If you’ve ever purchased two-by-fours at a big box store, you know very well that not all lumber is straight or flat. Although wood is hard, it can flex, bow, and bend as it dries or is exposed to moisture. Warped wood can be challenging 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 likely need to mill it—woodworker speak for trimming a board down to 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 one another, and trimming each end to your desired length at a right angle to the freshly straightened edges. Once I learned how to properly mill lumber, everything I built fit together better with less effort. It’s time-consuming up front, but well worth it in the end.
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 maximum precision in your wood, especially if you’re not gluing multiple pieces together or using complex joinery techniques. Consider my current project: a pair of floating shelves that just need to be close to 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 get it flat enough. So before you spend hours getting within 1/32nd of an inch, think about the level of precision you actually need. Sometimes close enough is good enough.
You can also fully mill wood with only hand tools, though that takes a lot of time and practice to do well.
Warning: DIY projects can be dangerous, even for the most experienced makers. Before proceeding with this or any other project on our site, ensure you have all necessary safety gear and know how to use it properly. At minimum, that may include safety glasses, a face mask, and/or ear protection. If you’re using power tools, you must know how to use them safely and correctly. If you do not, or are otherwise uncomfortable with anything described here, don’t attempt this project.
- Time: 1 to 4 hours
- Cost: None
- Difficulty: moderate
- Moisture meter
- Table saw
- Push block
- Crosscut sled (or miter saw)
- (Optional) jointer sled for the table saw
- (Optional) flattening sled for the planer
- (Optional) hand plane
1. Properly acclimate, dry, and store your wood. Wet wood warps. Drying wood warps. Wood that changes environments warps. If you haven’t properly dried and stored your wood, it doesn’t matter how square you get it. It will warp again.
When you first bring your wood home, test it for moisture content. Ideally, your planks should contain around 9 percent moisture or less. If they’re too wet, let them sit until they’re dry. No matter what, 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.
Don’t stack boards to dry directly on top of one another. This will trap moisture between them, which can cause additional warping, or even cracking. Instead, slide small strips of wood called stickers between each board to ensure proper ventilation. This will allow the planks to dry more uniformly.
To make my stickers, I cut approximately half-inch-wide strips from whatever scrap pieces of 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 a jointer, which is designed specifically for this purpose. Slide the board along the bed of the tool and over the rotating cutter head. Always use push blocks to do so, as you don’t want your fingers anywhere near the blades. It will typically take multiple passes to get the face completely flat.
If you don’t have a jointer, you can flatten wood with a planer. However, you’ll need to build a sled to do it. The reason you can’t flatten a board in a planer without a sled is because a planer doesn’t base its cuts off of a flat surface. Instead, the planer will follow the contours of the bottom 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 deformity. 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 yourself see when you’re done, scribble all over the face you’re working with a pencil. When all of the pencil markings are gone, you’ll know that face is flat.
- Note: For those without a planer or jointer, you can build a router sled to flatten the faces of your boards, but that’s more labor intensive, particularly if you’re milling a lot of wood.
3. Joint 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 a right angle to the flattened face. Again, the best tool for this job is a jointer. First, decide which edge to flatten. I typically pick the one that’s closest to flat already. If they’re both wonky, I cut the one that rides more securely 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 over the cutter head, trimming the edge. Again, this will likely take several passes. When you’re done, the edge should be perfectly straight and at a right angle to the face.
- Pro tip: Use a pencil to mark the edge and face you have flattened, drawing arrows that point to the 90-degree corner so you don’t lose track of what you’ve done.
- Note: If you don’t have a jointer for this step, you can edge joint a board with a table saw.
4. Flatten the second face. If you have a planer, this is straightforward. Simply run the board through the machine with the flattened face down. Again, scribbling with pencil 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 board’s bottom face, so you’ll get a uniform thickness. You can’t use the jointer for this because it isn’t capable of cutting parallel to the top face. If you try, the board will likely develop a front-to-back taper, which defeats the point of milling.
If you don’t have a planer, there are a few 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 only works if the board is small enough that your saw blade can cut it.
5. Cut the remaining edge. You now have two parallel faces and one edge that is at a 90-degree angle to both. The next step is to trim the remaining edge on your table saw. If you know the final width you need the board to be, set your table saw fence to that distance from the blade. Otherwise, set the fence to trim off a sliver of that last edge. By taking only a tiny amount of wood, you reduce the 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 jointed edge against the fence. This will create a cut parallel to that edge, which is also perpendicular to both faces.
- Pro tip: Anytime you use your table saw, check the angle of the blade with a digital angle finder. On some projects, like cutting boards, there’s a noticeable difference between 89.8 and 90 degrees.
6. Trim the ends to length. You can trim the ends of your board with either a miter saw or the crosscut sled on the table saw. The latter is generally my preference because I have better control with the table saw. Plus, 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 of your sled (if you’re using a table saw) or the fence built into the saw itself (if you’re using a miter saw). From there, trim off enough wood that the first end is perfectly flat. Then flip the board around and cut it to your preferred length (or simply slice off 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 properly milled the lumber to 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 forth and build.