Sunday, May 11, 2014

A Square Deal (7)

We return now to the furniture projects I'm doing for a client out in California. The first two pieces are a coffee table and side table, and I'm making them from a rare and costly slab of bubinga that I sourced in Pennsylvania.

I had cut the slab into a 40"x40" piece for the coffee table top, and next needed to reduce it down from a 3" thickness to a 1.5" thickness. There were a few options as for how to do that slim-down, as noted in the previous post.

After much head scratching, I decided that the risk/reward for slicing a 3/8"~1/2" piece off of each face was not favorable, and decided that the safe course of action was to mill material off of each face in a series of rounds, letting the panel rest and move for a few days between rounds, was, regrettably, the best way to proceed. Yes, half the panel went up the dust collector, but this method gave the best chance of the desired outcome of a flat and stable tabletop.

I could have gone the route of many when faced with processing a slab of wood larger than their equipment (jointer and planer) can handle, namely fabricating a leveling and support system and some sort of giant router sled. I decided to go another route however, and took the slab to a CNC facility in upstate New York. I know that this would produce the flattest result, and I don't process large slabs too often so the fabrication of a large specialized jig, plus the rather boring nature of endless passes back and forth to mill the surfaces, led me to take a pass on that option. I also sent the CNC place a CAD drawing of the completed panel, as I wanted them to also use a ball mill to cut relief grooves on one side of the panel, in the hope that it would dampen down any tendency for the board to cup or curl with seasonal movement. I provided a detailed milling schedule as well, and discussed the matter at the company with the very person who would be doing the work. And that person would remain the same for the duration of the project. Just trying to forestall potential problems that can come up when sub-contracting a project phase.

The milling took place in three steps, which meant three separate set ups on the CNC router deck, which made the work a bit expensive, however what came out in the end was a dead flat board:


Click on the picture for a larger view. The 'working' surface of the slab, shown above, is the face which is closer to the pith of the tree. The wood is looking quite spectacular, like a 'topo map'. If I had wiped it with alcohol for photographic purposes the figure would be more apparent to see, but I left off doing that.

On the backside, on the bark-facing side of the slab, the relief kerfs were milled just as designed:


The middle kerf is shorter as there will be a central tenon at that location. Please note that while the board is at finish width, it remains an inch long on each end at this stage.

Another view of the grooves:


I've applied a coat of anchor seal wax to the board ends to dampen down any moisture exchange and will let the slab sit for a while -several weeks at least- and keep an eye on it to see what movement tendencies it may have. Fingers crossed of course! I'm sending happy thoughts towards the piece - stable...stable...stable....

I'm contemplating making a slight revision to the breadboard end design, and am waiting and watching the board first to see what it has to say.

Thanks for coming by the Carpentry Way. On to post 8.

10 comments:

  1. Hello Chris!
    What grain pattern!, Maybe at a later time you could post a photo with a little alcohol (on the wood)?
    It is a shame to let the extra thickness go up the dust chute but, it's the end result that is ultimately paying the bills. Did you measure the moisture content of the sides/ends of the board or off cut? Just to see if there was any difference in the outer surface as compared to the inside? would give an indication of any potential movement.
    wher in upsate NY is the milling company? Name?... Look forward to the next step in this project.

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    1. Joe M,

      thanks for the comment! I do have alcohol kicking around so i could wipe the top to look at the figure and take a photo, so I'll try to remember to do that.

      I measured the moisture content on the ends of the board when i initially bucked the slab into shorter lengths, and it was around 10%, which is just fine. It was consistent from inside to outside as well, indicating that the material was properly dired. Bubinga dries well in thicker dimensions, unlike a lot of hardwoods.

      The company in New York that did the work is Four Corners CNC, located in Millerton.

      ~C

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  2. Chris, did you ever consider having the supplier saw you a piece off of the slab?

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    1. Jack,

      I considered many different options. the seller did not have adequate resaw equipment, and ripping a slice off the slab was one of the options I considered carefully. You need to allow room for the kerf and room for each sliced board to possibly move. When I added it up, I would risk the center slice (the table top board) warping or moving beyond what could be accommodated, for the gain of a couple of thin slices off of each face, each of which would likely move even more than the center board, and thus may not have yielded anything usable in the end. Maybe it would have worked out, but the risk seemed too high.

      I also looked at material from West Penn Hardwoods, who were willing to slice me a 40" length of 6/4 material (which would be ideal for the table top), but they were charging over $100/bd.ft, and I still would have had to source 12/4 stock for the table legs ,and West Penn didn't have any - which means another supplier, another log, and possibly mis-matched color and grain between the top and legs. That's why i encouraged the client to spring for the 16' long slab of 12/4, 50" wide, as it yields all the material for the two tables, and enough to do a sideboard as well if he should choose to move ahead with that aspect of the project.

      Thanks for the question.

      ~C

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  3. CHRIS;
    Beautiful wood!Since the table's top is so wide,why no batten's?Can't wait for the build!
    J.T.

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    1. Good to hear from you J.T..

      Why no battens? The function of battens primarily is to stiffen the top against bending. The aim here is to do a slab top for this table. The slab is 1.5" thick, and battens which would be effective at resisting table top movement with that thickness of material would likely have to be on the order of 2.5" tall or so. Kinda clunky.

      Battens work better, function-wise, when the top they attach to is quite thin, as it would be if I were making a frame and panel top. Having a slab though provides a more beautiful continuous surface, and is free from the expansion gaps necessary in a frame and panel design.

      Also, the breadboard ends (to be fitted) will serve to help resist the top bending/cupping, and will limit moisture exchange on the slab's end grain, which should dampen the extremes of any seasonal movement. Finally, I had the board milled with grooves on one side to limit the capacity of the material to bend/cup, as outlined in detail in the previous post in this series. My hope is that the cumulative effect of these techniques will allow the creation of a slab table top which has both accommodation for, and control of, movement. We'll see how it comes out.

      ~C

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  4. Hallo Chris

    What is annual air moisture content change at customers home ?
    Its hard to believe that such a big piece will stay flat after years.
    My oak table (made 10 years ago) is moving like bow every spring when moisture is low. I am sure that moisture was very low and wood fully seasoned,before it was made.




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    1. Priit,

      thanks for your comment. The relative humidity at the customer's home varies from between 60%~80% seasonally, meaning a wood moisture content moving between 13~16%.

      I'm thinking that the groove cut in the underside, along with the breadboard ends, will limit the tendency to cup. The change in width is accommodated in the joinery.

      It's not about starting with wood with extremely low moisture content, it's about milling the wood when it's moisture content is comparable to the environment in which it will be placed. I am letting the board sit in my shop for several weeks to keep an eye on its tendencies, and will assess after that.

      Bubinga is more stable than oak I would say.

      ~C

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  5. Hallo

    So small change in humidity looks OK.
    Still I do not understand completely how this joinery will avoid or compensate changes in with (I have looked drawings in Your first post) ,but it can be my misunderstanding.
    But one thing what I have seen with the old furniture is that dimensions still change and result is that some parts get during years a bit smaller !! (I am talking 1 to 2 mm/m). I can not explain this phenomenia,but is it possible that during moisture change in wood some internal parts get more compressed or recompressed (because outside layers are more dryier or wet than inside during season change) and therefore after some cycles dimensions do not "come back" to previous level.
    I recommend the test: Just make a simple panel wide enough to make measurements easier (1m or something), measure moisture content and width,then let it play 10 years or something and measure at the same moisture content dimensions again. My theory says that You do not get the same value in width.
    Solution: Avoiding all kind of design where dimensional changes will cause stability or visual problems.
    Best regards
    Priit Miljan

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    1. Hi Priit,

      you wrote,

      "Still I do not understand completely how this joinery will avoid or compensate changes in with (I have looked drawings in Your first post), but it can be my misunderstanding."

      I imagine you meant 'changes in WIDTH' - and made a typo.

      The design does accommodate table top movement - there is more to the corner joints than is obvious. All will be revealed in time, so please be patient.

      I have also seen old boards get smaller over time. I think your theory certainly explains some of that.

      I like working with solid wood because it forces you to design around its nature, which is to move.

      ~C

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