If you’ve ever purchased two-by-fours at a big-box store, you know full well that not all wood is straight or flat. Although the wood is hard, it can flex, bow and bend when it dries or is exposed to moisture. Warped wood can be difficult to work with: precision cuts will be more difficult and joints won’t be as strong.
Before you start a project with wood you just bought, you’ll probably need to mill it – woodworkers talk about cutting a board into a three-dimensional rectangle (aka a cuboid or rectangular prism). This involves flattening both faces, cutting the edges at 90 degrees to those faces and parallel to each other, and cutting each end to the desired length at right angles to the freshly straightened edges. Once I learned how to machine wood properly, everything I built fit together better with less effort. It takes time in the beginning, but it’s worth it in the end.
One note to keep in mind is that the steps below show how to achieve perfectly machined wood using modern power tools. However, you don’t always need maximum precision in your woodworking, especially if you’re not gluing multiple pieces together or using complex carpentry techniques. Consider my current project: a pair of floating shelves that just need to be nearly flat and square. Because the wood is too wide for my jointer and too long for my flattening sled, I used a hand plane to flatten it enough. So before spending hours getting closer to 1/32nd of an inch, think about how much precision you really need. Sometimes close enough is enough.
You can also machine the wood entirely with only hand tools, although it takes a lot of time and practice to do well.
Warning: DIY projects can be dangerous for even the most experienced makers. Before proceeding with this or any other project on our site, make sure you have all the necessary safety equipment and know how to use it properly. At a minimum, this may include safety glasses, face shield and/or hearing protection. If you use power tools, you need to know how to use them correctly and safely. If you don’t, or if you’re not comfortable with anything described here, don’t try this project.
- Duration: 1 to 4 hours
- Cost: None
- Difficulty: moderate
1. Acclimate, dry and store your wood properly. Wet wood deforms. Drying wooden chains. Wood that changes its environment deforms. If you haven’t dried and stored your wood properly, it doesn’t matter how badly you get it. It will warp again.
When you first bring your wood home, test it for moisture content. Ideally, your boards should contain around 9% moisture or less. If they are too wet, let them sit until they are dry. Either way, you need to let the boards sit in your store for at least a few days to adjust to the temperature and humidity of their new environment.
Do not stack drying boards directly on top of each other. This will trap moisture between them which can cause further warping or even cracking. Instead, slide small strips of wood called stickers between each plank to provide good ventilation. This will allow the boards to dry more evenly.
To make my stickers, I cut strips about half an inch wide from whatever scraps of wood I have lying around.
2. Flatten a face. Once a piece of wood is dry, flatten one side. There are a few ways to do this. The best way is to use a jointer specially designed for this purpose. Slide the board down the tool bed and onto the rotating cutterhead. Always use push blocks to do this, as you don’t want your fingers to be anywhere near the blades. It will usually take several passes to get the face completely flat.
If you don’t have a jointer, you can flatten the wood with a planer. However, you will need to build a sled to do so. The reason you can’t flatten a board in a planer without a sled is that a planer doesn’t base its cuts on a flat surface. Instead, the planer will follow the bottom contours of whatever you feed it. So if your board is warped, the planer will cut the top of that piece of wood to follow the warp. By using a sled, you force the planer to follow the milled surface of the sled, leaving a nice flat cut.
- Pro tip: To help you see when you’re done, scribble all over the face you’re working on with a pencil. When all pencil marks are gone, you will know the face is flat.
- To note: For those without a planer or jointer, you can build a router sled to flatten the faces of your boards, but this is more labor intensive, especially if you’re milling a lot of wood.
3. Join an edge. Now that one face is flat, it’s time to cut an edge. The goal is to get that edge perfectly straight and at right angles to the flattened face. Again, the best tool for this job is a jointer. First, decide which edge to flatten. I usually choose the one that is already closest to the apartment. If they are both wobbly, I cut the one that rolls more solidly along the bed of my jointer.
Place your board on the jointer infeed table with the chosen edge down and the previously flattened face tight against the fence. Push the board onto the cutter head, cutting the edge. Again, this will likely take several passes. When you’re done, the edge should be perfectly straight and at right angles to the face.
- Pro tip: Use a pencil to mark the edge and face you flattened, drawing arrows pointing to the 90 degree corner so you don’t lose sight of what you’ve done.
- To note: If you don’t have a jointer for this step, you can joint a board with a table saw.
4. Flatten the second side. If you have a planer, it’s simple. Simply pass the board through the machine with the flattened side down. Again, scribbling with a pencil across the rough side of the board will help you see when you’ve flattened every square inch of the wood.
The planer is the best tool for this job because it cuts parallel to the underside of the board, so you’ll get an even thickness. You cannot use the jointer for this as it is not able to cut parallel to the top face. If you try, the board will probably develop a taper back and forth, which defeats the milling point.
If you don’t have a planer, there are other ways to flatten this face. The first is to use a router sled, which was also an option for step 2. The second is to use a table saw, with the square edge down and the flattened face against the fence, but this method does not only works if the board is small enough that your saw blade can cut it.
5. Trim the remaining edge. You now have two parallel faces and an edge forming a 90 degree angle with both. The next step is to cut the remaining edge on your table saw. If you know the final width you need for the board, set your table saw guide to that distance from the blade. Otherwise, set the fence to cut off a piece of this last edge. By only taking up a tiny amount of wood, you reduce waste and keep the board more versatile for future projects.
[Related: Tune up your table saw the right way]
Pass the board through the saw with one face down and the seam edge against the fence. This will create a cut parallel to this edge, which is also perpendicular to both faces.
- Pro tip: Each time you use your table saw, check the angle of the blade with a digital angle finder. On some projects, like cutting boards, there is a noticeable difference between 89.8 and 90 degrees.
6. Cut the ends to length. You can cut the ends of your board with a miter saw or the crosscut sled on the table saw. The latter is usually my preference as I have better control with the table saw. Also, the mess accumulates near my miter saw and I have to pick it up every time I want to use the tool, when my table saw is usually clear.
Place one of the edges against the fence of your sled (if using a table saw) or against the fence built into the saw itself (if using a miter saw). From there, cut enough wood so that the first end is perfectly flat. Then flip the board over and cut it to your preferred length (or just cut it enough to flatten the opposite end if you’re not sure what you want to use it for).
- Pro tip: I try to keep the same edge against the fence for both cuts for consistency, but if you’ve milled the wood correctly in this step, it doesn’t matter which edge you use.
Now your board is perfectly square in all three dimensions and you have opened up a whole new world of woodworking. Go ahead and build.