If you’ve ever bought a two-by-four at a big box store, you know all too well that not all wood is straight or flat. Although the wood is hard, it can flex, bow, and bend as it dries or is exposed to moisture. Working with warped wood can be challenging: precision cuts will be more difficult and joints won’t be as strong.
Before you start a project with your newly purchased lumber, you’ll probably need to mill it: carpenters’ parlance means trimming a board into a three-dimensional rectangle (also known as a cuboid or rectangular prism). This involves flattening both faces, cutting the edges 90 degrees to those faces and parallel to each other, and trimming each end to the desired length at right angles to the newly smoothed edges. Once I learned how to properly saw wood, everything I built fit together better with less effort. It takes a lot of time at first, but in the end it’s worth it.
One note to keep in mind is that the steps below show how to achieve perfectly milled wood using modern power tools. However, you don’t always need the utmost precision on your wood, especially if you’re not gluing together multiple pieces or using complex woodworking techniques. Consider my current project: a couple of floating shelves that just need to be nearly flat and square. Since the wood is too wide for my jointer and too long for my flattening sled, I used a hand planer to flatten it enough. So before you spend hours getting close to 1/32 of an inch, think about how much precision you really need. Sometimes close enough is good enough.
You can also completely mill the wood with just hand tools, though that takes a lot of time and practice to get right.
Warning: DIY projects can be dangerous, even for the most experienced creators. Before proceeding with this or any other project on our site, please make sure you have all the necessary safety equipment and know how to use it properly. At a minimum, that may include safety glasses, a face shield, and/or hearing protection. If you are using power tools, you need to know how to use them safely and correctly. If you don’t, or are uncomfortable with anything described here, don’t try this project.
- Time: 1 to 4 hours
- Cost: None
- Difficulty: moderate
1. Properly acclimatize, dry and store the wood. Wet wood warps. Drying of wooden warps. Wood that changes its environment deforms. If you haven’t properly dried and stored your lumber, it doesn’t matter how square you get it. It will deform again.
When you first bring your wood home, test its moisture content. Ideally, your planks should contain around 9 percent moisture or less. If they are too wet, let them sit until dry. Whatever happens, you should let the boards sit in your shop for at least a few days to adjust to the temperature and humidity of their new environment.
Do not stack boards to dry 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 board to ensure proper ventilation. This will allow the boards to dry more evenly.
To make my decals, I cut strips about a half inch wide from whatever scrap wood I have lying around.
2. Flatten one face. Once a piece of wood is dry, flatten one of its faces. There are a few ways to do this. The best way is to use an edger, which is designed specifically for this purpose. Slide the board along the tool bed and over the rotating cutter head. Always use push blocks to do this as you don’t want your fingers to get close to 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 on a sledgeless planer is because a planer doesn’t base its cuts on a flat surface. Instead, the planer will follow the contours of the bottom of whatever is feeding it. So if your board is warped, the planer will cut off the top of that piece of wood to follow the warp. Using a sled forces 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, doodle around the face you’re working on with a pencil. When all the pencil marks are gone, you know the face is flat.
- Note: For those who don’t have a planer or jointer, you can build a router sledge to flatten the faces of your boards, but that’s more labor intensive, especially if you’re routing a lot of wood.
3. Gather one edge. Now that one face is flat, it’s time to trim an edge. The goal is to get this edge perfectly straight and at right angles to the flattened face. Once again, the best tool for the job is an edger. First, decide which edge to flatten. I usually choose the one that is already closest to the blueprint. If both are crooked, I cut the one that rides most securely along the bed of my edger.
Place your board on the jointer’s infeed table with the chosen edge down and the previously flattened face against the fence. Push the board over the trimmer head, trimming the edge. Again, this will likely require multiple 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’ve flattened, drawing arrows pointing to the 90-degree corner so you don’t lose sight of what you’ve done.
- Note: If you don’t have a jointer for this step, you can join the edges of a board with a table saw.
4. Flatten the second side. If you have a planer, this is easy. Simply run the board through the machine with the flattened side down. Once again, making pencil scribbles all over 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 a uniform thickness. You cannot use the edger for this because it is not capable of cutting parallel to the top face. If you try, the board will likely taper from front to back, defeating 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, square edge down and flattened face against the fence, but this method only works if the board it is small. enough that your saw blade can cut through it.
5. Cut the remaining edge. You now have two parallel faces and one edge that makes a 90-degree angle with both. The next step is to trim the remaining edge on the table saw. If you know the final width you need the board to be, set the table saw fence that distance from the blade. If not, set the fence to trim a sliver off that last edge. By taking only a small amount of wood, you reduce waste and keep the board more versatile for future projects.
[Related: Tune up your table saw the right way]
Run the board through the saw with one face down and the hinged edge against the fence. This will create a cut parallel to that edge, which is also perpendicular to both faces.
- Pro tip: Every 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. Trim ends to size. You can trim the ends of the board with a miter saw or with the crosscut sled of the table saw. The latter is generally my preference because I have better control with the table saw. Also, clutter piles up near my miter saw and I have to pick it up every time I want to use the tool, whereas my table saw is usually clear.
Place one of the edges against the fence on your sled (if you’re using a table saw) or the fence built into the saw (if you’re 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 the length you prefer (or just cut it long 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’s sake, but if you’ve correctly routed the wood in this step, it shouldn’t matter which edge you use.
Now your board is perfectly square in all three dimensions and you’ve opened up a whole new world of woodworking. Go out and build.