Well, I’m finally back with the next post. Up until now, I’ve covered storing wood, milling wood and allowing wood to acclimate to the shop before use. This time I want to look at designing with movement in mind.
No matter how long a piece of wood dries, it will always have some movement with changes in humidity. A poorly designed piece could result in joinery opening up or failing, or even the possibility of a part cracking due to pressures from expansion. Another rule of thumb is the smaller the piece of wood, the smaller the movement will be. If we take a look at a board which is 3 inches wide, its movement will be almost negligible, particularly when compared with a 20 inch wide table top. Why are we just considering width?
Width is the main issue when considering wood movement. There’s almost no movement across the width, and even less along the length. It’s the width which will grow and shrink. Think in terms of a dresser drawer. Ever had one that just refused to open in the summer, but worked flawlessly in the winter? During the humid summer months, the sides of that drawer are drinking in moisture from the air causing them to swell and bind. This kind of thing is what we’re looking to avoid.
With drawers, the solution is fairly simple. Just size the drawers around 1/6″-1/8″ smaller than the opening they’ll live in. There are other areas to consider this also though. Take a cross grain situation. By this, I mean a situation in which the long grain of two pieces runs perpendicularly. A classic example of this is the breadboard end.
The idea of the breadboard end is to keep a wide panel from cupping- yes, it’s a way of handling wood movement. The breadboard is placed on tenons at a 90 degree angle to the grain of the panel, usually a table top. It’s generally secured in place with pegs driven through the tenons. If you were to glue the breadboard all the way across its joint with the panel, the panel would have no room to expand across its width. The way around this is to glue only the center tenon and to oblong the peg holes in the outside tenons so the panel can move across the pegs without binding.
Another potential problem area is when attaching a table top to an apron. Because the aprons will expand vertically across their grain and the top will expand horizontally across its grain, the forces involved could result in catastrophic failure if the top is simply screwed or glued to the apron. There are several solutions to this dilemma. There are several commercially available fasteners sold for this application. Another possibility is to cut a groove in the aprons and make blocks with tongues to fit into the grooves. The blocks secure to the top with screws and are allowed to float in the groove. I prefer to keep things simple and cheap. I use oversized shank holes in the aprons. What does that mean? If the screw requires a 1/8″ hole for the shank of the screw to pass through, I’ll drill it at 3/16″ which gives the screw room to move back and forth with the top
Perhaps the most problematic thing when it comes to movement is the frame and panel. You want the panel to fit nicely into the frame without rattling, but if it’s too tight, the panel could push the frame joints apart or possibly crack if the joints don’t give way. This assumes, of course, that we’re talking about a solid wood panel. Plywood or engineered sheet good panels won’t move like solid wood, so they may be an option if you’re doing a flat panel. If you’re doing a raised panel, though, you probably are looking to use sheet goods. Ideally, you want to dry fit your frame first. Next, measure the height and width to the bottoms of the grooves. Once you have your measurements, size your panel around 1/8″ narrower than the overall width measurement and about 1/16″ shorter than your height measurement. But how do I keep it from rattling in the frame? That’s easy. Dabs of glue at the center of the top and bottom grooves will keep the panel from rattling. Because the panel is only secured at the center, it can still move across the width as it needs to. Perhaps one of the most important tip on frame and panel is to finish the panel before final assembly. If you don’t, you may end up with unfinished areas showing when the panel contracts. In addition, there’s the possibility of a film finish like polyurethane acting like a glue and rendering all of your careful design pointless.
I’m sure there’s more to know on this subject, but I think this is all I know about it. Hopefully I’ve explained everything effectively. If you still have questions, I’m happy to answer them. You can just drop me an email or leave a comment here. Don’t forget to check out my social media. If you prefer video, I’m on YouTube too. Until next time, always be a knothead!