Cant cut 12mm plywood

[ondrej-sestak]

Hello everyone,

i am having some trouble cutting thicker material. I have my lens focused in right distance, also focused in the middle of the material using 300mm/s speed with my 130W tube an 75mm lens. I am also using compressed air assistance, but i just cant cut thicker materials properly, probles starts at 8mm and up. For example I tried to cut 10mm birch multiplex with non water proof glue as i found in mailling list it should be good for laser cutting, but it cant cut the wood all the way through in every part of the project and even if its cutting through and the air exhast is running it burns up so much upwards, that i need to stop cutting to prevent air assistance connection from melting and also to protect the nozzle and lens. Also the material is very burned and edges are covered with smoke which stains a lot. Even slowing down the speed doesnt help at all.

Any suggestions where the problem is? The optics are adjusted totally accurate. Maybe wrong type of wood?

Thanks.

Ondrej

[Chris Uhlik]

That's my experience as well.  Above 6mm, cutting is highly variable.  For denser materials like some exotic hardwoods, even 4mm is too thick.  I've had some success running faster speeds and lots and lots of passes where each pass cuts down perhaps 1 mm at a time.  But this results in a wide kerf with a lot of edge burn-back.  Perhaps as much as 2--3 mm of kerf width, but it varies a lot.

Chris

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[Stefan Hechenberger]

Hi Ondrej, the cutting performance of plywood depends a lot on the
density of the wood and the kind of glue.

Also, how much air assist are you using. Too much is also possible as it
cools the cut area down.

I can cut birch multiplex at 300mm/min reliably with a 100W tube. See:
https://vimeo.com/home/myvideos

The plywood is this:
http://www.expresszuschnitt.de/Multiplexplatte-Birke-Multiplex
Denisity is about 650kg/m3
DIN 68705
Glue: BFU 100 (waterproof)

Higher density woods are also much harder to cut. To prevent any flame
from hitting the laser head you should cut faster and use multiple
passes.

--
Stefan Hechenberger
studio: http://www.nortd.com
project: http://www.lasersaur.com

[ondrej-sestak]

Hey Steffan,

so i was able to get exactly the same type of birch multiplex as the one you linked, but the result is still the same. I tryed different focus distances, different speeds, but the last mm of the plywood just cant be cut all the way through. Even on 200mm/s and 100% of my 130W tube the result is the same. Regarding my air assist: its very week, so i think this is not an issue. Multiple passe dont work as well i tryed to run multiple 800mm/s passes but i am still not through the material.

Its very confusing for me, any suggestion? My optics are fine, lense is clean the mirrors as well, output od the laser psu is set ok, focus distance is right, air assist is on, but I still cant cut anything more thin than 6mm. Should i try to buy 100mm lens for example?

Thanks for any suggestions.

Ondrej

Dne sobota 17. října 2015 17:28:04 UTC+2 ondrej-sestak napsal(a):

[Stefan Hechenberger]

Hmm ...not beeing able to cut the last part even when doing multiple
passes is usually evidence for a focus problem. Are you sure you have
the focus in the middle of the material? Have you tried lowering it a
bit.

Curious, what kind of tube do you have. The Reci S6? Have you checked
the laser PSU provides adequate output current? Maybe you are not
driving the tube at 100%?

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[Mark Melvin]

I would also make sure your lens is in the correct way up/down.  I just recently discovered mine was in upside down, and while the machine still worked, it was definitely showing signs of "low power" and I could not cut through materials even with multiple passes.  I've just been re-doing my optics and alignment and discovered the lens and questioned its orientation.  What a difference!

To view this discussion on the web visit https://groups.google.com/d/msgid/lasersaur/1445331341.3546.29.camel%40alpendohle.

[Steve Baker]

If you set the focus into the middle of a 12mm thickness - and don't
re-adjust it after each pass - then it's going to be 6mm out of focus by
the end of the cut...and that's definitely going to be a problem.

I think it would be worth adjusting the focus after each pass...or maybe
after each couple of passes.

12mm is right on the limit of what we're going to be able to cut - so I'm
not surprised that there are problems.

-- Steve

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[ondrej-sestak]

Hey guys,

Stefan: Yes i am sure i have it focused in the middle of the material and I also tried to use lower focus and also higher focu with no result, bascily there is always like not even a 1 mm to go through when i cut from the edge of the material to see.

My tube is Reci W6 130W rated power (150W max power) and yes I am sure my output is correct i have checked it with 3 different meters to be sure because there was a small difference with different ampermeters. http://www.cncoletech.com/Laser%20tuber%20long%20life.html my current should be 30 which it is right now.

Mark: My lense is up with the rounded side which should be correct, right?

Steve: Yes I did lowered the lens each pas, but basicly nothing happened.

Thank guys for any more suggestions.

Ondrej

[Steve Baker]

OK - so let's think a bit about this.

You make whatever passes at whatever power - and all that's left is a 1mm
thick layer of wood at the bottom of an 11mm tall 'slot' (the kerf left by
the previous cuts).

We know that the laser can, for sure, cut through a 1mm piece of wood if
the laser is focussed at that height. So if you followed my earlier
advice to refocus as you drill down - then 150 watt laser can vaporize 1mm
of wood at insane speeds - and you shouldn't have a problem.

The problem has to be the "slot" that it has to make it through on the way
down to reach that 1mm thick layer.

If we have a typical 75mm lens - then the laser goes from being about 10mm
in diameter at the top surface of the lens down to (essentially) zero
width - over a distance of 75mm.

So at the upper surface of your wood, you're 11mm above the 1mm thick
layer we're trying to cut - and the beam is going to be something like
10mm x 11/75 wide. So a 1.5mm wide beam is hitting the top of the
material - and only some of it is going down into the slot left by the
previous passes.

The focussed laser produces a kerf of around a third of a millimeter.

So we maybe have a situation where a 1.5mm diameter circle of light is
trying to get down an 0.3mm slot!

What percentage of the light will get to the bottom? The math to get this
right is more than my tiny brain can handle right now...but it's going to
be something like 20% I think...

So the mighty 150w laser is only getting *maybe* 30w down to the base of
the kerf...and a 30 watt laser has quite a bit of trouble cutting through
a 1mm thick piece of wood.

Not only that - but the light that doesn't make it into the curf is going
to be heating up the top surface of the wood and probably creating a lot
of smoke.

So I think you need some really GOOD air-assist to keep the puny 30 watts
that's reaching the bottom of the slot from being absorbed - and you're
going to have to take that last pass pretty slowly...because you
(effectively) only have a 30 watt laser!

I suppose, one thing you could do to help would be to deliberately cut a
tapering slot by cutting several lines parallel to each other in the early
passes to make a wider kerf that would allow more laser light to reach the
bottom in the final pass.

Cutting 12mm plywood - is really right on the limits of what we can
do...and possibly a bit beyond that.

-- Steve

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[Martin Mueller]

We have a 150W Boss Laser and our results on 12mm baltic birch are also intermittent. 

Sometimes a nice cut through, sometimes not, even on the same piece on the same path.

We haven't figured out why, so this thread is of interest.

Best Regards,

Marty at Gizmo-CDA

[Mark Melvin]

Hi Ondrej,

Yes, rounded side up is correct.

Mark.

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[Steve Baker]

Plywood is always a bit tricky:

Although the wood generally looks great on the outside layers,
the inside layers can be terrible quality with knot-holes and
large cracks.

Even high quality plywood, where you can see that they punched
out the knot holes on the outer layers and filled them with either
hexagonal or elliptical inlays of good wood, the inner layers
will typically still be pretty awful.

With knot holes, if the knot is still in there, it's very hard,
resinous wood - and the laser doesn't cut it very well.

If the knot falls out before they laminate the material, the
resulting circular hole in the inner layer fills up with the
super-hard epoxy adhesive...and the laser doesn't cut adhesive
very well either!

We VERY often find dense designs that we cut have an area where
the laser didn't get through that is about an inch across (and
usually elliptical in shape) - that's a for-sure sign that you
had a knot hole in the inner layers.

Cracks in the inner layers seem to be rarer - but when you find
one, it typically spans the entire sheet at right angles to the
surface grain - producing a quarter inch wide region that didn't
cut...and it's the same deal - the crack fills up with adhesive.

We haven't found a solution to this - if we go very slow to make
sure we *do* cut all the way through - we find that the laser has
burned out the dense knotty wood in the middle layer and left a big,
rough charred region that extends back into the middle layer for
a quarter inch or more. That's not acceptable to us.

So no matter what, we suffer this problem - and we can only solve
it by buying the best material we can find, and doing careful QA
work on the back end to discard any bad parts and re-cut them.

The percentage of parts that are affected is not important to us,
but the effort in checking them - and the negative effect it has
on our customers is a huge issue.

-- Steve

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[Steve Baker]

(And if you have a hard time deciding...you should be able to see a
reflection of your entire face in the upper side - just your eyes in the
lower side!)

-- Steve

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[ondrej-sestak]

Hello guys and fellow 12mm suffering cutters,

today i got 12mm special laser multiplex birch plywood, with the right glue and I must say the problem with cutting its just gone.

So everyone who has this problem, the biggest issue is the material and especially the type of glue. I saw some topics about this here, but i want to make it clear enough:

The right type of glue is Melamine glue - non water proof, or waterproof it does't matter i tried both with same results. We was able to cut wood with even more density than 650kg/m3 like this.

The wrong type of glue in plywood formaldehyde glue, which is unfortunately used in 90% percent of plywoods on the market (here in central Europe), it was really hard to get a supplier with right knowledge and right supply.

Also the focus depth came out quite diferent for me - in my case the best result was reached focusing on the surface of the plywood. I started to cut with focu in the middle of the plywood, but th kerf was to wide, so basicly we are able to cut all the way through without any problem on basicly any focus with 500mm/s (130W tube) but the kerf is the smallest  while focusing on the surface. We was also able to cut a 12mm plywood with 3mm plywood on top of it, also quite good.

So I just want to tell you guys what worked for me, I hope it will help anybody suffering with cutting performance, the glue is the key.

Ondrej

[Stefan Hechenberger]

Glad to hear you figured it out. Thanks for reporting back.

I am now wondering if Expresszuschnitt mixed multiplex ply with different glue in their stock.

[Steve Baker]

That's a good finding! Thanks.

I'm actually not too surprised that getting plywood with the right glue
helped - even cutting 3mm plywood, a blob of glue in the middle layer will
stop the laser from penetrating. Those blobs occur either because a
knot-hole fell out or there was a big crack in the wood in that middle
layer. But if this 3-ply wood is (in effect) having the middle layer
replaced with glue - then just 1mm of glue is enough to block our 100w
laser.

I presume your 12mm plywood had a lot more ply's - so a lot more glue layers.

Anyway - it's an interesting finding.

-- Steve

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[Mark Van den Borre]

Hi all,

I made this info, combined with some of my own research, into a wiki addition:

https://github.com/nortd/lasersaur/wiki/Choosing-the-right-plywood-to-laser-cut

Stefan, maybe this could be reworked into a nice addition to the manual?

Kind regards,

Mark

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