Double image, hot mirror
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Dan Funk
Nov 27, 2015, 7:42:00 PM11/27/15
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Jondale Stratton
Nov 27, 2015, 7:53:31 PM11/27/15
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This is simply an alignment problem. The double beam means it's reflecting off the end of the nozzle.
I'm assuming when you got the laser cutter someone had to go through the alignment process. Assuming it was properly aligned at some point, it looks like something has caused to lose that alignment.
Here is the entry in the manual on setting up the optics.
You will want the beam to hit the middle of your mirrors, be parallel with each axis, and obviously make it through the nozzle without hitting the end of it :) It's a frustrating process the first few times and maybe not super fun each time after that but it gets better and easier.
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Steve Baker
Nov 27, 2015, 8:04:54 PM11/27/15
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So let's take this a little at a time.
Mirrors get hot when the laser isn't being efficiently reflected by the
mirror.
There are at least three possible reasons why that can happen:
1) Misalignment - if even just a fraction of the beam hits the frame of
the mirror, it'll get hot extremely quickly.
2) Dirt on the mirror - dirt doesn't reflect light - it' absorbs it, so
even a relatively small amount of dirt will cause the mirror to heat up.
3) Scratches or abrasion of the mirror surface due to over-enthusiastic
cleaning. Same deal, the scratch or scuff doesn't reflect light very
well,
so it gets absorbed.
When the mirror gets hot, the most common first symptom (aside from not
getting as much laser energy onto the material you're cutting) is that the
little set-screw heats up. That screw has a soft rubber tip - which soon
melts - now the screw isn't tight enough - so the mirror falls out.
When the mirror falls out, the laser probably goes right through the hole
in the mirror holder and starts boiling the paint/powder-coat off of the
steel plate on the side of the machine. It's probably not going to cut
through it - but the outside of the plate can get extremely hot - and if
you have anything flammable leaning against the side of the machine, it
could catch fire.
OK - so that's what initially went wrong.
Now - very important step #1 in fixing the situation is to buy another
rubber-tipped set screw. The old one is junk...for 100% sure! Using the
old set screw will require tightening it up way too tight - and it won't
hold the mirror very firmly - so you'll have more alignment problems.
The double-image in the cut is another very well known symptom - it means
that your laser isn't properly aligned.
What happens if you're *NEARLY* in alignment is that part of the laser
beam makes it out of the hole in the nozzle - but part of it clips the
inside of the nozzle...burns off the powder-coat and then hits shiney
metal and reflects off at a glancing angle.
So some part of the laser beam is cutting properly - the rest is bounced
off a few millimeters to the side (typically) - and you get a double-line
when you cut/etch.
Typically, you'll find that this doesn't happen over the entire bed
(although it might).
If you're clever, you can see where the second ghost-image is offset from
the "real" image - and deduce which way the beam needs to be adjusted to
stop it from happening.
But in your case, because your #3 mirror's set screw is probably trashed,
you're probably getting wobble in the #3 mirror as the head is moved
around...and that means that getting everything perfectly aligned is
impossible.
Soooo....
1) Order a small bag of replacement set-screws (Thor Labs have them),
replace the screw in the #3 mirror.
2) Carefully inspect the #3 mirror - is it dirty? Can you clean it? Is
there scratching or haziness? Even if that wasn't the original cause of
your problems, the fall out of the mirror holder may have damaged it. You
may need to buy a replacement mirror...my advice is to buy several mirrors
so you always have some in stock. Mirrors and lenses are "consumable
items".
3) Look at the inside of the metal cladding on the side of your machine.
You'll probably be able to see a horizontal scar where the beam tracked
across it. Probably, it'll be OK - but if the metal is badly damaged, you
might want to either replace it or flip it upside down or something.
4) ALIGN THE HECK OUT OF YOUR MACHINE! Really - that's so important. A
misaligned laser can do tremendous damage - and any misalignment can
damage lens, mirrors, etc.
Mirrors get hot when the laser isn't being efficiently reflected by the
mirror.
There are at least three possible reasons why that can happen:
1) Misalignment - if even just a fraction of the beam hits the frame of
the mirror, it'll get hot extremely quickly.
2) Dirt on the mirror - dirt doesn't reflect light - it' absorbs it, so
even a relatively small amount of dirt will cause the mirror to heat up.
3) Scratches or abrasion of the mirror surface due to over-enthusiastic
cleaning. Same deal, the scratch or scuff doesn't reflect light very
well,
so it gets absorbed.
When the mirror gets hot, the most common first symptom (aside from not
getting as much laser energy onto the material you're cutting) is that the
little set-screw heats up. That screw has a soft rubber tip - which soon
melts - now the screw isn't tight enough - so the mirror falls out.
When the mirror falls out, the laser probably goes right through the hole
in the mirror holder and starts boiling the paint/powder-coat off of the
steel plate on the side of the machine. It's probably not going to cut
through it - but the outside of the plate can get extremely hot - and if
you have anything flammable leaning against the side of the machine, it
could catch fire.
OK - so that's what initially went wrong.
Now - very important step #1 in fixing the situation is to buy another
rubber-tipped set screw. The old one is junk...for 100% sure! Using the
old set screw will require tightening it up way too tight - and it won't
hold the mirror very firmly - so you'll have more alignment problems.
The double-image in the cut is another very well known symptom - it means
that your laser isn't properly aligned.
What happens if you're *NEARLY* in alignment is that part of the laser
beam makes it out of the hole in the nozzle - but part of it clips the
inside of the nozzle...burns off the powder-coat and then hits shiney
metal and reflects off at a glancing angle.
So some part of the laser beam is cutting properly - the rest is bounced
off a few millimeters to the side (typically) - and you get a double-line
when you cut/etch.
Typically, you'll find that this doesn't happen over the entire bed
(although it might).
If you're clever, you can see where the second ghost-image is offset from
the "real" image - and deduce which way the beam needs to be adjusted to
stop it from happening.
But in your case, because your #3 mirror's set screw is probably trashed,
you're probably getting wobble in the #3 mirror as the head is moved
around...and that means that getting everything perfectly aligned is
impossible.
Soooo....
1) Order a small bag of replacement set-screws (Thor Labs have them),
replace the screw in the #3 mirror.
2) Carefully inspect the #3 mirror - is it dirty? Can you clean it? Is
there scratching or haziness? Even if that wasn't the original cause of
your problems, the fall out of the mirror holder may have damaged it. You
may need to buy a replacement mirror...my advice is to buy several mirrors
so you always have some in stock. Mirrors and lenses are "consumable
items".
3) Look at the inside of the metal cladding on the side of your machine.
You'll probably be able to see a horizontal scar where the beam tracked
across it. Probably, it'll be OK - but if the metal is badly damaged, you
might want to either replace it or flip it upside down or something.
4) ALIGN THE HECK OUT OF YOUR MACHINE! Really - that's so important. A
misaligned laser can do tremendous damage - and any misalignment can
damage lens, mirrors, etc.
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Dan Funk
Nov 28, 2015, 4:23:22 PM11/28/15
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Thanks Steve, Jondale,
You were right. It was completely an alignment issue. I went back and started re-setting the mirrors from mirror one and it's all working correctly. Just didn't expect it to cut if it was so far off that it would hit the mount. But it is all working now and I'm not seeing the heat issue anymore or the double image. I'll order some more set screws and mirrors so I have them. And I'll replace that set screw. Thanks again for taking the time to help me. To view this discussion on the web visit https://groups.google.com/d/msgid/lasersaur/236562859518802a628a7573aee7f3cd.squirrel%40webmail.sjbaker.org.
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