Resawing

[Jeff]

It seems that every time I resaw a board I get cupping at least to some degree. Often, it's so bad that I can't even use the board. I read that there are two possible reasons for cupping. One: The center of the board is wetter than the outside and cups on the side newly exposed to air. Two: The board was not dried properly and cups due to relief of internal stresses (casehardening). Sometimes both contribute to cupping. You can tell which factor is at work by how long the board takes to cup. If it cups immediately after sawing, it is due to tension release. If it takes hours/days/months, it is due to the slower evaporation of moisture. It's possible to observe initial cupping due to tension release and more upon continued drying.

The cupping I get is always immediate (within an hour or two). No amount of stickering or clamping prevents it. And there doesn't seem to be much you can do to salvage a thin (1/8 inch, for example) board after it's cupped. If you run it through a planer or drum sander, it gets flattened while moving through the machine and you just end up with a thinner cupped board.

Is there anything that can be done to prevent cupping after resawing? The problem is really frustrating because thin wood is expensive and resawing allows you to generate your own stock for a lot less $.

Jeff

[workin for wood]

Only way to remove a cup is with a jointer, but if the board is 1/8 thick, you're done before you start. The stress in the board is most likely caused by the kiln operator..and if your experiencing it every time on every board you get from them, I say the cure is to buy from someone else. You're own personal diagnosis sure is good though. Another way of immediate stress in a board is if the tree was growing out the side of a hill, or the log was a branch from a tree. A branch goes out and then up, or just plain outwards from the trunk...there's alot of natural stress in that wood just to keep it from breaking off the tree, just like your own muscles would be stressed holding your arms out forever. A tree on a hill is stressed if it is growing on an angle or curved, but it isn't stressed if it's just straight up and plumb with the earth. So here we could say your boards were dryed properly but the tree was bad. It's tough to tell...if you can find any checking deep inside the boards, that'll help identify the kiln as at falt. I'm betting on bad kiln operating..they're rushing too much to get that lumber out the door. Shop elsewhere.

[GrayBeard Phil]

I agree with Jeff (Workin for Wood), it sounds like the seller / mill is at fault by trying to save a few dollars by incorrectly drying the wood. Proper kiln drying does cost money for the energy.

This should not be a problem if you purchase your wood from a real Lumber yard. Any real lumber yard would want to know about such casehardening problems that come from their supplier.

If you purchase from a home improvement store, well you are on your own. (Aside: from news stories on the departed CEO of the big orange, it might even be expected to purchase casehardened hardwood from them. It might improve now he is gone, but who knows.)

Just asking, before resawing, could you run your stock through the planner and take off a small amount on both faces? Then let the stock air dry for several weeks to see if it cups. If it does, let it dry for about a year, then try again.

Sorry, I know this isn't much help

Phil

[Paul S WI]

You might want to invest in a moisture meter. Allow the wood to air dry for a couple of weeks and then acclimate to shop conditions. Would save the headache/heartache of resawing for naught.

[lucky788scroller]

I inquired about resawing a year or so ago to Bill Peterson of petersons custom lumber as to the tendancies of wood cupping after resawing, and whats been mentioned here echos much of what he said.Keep in mind,no matter how the board was dried,the moisture on the inside will be different that that of the outside simply by the relative humidity surrounding it.Even without the internal stress being a factor, you should always expect some cupping. It doesnt take much for the newly exposed surface to 'dry' some once exposed to the air,causing cupping.I would suggest resawing a piece, and immediatly stack it back together like it was, and wrap around the edges with some clear packing tape for a few days to seal the air from it, and observe how much cupping occurs. Then, remove the tape,and watch it a few days.Thats when you might start noticing the cupping. Then, seperate the stack, and stand the pieces on end, so air can get all the way around them, and then see what happens.this will determine more if its moisture or internal stresses because the majority of the internal stess issues would have been addressed upon the cutting.
A couple weeks ago I resawed some 8/4 airdried walnut and didnt have any issues with cupping, but I only went as thin as 1/2 inch . Also when talking to the Petersons,I asked them why their scrolling wood is always so flat, and stays pretty flat, and they informed me that when they cut the logs into lumber, they cut it thin to begin with, so any cupping and warpage happens while drying (they have a kiln as well)and is removed through the planing and sanding so what you are getting is evenly dried and less internal stress.
The only other thing I could offer is resawing it thicker then needed, let it air a couple weeks, and machine it to your final thickness, which may be an excuse to go tool shopping for you. Dale

[Jeff]

Thanks for the replies. I ask because I'm taking a woodworking class at the local university and have temporary access to a large bandsaw. I've been trying to take advantage and stock up on thin wood by resawing my own. It's frustrating when the board cups. For example, I bought a nice piece of bloodwood at Woodcraft (wasn't cheap!) for another project and decided to resaw some of the leftovers for scrolling. I cut it into 1/4" slabs and the dern stuff cupped by the time I got it home. Depending on what you're making, you can often deal with the cupping, but it does limit the wood's usefulness.

Maybe I'm imagining things, but it appears to me that, at least in some cases, the slabs cut from the board faces bow more than those cut from the interior of the board. This would seem to make sense, irregardless of the cause of the cupping.

Jeff