# cuppage



## djg (Dec 24, 2009)

I've been wondering about cuppage (if that's a word?). Of boards that is. Years ago, I was told by a sawmill owner that lumber cut further inside the log, toward the center, was more prone to cupping than lumber from the outside of the log. He explained that the tighter the radius of the growth rings the less stable a board would be. I think he said the boards would cup toward the center, further shortening the radius. But if the distortion is real I would think that once the tension is released, the board would cup toward the bark. I believe he was talking about plane (plain?) sawing not of course quarter sawing.

This was one of the reasons he didn't mess with small diameter logs. The major reason, of course, was the low yield vs. time involved. But the tight radius of the growth rings of a small log, even the outer boards, might cup, making lumber useless. Walnut logs were the only ones that were an exception.

Lastly, my recollection of the conversation is vague, maybe I misunderstood him. It was at the end of a long day and, hum, a few beers.


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## TexasTimbers (Oct 17, 2006)

djg said:


> . . . I was told by a sawmill owner that lumber cut further inside the log, toward the center, was more prone to cupping than lumber from the outside of the log. . . .


It's just the opposite. As a general rule that is. Different species react differently to varying degrees as a percentage of movement, but boards the closest to the pith will cup less. 

Taken full stride you'd think that the very center board would be best but you don't want pith in it because that wood is the newest wood in the tree and is unstable.









As you can see the type of cut makes a big difference too. A perfectkly qtr. sawn board from near the edge of the log will not cup nearly as much as a perfectly flat sawn board nearest the center, though you cannot actually cut a *perfectly* qtr. sawn board from a round tree. You could from a square tree if the growth rings were also square and there was no taper in the log.


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## djg (Dec 24, 2009)

I guess I'm more dense than the wood, but I don't get it. Comparing the top flatsawn board in the diagram and the square directly below it, more toward the center. It seems that the board closer the center would be more stressed, and once cut, would want to spring back, outward to relieve the stress. Why would wood with tighter radius be more stable? What am I missing?


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## TexasTimbers (Oct 17, 2006)

yeah I said it just bassackwards. Closer to the pith more cupping because the rings have a greater degree of arc. I'm trying to program the cnc and change the dustbags and posting all the same time - bad idea. I ought to not think and type too, or is that backwards too? :wacko: 

Sorry for the confusion.


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## phinds (Mar 25, 2009)

I think part of your confusion stems from an apparent belief that the cupping occurs due to natural tension in the wood. I infer that from your statement about "once the tension is released". That's not actually the case. If it were, the cupping would occur as soon as the plank was cut, but a fresh cut plank will be flat until it starts to dry. The cupping happens because of the stresses that get set up as the moisture leaves the wood.

If you could keep a plank at the same moisture level it had in the tree it wouldn't change shape in any way. Of course, you couldn't DO much with it, but I guess you could always stand back and admire how straight it is. 

There ARE pieces of wood that have a natural internal tension (called "reaction wood" and it comes in two forms) but I think even reaction wood is flat when first sawn, it just has a lot more tendancy to go screwy on you as it dries.

Paul


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## Logger (Nov 26, 2009)

*pith*

Just wanted to say to that the pith is the oldest part of the tree, thats why when you saw a log you start seeing more knots the closer you get to the middle, the tree keeps putting wood on the outside, whats inside never changes. :smile:


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## djg (Dec 24, 2009)

Last questions
If distortion happens upon drying, does the speed of drying and temperature of drying effect the final outcome? For instance, is air drying, or at least down to a certain MC, better than a kiln, whatever type, which sucks the moisture out faster? Will the elevated temperatures within a kiln cause more distortion vs. air drying.
Lastly, I've seen some boards come off twisted, right off a circle mill; sycamore comes to mind. Is this what you called 'reactionary' wood?

There is a reason for all this discussion, not just idle chit-chat; but too long to go into.
Thanks everyone for your help.


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## Logger (Nov 26, 2009)

*drying*

From what ive found fast drying will cause more movement(twist/cup/bow/check),but ive also found you can dry lumber with alot of stress flat if you airdry 1st, the key is alot of weight on it, it needs to be cut and stickerd right and you will get good results. Hope this helps Mike (I put about a 1000lbs on top of my stickerd piles)


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## TexasTimbers (Oct 17, 2006)

Logger, I need to lay off the wine of the evening. :laughing:

Yes I know how the tree adds layers, but I had it stuck in my tiny brain that I had read that the pith, although technically the oldest part of the tree, takes in nutirents from the taproot and continues to have cellular regeneration. But I cannot find that anywhere now so I guess I dreamed it. I'm glad I mentioned it because it never made any sense to me.


DJ, Logger is right adding a lot of weight to a stack no doubt helps. I also like to boule cut and sticker and strap the whole thing, and tighten the straps every week or so. I only do that with certain species though. 

Eventually you get to where it will need tightening about once a month and once it gets closer to 15ish% you won't notice much more shrinkage until you stick it in the kiln and take it to 7% or whatever. there's quite a bit of shrinkage from EMC ~ 12% to 7% but if it's been weighted or strapped down to 12 - 15% a lot of that stress has been released slow enough and it will minimize it in the kiln, though not eliminate it totally usually where there's stress.


Also when you are building a lift, or stack of stickered wood, make sure to place the most valuable pieces on the bottom so they have the added benefit of the weight of the rest of the pile. 

The species I've seen benefit from rapid drying is the flame boxelder containing the dense gold wood mixed in with the red flame. You either have to dry it extremely slow, or extremely fast and a vacuum kiln is best for it.


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## Logger (Nov 26, 2009)

*drying*

Well said Texas Timbers :thumbsup:


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