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I was discussing sprung joints with a fellow woodworker the other day. I’ve heard of this concept before, but never practiced it myself.
This is a technique that is used when glueing up panels. The idea is this: you form a cup in the edge of one panel, and the other panel is jointed perfectly straight as usual. When you place the panels beside each other, you’ll see a gap between them (as if you did a bad job jointing). You glue up the panels and clamp them as usual. The difference is that, when clamping, the ends of the panels form a very tight bond. This is simple physics at work — if you place a curved piece of anything on top of a flat surface and then clamp them together in the center, the ends are forced down with great pressure. Supposedly, you need fewer clamps and less clamping pressure because of the “natural” clamping action of the panels.
It sounds like many woodworkers use this technique with great success, and I’d like to use it as well — if it will improve my projects. However, it feels strange to me. My instincts scream out that it will fail somehow.
Can someone explain if this technique really works? Wouldn’t there be excessive force on the center of the panel that would eventually pull the joint apart, especially as seasonal movement comes into play? Or does the wood eventually bend and conform to the right shape, reducing pressure on the center area? Don’t you risk cracking the wood with this technique?
It seems there are all levels of cupping you can form. I’ve seen pictures of sprung joints with very substantial gaps, enough to keep me awake at night if it were my project.
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I always plane my boards to leave a slight gap in the center something on the order of a 1/32” in a 3 foot joint. The often-stated reason to do this is to help counteract any shrinkage near the end. While this may be the case, I just find it a lot easier to work this way. The gap should be sized so that only minimal pressure is needed on the clamps to close it, so a pine board could have a bigger gap, than one of oak. I don’t even try to make one board straight, and the other have the “depression”, for me when the gap looks good, it’s time to glue. On thin stock, like drawer bottoms, I will make the effort to achieve a perfect joint, since these thin wide panels want to buckle when even a slight amount of pressure is applied. I have made several secretaries, that the partitions for the pigeon holes were only 1/8” thick, here a perfect joint is necessary since wood this thin is difficult to clamp (I use masking tape for clamps, in this situation).
*Matt,FWIW, I've been using sprung edge joints for years on table tops and other pieces where the length is around 5' or longer, haven't had a joint fail yet. As you said, it's simple physics.Dano
*Matthew --Ditto Rob's comments; I was taught that shrinkage at the ends of glued-up panels is the problem because the end grain, finished or unfinished, will always lose proportionally more moisture than the long-grain sides. I don't know the first thing about physics (my high-school physics teacher confessed to having failed it three times in college), so I can't say anything about your friend's and Dano's points.I usually cup both edges of the boards going into panels, probably somewhat less than Rob's 1/32" gap. Although I don't have the history that these guys have, none of my tabletops and other panels have shown any signs of splitting or cracking, so far.David
*Thanks everyone for your helpful and illuminating comments on this subject. Sounds like sprung joints are a very good practice. I do have to say that one example of a sprung joint that I saw had a gap of about 1/4", which seems excessive, after reading Rob's and Dave's comments. So I suppose the extra outward pressure in the center of the sprung panels doesn't affect them negatively?Dano, sometimes physics gives me comfort, sometimes it makes me worry. The interesting thing here, from a physics point of view, is that in the same joint you have physics working for you (at the ends) and against you (in the center). But somehow it more than works out.Woodworkers have built up a terrific store of knowledge, and it is excitig to tap into that.Thanks again.
*Matthew,I am certainly no physicist by any stretch. But, I was taught that the sprung joint has more to do with the i way the wood moves along the joint than with the pressures created, if you catch my drift. On raised panel work for cabinet and furniture doors I don't use them, unless I go over 4' or so.Generally speaking, my use of physics primarily concerns my golf swing and casting a fly these days. Though I will admit to using that knowledge considerably in College and the Army in obtaining extra pocket money at the local pool halls...b :)Dano
*Ok the question I have is how to put the cup evenly spread across the edge of one of the boards. Can you use a jointer or do you need to use a hand plane?Steve
*sbaia,You can use either. Both will require practice.Dano
*The cup is supposed to only be along a portion of the edge, correct? For example, if you have a panel 36" long, you would only cup, say, the middle 18", leaving 9" on either side of the cupped portion that is jointed straight/uncupped. Also, when you line up your edges, say on a table top, every other edge would be uncupped? That way, each cupped edge is pressed against a straight edge? Is this correct? At least this is what I observed in the examples I saw.
*I shoot for a more or less gradual hollow along the entire edge, with a gap of about 1/32” in the center of a 36” piece, of medium hardwood. Since I plane my boards while they are face to face in the vise, so both of them have a slight hollow. This gap does not need to be precise nor is necessary to measure it. As long as the gap closes with reasonable pressure on the clamps, you’ve got it right. I tried a sample joint in mahogany with the boards 6” wide by 36” long, and I was able to remove over half the gap with my body weight pressing down on them, and I’m what you’d call skinny and weak. I don’t think I’ve ever used more than 3 boards to make a panel, but if you have to use a more than three boards for your glue up, you might want a smaller gap, or do it in 2 separate glue ups. This would avoid having to apply too much pressure on the clamps
*There was an artical in a magazine ("Woodcuts") published a few years ago by Lee Valley, written by Ed Deak. His explanation suggested using a smooth plane or perhaps Jack, depending on the length of the joint and working from the middle out, lenghting the stroke each pass. Then going with a jointer plane end for end to reduce the hollow to 1/32 or 1/16 depending again on length. His view was such that if you put a clamp in the middle the end boards should not be able to be moved against each other. If they do, the joint is not correct.Greg
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