If you’ve worked with wood very long, you likely have found out that wood moves. Depending on what kind of woodworking you do, the movement will vary a little, or a lot. Turners will often start a vessel when the wood is green (not dried) and then allow the vessel to dry for a few months to finish later. When they go back to it, the vessel is often not round like it came off the lathe when they first turned it, but will be oval and irregular in shape. If you use a moisture meter you can determine with fair accuracy what the moisture content of wood is, and know about how dry the wood is at the time you’re working it. I often use a meter now that I have seen just what wood does when it dries.
My work is mostly flat work, I do turn a few things, but I’m not a turner like some are, where they do nothing but turning vessels. The one place that I found moisture changes affecting my work the most is chess boards. When doing that kind of project, there is more to consider than just moisture when thinking about movement of wood, although that is a primary consideration.
When wood dries, or looses moisture, it shrinks and loses weight. Some woods shrink more than others, and some lose more weight than others. There is a science behind knowing moisture in wood based on weight and size, and the wood industry uses this science more than meters to determine how wet wood is. (Kiln drying is where this science is most important) In one of the woodworking club meetings I attended some months ago, the presenter was a man that worked in the lumber industry doing grading of materials, both hard and soft woods. He explained many of the things that go into grading and knowing how dry wood is. I learned far more than I thought there was to know about wood movement, more than is viable to post here. But I will go into something that I have discovered that might benefit you. One of the things that all should know and understand, you should allow any wood you obtain to acclimate to your shop environment before you start milling it. If you’re expecting wood that has been in the heat or cold and work in a different temperature in your shop than its been accustomed to, it will move before you get a chance to put glue to it. Let it sit in the shop for a few days to a week or so and stabilize. If you’re not working in a heated/cooled shop, that acclimation time may not be as long depending on where the wood came from.
I made a chess board with Hard Maple and Red Heart. Red Heart is an exotic wood (use the link to go to wood database) and can be very beautiful when contrasted with the right other woods. What I didn’t do was check how much moisture was in the Red Heart when I made the chess board. After the board aged some I found that the maple squares were thicker than the red heart squares, both on top and bottom of the board (solid wood board). I figure that the moisture must have been about 20% when I made the board, as it was about 7% when I discovered the movement. I since ran the board through a drum sander and flattened it again, and seems to be ok now. The lesson is, make sure that the materials you’re working with are stabilized (should be about 7% moisture on average). If something isn’t below 10% moisture, you likely aren’t going to have good long term results from your project. A good moisture meter would keep you from making those kinds of mistakes. Buying lumber that is known to be dry is also a good way to avoid problems if you have a supplier that only sells dry wood (Woodcraft, etc). I buy from a seller that also has green wood available (mostly turning stock) but since there is some not completely dry wood in house, I use a moisture meter to find out if the wood I bought is ready to work or not. Air drying wood from gren to dry takes on average a year per inch thick the stock is. A board that is considered 4/4 (four quarter) is about an inch thick, and would take about a year to dry in an air dry (not forced) situation. A board that is 8/4, or two inches thick would take two years. Kiln drying is much faster, but there are down sides if not done properly.
Another something that is also worth mentioning here is wood density. Not all woods will work together, even though they look good together. To know more about that, there is a chart available from the wood database folk that shows how hard wood species are. This chart is available on Amazon here. The poster shows the Janka Hardness of many species of wood found around the world. Woods of reasonably similar harnesses work together well, some that are very different will need special handling when building a project to allow for movement of the two woods with temperature and humidity changes. Even after wood has been cut and dried, it still ‘breathes’ moisture in and out, so it isn’t going to be absolutely still sitting there on your coffee table.
Another reason to know how hard or dense wood species are is to know what kind of tooling it takes to work it. Some woods need carbide cutters, others are ok with tool steel. In doing turnings I found that carbide cutters work really nicely for the exotics and high speed steel is ok for most everything else. If you try to use high speed steel (tool steel) on exotics, it will dull the tools quickly and you spend about as much time sharpening as you do turning.
It is worth noting that I am in the south eastern United States, so my domestic species of wood include maple, cherry, walnut, oak, etc. The exotics that I mention come from other areas of the world, such as South America, Africa, Asia, etc.