Cutting thin birch plywood

[Kay Bengtson]

I'm trying to cut 1/32" and 1/16" birch plywood consistently with a 25W ULS 25ER laser cutter. On some sheets, it cuts like a dream but on many it won't cut all the way through. I realize that there is often denser wood in spots which is probably causing some of my problems.

Here is what I do to cut it. First, I focus at the surface then crank the lens down 1/4 turn for 1/32" ply and 1/2 turn for 1/16 ply. I cut at speeds at 1.6 and 1.2% full power and 350 to 400 PPI. I raise the material 1/8" off the engraving bed using 1/8" square brass tubes. To keep it from flaming, I dampen both sides of the wood.

This will work mostly for about 5 minutes but after that the airflow dries out the surface and flaming occurs. I then pause and with a wet paper towel, I dampen the surface and begin again praying that I haven't moved the wood.

In some of my sheets it can take 15 minutes to cut. The massively frustrating thing is that sometimes the wood takes up a lot of water and becomes soggy and the laser won't cut through. OR it won't take the water at all and it flames and when it flames, the laser won't cut through.

On dry warm days, with low humidity ( common here in California) the wood can be almost dry in 5 minutes. I try not to wet the bottom of the sheets too much because all I need there is to keep it from excessive burning but too much just saps away heat.

I always start with a clean lens too.

I can't drop down to 0.4% speed because the laser won't fire that slow.

I know about the waterproof glue that they use in this plywood and how it acts like a mirror to the laser beam. I wear sunglasses when cutting this stuff as a precaution. I heard that there is birch plywood made from a laser friendly glue but I cannot find a source. Does anyone know of one?

It is really maddening to flip over a sheet and find that it hasn't burned though. My only recourse is to cut the rest with an exacto knife and that looks terrible. The customer paid for laser cutting not laser marking and hand cutting.

Is there something I am missing here? Does anyone have a trick that makes this stuff cut reliably or is it simply a matter of more power? Or is this a job for an air assisted laser cutting machine?

Kay

[Larry Bratton]

Kay,
Something isn't right here. Cutting wood that thin, you should be able to cut it about 20s-25s at least w/100p. My machine is an Epilog-40watt and I can cut 1/8" at like 35 speed. Have you checked your alignment lately? I have found that one issue to be very detrimental to cutting and it doesn't take much either.

[Joe Pelonio]

I have 45 watts and cut 1/8" at speed 25, 1/16" at 40. Flaming is never a problem with air assist, I would get it if you can, but instead of wetting the wood, try applying transfer tape to the top, High Tac. Then mist that with water. The water will not get to the wood, but will minimize the flaming. When it takes that long to cut your moisture will dry out before finished cutting if a lot of small pieces, so you may have to pause a few times to mist again.

Air assist not only blows out flames, but keeps the smoke away from the lens. Without it I would expect to be cleaning the lens very often, as smoke leaves residue on it.

[Dan Hintz]

Kay,

You should be able to do that at around 15S... you need to check your alignment. Also, do not defocus for such a thin piece... a 2" lens has a solid 1/8" focal length, and at 25W you need all of the power you can muster on that top cap to blow it out.

Air through the lens assembly is a necessity to keep flaming down as water is going to be a real challenge to keep around for anything but the smallest of items (as you've encountered).

[Kay Bengtson]

Well, I checked the alignment and it is spot on. I did try cutting slower yet at 0.8% and with no water on the back side with a lower PPI setting of 120 instead of 350-400 and that reduced the flaming and allowed for a better cut. I got it to cut a piece of 1/16th plywood that was not cutting well earlier today.

Then I tried on some 1/32" ply and it didn't work. The incredibly frustrating part of this is that I do a test piece and it works then I put on a production piece and it messes up because the wood is so inconsistent. I figure that I will cut 1/32 at the same 0.8% speed and PPI as the 16th.

If I don't focus down in the material the cut line is very wide but if I do focus down, it sharpens quite a bit. Sometimes I wonder if my focusing tool is not at the correct height. With no red dot to work with it is a matter of trial and error.

Thank you for all your help. I will check out the tape idea too.

Kay

[Dan Hintz]

The red dot will not help you focus, it only tells you where the laser would hit if it was on. If you don't have a piece of anodized aluminum, use some painted metal gutter flashing from one of the Borgs... prop up one side of a several inch long piece, focus to the middle of the strip's height, and etch some lines. If it's not the thinnest right in the middle, your focus is off.

Also be prepared for the possibility that the tube may be bad... you should be running at nearly 10 times the speed you're currently at.

[Chuck Stone]

I wonder if adding a little glycerine to the water might help hold
the moisture longer? (not that this answers your question, I was
just wondering)

[Larry Bratton]

Dan wrote,
"Also be prepared for the possibility that the tube may be bad... you should be running at nearly 10 times the speed you're currently at."

that would be bad too..as I read it, the 25ER is no longer supported and if you can find a tube they are very expensive.

[Shawn Cavaretta]

where are you getting your plywood. i get mine from Northeastern scale lumber. with a 45 watt i cut 1/8 100 power 25 speed 1/16 at 80% power 40% speed 1/32 60 power 40 speed 1/64 50 power 50 speed i think you are using aircraft plywood which has a fuel proof glue in it and is a pita to laser cut

[Kay Bengtson]

The fellow I bought this from assured me that the unit has just a few months ago had a recharge. My cutting times for my balsa are in line with what other ULS 25ER machines make. I know this because my former laser cutter used one on my kits and I have some of his times for sheets.

The issue that I think is going on is that Phenolic glue layer in Baltic birch plywood that is like a mirror. When I cut that material the reflected light is so bright that I cannot stand to look at it even from across the room.

Cutting 1/8" lite popular ply goes really nicely at 1.6% speed. Then again, this is my first laser so I am hardly an expert. If you say that I should be cutting at 10 times the speed, then there must be something wrong with the tube.

I'll give that focus test a try.

Kay

[Dan Hintz]

With a 45 watt, I cut:
1/8" @ 25S/100P
1/16" @ at 40S/80P
1/32" @ 40S/60P
1/64" @ 50S/50P
Kay,

I cleaned up Shawn's settings as more proof of where you should be. His machine is nearly double yours in power, but he's still running over 20 times faster. I don't know what your last laser guy was doing... is this where you purchased the machine from? If so, his settings are going to be for a bum machine, too.

[Kay Bengtson]

Dan,
The speeds that I quoted are from his ULS laser that he still has and has used for 8 years. I bought mine used from someone else.

I did a search here for birch ply and found that others with 25W lasers have trouble getting consistent cuts. Like I said, some sheets cut fine but others have places where the laser simply won't make it through. They are usually thicker or have a knot or a burl like appearance.

One question that I do have is just how does the laser tube die? Does it slowly lose power until you replace it because it won't do the job or does it just fail to fire one day?

Another question, I seem to recall that a 30W replacement laser tube was available for the ULS 25ER at one time. Can anyone confirm this?

Kay

Kay

[Dave West]

It's probably the glue as you suspected. I have no problem going thru 1/8 and 1/4 poplar ply on my 25 watt, but the thin birch stock would rather burn than cut.

Dave

[Dan Hintz]

Kay,

Tubes can do almost anything, but the typical failure mode is either a surprise drop in power from one day to the next (usually a sign of the RF board going belly up), or a slow drop in power over a period of weeks/months (either the RF board going belly up or the gas needing a recharge).

[Richard Rumancik]

Kay, before you get too concerned about the tube, tell us how it cuts acrylic. Acrylic is quite consistent so if you can cut acrylic properly then it is not the tube.

If you are seeing extreme reflections when cutting birch then it is highly likely that it is the glue, not the tube. Maybe go to Mitchell's and get a small piece of birch ply - it will be expensive but good for some comparison testing. Then you need to find a source for plywood.

How did you get onto the wetting idea? I never wet wood when cutting. It is probably adding to inconsistencies. Keep in mind that some beam energy has to go into boiling the moisture out before the laser beam can raise the temperature of the wood to its degradation point. I think it will work against you.

[Kay Bengtson]

Kay, before you get too concerned about the tube, tell us how it cuts acrylic. Acrylic is quite consistent so if you can cut acrylic properly then it is not the tube.

If you are seeing extreme reflections when cutting birch then it is highly likely that it is the glue, not the tube. Maybe go to Mitchell's and get a small piece of birch ply - it will be expensive but good for some comparison testing. Then you need to find a source for plywood.

How did you get onto the wetting idea? I never wet wood when cutting. It is probably adding to inconsistencies. Keep in mind that some beam energy has to go into boiling the moisture out before the laser beam can raise the temperature of the wood to its degradation point. I think it will work against you.

I got into the wetting of the wood because when the fellow who I bought if from put my test balsa in the unit and it caught fire. Then he sprayed it with water and it cut fine. I read in the manual for the ULS 25ER that dampening the wood prevents charring so I went with that.

However, I got to thinking about the idea of boiling water off as I know how much energy can be sapped by boiling water and today I tried dry cutting various woods at much higher speeds and it works wonderfully. I was able to cut 1/32" birch plywood perfectly with no water and in less than half the time. Balsa cuts even faster. I am really overjoyed at this development. My kits now will cut in a fraction of the time!

However, the same can't be said for 1/16" birch plywood. I have not been able to find a power/speed/PPI setting that will cut through the wood without flaming. I think I may be forced to use dampened top layer for that. It is a major pain but now that the wood on the bottom is no longer wet, it will cut better.

I just cut some 1/16" acrylic at 60%, Speed 1.2% and PPI of 250 and it worked fine. This is not far from the settings suggested in the manual of 75%, 0.8% and 300. There were some notes written in the manual so that is how I got this setting. I have never cut acrylic before so I may be way off in my assessment.


Kay

[Larry Bratton]

Try 500ppi and see what that does. Epilog recommends 500 Frequency (Pulses Per Inch) for wood on my 40 watt. I cut acrylic at 5000 frequency.

[Kay Bengtson]

Try 500ppi and see what that does. Epilog recommends 500 Frequency (Pulses Per Inch) for wood on my 40 watt. I cut acrylic at 5000 frequency.


I tried 500 and 700 but the results were similar. Flaming is the issue. If I reduce the PPI to 120 and just slightly dampen the top of the wood, it will cut without flaming. However, of course this is on small tests, cutting a large sheet would still require redampening. The one thing that seems to be important in all is that the bottom of the sheet must be dry. It doesn't look as nice as when it is wet and the edges are still charred but at least it produces parts!

I am also going to look into using basswood as a replacement for some of the plywood applications. I know it is not as strong but for some things it may just be the right wood for the model.

On the subject of wood sources, I have tried Midwest birch plywood, BalsaUSA and Bud Nosen Models. There is a lot of variability in the wood too. One sheet will cut great and the next will just have knots that will not cut!

There is a member here that sells interior grade birch ply or at least he did in 2009. I have PM'd him and am waiting for a response.

Other than that, I can't find a source of interior grade birch ply. That stuff must cut great.

Kay

[Larry Bratton]

Kay:
I don't understand how you get a decent cut at 120ppi, but what works is what works I guess. Very low ppi's as such are approaching perforation instead of smooth vector cutting. It's too bad you don't have air assist, it is rare for anything to flame up when it's turned on.

[Kay Bengtson]

Kay:
I don't understand how you get a decent cut at 120ppi, but what works is what works I guess. Very low ppi's as such are approaching perforation instead of smooth vector cutting. It's too bad you don't have air assist, it is rare for anything to flame up when it's turned on.

Most of my parts don't have much definition, rectangles and gentle curves so the PPI letting seems not to matter too much there. I am hopeful that I can find some of that interior grade 1/16th ply someday. Then I expect to raise the speed and PPI settings.

Kay

[Larry Bratton]

Kay,
Have you tried Sloans. http://www.sloanswoodshop.com/plywoods.htm
They show they have 1/32" and 1/16" Finland Grade Birch-aircraft grade. Lots of people doing this type stuff buy from them.

[Kay Bengtson]

Kay,
Have you tried Sloans. http://www.sloanswoodshop.com/plywoods.htm
They show they have 1/32" and 1/16" Finland Grade Birch-aircraft grade. Lots of people doing this type stuff buy from them.

I contacted them by email today but they haven't responded yet. I have aircraft grade but what I need is interior grade. I'm hoping that they can get or have it.

Dan,
I made up an etching pattern of parallel lines spaced 1/8" apart and put a piece of straight wood 3/32" x 12" on a 3/8" steel bar and focused on the wood level. Then I let the one end lie on the engraving table and the other of course was up in the air.

I then etched the lines and just as expected the lines were different thicknesses with about two inches of very nice narrow lines centered 1.5" away from the neutral line. Doing the geometry that makes the focus center about 3/32" below where my focusing tool puts it.

This agrees very well with my practice of cranking down the focus before cutting but what I didn't do was keep it constant for each thickness. Well, I put in a few different thicknesses of balsa and ply then focused on each with the tool and then cranked the table down 0.03" per turn and so I used two and three turns to get from 1/16" to 3/32" deep. This is more than I have ever used so I was eager to see the results.

The results were interesting and consistent. I was able to cut faster for almost everything and now only need wet cutting for 1/4" balsa and 1/16" plywood.

Thank you for suggesting the test. It was really neat to set up and do. Now my cutting is simpler and better than ever.

Kay

[Dan Hintz]

Kay,

Wood is not an appropriate substrate for testing focus... it's too low resolution to tell you much more than 1/8" or so. You need a higher res substrate, like anodized aluminum (preferably) or evenly-coated painted metal (next best bet).

[Kay Bengtson]

Kay,

Wood is not an appropriate substrate for testing focus... it's too low resolution to tell you much more than 1/8" or so. You need a higher res substrate, like anodized aluminum (preferably) or evenly-coated painted metal (next best bet).

The lines were clearly delineated. I could clearly distinguish where the lines were sharper and cut deeper. The gradation was over 12 inches at every 1/8" linearly and a total height change of 3/4" The focus was clearly off and the results show that cutting has improved because of adjusting for it. I get sharper cuts and faster. Seems good enough for me.

Kay

[Dan Hintz]

Ah, missed the 12" long part of your post... not ideal, but should be enough for what you're doing.