One of the most important things we ever do as woodworkers is truing lumber because it is the foundation of what we will be working with. If the boards are not straight and true the whole project will be a struggle to put together and likely will be ill-fitting as well. It is very frustrating to try and build a quality piece of furniture or other woodworking projects, and have it come together perfectly ... when the wood we are working with is not true.
Watch it on Youtube: https://youtu.be/KQoY_fOxn2A
Of course, the first step in truing lumber is selecting the wood you will be using and that wood should be the best wood you can find for the price you are willing to invest. It should be a straight grained and as dry as you can find and are willing to invest in. If you need to let it dry a bit longer you can often find lumber that is somewhat lower priced.
Whether the lumber you select is planed or rough, kiln dried or wet, it may still require attention.
In a perfect world the first thing we should be doing is "truing" a face side of our boards, that is .. planing until flat and straight.
If the machine that you prefer to make your boards straight and flat is a jointer, then the wood needs to fit that machine, which may mean you need to trim it to width before you can true one face side. In my case, I have a 6 inch jointer so all the wood I want to surface needs to be no wider than 6 inches, which often means I need to trim boards down or ever cut them in half, which means I need to first make one edge of my boards "true" before I can make a "true" face side.
To make one edge straight and flat there are a few ways of doing this ...
1 - you could use your jointer
2 - you could mark an edge and use your bandsaw, then your jointer to get a flat straight edge
3 - circular saw with an edge trimming jig would also work
4 - or you could use an edging/tapering jig on your table saw, which is the method I prefer to use
Once you have one edge that is straight and flat, you may be able to use your jointer to "true" one face side, or you may need to trim it slightly narrower, as I did, so it will fit the blade on your jointer.
When the wood is slightly narrower than your jointer blade, you can go ahead and "true" a face edge until it is straight and flat. This will take as many passes on the jointer as required to achieve a straight, flat face.
Once you have a face that is "true", now you can go back and true one of the edges to ensure it is at 90 degrees to face edge and that is is now truly straight and flat.
At this point you will have 2 sides of your board that has been "trued" and they are both straight and flat, and they are at 90 degrees to one another, Exactly what you need to finish preparing the rest of the board to have a perfectly trued board.
The final steps in truing your board is to size it for thickness and width, but before you do this, it's best to check your board for moisture content, especially if you didn't do that at the beginning. I always check the moisture content on the face side of the board, after it has been planed. This will give the most accurate moisture reading.
If your wood is still slightly wet, that is ... anything above 10 or 12 percent, there is a high chance that board will continue to try, especially since you have already shaved some of it off, and during that drying process, it is common that the wood will move slightly by cupping, twisting or otherwise warping slightly.
For this reason, it is highly recommended that you allow the wood to sit overnight at a minimum (more if it is wetter) before proceeding the final steps of thickness and width trimming. If your wood moves, it most often will twist or cup on the face side. If it does, you need to once again run those boards through your planer until flat and straight.
Only after your wood is stable, should you proceed to the final steps of thickness planing and width sawing on your table saw.
If you follow these procedures you will have a much higher probability of ensuring your wood has not only been "trued", but that is is also stable to work with and will have much less chance of moving during your construction process.
These processes are what all woodworkers have been following for centuries and are the reason that furniture for bygone eras is still around, stable, and sought after for its construction and stability and it is these same processes that help modern woodworkers achieve higher quality woodworking projects and furniture.
Copyright - Colin Knecht