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I’m starting several tables, at least one of which will be trestle-style fixed top about 1″ x 38″ x 96″. Poplar, ash, walnut, and white pine are likely species. I really dislike the look of breadboards and my sense of symmetry shudders at the though of leaving the ends of the breadboard proud of the side edges. My design would be about eight, 1.5″ long x 2″ wide tennons from the top mating with corresponding mortises in the breadboard. I would pin the breadboard with pegs passing through oversize holes in the tennons to allow movement. I would likely put just a little glue on the very center tennon.
In the table tops made before, I did not use bb’s, but put the rings down (log heart below the bottom of the top) and fixed the top down with two hefty angle irons mounted in the exact center of the length of the top. The concept here is that any warping of the top would make the center cup upwards, this being negated by the pull of the angle iron/bracket screwed in the apron below. This was on a heavy ash slab and there has been no cupping in 12 years. Trestle design without an apron makes me pause, however.
My overall opinion of wood movement is that thermal contraction/expansion is just as much a factor as humidity, but whatever the source, significant force develops across the face of a big slab, so I will have to make some accommodation. Any thoughts greatly appreciated and needed.
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John,
There was a very recent and excellent discussion on bread board ends on the main page, < Obsolete Link > click here
As to "that thermal contraction/expansion" is a factor, that's a new one for this old dog.
Dano
*John-Do they work? Yes they do. Other methods also work, however. I suggest that if you really do not like the looks of breadboard ends, you look into alternate methods. Perhaps dovetailed crossbracing that allows for movement? You might also consider making the field of the table out of laminated material over a stable substrate.Hope this helps.Peace.-Rob
*I'm a little confused John. You seem to be saying you don't like breadboards but the rest of your post is talking about using them. Here's a couple of things that you may find helpful. First, breadboards are no where near as effective in resisting warp as the cleats that attach under a trestle table. These cleat exert much more force keeping the table flat than the breadboards.To help you determine how much expansion and contraction to expect, go to this site and click on the "Shrinkulator". Input your wood species, range of relative humidity, and panel width and it will calculate the expansion and contraction.http://www.woodbin.com/This site also has a "Sagulator" to calculate the amount of sag for various woods and engineered products.
*Interesting John. i "I did not use bb's, but put the rings down (log heart below the bottom of the top)" No need to use reinforcements such as right angle brackets or angle irons in the middle here surely? The middle will remain close to the rail, but the edges might lift because the growth rings tend to straighten.As far as I can tell, breadboards are mostly superfluous, but they are decorative. My opinion, for what it is worth, is just don't use them as long as you have suitable structural strength underneath to hold it all flat. Slainte, RJ.
*All,Thanks very much for the comments. Very helpful and thoughtful. Sgian, you're right-I reversed the image in my mind-the heart of the log would be, as you pointed out, above the table top. I have also copied below another reply that raises some interesting points and lists a very interesting site for engineering specs for wood: http://www.netexperts.cc/~lambertm/Wood/usdabook.htmlTo continue the thread: My chief question still remains, though my research is ongoing: If a table slab is sealed with poly or even oil, how can there be much change due to humidity? I don't see the vapor pressure and the ambient humidity is the same all around. With that in mind, doesn't thermal contraction across the face of a slab see to be the dominant force? A table in a sunning living room could easily vary by 50 degrees during the day, a seemingly great stress than humidity. Thanks again and any thoughts appreciated..John=======================Post from: http://woodcraft.infopop.net/1/OpenTopic?a=frm&s=471199022&f=381199022"I'm starting several tables, at least one of which will be trestle-style fixed top about 1" x 38" x 96". "1" is really too thin for a top of that dimension. I'd think at least 1½" would be called for.What are you trying to do with the breadboards? Normally they help disguise the exposed endgrain as much if not more than fight cupping. You're better off to fight the cupping by design instead of hoping that the breadboards will stop it."In the table tops made before, I did not use bb's, but put the rings down (log heart below the bottom of the top) and fixed the top down with two hefty angle irons mounted in the exact center of the length of the top. The concept here is that any warping of the top would make the center cup upwards, this being negated by the pull of the angle iron/bracket screwed in the apron below."That is an ugly (pardon) solution and not required if the top is designed properly. You can't FIGHT cupping, but you can design it out."My overall opinion of wood movement is that thermal contraction/expansion is just as much a factor as humidity,"Not true. Humidity changes swamp thermal changes. You can find the factors from this book: Wood Handbook, Wood as an Engineering Material a copy of which you can find at:http://www.netexperts.cc/~lambertm/Wood/usdabook.html
*John, no standard finish is completely water vapor resistant. An embedment in epoxy or polyester or encasing in paraffin can be considered completely sealed.That said, standard finishes have varying degrees of resistance to water vapor. Oils (BLO & Pure Tung)and wax have little resistance, oil/varnishes (Watco, ie.) sllightly more, standard shellac somewhat more (dewaxed however, is excellent in water vapor protection), lacquer and waterborne varnish somewhat more and oil varnish is the best. But varnish still allows water vapor to pass. Finishes just slow it up.The above means that you must always consider and allow for expansion and contraction. And, as was said, water vapor is a far more powerfull influence on expansion and contraction than is heat.
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