scattered beam directly from the tube

[skateboss3]

Hi guys, I have a new 150w tube and the beam is scattering even when you test with a piece of masking tape on the first mirror.

Has anyone seen this before? 

I am guessing that the tube has had it.

Regards Chris

[Jonathan Buford]

Is the exit of the tube clean without any grease or anything that might cause that?

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[Chris Lawrence]

Yes I checked with a torch and there is nothing on it. 😕

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[Steve Baker]

What exactly do you mean by "scattering"?

What are you seeing?

You should expect a spot size maybe a centimeter or so in diameter.

-- Steve

> On Tue, Oct 6, 2015 at 10:53 PM, skateboss3 <skate...@gmail.com>
> wrote:
>
> Hi guys, I have a new 150w tube and the beam is scattering even when
> you
> test with a piece of masking tape on the first mirror.
> Has anyone seen this before?
> I am guessing that the tube has had it.

-- Steve

[Luke Olson]

The burn on the tape looks roughly like a doughnut which is the shape of the intensity of the beam as it exists the tube. Looks good to me. Maybe try something with a clearer marking like thermal paper to get a better picture of what the beam actually looks like.

Luke

--

[Steve Baker]

Yeah - a doughnut-shaped beam is fine - I don't understand the subtleties
- but it's an expected thing for the kinds of laser tube we have. I see
that all the time.

-- Steve

>> <https://groups.google.com/d/msgid/lasersaur/52bc05bb-f0eb-40b6-a763-592afb4337ea%40googlegroups.com?utm_medium=email&utm_source=footer>
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-- Steve

[j. eric townsend]

On 10/6/15 10:53, skateboss3 wrote:
> Hi guys, I have a new 150w tube and the beam is scattering even when you
> test with a piece of masking tape on the first mirror.

How did you generate the test beam and at what power? My Jupiter power
supply has a Big Red Test Button that fires the laser at %100. Useful
for "does it work" but not very useful for "is it working correctly".


--
J. Eric Townsend, IDSA
design <http://www.allartburns.org>
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[Chris Lawrence]

That test was at 100% power with a split second push of the test button. The beam seems to pierce a hole both sides of the center rather than just piercing a single round hole.

If I do the same test on my other 40w machine I get a perfect hole in the center.

As I said this is on a 150w machine and I am struggling to cut a piece of 6mm mdf at 100% power @ 10mm/s.

Cheers 

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[Steve Baker]

Was the paper held a small distance in front of the mirror?

If so, then the laser would go through the paper in one spot, bounce off
of the mirror and go back out through the paper a little off to one side.
Might that not explain the problem?

I generally do these tests by removing the mirror and replacing it with a
25mm diameter plywood disk.

But I agree that you should be able to cut 6m MDF at 100% power and
2000mm/min quite easily, especially with a 150W tube. If you're having
trouble at 600mm/min, then there is definitely something wrong.

Here is a checklist of likely causes for poor cutting that I've been
building:

http://sjbaker.org/wiki/index.php?title=Reasons_for_poor_laser_cutting

Sadly, it's probably not an exhaustive list!

-- Steve

> https://groups.google.com/d/msgid/lasersaur/CAA4SruVxqzJcyc8ifCt%3DuzuLRbaHaJrR5AZPnU2idf_9CqapdQ%40mail.gmail.com.

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[Chris Lawrence]

Hi Steve, yes it was but that pattern looks the same at every mirror. I think the tube may have some kind of alignment issue inside. Do they have adjustable focal lenses on the front?

Cheers chris

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[skateboss3]

I removed the first mirror and held a piece of mdf around 6 inches from the laser tube and this image shows the result. Does it look normal?

We do get a clicking noise from the positive end of the tube when hitting the test button, I have checked that the connection is secure but still hear a click like an arcing noise this only lasts for a split second when hitting the test button. Our 40w tube makes no noise at all when testing.

[Steve Baker]

No there is nothing whatever that is adjustable on the laser tube itself -
the only thing you have control over is the beam current coming from the
laser power supply.

Don't go touching any of the bolts on the tube in an effort to do
something - that could allow contaminants into the super-pure CO2 inside -
which will permanently destroy the laser *instantly*.

If the beam pattern is the same size and shape at every mirror - then the
laser is still producing parallel rays of light. If the pattern is
entirely within the 25mm diameter circle of the mirror/lens then it really
doesn't matter how it's distributed because it all gets focussed down to a
point anyway.

Can you send us photos of the pattern? It's really hard to know what
exactly is the scale of your problem (or even if it's a problem at all)
from just a verbal description.

Did you work through the checklist I posted yesterday?

>> -- Steve
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[Steve Baker]

That picture looks very typical...and the 120 watt laser tube is quite
audible when it's firing (either single-shot or continuously), so it
doesn't at all surprise me that your 150w tube is similarly noisy!

I don't think there's necessarily a problem with the tube.

It's **MUCH** more likely to be in the realms of alignment and focusing.

-- Steve

skateboss3 wrote:
> I removed the first mirror and held a piece of mdf around 6 inches from
> the
> laser tube and this image shows the result. Does it look normal?
>
> We do get a clicking noise from the positive end of the tube when hitting
> the test button, I have checked that the connection is secure but still
> hear a click like an arcing noise this only lasts for a split second when
> hitting the test button. Our 40w tube makes no noise at all when testing.
>

> <https://lh3.googleusercontent.com/-uwaHx8k3OmQ/VhUB8yjNmtI/AAAAAAAALOE/kCAkSV7RZa0/s1600/scattered%2Bbeam%2B2.jpg>

>
>
>
>
> On Tuesday, 6 October 2015 15:53:33 UTC+1, skateboss3 wrote:
>>
>> Hi guys, I have a new 150w tube and the beam is scattering even when you
>> test with a piece of masking tape on the first mirror.
>> Has anyone seen this before?
>> I am guessing that the tube has had it.
>>
>> Regards Chris
>>
>

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[skateboss3]

Hi Steve, you dont seem to see my attached images on these posts. I have sent 2 images and one is attached to my last post.

Here it is again:

[skateboss3]

On Tuesday, 6 October 2015 15:53:33 UTC+1, skateboss3 wrote:

[j. eric townsend]

On 10/7/15 08:48, Steve Baker wrote:
> It's **MUCH** more likely to be in the realms of alignment and focusing.

I agree. This summer I wasted most of a weekend trying to fix an
alignment problem, turns out my laser tube *mounts* were slightly out of
align.

We (CMU) had a laser a few years ago that also produced a donut-shaped
beam, once I got that beam to the final focusing lens it cut just fine.

There are a couple of how-tos on the lasersaur site, I'd start with
those and do low-power cuts with something like bristol if you don't
have the ability to make Steve's 25mm wooden discs.

[Steve Baker]

Let's be absolutely clear about this: The 'donut shaped' beam is
absolutely normal for our CO2 lasers - it's not a problem or a defect of
any kind.

The technical reason for this is a bit beyond my knowledge - it's to do
with how the laser beam is polarized - the way that the electromagnetic
field oscillates relative to the beam direction - and there are various
distinct modes of oscillation involved. Our lasers are Transverse
Electro-Magnetic" oscillators ("TEM") - and there are various classes of
TEM called TEM-00, TEM-01 and so forth that describe more exactly how the
polarization happens to be.

Our lasers are "TEM-01 first order"...which for some reason that I really
don't understand means that the cylindrical laser beam (which is about the
diameter of a pencil) has all of the energy concentrated near the surface
of that cylinder - with very little in the center of the beam.

For our purposes, we really don't care where the energy is so long as it
stays within the pencil - because the entire thickness of the beam gets
squished into an tiny, tiny dot when the beam is focussed by the mirror.

So it doesn't matter whether the energy is concentrated in the center of
the beam (which would be "TEM-00") or on the outside (which is "TEM-01") -
or any other way...it all ends up in the exact same spot when we focus it.

In applications *OTHER* than laser cutting/lasic eye surgery/tattoo
removal, the way that the energy is concentrated into the beam actually
matters - and those people have to spend a LOT more money to get a fancy
TEM-00 laser.

We literally don't care what mode the light is oscillating in - or that
there is a donut hole - so we spend less money and get cheapo TEM-01
lasers.

-- Steve

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[skateboss3]

Thanks for all of your help Steve, I have been playing a lot today and I have it to where I can cut 6mm MDF at 20mm/s 100% power with 2 passes (JUST), there is something definitely wrong here.

4mm Acrylic just cuts with the same settings.

the beam ends up in the center of the final mirror before hitting the focusing lens and the beam is hitting the center of the other mirrors too.

So im lost as to what it can be.

The lens has a slight hairline scratch this may be a lot of the problem.

cheers chris

[Steve Baker]

A small scratch doesn't always mean that it's time to replace
the lens - but it could certainly reduce your power somewhat.

You're going to need to keep a couple of lenses "in stock" because laser
cutters get through lenses relatively quickly. In this case, I would swap
out the lens for a new one - and see if that produces improvement. If
not, put the scratched lens back into the machine - lenses cost around $60
- so you'll want to get your money's worth!

If you don't have a replacement lens, I'd try rotating the lens holder
through 90 degrees and see if that helps. Often the scratch doesn't go
dead center through the laser spot - and turning the lens around can buy
you more power.

The fact that the laser hits the mirrors dead center is only
the first step. It also has to appear *cleanly* out of the
hole in the nozzle - which may well require some further adjustments.

One of the most common problems we see here with insufficient power
is that the focussed laser beam is clipping the side of the nozzle
on the way out. It *looks* like the laser is properly aligned - but it's
not *quite* right.

Sometimes you can tell that this is happening because you get extra weird
etch lines when you're cutting - at other times, you can tell because the
focus tube starts to get really hot as it absorbs the energy that's not
making it out of the nozzle.

What I do to test this is to turn off the air assist and put a piece of
masking tape over the hole at the bottom of the nozzle - you can push
the tape up against the underside of the hole to leave a small dimple
in the tape. Give the laser a really brief test fire at low power and it
should punch a tiny pinhole in the tape in the center of the dimple you
put into the tape...actually, it should be smaller than a pin-hole.

If the hole is bigger - and especially if it has a weird shape or doesn't
emerge close to the middle of the nozzle - then it's probably
grazing the edge of the hole - or missing it entirely and bouncing off the
sides before it emerges from the bottom.

You might also have inadequate air assist or a badly inadequate
ventilation fan - smoke build-up inside the case attenuates the laser very
seriously.

You *SERIOUSLY* need to work down the checklist I provided here:

http://sjbaker.org/wiki/index.php?title=Reasons_for_poor_laser_cutting

-- Steve

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[Chris Lawrence]

The problem that I have with the whole thing is that the beam coming out of the tube seems to start as two beams around the center. I.e I get two pin holes either side of center when I hit the test button, this stays the same at each mirror. I end up with a donut shape if I hold it on for long enough but the initial press of the button leaves those two separate beams.

Cheers chris

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[j. eric townsend]

On 10/7/15 14:18, Steve Baker wrote:

> What I do to test this is to turn off the air assist and put a piece of
> masking tape over the hole at the bottom of the nozzle -

I do a similar test at the top of the tube to verify that the top mirror
is pointing the pre-focused beam in the correct direction.

[Steve Baker]

You're over-thinking it.

The beam is never perfectly even - and the response of materials to laser
energy is highly non-linear. So even a small difference in energy at the
sides of the beam might be enough to char where the rest of the beam
doesn't.

Again - 99% certain, your problem lies elsewhere. This isn't evidence of
a bad tube.

-- Steve


Chris Lawrence wrote:

> https://groups.google.com/d/msgid/lasersaur/CAA4SruVHDJ1t%2BoX62qCTvyT7w%2By3-vAo3x8v5vQ7NeSh1okt-A%40mail.gmail.com.

[skateboss3]

Here is a clearer picture of what is happening at every mirror.

I have found that the company who supplied the machine have neglected to set the power output on the PSU so they may have ruined the tube, when I got under the machine and looked at the digital readout on the PSU it was reading 48mw output. Everyone says no more than 28mw.

Does this look like something that would happen to an over powered tube or would it just decrease the life of the tube / P?

[Steve Baker]

"Everyone says 28mw"...FOR THE 100 WATT TUBE.

Therefore 28mW sounds low for a 150W tube.

If the 100W RECI W4 tube takes 28mW, then it follows that *IF* the
efficiency and supply voltage are the same for the 150w tube then 42mW
would be the right number. It's at least plausible that the 150w tube is
~15% less efficient than the 100w tube and actually needs 48mw from the
power supply. On the other hand, the 150w tube might be more efficient
than the 100w - and maybe you're right and the wattage is kinda high.

The trouble is that most of us are running the 100w or 120w tubes...so
this community doesn't have a whole lot of experience with the 150w
device.

But excess wattage should result in more laser power - at the cost of
shortening the tube life. Exactly how much life shortening for how much
excess power is uncertain.

I'm guessing you don't yet have many hours of use on the tube. So I kinda
doubt that this number of milliwatts is enough to have actually wrecked
it. If anything, I'd have guessed that this has increased the amount of
power rather than decreasing it.

I'm still betting that your problem is in the alignment or something.

-- Steve

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[Luke Olson]

For clarification it's 28mA not mW and that's for the 100W RECI tube. Not sure what tube you're using but if it's 150W then it will be more than 28mA load (probably somewhere around 42mA but check the manual or specs for the tube). What Steve said, you're over complicating this. The beam shape might be slightly different from the 100W RECI tube. Bottom line, does it cut stuff? If so then you're good to go. The fact that it's cutting through masking tape with an unfocused beam says yes, it's working fine.

Luke

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[Steve Baker]

@Luke: Ack! I knew that! Thanks for the correction. :-)

@skateboss3: I did some googling and found this table:

http://www.alibaba.com/product-detail/Original-RECI-Brand-80W-100W-130W_60104361072.html?spm=a2700.7724857.35.1.kyCJAj&s=p

Scroll down to:

"1. Specification of Reci Co2 Laser Tube 80W 100W 130W 150W :"

It says that:

* The Z2 (rated at 80w) tube needs 27mA
* The Z4 (rated at 100w) tube needs 28mA
* The Z6 (rated at 130w) tube needs 29mA
* The Z8 (rated at 150w) tube needs 30mA

It's a bit messy because the Wattages are the "normal operating" output.
They often brand the tube for it's "maximum" wattage - which is
significantly higher.

So my "120w" Z4 tube is really only usable at 100w if you want it to last
more than a couple of thousand hours - and the "24mA to 28mA" number that
the Lasersaur instruction suggest is about right.

The question here is whether skateboss3's tube is a Z6 (Max power=150w,
Rated power=130w) or a Z8 (Max power=180w, Rated power=150w)

There is a sticker on the tube that tells you that.

So, yeah - it looks like skateboss3 should dial back the current to around
30mA if it's a Z8 tube or 29mA for a Z6.

I'm surprised that it's so little - but evidently the larger tubes are
more efficient.

> https://groups.google.com/d/msgid/lasersaur/CAKy8TOJpM-niJqTvDPT%3DmcsaoQ5dNPA6%2BGwLcSRw-%2BEU-Hj_tQ%40mail.gmail.com.

[Chris Lawrence]

Thanks for all of the time and effort that you have put in. It is a 150w rated to a max in 180w so it's a z8.

I wound it down to 28mA so will go back to 30. I think that the machine suppliers are going to send me a new tube, mirrors and lens as the lens is scratched and the tube has been run over current at 48mA so has more than likely shortened it's life span which I dint see why I should foot the cost when it is something that should of been set correctly when it was built.

I may still play around with the alignment if I get a spare minute.

Many thanks Chris

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[skateboss3]

UPDATE:

They are sending me a new 150w tube but only after smashing the original tube as they wanted proof that I wasn't trying to get a free tube out of them lol.

Can I hook up a 40w tube to my 150w PSU if I turn the pot down and if so how many ma should a 40w tube run at? As I could use this as a temporary fix until the new one arrives.

Cheers Chris

[Jonathan Buford]

Hi Chris,

You would need to look up the specs for that specific tube, not all 40W tubes are the same requirement.

Best Regards,

Jon

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