I need a reality check — a verification that my thinking is reasonable and correct.
I am working with some extremely wide boards (solid wood) to make a two board bookmatched dining room table top. The idea is to take two contiguous/sequential boards from the same log, open ’em up, and create a center glue line running from end to end.
If I sneeze, the additional moisture/increased humidity in the atmosphere of my shop causes the boards to cup.
To stabilize this cupping (warp), I am thinking of laminating another matched board to the bottom of each side’s piece. I have to add a trim piece on the outside perimeter for appearances so what I am suggesting will not change the appearance of the table — only its final weight.
Rather than relaminating them in the same pattern as they came off the log, I want to keep them in the same sequence but flip the bottom board end over end (so that the butt end of the bottom board is now at the upper end of the top board, and the butt end of the top board now corresponds with the upper end of the bottom board).
What this will give me is a lamination with the pith sides out on one of the “halves” and the pith sides in on the other “half”. My hope is that a pith side to pith side lamination will counterbalance the tendency each board to distort. The expansion coefficients for the boards will be about equal across the lamination (rift on both boards’ edges with center section’s of flat sawn) so that the boards will more or less move in tandem.
I am hoping that the glue line between the laminations is of sufficient strength so that the warp will be restrained. It seems to be theoretically a good idea! Do other’s agree? Has anybody actually tried this?
I hope this is clear — I don’t think drawing it would help explain it. If need be, I can explain it with four cards from a deck.
Replies
Hey -
I'm way out of my league here, but if I were attempting to do something like this, my intuition would suggest the lamination (if I understand what you're thinking) would have the grain running perpendicular to the top. (?) What I'm curious about is the idea of running a trim piece around the perimeter. Aren't you binding the top pieces inside a frame which would be subject to the differential expansion of the wide top pieces and result in failure of the end trim pieces over time?
Just learning all this stuff so be gentle (grin)
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No Frame ! ! !
If I were to create a lamination with one lam running perpendicular to the face, then I would need a third lam that runs parallel to the face to balance the panel (as I would now have plywood). The perpendicular core would (ideally) restrain dimensional change but I doubt if the glue bond between lams would be sufficiently strong to hold.
For simpicity sake forget, for a moment, that I will be joining boards to make a top. What is most important is whether by flipping them and restacking/laminating them, I can create a situation of counter-balancing forces (regarding warp -- specificially cup).
Consider that I have two sequential straight grained boards from the same log or flitch. If I rotate one board (from the butt end) 180 degrees (so they are now end to end) and then slide one board on top of the other, I will have a situation that instead of spooning, they will now be oppositional.
My premise is that, with flat sawn boards, when the board changes (increases) moisture content, the side with more flatsawn surface will expand more than the other side and cup will occur. If the sides with more flat sawn surfaces, in a lamination, are both exposed and more or less the same grain pattern, will cup be eliminated (minimalized)?
Fundamentally, I am not trying to prevent the baords/lamination from changing dimensions; that will happen both across the piece and with regard to its thickness. What I am attempting to do is eliminate (minimize) it going non-planar.
sounds logical, I am sure you will hear from a true expert on this one (no-sarcasm, there are some smart folks here). Will make for a pretty heavy top unless you go "thin"
I am guessing that you will have to glue up your counterbalancing-unidirectional lams with a vacuum press to get a decent bond.
To the guy that recommended perpindicular - cross grain expansion would be a big problem.Steelkilt Lives!
Stanley- Out of curiousity, what is the species and how wide and thick is each board? Is it still in the rough? Have you checked moisture content?John E. Nanasy
English Brown Oak (Q. robur), 30"+ wide, 4/4, MC <10%.
"Heavy" is a relative term.
I need to do some minor final surfacing before lamination.
Yes, a vacuum bag.
Final top dimensions (oval) 56" x 84".
Appearance so far -- knock your socks off beautiful !
I had a similar problem with customer-supplied 4/4 cherry earlier this year. I had to use all the wood and some was badly twisted. The only way I could use the wood was to make twenty or so cuts about 2/3 deep every 3" roughly perpendicular to the length. I then had to fill the cuts with autobody filler and paint the underside flat black. Worked great actually-really smooth.
IMHO, glueing the boards together will work, but I would relieve the stress in the same way as flooring gets milled to do-with grooves in the underside to reduce the wood that supplies the strength to make it cup. How much for 4/4 oak? I don't know, maybe 1/2" or less. The grooves would be hidden but for the ends; you would have to glue and fill with same-wood wedges after gluing up. Much care would have to be taken to have the glue table dead flat so you would not put some twist in the top. I would find some concrete floor that I had checked with a level/big straight edge to rest it on.
That might take care of the cupping, but I just don't know about the normal expansion across grain. Would cutting grooves 1/8th wide the length of the board, space every 1 or so, make the wood more likely to check on the face? Can't really say. Keeping it out of direct sun and air flow would be a given. Candidly, I would really try to have some kind of cleat underneath, with slots for the screws, to provide some kind of bracing. Also breadboarding the ends might help, though I can't speak from experience here. With 8/4 oak you have some real tension to consider. This is a pedestal table, right?
For what its worth,
John
How much are the boards warping now? A small amount of warp might be constrained by the glue (between the top and bottom boards) but a lot of warp might be too much and the boards will tear each other apart. If you do the gluing work very quickly in a controlled atmosphere you could get a few coats of finish on the top before the warping took place. The finish would minimize future movement.
" ...Fundamentally, I am not trying to prevent the baords/lamination from changing dimensions; that will happen both across the piece and with regard to its thickness.
What I am attempting to do is eliminate (minimize) it going non-planar."
Well, I *did* preface my whole comment with the disclaimer! (grin). Fortunately (for me) I learned something in the process.
Dennis in Bellevue WA
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There's also the possibilty that the drying and swelling forces of the end-opposing lam could produce wind along the length. Have you considered that?
Dave:
I have been attempting to visualize grain directions and your comment about possibly inducing wind has totally baffled me.
It is like I am maybe missing something and I would appreciate if you could share with me your thoughts -- I believe you had a good reason/idea for raising the issue so I don't do something (again) that is stupid!!
Thanks
Well, if you have two boards from the same log expanding and contracting at possibly different rates on each end, and you turned one board 180 degrees and laminated them together the unequal shrinkage rates could result in twist over the lenght of the plank. Now, I don't know for sure that this will happen, it may or may not, but it is one scenario I believe could come into play. I have no idea of the grain characteristics of the wood you are using, but it's something to think about.
Stanley,
I don't know. However, I think Kim Carlton Graves has made some big table tops and it might be worth an e-mail to him.
Regards
Frank
My understanding of him is that he is more of a veneer type guy. From what I've read of him, he doesn't work with solid wood on such a large scale(large table tops), but I'm sure he will have some valuble input.
That's an interesting question Stanley, and all I can think is that it might work. I've done somewhat similar with success, and not so great success. In one case that worked we had some very interesting timber, all spectacular interlocked grain and badly warped. I got the stuff flat, but the planks were pretty thin by then-- ~6- 7 mm (1/4"+.) I machined 'plain' planks of the same species all to 20 mm (~13/16") thick and glued our 'interesting' stuff one at a time to a plank which gave us pieces all about 26 mm thick. We tried to select similar end grain pattern, i.e., all cupped the same, which wasn't easy. We then treated the resultant planks as a solid timber edge glue up and used biscuits to aid alignment. We disguised the glue line in the edge by starting a chamfer on the underside at this point. The whole lot was held flat as a table top by the normal four legs and rails with stout bearers added between the long rails.
In another case that rather failed, the top warped quite badly, but it wasn't well restricted as in the first case. We ran a series of router cuts along the length on the underside stopping just short of the ends-- where we ran in a dovetail saw cut here and there as needed--- squashed the top down on a flat surface, individually sized and glued in fillets with urea formaldehide glue, chosen for its non creepines. (Someone else in this thread mentioned this idea, but I don't now recall who.) This resulted in a bit of a wash board effect on the top face, so we planed it flat and re-finished. This seemed to work okay. Both jobs were done in the '80's, when I worked for someone else, so 15- 18 years on I can't tell how successful either job has been over the longer term.
Why not just book match your two boards, run a series of grooves on the underside as mentioned above and by others, glue in the fillets, and attach your lipping to give the impression of thickness? Now you have a spare pair of boards to make another spectacular table top <g> Perhaps you don't have much supporting framework-- a trestle or pedestal table or something? I think I might remember this project being mentioned to me a while back? Slainte.
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Edited 9/14/2002 3:21:49 PM ET by Sgian Dubh
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