Preparing timber for acclimatising - drying the wood out.
Don't start zipping your wood through your planner thicknesser just yet!
This is an important step for trouble free woodworking, you’ll be kicking yourself for if you miss it out. This is why:
The timber will, more than likely be Kiln Dried. This theoretically reduces the moisture content to less than 20%. If its been transported and stored well between the kiln and the your shed, the moisture content should not have risen. However this doesn’t mean it’s a consistent 20%, the inside could be much higher and the outsides and ends much lower. It's not ready yet for any serious woodworking.
Timber in your house will dry out to an average level through its full section at between 8 and 12% moisture.
Wood lives, it expands and contracts in relation to the moisture content. A working assumption is the thickness and width will decrease by 0.25% for every 1% decrease in moisture content below 20%.
So if you make something now, by the time it’s got used to your cozy dry house it will be at least 2% smaller. Think about it a 100mm piece will be 98mm, a meter wide table top will be 20mm short. And that’s without any warping or twisting!
Before you use your Planer Thicknesser,
The trick is to make sure the wood is acclimatised to the conditions where the finished piece will be spending most of it’s time. To do this:
1. Rough cut the pieces with a table/bench saw or band saw. Oversized enough to plane down to size using a planner thicknesser.
How oversized is a bit of best judgment, dependant on the section dimensions, how rough your saw cuts are and how much the wood might warp.
As I mentioned earlier I’d add 2mm to each side on a 50x50 section a meter long. Add more to thicker sections or longer lengths dependant on the grain / growth rings. Err on the side of caution if the amount of planing doesn’t put you off.
2. Any off-cuts, cut in to 10 x 10mm (ish) sticks to use as spacers.
3. Stack the wood, preferably somewhere that replicates the conditions where the finished piece will be. Stack it using the spacers to ensure a good airflow. On long boards put the spacers a quarter of the length in from the ends. This prevents bowing under gravity, the next layers spacers have to be over the previous layers.
4. How Long to acclimatize the wood? This is difficult to answer as there are so many variables. Small sections might only need a couple of weeks, thick sections might need a month or two.
The cheapest easiest method is to weigh a piece regularly. When it stops getting lighter its ready.
You can get technical, weighing a piece and drying in an oven etc. but that’s getting extreme for our purposes.