Question on bending wood?

[Noklin]

Assume I steam bend a section of ply into the desired curve and mount it so that it is held in that shape. If I come back later after it has set up and cut a section of that ply out of the middle, will it maintain that set curve or will it tend to straighten out over time? Would ply and solid wood respond the same in this situation.

[PeterP]

Plywood is a lamination I would not try to steam bend it. I don't think it will retain the shape because the grain orientation is optimised for flatness. Rip up your plies and glue them up to the shape you require. The thinner your plies the less spring back you will have. PeterP

[PAR]

The plywood will retain a good bit of the curve, though some "relaxation" will occur, which will be more then solid lumber. Plywood's veneer orientation is intended for cross grain strength, which is poor in solid lumber and to "stabilize" the product's moisture gain/lose dimensions, which can be dramatic in solid lumber.

Steam bent spring back (relaxation) is predictable in most solid wood species at around 10% of the radius of the curve it's bent to. So, if you desire a 10' radius for the finished piece, the mold is built to a 9' radius, knowing it will spring back to 10' once released from the mold. In a laminate you can decrease the amount of spring back or forget it completely if enough lamination layers are use and the finished piece is "captured" by other structure.

As a rule plywood will only bend so far, which is considerably less then bending solid lumber. An example would be 1/4" plywood can reliably bend to a 24" radius (4' circle) with the grain and a 20" radius across the grain. There's an old rule about bending plywood. Once it starts "talking" to 'ya, you've maxed out it's ability to bend further.

[Noklin]

OK, I understand about 'spring back'. My question is if the wood has been allowed to dry and set in its new curve and then it is taken off of the form will it maintain its shape over a long period of time or will it tend to straighten out.

[Tom Lathrop]

OK, I understand about 'spring back'. My question is if the wood has been allowed to dry and set in its new curve and then it is taken off of the form will it maintain its shape over a long period of time or will it tend to straighten out.

You will have to experiment if you really need to know the answer.  Having bent a fair amount of plywood, I know it is fraught with danger if you bend too far.  For instance, bending too far to work in tight curves, even in the easiest direction) can cause the plies to delaminate near the edges.  Wetting, steam or whatever, while it can be done, is of limited usefulness in plywood.  More plies makes it more difficult.

Do the experiment and report back.

[Ken_Potts]

  It will tend to straighten out over time but I don't know if it would ever be as straight as it originally was.

[PAR]

Plywood will "take a set" just like solid lumber and when released, it will relax to some degree. How much will be a guess. I have planks pulled from a lapstrake a year ago and they initially had quite a bit of set, but now have relaxed a lot, though still are very curved.

[Hirilonde]

What exactly do you want this piece of bent plywood or wood to do? 

In some cases, wood or plywood is prebent, steamed or what ever to make forming it to a situation work, such as a cockpit combing.  Once in place, it does not need to hold its own shape.  The deck opening it is attached to will do that.  In another situation, like the frames of a plank on frame boat, they need to end up capable of helping to support a shape.  In this situation the ability to hold a shape on its own afterward is important.  This is where calculating the spring back, and incorporating it into the mold for steam bending comes into play.  Some seriously bent frames are kerfed in the area of the tightest part of the bend to make the bending at that point easier, and to end up with a laminated section  which when fastened will hold the bend better when done.  Probably the most stable bent construction is the glued up piece done over a mold.  With enough layers and thickness the result will be virtually immovable in shape in any environment.

So what ever this "bent wooden what ever it is" is expected to do when completed will help determine what material to use, and how you should bend/create it.

[Capt Nat]

Make your own plywood in any shape you want; the process is called "cold moulding". This can be done with a male or female mould, with or without the use of a vacuum. No more (probably less) complicated than the set-up for steam bending, plus the results are predictable.