Resawing - Stress relief? Moisture uptake?

[mrbmcg]

Hi Folks

I've seen this topic covered on a few threads and have a few questions for anyone who might have the answers.

I recently resawed a 50mm board of cherry right down the middle. It had been sticked in my workshop (heated and dehumidified) for nearly a month and should have been acclimatised (I have two thermometers/hygrometers - one in my house and one in the workshop and try to keep the rel hum and temp within about 5-10% of one another)

As soon as I resawed it it started to pinch together and by the time I got through the end of the board (about 1700mm long) the two halfs were bananna'd together with a gap of about 20 mm between them in the middle if both ends were held together. Now given that I wanted two 20-22mm boards I now have a bit of a problem

If I leave them to settle then I probably will only get about 15mm thickness per board which isn't any good. Is there anything I can do about it?

So, what actually happened? Was it stress relief? Was the board case-hardened (or reverse case hardened in this case?) or was it just caused by moisture release?

I can't see how the board would bow that much off the saw via moisture release, surely it would take a bit longer than that to develop? :?:

Stress release is my favourite choice currently, but isn't this rather a lot? I've had boards that bowed when resawed before but not this much. It doesn't say much about the quality of this timber.

Anyway, is there anythin at all I can do to straighten these boards either by clamping, adding moisture on one side or whatever? Its far too expensive to throw away? I spotted Noelys dew suggestion, could this be done by wetting the board on one side? There aren't many rain-free days north of the border in October.

Help!

Cheers
Bob

 

[Noel]

Bob,

Bad luck. Ref the dew business, it does work. I've never tried it personally and it's really only best to try it in the summer months with our climate. I'm sure there'll be plenty of informed and honest opinion on your next plan of action. I've clamped boards that have been in a similar state (basically clamp out the curve so you end up with the opposite curve and leave for a week or so) but I may've been lucky. Hope you get it sorted.

Noel

 

[Sgian Dubh]

Is there anything I can do about it?

Probably not for the purpose you intended Bob. That's one of the dangers inherent in resawing thick boards to create two thinner boards. It's often attempted to achieve a book match, but it's not uncommon for it to fail. A simple test should be carried out before attempting what you did-- I'll get to that later.

By splitting the board down the middle you released the stresses that were built-in during the the drying. This happens because case hardening has been induced through an innappropriately applied moisture gradient which the operator used (probably) in the kilning process.

Air dried timber can suffer from case hardening too where the stuff is dried too quickly setting up the outer shell of the board in tension which puts compression on the inner core.

Eventually a situation can develop during particular faults that can occur during drying stages, in either air drying and more commonly in kilning, in which the forces reverse wherin the inner core is in tension and the outer shell is in compression resulting in stage 3 case hardening. Splitting the board down the middle as you did results in cupping across the width and bowing along the length.

A further development of these case hardening stages results in honeycombing where the in-tension core splits to relieve the stress on it. (I haven't even gone into other drying faults such as surface checking and core collapse because I want to keep this answer reasonably short, ha, ha.)

The best thing to do in the future if you plan to split a board in half as you did is to conduct a fork or prong test to establish if the piece is stress-free.

Cut a length section out of the plank about 25- 30 mm long. Turn the end grain of the just sawn section onto the table of the bandsaw and cut a line about 6- 10 mm in from from one plank face to within about 25 mm of the plank edge. Cut another line 6- 10 mm in from the opposite plank face to the same point near the edge and remove the waste in between. On particularly thick planks you might cut another fork in the middle

If the forks close towards each other you have case hardening. If they open up, you have reverse case hardening. If they remain about parallel the piece is relatively stress free and likely to resaw satisfactorily. This test is worth its weight in gold because it can save you a lot of wasted time and costly wood.

The pieces you've got as a result of reswaing can probably only be used to good effect in shorter lengths. In future buy boards closer to your desired thickness and plane off an equal amount from both sides to keep any stresses in balance, or at least do the fork test before commiting yourself.

I can highly recommend the book, Understanding Wood by R. Bruce Hoadley if you want to get into the down and dirty nitty-gritty stuff of wood technology. Slainte

 

[Aragorn]

That's interesting reading Sgian Dubh, thanks for that.

Bob - you may be able to considerably reduce the bow by warping it back the other way as Noely suggests. When I do this, I warp it back by double the offset, so that when it springs back after a few days in the clamps, it ends up straight (or there abouts).
Bad luck though. You can never totally predict the stress relief along a length of wood as far as I know, but SDs test sounds like a useful tip.

 

[mrbmcg]

Thanks for the advice guys.

I actually have the Hoadley book and just took a look at it and it describes the case hardening process and the test forks in detail. I really should read the books I buy

In the meantime I have actually managed to get two 20mm thick boards from the two planks, I remembered that I cut the board about 200mm longer than required to get round a knot and this reduction in length made it possible to reduce the bow quite a bit.

It's an ongoing learning process this sawdust-making isn't it?

 

[Bean]

I had I simular problem with a piece of walnut, my method to salvation was to soak the plank in water then to clamp it as Aragorn has suggested, then by the time it had dried it was close, although they were tension filled days.
Your right though It a bit of a :wink: this woodworking


Bean

 

[Sgian Dubh]

Those old tricks do get mentioned quite often, and they do work reasonably well from time to time, particularly on old table tops, shelves or man-made board materials that have become warped over time due to weight distribution.

They're less likely to work on planks that are case hardened due to an improper kiln seasoning schedule because the stresses are 'seasoned' in. Sometimes planks with lesser case hardening problems can be fixed with a proper schedule of heat and steam in the kiln to relax the lignin allowing the fibres to re-arrange themselves. But truly badly seasoned wood is usually beyond repair and fit only for the fire-- It'll never remain flat which can cause all sorts of problems such as doors that'll never stay true, etc..

I suppose the real answer is to do the fork or prong test with every batch of wood purchased to establish beyond doubt that the wood is seasoned either properly or improperly. If it is case hardened it really should be returned to the supplier for a refund. If enough people return badly seasoned wood the timber yard will stop using the kiln operators that do a poor job. Slainte.