it’s fall. there are campfires in the air and leaves make it so difficult to walk without falling i’ve mostly taken to getting my exercise in the middle of the street. the season of foreboding is here — in the northern hemisphere, it’s about to get so cold those who possess external reproductive parts will suddenly find them solidified by temperature when they venture outside and we will all truly begin to debate the potential wisdom of migration to morocco. personally, i’m all for it, though my spoken arabic is less than perfect, despite all my attempts to improve — it’s just got so much gender in it. but i digress. it’s definitely fall. so i think it’s the perfect time to talk about spring joints.
if you know what a spring joint is, that’s great. but explaining it to someone who’s never seen it done before can be a little confusing. so let’s take a scenario. you want to build a table and the top is wide. you aren’t likely to find a single board that wide so you have to glue two thinner boards together, edge-to-edge. the standard procedure is to take those boards, joint and flatten an edge on each then apply glue and an absolute multitude of clamps to try to get uniform pressure along the entire length of the board — remember, this is a tabletop we’re talking about so it could easily be a couple of meters long or more and the glue-joint spans the whole length. this can be cumbersome and imperfect. with modern pva, you need serious clamping-pressure to get a good joint. yes, you can definitely use epoxy but that’s expensive, messy to clean-up and takes so long to dry by the time you’ve got the table finished you might as well reuse the boards for a coffin. there has to be another way.
well, there is. perhaps not better but definitely more convenient. and it solves two potential problems. one is that this joint must absolutely be flat and true or any gaps will simply cause the table to split as the seasons change. the boards will pull away from each other unevenly if there is even the thickness of a piece of paper so you have to be accurate with your handplane or you’re … screwed. and not in a nice, get-out-the-drill sort of way.
the other is that you need more clamps than you can possibly own. often very large ones. so you end up going down to the hardware store and spending a wedding’s worth of investment on clamps you’re only going to use three times in your life unless you actually build tables for a living. or you get cheap pipe-clamps and end up torquing the table out of flat and it takes you nineteen-thousand days with a plane to get it surfaced after it’s glued.
the answer is the spring joint. imagine your two boards sitting on-edge clamped to your bench — however you clamp them. i do it between a dog in my end-vise and a dog on the bench and this is how i suggest doing it but you can do it any way you like. heaven help you if you’re trying to plane an edge on a rough 2m board with nothing more than a planing-stop, though. you’re likely to faceplant into the benchtop with your plane when the board starts to tip. but it’s your face so i’ll leave you to it. i use a vise. just to be clear, i only work on stock that’s firmly attached to the bench. i don’t do wedging things in a single direction. i’ve learned that lesson too many times. you will, too. or you can take my word for it and clamp everything securely now and save the pain. up to you.
you’ve been taught to take your jointer-plane and make sure you get a single, continuous shaving the entire length of both boards, preferably simultaneously. once you have that, you know the joint will go together smoothly — even if it’s not perfectly right-angled, it will still work because of the joys of math. which i’m not going to explain but assume it’s true. there’s nothing wrong with this — except clamps from here to the afterlife. to create a spring-joint, though, intentionally create a slight dip in the center by taking a (very, very light) pass in the center, another a little longer, continuing until you get to the outside. you should now have a valley in the middle. what you’re aiming for is invisible, just something you can feel, not something you can visibly determine is there unless you hold the boards together on-edge and shine a light through.
and that’s the easiest way to check. stick the boards together and make sure you can see a tiny sliver of light at the center that tapers to the edges where there’s absolutely no light. what you’re aiming for is perhaps the thickness of a single playing-card or a couple of sheets of printer-paper. this is a very slight joint. it’s not like making clamping-cauls. which i will talk about in a second but isn’t the same thing.
the idea is that you now only need to put pressure on the center of the joint and, as you clamp it more and more tightly, it will actually distribute the pressure along the curve. you can spring both sides of the joint or just one board — the effect will be the same. i do both sides because it’s easier to get a uniform taper that way but it works mathematically in either scenario. when you apply glue and clamp the boards in the center you’ll see the glue squeeze out at the ends then gradually start to do it more and more as you get closer to the center. when the glue squeezes out the center of the joint, you’ve got enough pressure and you can leave it to dry for as long as the glue takes to do that — usually overnight or whatever. take it out the clamp (yes, the single clamp — amaze!) and it’s done. now you can glue panels without the massive clamping investment. and you can even do it in your vise if you want — no clamps required. just do it flat on your bench and pinch the center of the boards between the dog in the vise and the dog on the bench. as long as it’s in the center, it should work the same way. the only thing to keep in mind is you have to make sure the boards don’t angle up or down when they’re being clamped. you can always add a few on the edges to balance the force in the direction of either face but you’re still not going to need the one-every-fifteen-centimeters kind of quantity you’d need for a flat joint.
so that’s a spring-joint. i wouldn’t bother for a short panel (like less than a half-meter) but it’s an excellent way to join large boards, especially when you are limited in terms of clamp availability. but you’ve probably thought you’ve heard something similar before — something i’ve talked about more than once. clamping-cauls. and those are awesome. and they work on a very similar principle so let’s just take a brief look at those while we’re on the subject of clamping without lots of clamps (or in the vise, as i often do for complex joints).
when you want two pieces to have uniform pressure, there are various ways to do it and a spring-joint is definitely one option. but, if you want to have straight mating-surfaces, you can add the pressure-distributing curve on the outside, too. in this case, it doesn’t mean you clamp with a single source of pressure in the center but two — one on each end. this can certainly be used for things like veneer but for simplicity let’s look at it being done for the same purpose we’ve already discussed — tabletops. in this case, you don’t need to plane the boards differently. just make sure you have two other boards that are smooth, level and square at least the length of the tabletop — it’s usually pretty easy to take a few long scraps and make these cauls. they don’t have to be nice wood. actually, you’re likely going to do them a fair amount of damage and they’ll be warped and cracked if you keep using the same ones for this. but you can use the same ones for many glueups.
take the tabletop boards and glue them together as normal, edge-to-edge. on the outside edges of the panel, add your sacrificial caul-strips. at this point, it just feels like you’re adding another board to the tabletop on each side. now take a small spacer — a scrap of wood maybe a couple of millimeters thick at most — and put it in the center between the caul and the next board on each side. now when you clamp down, the pressure will begin in the center and distribute itself along the entire length of the board. when the ends are pressed flush, you’ve got a complete curve of pressure-sharing. the thickness of the spacer will vary depending on the material but it’s probably going to need to be thicker than you imagine — wood really does bend a lot more under pressure than most people think possible, even maple or oak. if you’re making cauls from pine, just remember how warped those boards get simply from hanging off the ends of the shelves in the lumberyard and you’ll have a good mental-image of just how much you need to compensate for the internal flexibility to make a clamped curve.
anyway, glue isn’t generally all that exciting and, unless you’re not like me, this is the part of most projects that tends to be the most overwhelmingly boring. but it’s necessary. unless you regularly buy boards a half-meter wide, you’re going to end up with many projects (like tables) where a panel simply has to be glued together. or made from sheet goods. and while i love sheet goods this is generally not the best application for them unless you’re aiming for a veneered surface. i hope this is useful. enjoy your fall. but mostly enjoy your spring. thanks for reading.