Making 45º or miter cuts and joints doesn’t seem very difficult. After all, what could go wrong? Well, pretty much everything. You need to be very precise with your measurements and cuts to ensure the joint is decent.
The first thing to consider is that the pieces of wood you want to join should have the same thickness and width. Simply buying them with the same dimensions or from the same store isn’t enough. Sometimes, even pieces cut from a longer board might not match perfectly. It’s best to cut the pieces two or three centimeters longer than needed and check, by placing them together, that they have the same thickness and width. This won’t affect the joint itself, but you’ll see that in the inner corner of the joint, one piece will stick out, or some faces of the pieces won’t be flush with each other. You probably know this will mean a lot of sanding to even them out, so it’s best to level the four pieces beforehand with a thickness planer. I didn’t have one, so I made a homemade thickness planer.
The second thing is to make the cuts as perfectly 45º as possible. One option is to use a miter saw, which must be well-adjusted for cutting angles. Another option is to make a guide or template for miter cuts with the circular saw, which is the topic of this post.
To make the miter cutting guide, the first thing you need to do is make a guide for 90º cuts (you can also see how to make that 90-degree cutting guide), which we will modify for 45º cuts. Two things are important for that 90º cutting template: that the vertical cut is as close to 90º as possible and that the cut it makes through the wood pieces is also as close to 90º as possible. Now we will modify it as seen in the video, for which it’s crucial to have a square as perfect as possible. The most reliable and cheapest I found was one of those green drafting triangles from Faber-Castell. It will come in handy in the workshop as it’s more precise than my carpenter’s square and the other transparent plastic square I have.
The third thing, which we sometimes overlook, is that even with perfect 45º cuts, the joints might not fit well. If you want to make a frame, square or rectangular, it’s not enough for the cuts to be at 45º; the opposite sides must also be exactly the same length. No matter how accurate your measuring tape, how sharp your pencil, and how good your eyesight, getting identical pieces will take time and practice. The best option is to measure one, place it in the cutting position, and set a stop, as seen in the video, so when you cut the other piece, it will be exactly the same as the first one.
Keep in mind that this guide can be dangerous, and sometimes I don’t use it in the safest way. Check out this post on kickback, the problem of kickback or backlash with the circular saw.
(July 4, 2013: A tip: Making a miter joint can be much more complicated than it seems. This guide works as well as you can make it. The theory is very simple, but every little error multiplies by four. Poorly fitting miter joints can drive you crazy, me first. If this is a hobby for you, as it is for me, maybe you should think, … well, it’s good enough for me, I’ll keep improving. Sometimes it’s better to be a bit of a perfectionist than to abandon one of our woodworking projects. And if you are a professional carpenter… well, you already know that mastery requires knowledge and lots of practice).
Clamping and gluing the joint can also be tricky. While we don’t make something for it, I use Wolfcraft ratcheting straps. They are very useful, sometimes more convenient than clamps, and can be used for other tasks where you need to hold something, like securing large items in the car.
Finally, why didn’t I just put one strip in the template for cutting at 45º? You could cut one side, flip the strip (so the face that was up is now down), and then cut the other side. The problem is that sometimes you will cut moldings, which will have only one flat face to rest on the baseboard, making it impossible to flip and rest the molding side on the baseboard. That’s why I think it’s better to have the option to make both cuts with the same face of the strip resting on the baseboard, and for that, we need both strips. Although working with the wood on this side of the guide can be more dangerous than placing the wood on the other side.