A Square Deal (5)

This is the fifth post in a series describing the design and build of a coffee table and a side table. Post 1 in this series can be found here, with subsequent posts linked at the bottom of each entry.

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The bubinga slab has been paid for, and will ship tomorrow from Pennsylvania to my shop in western Massachusetts. I expect to have in hand - hah-hah - it by early next week. I had some concerns in regards to how I would move a 1200lb. board into and around my shop single-handed, however I've pretty much got that figured out now and am not anticipating too much pain and agony in that regard. The arrival of a piece of wood of this scale and weight IS something one should plan in advance for, in my opinion, particularly if there is no forklift on site, as it the case with my shop.

With that concern receding into the background, and a cutting plan mostly worked out (conditional upon inspection of the board of course), my thoughts have now turned to the next stress-inducing aspect to the project. And I mean that literally - the stresses hidden in the board concern me. Of course, at this point I have no idea what stresses may lay hidden within the board, but I do know that I am receiving 12/4 (3") stock and yet the tabletop I seek to make needs to finish out at 6/4 (1.5"). Crosscutting to obtain a blank which is 40" on a side is not likely to be much difficulty with this slab, but reducing the piece in thickness is an aspect filled with unknowns.  And given the cost of the board, these concerns are of altogether a different level than if I were dealing with some other, rather more mundane material. I mean, all wood should be treated with a certain degree of reverence and care in how they are utilized, but some boards are more special than others of course.

I have given this matter a fair amount of thought over the past several days, and I came up with 3 potential approaches to making a 12/4 board into a 6/4 board:

1. Remove material from each side of the blank in even amounts, taking a break, and then removing another layer, again, in equal amounts from each side of the board, ans repeat until finish dimension is reached. One could imagine starting by taking 3/8" from each side (leaving a 2.25" thick slab), let the board rest a day or three and then take 1/4" from each side (leaving 1.75"). A final pass a few days later taking that last 1/8" from each side would leave the board at 1.5". This approach is likely the safest one to take, as it allows the board to gradually equalize as material is gradually relieved in a balanced manner from both sides. The downside is that half of the board is turned into chips, and it is not especially palatable to turn some $840 of material into a waste product. This option is the least appealing for that reason.
2. Have the board resawn in a balanced manner, slicing off about 1/2" or so from each face. The core remaining would be a little under 2" thick, and this would be brought down to dimension in the same manner as in option (1) above. The sliced off 1/2" thick boards are clamped flat with stickering to allow air circulation in the hope that they remain flat. If all goes well, the table top board is obtained and a couple of really wide panels as well. If, on the other hand, the two offcuts do not stay flat, then they could be ripped down into narrower panels - hopefully something would be obtained from these offcuts.
3. Have the board resawn in an unbalanced manner, taking a 1" slice off of the slab, leaving a table blank about 1.875" thick. The 1" board is clamped flat, and the table-top board is allowed to move as it will, then brought down to dimension in a series of balanced passes as in option (1) above. The hope here is that a thicker secondary slab would be produced by the process. Worst case though is that both pieces warp beyond the point of salvage-ability.

All the above approaches rely more or less on removing material from the slab and letting natural stresses work themselves out - along with any stresses that may have been induced by kiln drying, including case-hardening. Bubinga dries well and based on my previous experience working wood from the same log as from which this soon-to-arrive slab comes, I do not expect the wood to move too much when resawn. But you never know- movement is an unknown. If I go with either option (2) or (3) above, the first behavior of the first board which is resawn from the slab will tell me a lot. When I make that first crosscut of the slab to section off the table blank, I will be able to check the moisture content at the core of the stick along the fresh cut, and the results of either process may lead to a change in plans.

At this point, the option (2) listed above seems like the best strategy. It should lead to a reasonably stable table top board, and if I'm lucky I can obtain some panel stock from the offcuts. There are some additional strategies I can try if the board cups more than I would like, taking advantage of grain pre-compression, but I'll cross that bridge when I get to it. I'm rather hoping the board behaves nicely.

Now, here's the thing: there is the aforementioned movement which can take place during and  shortly after cut out, a behavior with predictability akin to a flock of sheep, as it were, where at best I can strive to corral and control things in a general sense.

After the table is made there is the movement in service of the same board at the client's location. In this case, I will be sending these tables (I'm making a side table as well as the coffee table) to a climate-controlled space, but I cannot of course be assured that seasonal ambient humidity will remain constant. And if it varies, the table top board will move relative to its cut of grain.

This board was sliced, from what I am lead to understand, from a 50+" log, about 14~16" or so up from center. I could expect the board to have a grain pattern somewhat like this then:

Actually, I believe that the board that I will get is less flatsawn than the above sketch. Let's call the above the worst case scenario. The bottom line is that the middle portion of the board, probably a strip of material at least 12" wide, with be primarily tangential grain. As you move out toward the waney edges, the grain will become rift orientation.

The board's flatsawn central zone will have a propensity to cup towards the bark side if it were to lose moisture, and cup towards the pith side if it were to gain moisture. A tabletop could gain moisture on one side, for instance, if water was spilled on the surface and not wiped up fairly promptly. Or problems could occur if one side of the table had ample air circulation while the other side did not.

Flatsawn boards tend to cup towards the bark side in practice because it is more likely that as the wood is dimensioned down from the starting stock, the exposed wood will have greater moisture content than the original surface. Particularly with thicker stock and conventional kilning, and especially if no stress relieving was done at the end of the cycle, a board can be quite dry on the outside an a few percent wetter at the core. Once exposed, the core wood will begin to equalize with ambient humidity, and as it equalizes, the drying wood shrinks and the flatsawn board cups across the grain.

A finish of course will dampen seasonal moisture fluctuations to a certain extent. Short of encasing the tabletop in a thick coating of resin, a finish slows, but does not stop, moisture gain or loss on the wood itself. With a board such as I have to work with, any loss of moisture at the installed location will cause the tabletop to slightly cup, as this sketch showing the shrinkage behavior of a flatsawn board:






























To the lower right of the sketch is shown what can happen if a cupped board is forced flat (or is fixed flat while it is trying to lose moisture - a crack may propagate in the bark side surface.

To control this cracking, one strategy it to pre-cut a groove - make a kerf - along the run of the grain in the bark side surface, so that the tangential grain running around the growth rings is severed and thereby has a greatly reduced mechanical capacity to distort the board into cupping. You will see this done commonly on boxed-heart timbers in Japanese carpentry, a practice termed se-wari, or 'spine-cutting'. The kerf 'absorbs', so to speak, the shrinkage, widening to a pie shape while most of the stick's faces remain free of checking.

A similar practice is used in Japan in the fabrication of sliding track stock when using flatsawn lumber - the grooves are cut in the bark side of the stick, as this helps control cupping:

I was thinking about this a little further and remembered that the cutting of grooves or kerfs into the bark side of a flatsawn finish floor boards, or a door sills, was and still is, practiced. Just because it is practiced however doesn't mean that people know why it is done, or do it in the belief that it serves other ends. Indeed, if you look around the web you'll come across a diverse range of explanations for why floor boards have relief grooves on their underside. some say it is to save shipping weight, or to mitigate case hardening, or to allow the board to more easily conform to an uneven sub-floor , or even for air circulation. I don't find any those explanations especially convincing.

While I'm not 100% sure - this is more a theory - it seems likely to me that many years ago, before the advent of high speed molders and where the wood might have been imperfectly dry at times, the person running a stick through a shaper or molder in order to produce floor boards had a moment to inspect the board before stuffing it in the machine. If it was flatsawn, the board could be flipped bark side down and that side would have a kerf or two cut in it to thwart the mechanical tendency of the board to cup towards the bark side as it dried. Flooring pieces which were flatsawn would be laid down pith side up. A quartersawn board would not require any such grooving of course. In time, the presence of the grooves indicated better quality flooring which would tend to lay flatter throughout the year. And it wouldn't be very hard for other flooring outfits to copy those grooves, whether or not they understood the function of those grooves.

Given that the business of making solid wood flooring is very much a game of throughput, the bigger and faster the molder you have the cheaper the price per linear foot, any time once taken to sort the boards for grain would have fallen by the wayside as an inefficiency to be cut. But the grooves need to be there now as a sign of quality - or at least they are what looks normal, they are what are expected - so they get grooved all the same. with this confluence of factors, unsurprisingly there is a 50-50 chance that the grooves will be placed on the wrong side of the flatsawn board, as in this example:

The above pic comes from a Japanese website, so someone over there seems to have misunderstand the purpose of kerfing in this application.

Kerfing the back of a board is a common carpentry practice to make a board pliable for bending:

(image from: http://www.shopsmith.com/academy/tblsaw_spops/index.htm)

The kerfing idea makes sense to me as a means of controlling the tendency of a large tabletop to cup at certain times of the year. One wouldn't want to just run a sawblade across the surface half a dozen times as that would look a bit crude. If the kerfs were made with a ball end mill on the other hand, it would look fine (especially since the only way to actually look at it would be by laying on one's back on the floor by the table). So, my plan is to place a series of grooves on the underside of the coffee table, orienting the board bark side down:

Another view, which shows the grooves more clearly:

Depending upon how wide the band of flatsawn wood is, the number of grooves could vary from the above conception. The ball end mill cut leaves a rounded internal groove profile, which will not tend to propagate cracking.

There are other means to control a board from wanting to cup, namely battens and breadboard ends. Battens, which would be attached to the underside of the table with a full-length sliding dovetail, make sense when the table top is relatively thin, as it would be in frame and panel construction. With a 1.5" thick top however, the batten would have to be pretty hulking in section to adequately resist the top, and that just isn't going to work with this design.

This table top does have breadboard ends fitted of course, which will do their bit to keep the top flat over time. They also dampen the rate of moisture exchange from the end grain portions of the table slab, which also helps maintain long term stability and a reduces any tendency for cracks to propagate inward from the end grain.

I made a slight design revision to the junction at the mitered ends, making the inside corner of the joint interface rounded to help diffuse a potential stress riser:

And, as mentioned above, the client has opted to have a small side table with single drawer, which I will be constructing to a similar pattern as the coffee table:

One more of the side table, showing the back side which will have a demountable framed panel:

In the next day or two I'll plan the rough cut out - a map of sorts - from the slab, as more material will need to be chopped off after the table blank. I'm thinking I will hold off on any second crosscuts until I've seen what happens with the first, and the subsequent re-sawing adventures that await. Fingers will be crossed, and incantations made if necessary.

Thanks for coming by the Carpentry Way. On to post 6.