Bent lamination strength question.

Hi. I am wondering about how to make the strongest bent lamination. For this example, let us think of a rocker for a rocking chair. Let's say I want a final size to be four feet long, and then one inch wide and one inch high in cross section. I don't know if the bend matters, but let's say it has a forty two inch radius. I am thinking that I will cut eight bands of wood four and a half feet long by one inch wide and one-eight inch thick. My question starts here. In which direction should the grain orientation be? If looking at the end grain of the band, should the grain be parallel to the 1/8 inch dimension (like quartersawn wood)? Or should it be parallel to the one inch dimension (like plainsawn wood)? Or should I think of it as plywood and have the grain of each layer run in the different direction as the two layers that it is glued to? Please remember that I am not looking for good enough, I would like to know whats the strongest. Thanks for your help. Pete

I have made a number of tables and rocker useing bent lamination. Quarter sawn wood is not used often here do to the fact it tends to split easier(..it is more stable then flat sawn lumber, but is more prone to splitting). I use lumber that the grain runs from one corner to the next(picture the end grain of a board...you want the grain to either run from the top left corner towards the lower right hand corner of the board(any direction is fine...just so long as it runs corner to opposite corners. This type of grain orientaion is the most durable. Couple that with the adesive and you will be set.


DonnieR

Interesting question, Pete. I like Donnie's idea...perhaps rift-sawn (you know, half-way between quarter and flat) might work, too.

I think the thing that make this very strong may not necessarily be the orientation of the wood so much as the adhesive you use. The laminations are so thin that I think it may not matter how the grain is oriented but for the specific question you are asking, I think Donnie's suggestion makes sense.

Perhaps a sandwich of various orientations might be best?

Hi Chris, You brought up something else that I don't know about. What type of glue would help to make the lamination be it's strongest? Also it has to have a somewhat long working time. Long enough to spread the glue on each band and get into the forms. Pete

I use a slow setting plastic resin glue that I buy throug my local "paint" professional shop. I do not recall the name brand.

Chris,
You make a good point about the thinkness of the strips...and to be honest..I have never done it any other way(the application i suggested was the way I was taught) and it makes perfect sense to me also. Why dont you try and let us know how it turns out.... :D :D .....


DonnieR

I would take a look at Unibond 800, a 2 part urea resin from VacuPress, in Maine. It has agood working time, and is tintable, and pretty waterproof. It is a non-creep glue. It can be worked with tools, without trashing the tools. I comes in rather a large quantity, but is not expensive, and if you keep the 1.2 gallon of resin in the beer cooler, will last 2+ years.

I would use a poly glue myself...GG (Gorilla Glue) to be specific (since I have two large bottles of it! :D ).

For whatever reason, I have done a fair amount of laminations lately of mostly 3/4" ply to 3/4" ply and I just prefer to work with GG.

The open time is nice, if you heat it up in the microwave to a decent "hot-shower" warm, it flows very nicely and spreads very well and thinnly with a credit card.

Hi Pete

A good marine epoxy like West System or Evercoat would be my choice in glue. With a slow hardener you will have good open time. Use some sort of filler to increase the strength.

The way of epoxy is to:

"Wet" the surfaces, not water, but with epoxy.

Apply a coat of thickened epoxy over the "wetted" surface and clamp the whole thing together.

I would use strips 1 1/4" to 1 1/2" wide and then trim to size once the glue cures.

Epoxy is slippery stuff and you should do a practice glue up with some shorter pieces and a nice bend to uncover any concerns that may turn to panic when you are working on the big pieces. I would suggest that you run a test with any glue prior to final assembly. Putting those pieces in tension with glue between can lead to some unexpected results.

Thanks

Ha, ha...I had to smile at Richard's "panic" comment. Been there, done that! :D

You could even just practice glueing them up dry, too. It'll give you feel of where to put your clamps so they are easily reachable and where you need to clamp down.

You'll also want some waxed paper to use. As Richard said, the wood becomes slippery and it'll slide all over the place. You'll want to think about what to do to keep the strips somewhat in line with each other.

Definately make the lamination wider and longer than you need. It might take several 1/16" cuts at the TS to true everything up. Glue squeeze-out can mess up your nice reference against the fence. A nice heavy router flush-trim bit and template is also good for cleaning up the edges from glue. Usually the face of the laminate stays pretty clean of glue (hint, use blue painter's masking tape to ensure this).

After trying several different poly glues and T-88 epoxy, I've settled on DAP plastic resin for bent laminations. As to grain orientation, I guess I've never really paid that much attention to it. I cut 'edge' or 'face' pieces, depending on the size of stock I've got. And I think you'll be fine at 3/16 or just a hair less on the thickness. If I start getting much below that, the Performax starts sanding the conveyor belt. :(

KC

Thanks for all of your replies. They make me think of another question. In looking at a couple of glues, I see that they can dry to a color. The plastic resin glue is tan in color and I think the gorilla glue dries to brown, is this right? Anyway, I wanted to make this piece out of hard maple. I was wondering how the glue lines would look against the color of the maple. So which glue is strong and has a color that is close to maple? Now I just thought of another question. Is maple a good strong wood for bent laminations? Thanks for any thoughts. Pete

Maple should be fine. Just about any wood would be fine...remember it is the adhesive that makes a laminate strong. Maple is plenty strong.

The glue color is also an interesting question. I lamintaed 3 shets of maple ply with GG. I didn't notice any unusual coloring effect from the GG.

BTW, the GG dries to a light creamy yellowish color so might actually work well with the maple.