Get the step-by-step plans to build 5 Shop-made Clamps (including this frame clamp) from WoodsmithPlans.com.
The plywood corner blocks capture and draw all the joints together at once. The blocks are joined with lengths of threaded rod you can get at the hardware store. I used tip nuts to speed up the process of adjusting the knobs and blocks.
Making the Blocks. Each block consists of three layers of plywood. The lower layer serves as a platform for holding the corner of the mitered frame. I clipped the inside corner of the platform to ease the edge. The top is glued up from two layers of plywood and cut into an L-shape. It accepts a pair of threaded rods. The key is making sure the inside corner is 90°. The holes in the top aren’t identical. One is stopped, and the other is a through hole. When drilling these holes, be consistent. The blind hole in one block should align with the through hole in the adjoining block.
After gluing the layers together, you can apply cork pads to the inside face. This keeps the clamp from marring the frame and provides a relief in the corner. Finally, be sure to wax the platform before use so the frame isn’t glued to the clamp blocks.