what's the general rule of thumb for "oversizing" for acclimation?
say I want a board that's final dimension is 3/4x5x12, what would be a good rule of thumb?
My instinct would be that it does not and cannot exist. Every species is different from another, every geographical location is different from the next, and every season within that area, every room every part of the room be it a door or a floor or a wall would create different situations. The way the lumber is manufactured, the way it is initially dried and to what degree.
I just don't think its possible to have a rule of thumb with so many infinite variables.
I'd just add a 1/2 to the dimension, to account for shrinkage, warping, etc.
Edit: Don't know what I was thinking when I posted this. Spelling errors ::) :D
1/16" shrinkage for each 2" in width if working with green (flat sawn) to 12% MC is a rule of thumb. I've calculated the numbers for a few species of hardwood and there isn't much variance. To get a good estimate, you need to know specific gravity in green condition (Gg) and volumetric shrinkage from green to oven dry condition (Vsh%),and the tangential shrinkage (Tsh%) for that species (radial shrinkage for quarter sawn). You can find partial shrinkages from fibre saturation point (FSP) or green to oven dry condition. Previous numbers are in the wood handbook for several species.
FSP = Vsh%
Gg
Why calculate FSP? because anything below that MC shrinks, anything above that doesn't move. ;)
Take a moisture meter and see what the actual moisture is of your wood before milling.
But as Jeff indicates, you have to know the application and then know that wood is going to move from season to season. Also know that a high dew point temperature indicates high moisture, compared to RH which only indicates how close you are to precipitation.
In my area a target MC of 12-16% will be pretty stable.
that's kind of what I was afraid of...
I'll just keeping adding "a little bit" :)
First you need to know the moisture content range for the environment. In our house up in north Missouri the swing was from 7 to 9 percent for red oak. The range for our home in Arkansas is 6 to 12 percent. The reason for the big difference is because in Missouri we used a heat pump year around...sometimes a wood stove in winter. In Arkansas our heat is a wood stove with a window air conditioner in summer...which doesn't control humidity well. I found these values by testing a sample board with moisture meter for a year. After you have the swing use the Shrinkulator (http://www.woodbin.com/calcs/shrinkulator.htm) to predict how much the wood is going to move. Like Jeff said...no rule of thumb, but it is easy to get the correct answer with the internet.
True story...I built cabinets in Missouri for our home in Arkansas. They sat in the shop for well over a year with no problems. Brought them to Arkansas and the first summer all the doors were stuck...no fun to make adjustments on cabinets after they are finished.