Lens size and cutting baltic birch plywood

[Richard Link]

I've been having very inconsistent results cutting baltic birch plywood in the 1/4" thickness. I'm using a 45 watt Epilog with a 2" lens. Cutting settings are 100% power, 5% speed with frequency of 500 using air assist. I've aligned my bed so that it appears flat relative to the cutting head throughout. The bed size is 24x36".

The laser cuts easily through the plywood in some areas but not in others. I suspect that this has to do with some of the inevitable warping so that the surface is not actually flat throughout the entire sheet. Despite the apparant flatness of the vector cutting table, the actual surface of the plywood material differs in height somewhat due to warping. In many cases, the areas that aren't cut through are "slightly scribed" on the bottom. And yet, I'm wasting a lot of material with these failed non full thickness cuts.

Would I be better off doing vector cutting with a 4" lens for 1/4" material? What do I give up when vector cutting using that lens vs. a 2"? I was under the impression that 1/4" material is nowhere thick enough to require a 4" lens but maybe with the warping that isn't true....

Any other suggestions other than chopping the sheets up into 1 foot sections and breaking up the cutting into many of them? I guess that would work but it seems kind of stupid since I have a 36" wide bed...

Richard

[Joe Pelonio]

I have had great results with 45 watt epilog cutting the same material in smaller sizes, carefully choosing it for the least warped. One trick is to add a couple of small holes in the cut file between parts, then wire through the vector grid to pull the piece flat before cutting the rest. You could also try adjusting the focus and making a second pass in the high areas. I haven't tried a longer length lens, if you have one it may be worth trying, but is a somewhat costly solution.

[David Fairfield]

Just curious, do you get an orange flame and hear a sort of sizzle, on areas that don't cut through completely? Its something I've noticed here, wondering what the cause might be.

Dave

[Rodne Gold]

The 4" lens would cut better due to its increased depth of focus at its maximum energy density , ie it will maintain its smaller spot size more as the beam diverges away from its focal point for a greater distance either side of the focal point.
However you cannot get as small a spot with a 4" as with a 2" and thus to get the same cut with a wider kerf (thicker cut line) you have to put in more energy to get the wood to vaporise. You might actually NOT have enough energy for the bigger spot size of the 4" to actually get the material to vaporise and thus might actually get a WORSE cut. My suggestion would be to got to either a 2.5" or 3" lens based on the power of your machine or to make sure the material is dead flat for your 2". If you are focussing on the top of your material right now , I would try to focus deeper , ie 2mm INTO the materal , ie get your focus at the top and then raise the table 2mm.

[Michael Kowalczyk]

Richard,
I just cut some of my 6mm Baltic Birch last night with my new 1.5" FL (I will post the story on how I ended up with a 1.5" lens soon) and had no problem. Sometimes grain direction can make a difference also. I typically use vertical grain when I want it to lay as flat as possible, especially when cutting lots of parts out. When I focus on a sheet, I will start at say X1, Y1 and check the screen, on my Trotec job control, for the Z height. Then go to the opposite corner and focus again. If the #'s are close (.001" to .003", I will leave it. If there is a slight difference, I may split the difference. If it is a lot, then I may rotate the sheet 180 and try again. If that doesn't work then I get a new sheet an put the old one under some full paint cans to flatten it for another time. On thinner material sometimes you can bend it in the opposite direction, to counteract the materials memory, and load and cut it quickly. Sometimes a weight can help if positioned in the correct place.
***** MAKE SURE IT DOESN'T INTERFERE WITH THE LASER'S PATH ***** Someone here mentioned, a while back, hanging a sinker by a hook. Look around the shop and be resourceful.

Hope this helps and ...

[Craig Matheny]

"Someone here mentioned, a while back, hanging a sinker by a hook." It works very well have a 4o hook and a 4 oz weight hook the edge of the wood and let the weight pull it down.

Richard cutting 6mm BB on a 45 watt laser will take everything it has to cut it and what grade is the wood you are cutting that makes a huge difference?

[lee chitwood]

One of the "big" things about 1/4" Baltic Birch is the glue and more so ply filler in the plys. Instead of footballs they sometimes use a filler something akin to "rockhard water putty". Usually this eats laser power...your cut will be through the top ply and interupted in the core plys. Its enough to ____ a fellow off!

[Michael Kowalczyk]

Hey Lee,
Just to clarify your statement above; 6mm B/BB Genuine Baltic Birch does not have fillers that I am aware of. It is all veneer core with a CARB II certified glue, at least the ones we get. It will have some footballs on the BB side and the glue is what you choose. All ours is interior. If you use exterior glue, you will have issues. If you have putty for patches that is because you have selected a lower grade of plywood. Most likely a C or possibly a CP grade if it is sanded.

Also make sure you have the correct stamp on the side or in the corner showing the grade. Not sure what the Chinese ply has on it, because we don't buy it, but Genuine Baltic/Russian Birch will have something like this in the corner and some have a stamp on the edge now.

[George M. Perzel]

Hi Richard;
I cut a lot of plywood-some 3mm and 6mm baltic birch but mostly 1/8 and 1/4 inch luan mahogany.-mainly with a 2 inch lens. The biggest issue, by far, is keeping the stuff flat. Use Joe's method or any other way you can think of to get it as flat as possible-or cut it in sections with refocusing in between. It may be perfectly flat when you start, but heat from the laser and cutting crossbanding release tension and causes warping.
The second problem is the glue-varies greatly between vendors or even between sheets from the same vendor. Some stuff cuts like butter and others like vitrified cow dung. Also, back off on your PPI setting-500 is too high-go to about 150.
Good Luck
Best Regards,
George
Laserarts

[Richard Link]

Just curious, do you get an orange flame and hear a sort of sizzle, on areas that don't cut through completely? Its something I've noticed here, wondering what the cause might be.

Dave

No. My air assist seems to be minimizing that. Everything cuts with normal appearance, just not all the way through everywhere.

[Richard Link]

Hmm. I've not had that experience. My laser seems to cut the 1/4" BB like butter when it is flat and I focus on a small area. The problem seems to be warping on larger sheets and trying to keep things flat. Lots of great ideas here from everyone. Thank you!

I've bought several types of 1/4" baltic birch from Rockler recently. The higher grade material (good both sides) cuts very well provided everything is flat (although I am having the problems above with larger sheets). I also bought some lower grade stuff (on discount) which has only one good side and that stuff barely cuts halfway through. I suspect the glue is mucho tougher to cut on that stuff. Good thing its still ok for drawer bottoms.....

Richard

[Richard Link]

Thanks George. I've never tried cutting with a PPI as low as 150. Would be worth a try.

Richard

[Mike vonBuelow]

Richard,

FWIW, using the birch I purchased from Mike Kowalczyk has made a difference with me, with wood from other places I've bought from. Easier to cut with my 25watt'er with 2" lens

3mm - 3% speed / 100 power - 1 pass
6mm - 3% speed / 100 power - 3 passes

I have very few un-cut spots, most are usually easily score-able with my xacto blade