Unfocused laser burn mark
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Jacob Kritzinger
Aug 19, 2014, 11:34:02 AM8/19/14
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Hello everyone,
I've been trying to sort out laser power loss and have upgraded all mirrors, and even replaced my lens in hope that this would solve the problem. Yet after all that, and double checking alignment, I still have a large loss in cutting ability.
My question to anyone who can help is as follows:
The unfocused burn mark looks like a circle, not a solid dot. So instead of a solid pencil sized dot I have a circle cut forming if a piece of paper is held in front of the tube while firing. Is this a sign of a dying laser tube? I have the correct Amp reading coming out of the tube, which would let me think its ok? But could it be a problem with the electrode?
Any help would be greatly appreciated!
Kind regards
Jacob Kritzinger
Chris Uhlik
Aug 19, 2014, 11:47:37 AM8/19/14
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Steve Baker
Aug 19, 2014, 12:27:00 PM8/19/14
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I don't think that's incorrect...both of my lasers produce a ring-shaped
mark when unfocussed. You don't always see a ring when you zap a piece of
wood with the unfocussed beam because if the power is high enough, even
the center of the ring has enough energy to blacken the wood...but if you
dial down the power until you get a soft brown scorch - you can usually
see the ring quite clearly.
My understanding of this is that it relates to the "TEM mode" of the laser
- the best explanation I've seen is:
https://en.wikipedia.org/wiki/Transverse_mode#Laser_modes
...but it's still not enough for me to be able to understand it!
I suspect from reading around that the "TEM mode" depends on how well the
mirrors inside the laser tube are aligned...ours, being rather cheap
lasers, aren't all that great - so you get the ring.
But when you think about it, it doesn't matter how the laser energy is
distributed through the diameter of the unfocussed beam - so long as you
are correctly focussing the beam down to a tiny spot at the end - because
all of the light from the beam eventually converges on that one spot
anyway.
Anyway...causes of power losses that we know about:
(we need a FAQ someplace!)
1) Incorrect laser power supply (eg you got the 240v version and have a
120v wall outlet)...there is a solder bridge inside the power supply to
let you switch this.
2) Insufficient current being delivered by the laser power supply...you
can measure and adjust this. Don't set it too high or your tube will wear
out extremely quickly!
3) Old laser tube, gradually losing power (seems to start declining at
about 3,000 hours use or around 2 years even if it's just sitting there).
4) Dirty exit window on the end of the laser tube itself. (Be VERY
careful how you clean it!)
5) Dirty, scratched or otherwise non-shiney mirrors.
6) Dirty, scratched, cracked, discolored or otherwise damaged lens.
7) Poor alignment of mirrors causing the beam to partially hit the side of
the nozzle on the way out. (this usually manifests as weird-looking cut
lines - and often is only evident over some areas of the bed).
8) Poor focussing of laser onto material. (should be focussed to a depth
midway through the material for wood - should be focussed to the upper
surface of the material for acrylic). Warped material that's not laying
flat on the bed will be out of focus in some areas, causing poorer cutting
there.
9) Poor smoke venting allowing smoke to obscure the path of the beam (this
usually manifests as significantly worse cutting when the head is furthest
from the origin when you're getting good results close to the origin).
10) Inadequate air-assist (technically, this isn't affecting laser power -
but it does affect the ability to cut).
By far, (5),(6),(7) and (8) are the most common causes in a lasersaur
that's been functioning OK in the past.
(7) and (8) need to be checked in all four corners and in the center of
the bed.
I've also heard that clamping the laser tube too tightly can subtly bend
it - resulting in a much worse misalignment of the internal optics. But
the lasersaur design only clamps the tube at two points - so there isn't
any opportunity for bending it.
-- Steve
mark when unfocussed. You don't always see a ring when you zap a piece of
wood with the unfocussed beam because if the power is high enough, even
the center of the ring has enough energy to blacken the wood...but if you
dial down the power until you get a soft brown scorch - you can usually
see the ring quite clearly.
My understanding of this is that it relates to the "TEM mode" of the laser
- the best explanation I've seen is:
https://en.wikipedia.org/wiki/Transverse_mode#Laser_modes
...but it's still not enough for me to be able to understand it!
I suspect from reading around that the "TEM mode" depends on how well the
mirrors inside the laser tube are aligned...ours, being rather cheap
lasers, aren't all that great - so you get the ring.
But when you think about it, it doesn't matter how the laser energy is
distributed through the diameter of the unfocussed beam - so long as you
are correctly focussing the beam down to a tiny spot at the end - because
all of the light from the beam eventually converges on that one spot
anyway.
Anyway...causes of power losses that we know about:
(we need a FAQ someplace!)
1) Incorrect laser power supply (eg you got the 240v version and have a
120v wall outlet)...there is a solder bridge inside the power supply to
let you switch this.
2) Insufficient current being delivered by the laser power supply...you
can measure and adjust this. Don't set it too high or your tube will wear
out extremely quickly!
3) Old laser tube, gradually losing power (seems to start declining at
about 3,000 hours use or around 2 years even if it's just sitting there).
4) Dirty exit window on the end of the laser tube itself. (Be VERY
careful how you clean it!)
5) Dirty, scratched or otherwise non-shiney mirrors.
6) Dirty, scratched, cracked, discolored or otherwise damaged lens.
7) Poor alignment of mirrors causing the beam to partially hit the side of
the nozzle on the way out. (this usually manifests as weird-looking cut
lines - and often is only evident over some areas of the bed).
8) Poor focussing of laser onto material. (should be focussed to a depth
midway through the material for wood - should be focussed to the upper
surface of the material for acrylic). Warped material that's not laying
flat on the bed will be out of focus in some areas, causing poorer cutting
there.
9) Poor smoke venting allowing smoke to obscure the path of the beam (this
usually manifests as significantly worse cutting when the head is furthest
from the origin when you're getting good results close to the origin).
10) Inadequate air-assist (technically, this isn't affecting laser power -
but it does affect the ability to cut).
By far, (5),(6),(7) and (8) are the most common causes in a lasersaur
that's been functioning OK in the past.
(7) and (8) need to be checked in all four corners and in the center of
the bed.
I've also heard that clamping the laser tube too tightly can subtly bend
it - resulting in a much worse misalignment of the internal optics. But
the lasersaur design only clamps the tube at two points - so there isn't
any opportunity for bending it.
-- Steve
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Jacob Kritzinger
Aug 19, 2014, 12:32:50 PM8/19/14
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Thank you Steve,
I've wondered about the laser power supply, if it's set to the incorrect Voltage input would it show the correct amp reading on the output of the tube? We run on 240v but ordering the power supply from the states I thought it may be a 120v power supply?
Kind regards,
Jacob Kritzinger
To view this discussion on the web visit https://groups.google.com/d/msgid/lasersaur/de4053b6c8a25d09bde7de25e7509498.squirrel%40webmail.sjbaker.org.
Stefan Hechenberger
Sep 2, 2014, 5:47:05 AM9/2/14
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As for the power loss ... your issue is that you had a working system that decreased in cutting performance? How quickly did this happen and after how long of normal operation?
-stefan
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