Bending (short) plywood

[thealley]

Good Evening,

I have a couple of queries about bending maybe someone with experience could help me. 

Basically I have these six pieces which I have to bend so that they could be attached to the semi-circle-like base in the attached pictures. I did some research and I think the best option that came up was bending through kerfing by cutting 1/2 deep lines near each other so that the wood looses strength, however my designs are fragile, and kerfing could be damaging. It seems bending with steam is not recommendable for plywood. I thought of posting here to check what the experts think... 

As a matter of fact, I always stick the design to the wood with the lengthy part in parallel with the wood vein (is it called a vein??) so that it would be less prone to bending by mistake. As a result, the images in the first picture are more difficult to bend. Can you give me your opinion on what is best to do when you need to bend (i.e. stick the design in favour of the vein, or not)? I hope I was clear in my unprofessional explanation hehe 

Thanks for your help.

[thealley]

By the way, this is the image of the whole design:

[Rapid Roger]

You are dealing with a REAL IFFY PROBLEM here!
If it was solid wood, steaming or soaking in water would be my answer but, do NOT try to steam or use water with plywood as it will delaminate so fast that it would make your head swim.
If I were to try bending this (I am assuming that it is 1/4" or less thick) I would find something round like a can or bottle a bit smaller than what you need for a radius and try wrapping using my hands it around as far as I felt comfortable with, to see if I could at least get it started into a curve without breaking.
(Now that I think about it maybe start with something larger than the final radius and work your way down smaller in steps.)  Then wrap some rubber bands around it and leave it overnight so that it keeps tension on it.  (overnight or longer if necessary.)
The next day unwrap it and see if it will hold a curve. If it is still too loose go to the next smaller size as you will get some spring back for  sure. When you can get it to hold a curve close to what you need I would go ahead and glue it to the base piece and let the glue hold it in place.
I'm NOT at all sure this is going to work (I've never tried it) but, it is the best suggestion that I could come up with on the spur of the moment.

Rog

PS It is called "grain" not "vein" of the wood.  Which with plywood is only on the surface layer and every other  layer there after. The other layers are running perpendicular to the first to create strength and that is why it is so hard for you to bend. If you have five ply plywood, the top, middle and bottom layers the grain will run in one direction and layers 2 and 4 run in the other direction.

[rrjwright]

Nice project...  Perhaps try contacting the owner of this pattern  for their suggestions. Maybe they have a trick they can share??

Jim. . .

[don in brooklin on]

I was at a demo the other night and the person bend a piece of oak and a piece of cedar using a hair curling iron.  Used a real big one that was as hot as it would go.   

Don't know if it would work on plywood as slated above it might delaminate.  You might want to test first.

Don

[EIEIO]

THIS IS JUST A GUESS: You might try a scrap piece of plywood in the microwave to see if you can soften the glue between layers. If the glue allowed the laminations to slip then it should bend easier.

I think the thin kerf is the best idea but it should be done before doing the fretwork. Maybe next time?

Also next time, it might be better to put the grain perpendicular to the direction of bend. 1/8" BB Ply is pretty bendable perpendicular to the surface grain.

[Rapid Roger]

I think that Don has a good idea of using heat.
You might try making the outer layers damp by just wiping a bit of water on both surfaces (DO NOT soak or get too wet) and use a heat gun (old hair dryer in my case) on one side only and see if you can make the plywood warp a bit. Of course you need to experiment on some scrap first.
If you lay the face of the wood on the bench wet and heat (dry) the back side of it, it might curl up a bit. I have seen this method work in reverse to flatten solid wood.
The only time plywood will warp is when you don't want it to.  

Rog

[BilltheDiver]

http://www.woodmagazine.com/materials-guide/lumber/sheet-goods-selector/?page=4

The above is a link to an article from Woodmagazine on bendable plywood.  It might help you locate this product which should provide what you are after.

[thealley]

Hi,

Thank you all for your suggestions. I began with Rapid's suggestion by bending it slightly in smaller steps... let's see how it goes, even though I already noted some small cracks due to the small size of the designs.

I will also try the hair dryer thing. What I did not understand is the use of the hair curling iron and how you can apply it on wood. Did you mean to touch it against the wood? Isn't it better the hair dryer since it blows hot air?

Thanks again!

[don in brooklin on]

Hi

With bending on a curling iron it is a safer way to do pipe bending. I understand the demonstrator used to heat up a pipe with blow torch and then wraps the wood against the pipe.

Since you can't use an open flame in a demo he started to use a curling iron.  It has to be as hot as it can go and then wrap the wood against the rod.  Took a few goes to get it in the shape of half a heart. 

As I said I am not sure it would work with plywood but worth a test.

Don

[thealley]

So just to be sure, the wood needs to touch the curling iron, right?

[don in brooklin on]

The wood touched iron so you better not let the boss see you.

Don

[Dakota Saw Dust]

While streaming will delaminate the interior glues used, if a very damp wash cloth is placed on the "outside" of the intended curve -of the fretwork- and a steam iron applied heat to that, the tops layer/layers of ply. swell an assist in the bending - to a point. When the piece dries it may crack if the circumference of the bend was to tight for the ply thickness. An additional assist to this would be vertical "V" cuts on the back side/ in side of the curve to reduce the tension of the back ply.  Unfortunately, the vertical cuts should be made on the blank before all the fretwork is done so the verticle cuts pass through all parts of the fretwork in alienment  [requiring a sacrificial bottom blank for support when cutting].    PS -the vertical cuts and the base line of the pattern MUST be at 90 degrees to each other. If one really wants to go the extra mile. There is an old cabinet makers formula for depth of the "V" cuts in relation to the material thickness and the spacing between those "V" cuts. More than you really wanted to know