Thread: Timber bows after machining

Originally Posted by JillB

Is there any way to straighten this bowed piece, like clamping it to a strong straight edge ( 6mm Al angle) for a few days?

That might work if it was placed in a steamer while secured to the angle, but even that is very problematical and would still require jointing afterwards, so I wouldn't bother with that. Better to joint the timber as is. Do you have access to a jointer or a router table with an offset fence?

Doug

The important thing to note is that a thicknesser and a saw do not straighten, they only size. The 'correct' process is to select then dress the face side being careful to remove any twist that may be present. Then select and straighten the face edge at 90 degrees to the face side. Next, if the stock is significantly oversize it should be run through the saw (dressed face/edge to the fence/table removing the undressed face and edge) slightly over finished size. Finally run the timber through the thicknesser to give the finished size.

This is the correct process in the workshop where I did my cabinet making apprenticeship. Other workshops may well have equally valid correct processes.

Philip

As Phil says, first, you need a straight, flat face side and edge, that you'll get from a jointer or plane.
If you can, it's not a bad idea to machine it a little oversize, and leave it sitting on edge for a couple of days for moisture and stresses to even out.
As you haven't had to take much off the thickness, I'm wondering why it has bowed. Did you leave the pieces lying on their faces rather than on edge? This can cause uneven distribution of moisture and stresses.

I bought the timber already dressed, but oversize, as I hadn't finalised the design of the mirror frame.

I did check each piece I bought against a straight edge to get the straightest side before ripping. But the max error would have been less than 0.5 mm. I didn't think that I would achieve much better by planing by hand. But none of this error would have made a 2.5mm bow on one 1200 long piece that I ended up with after ripping and thicknessing.

Thanks to HuonPhil for the detailed process. In future I will be starting off with one true face, and then checking the timber ( even the dressed timber) for straightness more carefully at each stage, before and after jointing, before ripping and after ripping.

AlexS, I was unaware that the timber should be laid on its edge, not on the wider, flatter side. I have laid both pieces on their flat sides on a flat MDF board.

When you cut a piece of timber you can release or vary the stress patterns in the wood. Better to cut over size and leave for a short while to let it settle then cut to final size.

I am going to hijack this thread with a similar problem.

I recently got a whole lot of off cuts and cut them down to the same length and width. About 450mm long x 100 wide. I made sure the individual strips were perfectly by running them through the jointer and planer. Then I glued them together, stacking them one of top of the other into a brick like shape. The height was about 150mm.

Left it overnight to dry, and the next day, the brick looked great; like a multi coloured cake. Exactly what I was after. Cleaned it up on the planer, ripped some slices on the band saw. and then put them through the planer.

The next day, I noticed that both the two thin slices (about 8mm) were bowing down the length on one side. A few days later, even some of the thicker bits are doing the same thing (see picture.)

What is causing this, and how can I stop it happening?

Jill
If you don't have a jointer, then you are going to have a helluva job getting it straight and you will lose size.
When you ripped it, you released the internal stress in the timber.
Leave it for a couple of days and see whether it straightens itself out again.
Something that worked for me once, was to lay it on a flat surface with the convex side facing up, pour hot water over it, cover it with a straight length of timber and put heavy weights (like besserblocks) on top to force it down.
Repeat every day until it has straightened out.
It took about a week of repetition to get it straight, then I left it for another week to see whether the bow returned.
On that piece it was minimal bow in the end.
If you don't know anyone with a jointer, take a drive to Tully Heads and get it fixed

BTW, did you get the framejig?
Cheers
Wolffie

Torb, when you stack them, you should put stickers between each one, to allow air to circulate around them.

Aside from when the wood isn't properly dry, cupping and bowing is almost always caused by uneven exposure to air or sunlight - usually air. Allowing circulation avoids or minimises this problem.

Back to the OP's question of how to avoid this in the future. You really need to dress the timber equally off opposite sides. What you did was remove 14mm off one side and 1mm off the other (or something like that) if the timber is perfectly straight grained and has the same moisture content all the way through this would not be a problem. Butn since we work with a natural product (hell, even particle board and MDF will move when ripped) it will never be "perfect".
So if say the outside 5mm has a lower moisture content (MC) than the rest of the timber, by taking all that lower MC timber off one side and leaving most of it on the other you now have different tensions on each side of the piece. So it bends! The narrower the board, the worse the effect.

Jill

Just to pick up on some points others have already made.

Young timber has growth stress contained within it. Even if the board has been straightened in the milling and machining process further stresses will be released when you rip the board down again. This I suspect is what happened in your case.

The bow can be corrected on a jointer and as HuonPhil mentioned for a thicknesser to produce straight timber it has to be used in conjunction with a jointer.

Really the only way to correct the straightness, or lack of it, is to plane off the bow on one side and the curve on the other. This could be done with a jointer or a hand plane. The longer versions of either tool make this task easier. However even a shortish handplane will sufffice by planing down to a line and constant checking with a straightedge (long spirit level or similar).

Clearly the boards will end up narrower. If this is a problem you could try joining the boards as they are and pulling them in with clamps. Use strong glue of course. Check for distortion in the other plane as you are clamping. 40mm wide is not much and I think you should easily be able to achieve that. Even if that doesn't work you haven't lost anything by trying except your time and some frustration.

Regards
Paul