,
you’ll just keep wrecking optics,” Donner said.
He added that if the optics aren’t damaged, they
may have a film on them. This comes from the liquid
nitrogen used for the bellows purge, which is why
Donner recommends the delivery optics be cleaned
every three months (see Figure 4).
After this, it’s on to the CO2 laser’s resonator and
the quartz tubes (see Figure 5). If they’re not clear,
there’s a problem that’s probably robbing the laser
of power. Contaminated quartz tubes can appear
black or white, depending on the resonator. In extreme cases, pieces of quartz can break off and get
caught inside the tube.
“It often means the operator didn’t change the
laser gas bottles when he was supposed to change
them,” Donner said.
This goes back to simply following the manual
that came with the CO2 laser machine. When the
carbon dioxide, helium, and nitrogen laser gas
pressures in the bottles fall below a certain level as
specified by the machine manufacturer—like 500
PSI for nitrogen and helium and 300 PSI for carbon
dioxide—you need to change out the bottle. Donner
recommends shops refer to their machine manuals
for the exact pressure requirements.
“If you let the bottles run down to, say, 50 PSI,
you’ve already done damage to your resonator,”
Donner said. “Below 500 PSI [or as otherwise speci-
fied by the machine maker], the impurities in the gas
bottles get in your resonator. This is why I recom-
mend shops buy alarms for their laser gas bottles.
Below a certain pressure the bottles start chirping,
and this forces you to go over and replace your laser
gas bottle. This helps protect your investment.”
If a shop is concerned about laser gas usage, Don-
ner recommends leak-testing all the lines between
the laser gas and resonator, simply by pressurizing
the lines and spraying soapy water on them. “I have
a machine that’s on its second bottle of CO2 in nine
years. My nitrogen bottles are lasting me three to
four months, and my helium is lasting one month to
two months,” he said. “It’s all leak-tested. I have no
leaks in my lines.”
When replacing the laser gas bottles, clear out de-
bris by cracking the bottle before hooking it up to
the line. At his shop, Donner uses purge valves on
the lines before the regulator. After he cracks the
bottle and closes it again, he hooks it up to the line,
opens the bottle full-on, then releases the purge
valve to clear the airstream of any debris. Only then
does he open the line to let the gas flow into the
After the quartz tubes, Donner also inspects the
resonator’s output coupler, checking for haze, dirt,
and debris. He then does the same inspection routine for the beam-delivery optics all the way to the
Next, he uses a power puck to take a power shot
at the cutting head and resonator, testing for power
loss. If resonator optics are damaged, all the available power almost certainly isn’t making it out to
the cutting head, no matter how pristine the beam-delivery optics may be—and vice versa: A shipshape
resonator won’t send all its power to the workpiece
if the delivery optics are dirty.
“A power puck test is essential,” Donner added.
“Just because the screen [on the control] says you
have a certain amount of power from your resona-
tor doesn’t mean you truly have that power.”
With beam-delivery optics in optimal condition,
typical CO2 lasers should have at most a 400-W pow-
er loss between the resonator and cutting head.
However, with suboptimal optics the loss is much
greater, because they are absorbing (instead of re-
flecting) more power than they should be.
Donner reiterated that he has never had to meticulously inspect the beam-delivery component
on his own CO2 laser for nine years. His secret? “I
keep my machine clean,” he said. “For instance, the
The film on this delivery optic, from a system with a
liquid nitrogen purge, shows why optics should be
cleaned every three months.
This quartz tube mounting block is coated white, a sign
of serious contamination. A piece of quartz has even
Corrosion in this shield (inside the beam path) is a sign that
the chiller water hasn’t been properly filtered, flushed, or
maintained at the right water-to-chemical ratio.
Dirty water in the cooling system can contaminate the
cathodes. If the chiller water is properly cleaned, the
cathode should remain clean, as shown by the top part
in the bottom photo.