Thursday, February 7, 2008

Something in the Way Wood Moves or Nails and Screws and Dowels, Oh My!

Clients often ask me whether I use mechanical fasteners (nails, screws or dowels) to join wood. People seem amazed when a piece of furniture isn’t held together by screws. I guess it is pretty amazing. Screws have their place in fine furniture (for instance, sometimes I might use them to help align pieces of wood in a certain way), but let me be clear: fasteners such as screws or nails or dowels should not be used to join wood because they are inferior to traditional wood joints.

To explain why, I first have to explain the concept of wood movement. When I describe wood movement to civilians (non-woodworkers), they sometimes look at me like I’ve got three heads. It’s kind of complicated, but here goes.

When you don’t have to consider wood movement, such as when you join pieces of plywood together, you can use fasteners and they work just fine. But if a piece of furniture is made of wood, fasteners like screws and nails and dowels will not last over time. That’s another major point. When I talk about making something to last, I’m talking about making it last a hundred years or more.

Let’s go back to wood movement. Wood moves. All wood. Some woods move more than others, but wood expands side-to-side with the humidity of summer and contracts to the dryness of winter. Any change in length is negligible, if it happens at all. This is by far the most challenging aspect of furniture making. If you’ve got a board that is 12” wide, it might be 12 1/8” wide during the summer and 11 7/8” wide during the winter. That doesn’t sound like much, but think about a table that is 36” wide – made of three 12” wide boards. It would expand 3/8” in summer and contract 3/8” in winter. That’s ¾” of change during the year! If you were to screw those boards in place so that they couldn’t move, an enormous amount of pressure would build up. The pressure wouldn’t have anywhere to go, so it would cause the wood to split. For this reason, you can’t screw a table top firmly to a base without making the table top split.

Look at any antique table with a hardwood top (be sure that it isn’t veneered plywood). If it is old and hasn’t cracked, then I will guarantee you the top wasn’t screwed down to the base. Actually, I need to clarify something here. Screws are used to hold down table tops, but with a special trick.

Remember that wood moves side to side. If the table top were screwed down firmly to the aprons (with the screw going up through the apron into the table top), the table top would split. One way of securing the top but making sure that it won’t split over time is to make an elongated hole for the screw to pass through. This way, the screw sticks into the table top (holding it down and in place), but the elongated hole through the apron allows the screw to move side to side as the wood moves. There are other, better methods for doing this, but the main point is that wood movement is a major consideration for furniture makers.

Regarding dowels, think of a hole drilled into a piece of wood to accept a dowel. The hole starts off round. Over years and years of expanding and contracting, the hole will become oval and lose its grip on the dowel. The same thing happens to screws and nails.

If you're having someone make a custom piece of furniture, ask how he or she plans to handle wood movement. That's how you make a furniture maker's day. Nerd alert!