Step 3: Mitering the Corners

This is the most important step of the project. In this step, we’ll cut the actual corners for the frame. To start off, cut the 2 big pieces you have down into the 4 pieces we’ll work with. Make them longer than necessary, since we’ll miter the corners. It’s easier to work with 4 shorter pieces than 2 long ones. Since the frame will be 20″ x 17.5″ (outside edges), cut each long piece into one piece that is 26 inches long, and one that is 24 inches long. This will give you 2 of each, with 6 inches of extra material on each piece, with 10 inches left over from each of the two big pieces you started with.

A miter joint is a 45-degree cut on both ends of two pieces which will join to form a 90-degree angle, as shown in the illustration to the right.
Miter Joint
In order to get perfectly mitered corners in a picture frame, you need to make sure of the following two things:

  1. The angle on each end of each piece must be exactly 45 degrees.
  2. Opposite side pieces must be exactly the same length.

I’ll show you some tricks I’ve read about to guarantee perfect corners.
Measuring Exactly 45 Degrees
Setting the miter fence to exactly 45 degrees
To ensure rule #1 is true, use a drafting triangle to set your miter gauge on your table saw to exactly 45 degrees. Don’t trust the scale on the gauge itself, particularly if your saw is a basic home-workshop model, like mine. If it is off by even a couple of degrees, you’ll get gaps in the corners of your frame. Using a drafting square will make obtaining a perfect 45 degree angle foolproof.
Watch out for the teeth on the sawblade. When setting the angle, make sure that your triangle is pressing against the blade face, and not touching the teeth at all, or else the angle won’t be perfect. My table saw has a 10″ diameter blade, so I bought a drafting triangle that is only 8″ wide, so the entire triangle can rest against the face of my blade without touching any of the blade’s teeth. This is a cheap tool (I got mine for $5 at Staples) and will substantially improve the quality of your frames.
Once you’ve set the angle, cut one end off each of your 4 pieces. Pay attention to the angle you’re cutting, be mindful of which edge is the inside edge of the frame, and which is the outside. After cutting, the shorter edge of the wood must be the inside edge. That is, the edge with the rabbets cut into it. The rounded edge should be the longer edge.
Cutting the Pieces to Length

When you’ve cut one end off all the pieces, gather two opposite side pieces together. We’re going to cut the other ends now, in a way that will make sure rule #2 is true. The trick is to cut the sides in pairs. That is, the two pieces that make up opposite edges of the frame are cut at the same time.
Take one of the two side pieces and make a mark at 15.5 inches on the inside edge. This is where we will want the blade to cut. Position this piece against the miter fence, such that the mark will touch the edge of the saw blade at just the right length. Again, pay careful attention to make sure you’re cutting the angle the right way. I’ve wasted a few pieces of wood making this mistake.
Once the first piece is in just the right place such that the saw will cut it exactly at the mark you made, stack the other side piece on top of the first one and slide it along the bottom piece, feeling the already-mitered ends with your finger to make sure the pieces are perfectly lined-up.
Ensuring opposite-side pieces are exactly the same length
Clamp the pieces to the miter fence
When the mitered ends are perfectly flush with each other, clamp the pieces against the miter fence. Use a piece of scrap wood to prevent the clamp head from touching the actual wood, or it may leave a mark. Make sure your saw blade is raised high enough to cut through both pieces. Fire up the saw and make the cut. The pieces should be exactly the same length, and the angle should be exactly 45 degrees.
Repeat this process for the frame’s other two edges, which should have inside dimensions of 18 inches.