Oh hey, wood moves! (Aka, don't glue things cross grain)
Hey guys.
So, my awesome new sloyd bench, which I planed dead flat 3 months ago, is now cupping with a concave on the top.
But, I realized the reason why, and just planing it flat isn't the solution.
If you look at the design of sloyd benches like I built, you'll see that on the underside, there is a "width wise stretcher" attached to the underside of the bench where the leg assemblies go, to raise the leg assemblies above the vice hardware. Well, I glued those on cross-grain, like an idiot. You can imagine what happens as winter comes and the humidity drops off drastically: the width wise stretchers don't shrink, staying the same length, but the width of the bench (long grain) shrinks. Because the top of the bench is able to shrink in width, and the bottom is not (glued to the long grain stretchers) the entire bench top, over 2" thick, cups! And in so doing, it even bends those stretchers along with it, meaning they've become slighly convex on the underside, which is not great for width-wise stability, allowing some potential see-saw like rack to take place.
If I want to correct this, I need to remove my vise hardware and rip those stretchers off and replace them. Yikes.
This doesn't just happen with this bench though. I inspected a quick and dirty benchhook I made, which I also glued cross grain. Same thing, cupping! And each end cups a different direction, because the stops are on opposite faces.
Wood shrinks in one direction and not the other. Plan on it. We all know this, of course, and I think all of you more experienced guys don't need to hear this, but I've been doing this on and off for years now and still made these huge, and very rookie mistakes. So, to all my fellow rookies, take this seriously! You can't just overcome the power of wood with glue and fasteners and expect to tame it. It'll move however it wants to, so you had better count for that and work with that tendency rather than against it.
