Is wood bowing predictable?

[rudy de haas]

I am putting solid edging on 12mm (about 1/2") baltic birch. To do it I used the shaper to cut a 1/4" dado down the center of both 12mm: x 96" edges of the plywood (leaving about 1/8th on each side). To make the tongue part I bought 1 x 6": maple in 96" lengths, cut it down the middle to get two pieces each 1 x about 2 3/4ths x 96 and then cut these down the 1" side to produce two strips - one about 12mm thick, the other about half that. I then run the 2.75" wide (minimum width for the power feeder seems to be around 2") piece through the shaper four times to make two tongues and finally cut the strip twice to get the 96" x 12" x .6" strip that has the tongue part and f1/4" of solid edging.

Notice that this process starts with two long cuts at right angles to each other - and, later, a third cut that parallels the first one. What I've noticed in doing this is that the resulting strips all developed significant bowing - despite the act the boards started out pretty close to straight. The intermediate products (about 2.75" x 96" 12mm) bow as you would expect, but the strips when cut off tend to bow at right angles to that.

Does anyone know why? and,if you know why, can you tell me how to predict/minimize this? (In practice, they can be forced straight pretty easily during glueing - but why does it happen?)

[Larry Edgerton]

The board you started with is dryer on the outside than on the inside. Normal, extreme cases are referred to as case hardened and are almost unworkable at times.

I recently had a conversation with the kiln operator at a place I quit buying from for this reason. He said the owners got busy and started forcing him to push the drying schedule, hotter for less time. The result is case hardened wood. Every time you run it through the plane it bows toward the face that was planed.

Some is normal, even on wood that has been dried properly as it changes moisture content according to the area it is stored in. Dry conditions it dries on the outside first, moist conditions the opposite. its just something we have to deal with. Splitting wood to make thin stock exaggerates this condition as you have the cut center of the board on one side and the original on the other. Given a couple of days they will stabilize as the moisture content evens out a bit.

Sorry, no cure and it is not anything you are doing wrong.

Larry

[rudy de haas]

The board you started with is dryer on the outside than on the inside. Normal, extreme cases are referred to as case hardened and are almost unworkable at times.


Larry

ah - that makes sense.

[Wayne Lomman]

There are other reasons as well but not necessarily any more curable. All but the very finest timbers will have some form of internal stress. These stresses are a natural effect of how the tree grew, where it grew etc and how it was subsequently milled and dried. You can dress a piece of timber perfectly straight, saw it down the middle and end up with two boomerangs. That's why its best practice to rough saw to a bit oversize and then dress to finished size. With thin strips this is problematic. However, being thin means you can still work with it most of the time as it will bend to your will to some degree.

Another reason is too much machining with pressure applied several times in one direction and on one side. This can effectively change the density on one side vs the other. You deal with that by keeping machine runs to a minimum and where possible flipping from side to side if thickness planing. This can't always be done of course but as I said, here's more reasons but not much comfort. Cheers