Think about the last time you looked at a basic wooden picture frame and thought, "That thing is perfect!" Never? Yeah, probably never.
When four-sided wooden frames are neatly put together — where the grain wraps around corners almost seamlessly and there's no exposed end grain — you barely even notice them at all. They're just clean and neat ... and perfect. That's because the miter joints are made the right way. It might look easy, but it takes know-how to pull it off. That's what we're here to talk about.
For the purpose of this tutorial, let's use that four-sided picture frame as an example. Before walking through the process of cutting miters, let's discuss the pesky reasons why a frame can turn out not-so-great.
Things That Can Mess Up Your Miter Joints
Getting the angle wrong, even a teeny bit, is one of the biggest culprits that can lead to bad miter joints. Four-sided mitered forms should have cuts at 45 degrees, and even a small degree of error can create gaps at either the heel or toe of the joint when you assemble it. If each miter is at 45.2 degrees, the compounded error will result in a 1.6-degree error, which will be pretty visible. In the photo above, notice the gap at the top right corner, caused by cutting the joints at too shallow of an angle.
It's easy to check your mitering setup with a measuring tool like a speed square or a combination square, but the truth is that no matter how carefully you measure, these methods won't be all that accurate.
Setting up a saw the way it's done in this photo will only get you in the ballpark.
A better way to verify the angle is to make enough test cuts so you can put together a closed shape, then see how it fits.
While we're talking angles, we should examine the other angle direction: the bevel. If the cuts aren't perpendicular, you'll end up with problems. When you put two parts together on a flat surface, you'll be able to see an even gap across one surface. Now if the error is fairly small, you can usually use clamps to close the gaps — at the cost of pulling the frame out of flat.
The cuts in this image were made at a slight bevel (the angle from top to bottom). So even though the joint forms a 90-degree corner and appears tight on one face, it gradually opens up towards the other face.
When you want to create a good miter joint, the length is just as important as the angle. If the parts have different lengths, the joints won't fit well even if the angles are perfect.
Luckily, length is easy to regulate. Stop blocks give you accurate and repeatable positioning of the material. Square frames need only one stop block to be set, while rectangular frames need two.
3. Straightness of Cut
It's hard to get two uneven cuts to meet perfectly. What you want are impeccably straight cuts.
Most miter saws and table saws can make straight cuts, if they're set up the right way and have a good-quality blade. It's important that the stock doesn't move during the cut, so it can help clamp it in place.
If the tool you're using doesn't make straight enough cuts, you can clean up the resulting surface with a disc sander, a hand plane coupled with a shooting board, or even a dedicated miter trimmer. But cleaning up the surface after cutting alters the length, so be sure to check the fit afterwards and make adjustments if you need to.
Even with a good-quality miter saw and a sharp blade, the cuts you make can range significantly; the worst one is on the right in this photo. It's best to clean up the cuts before assembling the miters; that's what makes the sample on the left look so much better.
How to Cut Miters
Let's take a look at the process for making a four-sided frame. But before you start, make sure your material is accurately milled. This is important for getting great results.
1. Set the Angle
Set your saw to 45 degrees. Verify your angle by making two test cuts and checking the resulting corner with a reliable square.
Use a reliable square to verify whether or not your test cuts meet at a right angle, as in this photo.
2. Miter Cut the Ends
Make a cut on one end of each of the four frame pieces. Reserve an offcut for use as a stop block.
In this photo, the angled stop block provides greater registration for the workpiece and protects the fragile corner.
3. Cut the Other Four Miters
With the mitered offcut clamped in place and acting as a stop block, you can now cut the remaining four miters. Sometimes it's hard to position the stop block exactly where you need it right away; it helps to make a few test cuts, moving the stop block closer to the blade after each cut.
If you follow this process and practice a few times, you'll start making perfect mitered joints like it's no big deal. Once you get a feel for the right (and wrong) way to make miters, creating clean joints becomes easy — and fun, too!