secret dovetails?

[estimated reading time 6 minutes]

i’ve been on a bit of a trend of beginner woodworking articles lately so i wanted to tackle something a little more involved — sliding dovetail battons and table glueups. this is something that’s extremely popular, especially with students looking to go beyond their basic projects to make something truly useful for their homes. a dining-table is often quite large — 1m x 2m, sometimes closer to 1.5m x 3.5m if you have a really large room. even on the smaller end, though, you’re not likely to find a single piece of wood that’s wide enough for the tabletop. of course, there’s a huge amount of joinery involved in making the base — not to mention the complexity of attaching the base to the top and that’s a discussion of wood-movement that will come in another article. but everyone thinks the simple part is gluing up a few boards to make a tabletop and just moves on. that’s not necessarily the case.

there are a few different ways this can be done but there’s one main question that needs to be answered — that’s one huge, wide piece of wood — how are you going to keep it flat? wood moves. and wood moves in such a way it will bend unless the grain is perfectly straight and flat. trees are round. if you ever find a board where the grain is, without exception, straight and flat, consider yourself very lucky. most boards, no matter how straight the grain is, are not absolutely perfect. they will move with moisture change and that’s not necessarily a bad thing. it’s just something you have to be aware of. but that means if you have a tabletop made of, let’s say, flat-sawed cherry a meter wide, that piece swinging between 12% and 18% moisture can vary in width by 15mm. if you’re finding that hard to picture, that’s loosely the width of your thumb. this is not a trivial amount of wood movement. and that can easily make the piece banana either up or down, depending on the orientation of the grain, a good 10mm — potentially twice that if the tree was particularly tense before it was cut down (sometimes trees have stressful lives like the rest of us).

the solution to this is to restrain the tabletop in some way. there are various plans and they don’t just work for tabletops — you can do frame-and-panel construction. let the thing ride in a groove and float along its width. that’s not nearly as common for tabletops as door panels (think shaker or arts-and-crafts furniture) but it’s definitely done sometimes. you can add aprons around the edges of the table and that will definitely hold it to a certain extent. but the most popular method for something this large is to add batons under the table — either in addition to stretchers or without them. the stretchers are an aesthetic choice. whether you have them or not will impact how the table looks and you shouldn’t be adding them for strength. adding batons where they can’t really be seen, though, is likely necessary for anything larger than about 50-60cm even if you have aprons. if you only have an apron at each end of a long piece, that’s simply not going to be enough to keep it flat and trying to do that will likely mean it will warp the aprons, too. or at least separate from the aprons. of course, you could just make the thing from stable material like plywood and veneer it. and that’s a fantastic and historically-accurate option in many cases. but if you want a solid top, you’re getting into the land of batons.

there are many ways to attach batons. you can screw them to the bottom of the table. this is functional, fast and generally looks like shit. yes, screws are definitely useful in fine woodworking. but if you’re using them in a way that’s either visible (like screwing through the face of the board into the bottom of the table) or obviously-plugged (come on, ikea, you can do better than that and you’re setting a terrible example for the rest of us cause this is definitely not an aesthetic component of danish modern) you’re probably just not thinking your joinery through or “fine” might not be the most important part of your style. as yoda would say, functional they are — beautiful they are not. there are other options, though. i’ve even seen (and used a couple of times) metal c-channel to brace the bottom of a potentially-warping table, though this is usually done with slab-top tables rather than glueups. but here’s my favorite — sliding dovetail batons.

why are they so awesome? they let the wood move side-to-side without letting it warp. they require no hardware of any sort. and they look impressive despite being really easy to make. but here’s the problem — people install them as afterthoughts and make a horrendous design error when they do it. it all comes down to when you add them. a sliding dovetail has to enter the tabletop somewhere. so the typical way this is done is to glue up the tabletop — four 300mm-wide boards, for example, to make a 1.2m-wide table. then you make a series of dovetail batons and space them evenly along the table, inset from the edge a little but spanning the majority of the width of the table. you cut the dovetail then you route the dovetail slot from one side of the table, cut a filler piece and slide it in, attach the filler and you’re done. i know what you are probably thinking here — it’s what i first thought. how hard can it possibly be to cut a stopped-angled dovetail slot in the middle and drop it in. but you’ll quickly realize the cutting of the slot isn’t the hard part. if you can get it in, it’s useless. the whole point is that the dovetail is wider on the inside than the outside (in the middle of the tabletop’s thickness rather than the bottom face if that’s not clear). you can’t slide it in if the groove is stopped. you just can’t. if it goes in that way, it’s not a dovetail. it’s a tenon. and there is a way to do this but it’s far more complex than it needs to be — you cut the thing as a very long tenon, cut the groove in the tabletop as a dovetail mortise then you put a wedge in the center of the tenon the entire length of the baton and drive it in. it expands as you put it in the mortise and becomes a blind-wedged-tenon shaped like a dovetail. it solves the problem but wow is it ever overengineered!

there’s a simpler solution, though. forethought. and every time i see dovetail-shaped filler pieces on the edge of a table i die a little inside wondering how many times people are going to do this and have to cover their mistakes. i even see it in articles and videos as if this is an intentional technique rather than a failure of design planning. this is not a good look. it’s sloppy in the same way plugging screwholes is. some joinery is meant to be seen. this is meant to be secret — you’re compensating for the fact it’s wood rather than glass or steel, which doesn’t move and doesn’t need this support piece to keep it flat. if the wood behaved as you wanted, it would be unnecessary. we hide those details.

it’s simpler than it sounds. take your tabletop and glue it up in two halves. if it’s four 300mm boards, glue two sets of two and you’ll have two 600mm panels. now make your dovetail batons. route the slots in one side of each panel — from what will be the center of the table, half the length of the baton (plus the gap for potential expansion). drive the baton in and, if you want, glue just the center 100mm or so. the glue is unnecessary but it might give you some extra piece-of-mind nobody’s going to be hitting it with their legs under the table and wondering why things aren’t secured. only the middle 100mm, though. any more than that and you’re not letting the tabletop move. now you’ve got all the batons attached to one side. slide on the second side and add the glue to attach the two panels together. you have dovetail batons attached in the middle with no relief slot cut from the outside of the table — they’re totally hidden unless you look at the table from below.

of course, this sounds like an incredibly complex technique and it’s not. you’re just routing a couple of dovetail grooves and slipping in some batons, which is why i always find myself getting annoyed when people don’t think the whole process through enough to do this at a better stage in the process.

there’s a repair issue, of course. if you ever end up having to replace one of them, you might be thinking having no easy access is a real problem. but if you’re dealing with a broken baton, that table is in a sad state and there’s a simple way around it. rip the table down the center seam, take out the baton that’s broken, glue the whole thing back together and ask yourself exactly how many people were dancing on the table to crack a 100mm-thick dovetail baton from above. cause it was either an elephant or an army.

anyway, that’s a truly simple trick to take a process we do on a regular basis when making tables from awkward to professional-looking. you can do this even if the top is going to be surrounded by aprons, by the way. those just get attached after the thing is glued up, batons-included. hope some of you find this as useful as i did when i first started doing it! thanks for reading.

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thank you for reading. your eyes have done me a great honor today.