Wood movement coefficients
How to estimate movement in your project:
dimension x coefficient x change in MC = estimated change in dimension
If your piece has both flat sawn and quarter sawn areas, make an educated guess on what the coefficient should be. If a cherry board is approximately 50/50 quarter/flat sawn, I would choose a value of 0.00200 which is slightly past the middle between the QS and FS coefficients. This gives me a bit of wiggle room, and it is easier to use than 0.001855 which is the exact middle value between the two. No need to be that accurate, really.
The coefficients for different sub-species might differ slightly. The USDA research papers has a lot more listed, but I think these values are perfectly adequate.
Example: Pine has the subspecies lodge pole with a coefficient of .00148 (QS) and slash has the coefficient.00187. For a 100mm wide board, the difference between the two is negligible – 0.195mm. That’s 1/128”. We’re way past the measurement uncertainty here.
CAVEAT: The values listed here is meant as guidelines, not absolutes. Err on the side of caution! Also remember that the calculated movement is the total, not how much your part will change from here on – unless you happen to be at either end of the moisture range.
|HARDWOODS||Quarter sawn||Flat sawn|
|Softwoods||Quarter sawn||Flat sawn|
|Pine (eastern white)||.00071||.00212|
|Pine (western white)||.00141||.00259|
|Relative Humidity||Moisture Content|
Different cut types
When we cut a log into planks, there are three types of cuts:
- Quarter sawn: the annual rings are perpendicular, or 60-90º to the surface (the closer they are to 90º, the better). The planks will be very stable, but QS wood requires a lot of work. There are very few boards that naturally will be QS. In the image below, the quartersawn planks are light brown.
Quarter sawn wood will have lots of ray flecking for certain woods such as oak.
- Rift sawn: the annual rings are 30-60º to the surface (45º is the optimum angle). Also very stable, and the grain pattern will be pretty uniform on all sides. In the image below, rift sawn boards is shown in dark brown on the upper right hand side.
Rift sawn wood is time consuming to cut.
- Flat sawn: Prone to warping and cupping. Will contain some areas that are rift- or quartersawn. Known for the “cathedrals” – the oblong, sweeping arcs that resembles windows in the old cathedrals.
Flat sawn gives the most yield and is the easiest form for resawing a log.
Remember: you are working with wood, not metal. Your 100.3954 wide board won’t be 100.3954 tomorrow. Translation to the dark side (imperial): your 3 22/128” wide board today won’t be a 3 22/128” board tomorrow…
Or if you sneeze on it.