I am re-posting this from the Woods and Materials Forum.
I am building a solid wood cabinet top of cherry, about 21″ wide and 1-1/8″ thick. I am concerned about warpage over time, but don’t like the look of breadboards for this piece. I am thinking of using sliding dovetails installed without glue on the underside of the top and then planed flush. Has anyone had any experience with this approach? Would the dovetails be stiff enouigh to prevent warpage? Would wax help the installation of the sliding dovetails?
Replies
I would not be too concerned about the top warping if the wood is dry to begin with. A 21" tabletop is not particularly wide. You should use cabinetmaker's buttons or those steel figure-eights to attach it to the aprons. This will allow for wood movement and tend to keep the top flat. There are plenty of tables with 21" wide tops out there that have no bracing to keep them flat at all. If you are really determined to add crosspieces, you might consider just screwing them to the bottom of the tabletop with slotted openings for the screws to allow for movement. You can use sliding dovetails if you want to, but you will see the ends of the dovetails on the side of the tabletop. Hope this helps.
Jim
Why?
Rounded,
I don't like to rain on anyone's parade, and if you really want to do the sliding dovetail thing I can't see that it would hurt. But it's really uneccessary. And in general, if something has been done traditionally in a certain way for hundreds of years by countless craftsmen, then I usually say to myself that I'd better have a damn good reason to do it otherwise. So briefly, why not attach your cabinet top in any of the tried-and-true ways that have been around forever?
Sliding dovetails are, in some sense, one of the traditional ways to attach tops to cases. The Goddards and Townsends, and Boston cabinetry of the same period did cut sliding dovetails on the tops of case sides to attach the tops. or used a dovetail key to hold the rear of the top to a rear cross stretcher. But I don't think that is what is being proposed by the OP. But unless there is a hugh overhang I can't see the need for either breadboard end, or cross grain sliding reinforcements.
The amount of wood in your sliding dovetails, it seems to me, will be so thin that it won't be able to keep the top from warping. I use wooden buttons suggested above all the time.
By warping, are you worried that the individual boards in the top will cup, or that the entire top will cup:? If your intended finish is pretty moisture resistant, and at least a couple coats are applied to the underside of the top, there should be less moisture movement which would cause boards to cup. Quartersawn cherry boards should not try to cup, so that would help. Extreme flatsawn boards will cup the most. Especially with flatsawn lumber, alternating the boards bark side up and pith side up will alternate the direction of cupping, , so that the entire top doesn't cup one way or the other. And the amount of cup in individual boards can be reduced by ripping wider boards into two narrower one. Finally, just to cover all the bases, the lumber for the top should be well dried and acclimated before milling and gluing. If the boards have a higher moisture content because they were stored in an unheated area or outside for more than a few weeks, they will want to cup as they dry to equilibrium moisture content. Hope this helps.
I use sliding dovetails quite a bit, including attaching tops. A great joint that won't pull apart and allows wood movement. They can help keep a top from cupping but lumber selection and assembly techniques are more important in that regard. I use straight sliding dovetails most of the time. Getting the fit just right is a challenge, too loose and they don't work as well, too tight and you can't get them together.
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