Loose Tenon Joinery for Breadboard Tabletop Ends
Here's a detailed discussion of breadboard joinery, focusing on whether loose tenons will serve the purpose in a Mesquite table top.March 28, 2012
I'm making a 9' long farmhouse style dining table 44" wide out of 8/4 solid mesquite with breadboard ends that will be approx. 8" to 9" wide. I will be using loose tenons to connect the table to the breadboards because of the available length of mesquite, (long lengths of mesquite is not common).
Does anyone have any suggestions on how to utilize the loose tenons and still allow the top to move for seasonal changes? I am aware that mesquite does not have a great deal of seasonal movement as compared to other woods. Does anyone have any experience with simply epoxying everything in place (without the top cracking, splitting, etc.)?
(Furniture Making Forum)
From contributor W:
We do 40" plus wide 8/4 tops pretty frequently. We always create a tongue on the panel instead of using loose tenons. We debated about using loose tenons but the thought of someone sitting on the breadboard portion that is unsupported made us decide to keep the tenon integral. The breadboard would probably split before the panel but I could see the panel splitting on the bottom of a loose tennn with enough weight.
Our tenons are 2" deep so you would only lose 4" from the overall length. You can always notch the tenon short of the ends so it's not visible. Just leave enough room inside the groove to accommodate movement. I would avoid gluing it all together even with mesquite but I have seen it done before.
From the original questioner:
The reason that I'm using loose tenons is because the usable lengths of mesquite are at 92" and I simply don't have enough length to form an integral tenon. The joint where the table top and breadboards meet will be supported by the 6" X 6" square legs and apron assembly. I had planned on using System Three T-88 epoxy to join this together. I wouldn't normally consider doing this with any species of wood other than mesquite. I would like to hear from other woodworkers who have glued together tops/breadboards and the results over a period of time.
From contributor W:
As long as the end is supported I would be comfortable using loose tenons. You could probably get away with 3 large loose tenons say 8" long. I'd glue all three into the center panel and then only glue the middle one to the breadboard end. I'd leave glue off the outer two and place a peg with an elongate hole in the loose tenon to keep it tight. If you don't want the peg to show on the top you could drill up from the bottom and stop 1/8" or so from the surface and still have enough wood to prevent it pulling loose.
From contributor J:
I haven't worked w/mesquite, but the numbers say a 44" wide top should come and go about 1/8" with a typical 3% seasonal MC change. That's not much, but it might be enough to cause some stresses, weaken the joint and/or force a small crack. If the table were moved to a different part of the country where humidity levels were very different, the problems with a fully glued joint would worsen. Contributor W’s method sounds pretty good to me. I don't understand why you're considering gluing both sides of the joint. The risks with mesquite are smaller than average, but I just don't see what you'd gain.
From Carl Hagstrom, Systems Administrator at WOODWEB
Using some of the most forgiving values for shrinkage/expansion points to a seasonal movement of 3/8" for a 44 inch top. You may want to see for yourself using our shrinkage calculator (link below). Mesquite isn't available on our list, so I used redwood as the species (one of the most stable species), chose a 5% MC change (which is typical from summer to winter), and entered a width of 44 inches. In my opinion, 3/8 is the *minimum* the top will move. Failure to accommodate this seasonal movement will only result in problems.
The calculator is Wood Doctor Approved. I worked closely with Professor Gene Wengert many years ago developing the calculator, and can attest to its accuracy.
WOODWEB's Shrinkage Calculator
From the original questioner:
Thanks to all who have responded. In the past, I've never glued up an assembly like this, always allowing for seasonal movement. Down here in San Antonio there are a lot of woodworkers working in mesquite. Aside from its natural beauty and hardness, it is one of the few woods in the world that has very small movement or shrinkage, (any movement is pretty much equal also), and is very stable. In fact, some of the streets in downtown San Antonio were paved with cross-sections of mesquite.
Some woodworkers have constructed tables with crossmembers perpendicular to outer bands, etc. I was curious as to how these tables held up over time. Having said that, I do plan to construct the breadboard ends pretty much as Contributor W suggested.
From contributor F:
Your loose tenons will work fine. Glue them into the end of the panel/top so you have a long grain glue joint. A good method to drill the holes is with a plunge router and sprial upcut bit. Assemble the breadboard onto the end and pull up tight. Clamp a straight edge to the top for a guide and punch the holes in the desired locations you will need to secure the router when doing this so it doesn’t skate.
After cutting all the holes, remove the breadboard, plunge back into the holes using the straightedge as a guide (still clamped in position) and elongate the holes slightly. This makes for precise work. You could use a shim between the router base and straight edge on the first bore and remove it when you elongate the holes to have a drawbore effect as the table creeps. As others have pointed out, leave the center hole round and glue the tenon into the breadboard. when driving the pegs through the dry tenons, I drive them all the way through then paint a little glue at the base and drive it in a bit further stopping short of where the glue would be drawn into the tenon.
From contributor Y:
Any of the methods of attaching BB ends will work as long as you allow for the width of the table to shrink a bit in winter. If you prefer fixed tenons to loose but don't want to cut tenons on your table boards, you could mortise the long boards and cut the tenons on the BB's.
Regardless of how you attach them, the problem you'll have with BB's is aesthetics. If you don't want to hear your neighbor's or father-in-law's critique of your woodcraft skills you need to think about it at the design end of the project. You might consider making the BB's intentionally wider than the table by half an inch or so at either end and melding the tabletop and BB's together with decorative splines.
In fact, you could do the ultimate loose tenon - spline the BB's to the table from end to end, and chamfer and ebonize the spline ends that stick out. That may be easier than tenoning the middle and adding splines too.
One other thing while we're talking about wood movement - be sure to finish the underside of the table! Whatever you put on the top, do the same on the bottom. It's easy to forget to do this. It won't keep the top from shrinking and expanding laterally but it will ensure you don't end up with a tabletop shaped like a potato chip. I made this mistake once with a 3" thick hard maple top. "Once" is the operative word there.