Jon/All,
I just resawed some 8/4 cherry, intending to book match the resulting roughly 4/4 pieces for a benchtop (the sitting kind). The piece started off rough sawn and I trued it before resawing, trying to remove shavings equally from each outside face. There was no issue with resawing. Each half came out in equal thickness and fairly true. I stickered the two halves and weighted them down for a couple weeks.
Both pieces cupped toward the inside faces. In other words, when putting the two pieces back together as they were before resawing, there was a hollow down the length of them. It clearly isn’t caused by tangential shrinkage, as the shrinkage is more along the tangential rings on one piece and along the more radial in the other (again, toward both inside faces). I would have thought the moisture content would have been greater on these inside faces. Are they cupping because they were cut straight when the moisture was greater in the inside and now are contracting as the moisture equalizes with the drier outside faces?
Is there anything to do to remove the cup, apart from planing it out? Much planing will compromise the bookmatch appearance and reduce the thickness more than I want. Did I invite this behavior by not letting the timber acclimate to my shop long enough? My west coast shop has been fairly dry all summer. Am I referring to something painfully obvious to the practice of resawing? How is resawing done to yield appreciably thick bookmatched surfaces?
Thanks to all,
Greg
Replies
It is the moisture difference as you suspect. Even though the wood in the shop will dry to a certain %age water, it is not evenly distributed throughout the board. Rule of thumb is 1 year of air drying per inch thickness. At this point you'll have to face joint the cup out. If the cherry is wide enough, you could rip it down the middle, then joint, and lose less thickness and keep your bookmatch.
Greg, I think you are dealing with a classic case of case hardening. The wood was not properly kiln dried. In other words, toward the end of the kiln schedule the steaming procedure that allows the wood to relax internal stresses was not conducted properly...I'm not suggesting that you sue the mill, in that this happens more often than not when trying to kiln dry heavy stock (8/4 and above).
Cupping caused by case hardening cannot be cured by acclimating the wood so that its moisture content is uniform throughout the wood tissue. You would have to steam the wood to a high enough temperature (of the order of 190F, to resoften the wood's lignin) and basically re kiln dry it to the correct schedule in order to remove the stresses. This is obviously an impractical alternative...so, the only real solution is to let it acclimate in the shop for a few weeks (just to allow it to attain relatively uniform moisture content) then join the stock so that it is back to square. You need a joiner for this (unless you want to try to do it the old fashioned way, with a hand plane).
Running it through a surface planer won't work, unless you feed it cupped-face-down, on the first pass, with a shim strip running under the center of the cup. The pressure rollers in a typical surface planer simply press the cup our of it while it passes by the knives, and the cup springs back as the wood exits the planer. However you do it, you're going to lose thickness.
Sorry I can't offer a happier solution...but there aren't any miracle cures for this problem.
Edited 9/10/2004 3:51 pm ET by Jon Arno
Jon,
Thanks. Of course, you're right. I thought the piece was case hardened after cross cutting a little of the end off, before cutting the portion to be resawn to length. It showed honey-comb checks somewhat similar to those in Hoadley, pg. 149, photograph A. I'd never worked a piece of case hardened timber before and wasn't sure what would happen as I pressed on. The question is, shall I carry on with this piece or scrap the whole thing and start again. Hand planing isn't a problem, but I don't want to spend a lot of time at it if it will continue to change dimensionally if the stresses continue to change as I remove shavings. Is it worth it to go forward? Will it fly apart or crack later on? Someone's gonna sit on this thing.
I'll probably speak to my supplier tomorrow (MacBeath's in San Francisco), although I don't know how much good that will do. I looked through every plank in their 8/4 rack to find this one. It was the right length and width, seemed fairly straight and had the fewest defects in the portions of length I needed for the project. Of course it was all rough sawn, so not too much could be seen apart from its gross aspects. A little gun shy now. Really want not to repeat this. Will read more Hoadley.
Thanks again.
Greg
Greg, it seems to me you've got some subjective choices to make. How important is the book matched grain pattern to you? If you are willing to cave on that appearance attribute, I'd suggest that you go out and buy 4/4 for this project. If it's going to be a seat, you'll have to meet structural requirements that won't allow you to sacrifice a lot of thickness.
You can save the cupped, case hardened stock, that you've already resawn for a project requiring narrower pieces. Once ripped to narrow widths, it will require less joining to square it up and you won't lose as much thickness. Also, any remaining tendency for the stock to cup with changes in humidity wont't be as noticeable...but this shouldn't be much of a problem anyway, since the initial cupping has relaxed some of the stresses.
If you feel the book matched appearance is a key style feature, you'll have to alter your design to accommodate the structural shortcomings of thinner stock...either by laminating it to a secondary wood, or changing the bracing spans...and that can get to be a lot of work.
Thanks Jon
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