Clamping tricks for mitered frames
To help clamp miters, Craig Thibodeau uses small triangular offcuts from the mitering process as clamping cauls
When it comes to gluing miters, there are two big issues. First, this is a weak end-grain joint, so you have to take extra steps to ensure a good bond. Because the end grain will soak up glue quickly, I recommend double-coating the joint. Apply glue to all the end-grain parts, wait a few minutes until the first application of glue has soaked into the end grain, and then coat them all again before assembly.
To ensure a long, gap-free life, I reinforce miters with Domino slip tenons (biscuits and splines work, too). The Dominos also keep parts aligned while clamping, reducing the need for sanding or planing later.
Pressure where the miters need it
The next challenge with miters is applying pressure at 90° to the mitered surfaces, and in the center of the joint. To help, I use the small triangular offcuts from the mitering process as clamping cauls. Sometimes I glue them to thin 1⁄4-in. MDF strips. I clamp a pair of these at each corner of the frame during glue-up. These work very well for picture frames, but I take a different approach for situations where it’s not convenient to clamp them to a frame from the inside, such as a frame-and-panel assembly. In these cases I just glue the offcuts to the frame pieces and then bandsaw and plane them off after assembly.
Glued on cauls
Solution For Shapely Frames
When I have a frame with molded edges, I make cauls that are roughly the reverse of the molding profile. They don’t need to be exact, but they should make enough contact to be glued securely in place. Once the caul is shaped, glue it in place with a bit of yellow glue, making sure the clamping surface lines up with the miter. After the frame is assembled, these blocks will be cut off using a bandsaw or handsaw. Then the frame gets planed and sanded smooth.
This is an excerpt from Craig’s article “Tablesaw Sled for Miters” in FWW issue #257–Nov/Dec 2016