# Wood bending problem...



## Lambo240z (Jul 24, 2010)

Hi all. I have a little problem with the bending of the wood I am working with.
I needed to bend plywood (birch) in a 63 cms radius (almost 25 inches) and I decided to go with the thin sheets and glue method instead of the steam one since the parts are a little big.
I got birch sheets 5.2 mm thick and glued 3 of them and bent them with the jig I made. I used Titebond II glue and left it to dry for 24 hours.
The problem is that after releasing the clamps and removing the wood from the mold, the wood returns slightly to its original shape so the end result is always a little less curved than it should. For the pieces I did, it was ok since I worked around it, but I will have to make more later and would like to know exactly how they are going to turn out.
I am posting the pictures of the mold I did so that you can get an idea and tell me if you see anything that could be causing this.
By the way, I tried it during hot days and colder days and still the same. Sometimes it is a little closer to the original shape of the mold and sometimes a little more out of shape but never exact.
Thanks a lot for any insights you may have.
Leo

This is the mold I Used:










This is the early mold in use. I added a third section in the middle as seen in the first picture and used pieces of wood on top where the clamps were touching the wood that run the width of the wood to avoid any unneeded bending.


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## woodnthings (Jan 24, 2009)

*I'm thinkin' it's the plywood*

Because plywood is not a uniform material as a solid wood, the various layers are returning to their straight/flat form. If you kerf the plywood across the bend radius you'd have better luck. I don't know if the kerfs will show
on all but the inside surface which you could leave plain or unkerfed. JMO :blink: bill
The only other solution I see is to over curve the mold and account for the spring back that way.


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## Leo G (Oct 16, 2006)

It doesn't matter if you use plywood or solid wood it will always have a small amount of spring back. You need to make the molds slightly more curved than the pc you want to have come out of it. It can be a complex formula. The thinner the wood the less spring back there will be.


http://www.woodweb.com/knowledge_base/Springback_formula_for_bent_laminations.html


> The basic formula is y=x/n2 (n2 meaning n squared), (x is the vertical distance between the chord and the top of an arc), (n is the number of laminations) and y is the amount of springback, or the amount x will be reduced after the laminations come off the mold.


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## cabinetman (Jul 5, 2007)

Bending sheet stock isn't that hard to do, but you need to make a press. The mold you made has too much gap, and the clamping method doesn't afford much widespread compression. In addition to what you made, you would need a top half to clamp down on the sheet stock making the sheet stock a "sandwich".

First, take a piece of 1/4" plywood and draw the whole press on the face. This will include whatever you need for "legs" if any when the curve straightens out. Draw the bottom half. Then allow a cut line (for a jig saw, or band saw) for the thickness of the stock to be bent. Then draw the top half with the line above the sheet stock allowing for the same cut line with a jig saw or a band saw.

Then cut out the two pieces. From that, cut out the two forms for both halves. Then use 3/4" material, either MDF or industrial particleboard to make the pieces for the two halves. You need to cut enough of them to stack them tightly together to form a solid form.

For example; for a 12" wide section you would need to cut 16 pieces to make it 12" across. Rough cut one piece from the original pattern, and smooth the moulded edge. Then use it as a pattern to rough cut the rest, and use it as a pattern to rout with a flush trim bit.

When all the bottom pieces and top pieces are ready, glue them together and clamp up. When done, you have made a press with a bottom and top half. As for the plywood stock to be used, kerfing the inside surface may help relieve the stress, but orient the face grain to be 90 degrees to the mould. IOW, it's easier to bend with the grain giving. You don't want long face grain in the mould. 

Next, I would suggest using a urea formaldehyde glue, like Dap Plastic Resin Glue. You mix it yourself with water. It gives a longer open time...could be 45 minutes to an hour, and cures with a more rigid glue line than any of the Titebonds.

You need to use a lot of clamping pressure. The clamps you used are inadequate. I would suggest 3/4" pipe clamps. If your press is made with a flat absolute bottom and top your clamps can be used all around the edge.

Allow the press to sit for 24 hours. From your initial lines for where the curve ends, You can draw a line and cut off the unneeded amount. It's a bit tricky to figure out how long the sheet stock has to be to fit the form and end where you want. That's why I leave the "legs" a bit long so I can trim off the waste.












 











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## ash123 (Mar 14, 2010)

Why not just use bending plywood? Or use Kerfkore and veneer it?


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## cabinetman (Jul 5, 2007)

ash123 said:


> Why not just use bending plywood? Or use Kerfkore and veneer it?



There would still be a need to have some sort of form or structure to keep bending ply in the desired shape. Bending ply or Kerfkore can do bends nicely if applied to a curved formed surface of some sort. This may have been just for the experience of this type of procedure. 












 









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## Lambo240z (Jul 24, 2010)

Hi again and thank to all for the replies. Ok bending the wood more to account for the spring back could be done but I dont like it completely because is still too imprecise. In the worst case scenario I will have to use that formula and apply it. Thanks for the formula by the way, I didnt know it existed 
I know the mold I did was not the best and I could do a complete top and bottom as suggested and I think I will try that specially if I have to make 40 or 60 more of those bends. That way I make sure they come out as good as possible. And I will see about the price of the formaldehyde glue, I hope is not more expensive than the Titebond?
As for the clamps, they had enough pressure to keep the wood tight against the mold with no problem, so I think they would be ok? I will buy more anyway.
And yes I used 2 of the 3 sheets with the grain running 90 degrees from the mold and 1 with the grain just to give it more strength. At least thats what I read it was good to do?
Thanks again for your help everyone...
Leo


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