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Would you like to add information to this article? Interested in writing or submitting an article? Have a question about this article? Mitered Frame and Panel Table Tops 
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And yes, it is embarrassing to have those miters open, but open they will if you use solids. Do a search in this flavor in the Knowledge Base and you should find a few excellent past threads on this subject. So-called primitive people understood the movement of wood to such a degree that they would dry wood wedges down as dry as possible, drive them into little holes chiseled into stones along a line, then wet the wedges to cause them to swell. The resulting pressure would crack off a 20 ton slab of stone - perfect for a pyramid or temple or whatever. Wood will move - there is nothing we can do about it but acknowledge it and act accordingly.
From contributor R: If you are building this top with a lumber panel, it is essentially the same as a frame and panel door and you must leave room for expansion and contraction of the panel. As stated, there is nothing wrong with a veneered top if done correctly. Much of the finest furniture in the world has veneer on it. The veneer does not make it a lesser product. Veneer helps solve some of the inherent woodworking problems we have when stretching the limits of what the wood can do before failing. If you still want a lumber panel, make it like a tongue and groove joint on all 4 sides of the panel with the panel flush to the frame and leave 1/16" - 1/8" clearance on all 4 edges. If you do use veneer, tell your customer the table is made from solid mahogany and premium mahogany veneer. From contributor M: This might echo the other responses, but I would go with a floating panel if you use solid lumber. Another thing I do is use pocket screws to reinforce the miters. I know most wouldn't consider that fine furniture joinery, but it works and depending on the location of your apron and legs, they would probably never be seen underneath a table top. From the original questioner: Thanks for the quick responses. It's not a floating panel, but a 4 sided top with 4 miters. Say the frame is 3 inches wide, the inner panel would be about 12 inches, all glued up together. If it was a panel just as I do raised panel doors, I give a 16th on every side as most do, but this is a solidly glued top. So if I used MDF as a core, veneered both sides for balance, and glued it up in the same way with splines on the miters, it shouldn't move or add pressure to the frame that much? I'm assuming that, because MDF in my experience (besides bowing/warping) doesn't expand and contract as solid does. I love the look of mitered tops, as I like hiding end grain in some woods. It is great to finally find a solution to the problem. From contributor R: You have it correct. I have done many mitered frames for a lot of different purposes. A 1/8" thick spline in 3/4" lumber, 1/2" - 3/4" deep into each piece, will stand up to the test of time. From contributor D: Regarding splined miters, or blind splined miters, the long grain of the spline should span the joint to take advantage of the inherent strength in the wood spline. Too many folks make the spline and run the long grain parallel to the joint, then wonder why the reinforced miter failed. As for MDF and movement, I have made up to 8' MDF or particleboard layups and have not seen any movement, using Unibond 400 or other rigid type glues. And just in case you would consider it, don't use contact cement for anything other than HPDL.
From the original questioner: Thanks again. Yeah, I've heard the downsides of using contact as a universal adhesive, and I've noticed that it doesn't fully bond or solidify, thus it allows some movement. Even if a little, it becomes noticeable with weather changes and over time. From contributor J: I don't think I'd seriously consider framing a solid wood top at all. If your miters opened because the panel expanded, then you have not built something really nice. Your embarrassment is warranted. An expansion joint looks fine on a raised panel door, but collects bread crumbs and sesame seeds on a table top. Why not a breadboard end instead? If you really want to eliminate the end grain entirely, you can miter self returns to the ends. Contributor M, pocket screws and metal fasteners don't degrade the integrity of your work. Understanding when, how, and why to use them makes you a better craftsman. The only reason that early craftsmen didn't use them is that they weren't readily available.
From contributor T: Keep it simple, self edge. Or mitre returns in multiple wedges to create a face grain bullnose, or not. From contributor C: When I want to do a mitered frame around a solid panel, I T&G the frame and panel with a fat 1/16" gap for expansion. After the piece is fully stabilized and the finish is dry, I fill the top side of the open clearance groove with a good silicone caulk or 3M 5200, wiping to a nice concave fillet, and let it dry thoroughly. These mitered frame panels are in all my jewelry boxes and chests, some 25 years old with tight mitered frames still. Have you reviewed the related Knowledge Base areas below? 
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