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Author Topic: Can AD oak case harden?  (Read 2056 times)

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Offline brdmkr

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Can AD oak case harden?
« on: February 28, 2006, 12:16:35 PM »
 I was reading a woodworking forum about a guy that was having trouble feeding popular through his table saw.  He was asking what would cause that, and one of the suggestions was that the wood was case hardened. 

This reminded me of some water oak that I cut and tried to rip after it was air dried.  A few boards really wanted to pinch the blade of the table saw.  I didn't really think that much of it at the time, but it has me wondering.  Is it possible for AD oak (or other lumber for that matter) to case harden?  I have always thought that was something to worry about when attempting to KD.

Lucas 618  Mahindra 4110, FEL and pallet forks, some cant hooks, and a dose of want-to

Offline DanG

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Re: Can AD oak case harden?
« Reply #1 on: February 28, 2006, 12:40:48 PM »
You probably shouldn't consider that oak to be air dried for at least a year, assuming it is about an inch thick. That may or may not be the problem.  Sounds like the table saw was releasing some tension in the wood, causing it to move as you were sawing it.
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Offline beenthere

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Re: Can AD oak case harden?
« Reply #2 on: February 28, 2006, 01:33:50 PM »
Probably early and fast surface air drying can lead to case hardening. The surface dries (and oak is one of the worst ones for that and surface checks can happen with oak if just left in the sun while going for coffee break or lunch) causing shrinkage and stretching of the wood fibers, and even failure meaning checks. Then later on, when the center of the board finally dries and shrinks, the surface doesn't come back to the same dimension (hysterisis) as it has been stretched. So the surface is in compression and the center ends up in tension.  If I recall correctly, this is case hardening, but it will cause the pinching that you experience.
I believe the drying manuals will explain it better. 

And brdmkr, its 'poplar' not popular  ;D  (just in case this gets sent through one of the language translators  :) ).
south central Wisconsin
 It may be that my sole purpose in life is simply to serve as a warning to others

Offline brdmkr

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Re: Can AD oak case harden?
« Reply #3 on: February 28, 2006, 11:36:55 PM »

And brdmkr, its 'poplar' not popular  ;D  (just in case this gets sent through one of the language translators  :) ).

I'm not one to disagree, but this wood really was popular.  All the other wood really liked it and enjoyed having it around :D

Oops ;D
Lucas 618  Mahindra 4110, FEL and pallet forks, some cant hooks, and a dose of want-to

Offline jimF

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Re: Can AD oak case harden?
« Reply #4 on: March 01, 2006, 08:33:59 AM »
Maybe it was the way beenthere wrote it, maybe the way I read it or maybe my mood today :( but not everything came across to me quite accurate.  Air drying does cause casehardening.  While oak is one of the worst species for surface checks all species develop casehardening, oak is not the worst for this.
As far as saw pinching,  was this while you were ripping through the thickness where it was pinching because the end was closing up or through the width where it was pinching on the two edges?
 If it was the first, it was because of growth stresses or differential longitudinal shrinkage which is not related to casehardening but a small amount of relief can be acheived by conditioning.
If it was the latter, it was do to casehardening.  If air dired, it needs to be finished off in a kiln and then conditioned.  Another way is for it to set  for a long time for the stresses to be relieved by viscous stain movement.  This is not related to drying for a year for every inch of thickness.  It takes alot less time for the water to come out of the wood than that tradition. IWhat i am talking about is something like the oozing of putty over time.  The wood does this but at a slower rate and smaller degree and the drying stresses can be relieved this way.

Offline brdmkr

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Re: Can AD oak case harden?
« Reply #5 on: March 01, 2006, 10:13:16 AM »
Jim

I was ripping the oak, so it sounds like the growth stress/shrinkage that may have been the culprit.  When you say that these stresses can be removed by allowing a 'long time' for viscous stain movement, I am wondering if there is a rule of thumb for how long this takes?  I have noticed that some of the first oak that I cut (considerably less than a year ago, maybe more like 6 - 7 months) seems to have reached a moisture equilibrium (not loosing any more weight).  I assume this wood is now air dry, but what about stress relief without a kiln?

Thanks to all for their input :)
Lucas 618  Mahindra 4110, FEL and pallet forks, some cant hooks, and a dose of want-to

Offline jimF

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Re: Can AD oak case harden?
« Reply #6 on: March 01, 2006, 02:09:09 PM »
I don't know of growth stresses being relieved through viscous strain only drying stresses.  And that would take yearrrrrrs.  Remember growth stresses show up when you are cutting say, the one inch direction in a 1 inch thick boards.
The very center of your 6-7 month boards may be wet.  You need to use a meter and penetrate to the center and get a reading.

Offline SwampDonkey

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Re: Can AD oak case harden?
« Reply #7 on: March 01, 2006, 05:02:14 PM »
Was just reading up on case hardening in the text.

The surface of case hardened wood is softer than the 'core'. This is because the cells on the surface dry in a partially stretched condition. As a result the wood in the 'shell' (out layers of cells of the piece of wood) is less dense and softer than if a full amount of shrinkage accurred. Case hardening can lead to honeycombing as well. For details on how to releve case hardening obtain:

Rasmussen, E. F: "Dry Kiln Operators Manual," U.S. Dept. Agric. Handb. 188, Washington, D.C., 1961.

And before anyone asks, I'm no Wood Tech. ;D

As Joe Friday would say, "Just the facts maam". ;)

Pre-commercial thinning pays off. :)

'If she wants to play lumberjack, she's going to have to learn to handle her end of the log.'
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Offline jimF

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Re: Can AD oak case harden?
« Reply #8 on: March 02, 2006, 08:37:56 AM »
Did the DKOM specifically say the shell was less dense or softer?  I have not read the 1961 version lately by I don't remember ever reading anything like that as a fact.  Just because the shell fibers are stretched does not mean they are softer or less dense.  When the fibers are stretched they become narrower maintaining the same volume, therefore the same density.  Just like a rubber band becomes narrower as it is stretched.

Offline SwampDonkey

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Re: Can AD oak case harden?
« Reply #9 on: March 02, 2006, 01:01:20 PM »
Read the last line before the reference. Anything else has nothing to do with DKOM. The first four lines are from the Wood Technology Text, I can only assume it's well researched by them. I'm not going to argue about it one way or the other, refer to the second to the last line. All I can say is take up any issues with Panhin and De Zeeuw, 'Textbook of Wood Technology:Fourth Edition', 1980. ;D  ::)

Pre-commercial thinning pays off. :)

'If she wants to play lumberjack, she's going to have to learn to handle her end of the log.'
Dirty Harry


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