Help -- Resawed boards bowing like chair rockers

[DR Buck]

Got a call from a customer today with a problem I've not ever encountered.    He's resawing 5/4 heart pine to 2 half thickness pieces.  Booth pieces are bowing almost right away and won't stay flat.   He says measured MC is 6-8% on the outside and ~10 inside where he is halving it. 

I was thinking it's because of the MC differential.   Is this correct?   Will it flatten back out if stacked, stickered with weight on top and let dry to an even MC?


If I kiln dry this stuff before resawing how do I get the inside to dry down without over drying the outer portion?   Going in MC is measured at 19% and I tried to run the DH kiln to 120° with a RH of 30% but I could never get the RH up to anything even close to 30%.

Do I need to do some conditioning after running the temperature up to 160° to set pitch?  What's the best way to do this?   Should I hose down the lumber stack or just set a couple buckets of water in the chamber to help raise the RH?

[beenthere]

There are stresses set up in the halves that should (hopefully) have been relieved during drying. May or may not flatten back.
Not sure what the plan should be at this point. Taking a good set of sample boards as the load is drying to check for the stresses is important, if I recall the kiln courses correctly.

Always risky to resaw kiln dried material and expect it to stay flat. Best to resaw and then dry for good results.

If the customer stickers the lumber and lets it come to some equilibrium over time, then he may get that mc to even out and have better luck resawing.

[scsmith42]

Got a call from a customer today with a problem I've not ever encountered.    He's resawing 5/4 heart pine to 2 half thickness pieces.  Booth pieces are bowing almost right away and won't stay flat.   He says measured MC is 6-8% on the outside and ~10 inside where he is halving it. 

I was thinking it's because of the MC differential.   Is this correct?   Will it flatten back out if stacked, stickered with weight on top and let dry to an even MC?


If I kiln dry this stuff before resawing how do I get the inside to dry down without over drying the outer portion?   Going in MC is measured at 19% and I tried to run the DH kiln to 120° with a RH of 30% but I could never get the RH up to anything even close to 30%.

Do I need to do some conditioning after running the temperature up to 160° to set pitch?  What's the best way to do this?   Should I hose down the lumber stack or just set a couple buckets of water in the chamber to help raise the RH?

I'm with you - the movement is related to the difference in MC%.

Pine is usually pretty forgiving to dry, although you would need to condition it at the end (by adding in steam or water vapor) in order to bring the shell MC% back up to the interior.

[T Welsh]

I call it stored memory, wood has a mind of its own and any time you change its habitat it gives you fits. your customer just opened up a new board because of the pitch. hardwood act in a similar fashion some times. Tim

[5quarter]

Definitely MC related. not as much of a problem with hardwoods, but at 1/2" thickness, pine does not have the beam strength to resist bowing. restacking with weight will work, by in large, but it will take time. conditioning, resawing and sticker stacking will also work. alternatively, your customer could use the redneck method, which of course is to lay the cut boards out in the sun for a few hours wet side up. have to watch them though. too long and boards could begin bowing the other way!