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Most improvised corner clamps just pin a joint at 90° without applying any closing pressure across the miter itself — this one is built specifically to squeeze the two mitered faces together, which is what actually closes a gap in a loose-fitting miter.
Slots cut into the plywood base let the jaw sections flex inward when you tighten the wing nuts, drawing the two sides of the joint together rather than just holding them in place. That flex is the entire mechanism — there's no moving hardware beyond the bolts themselves.
Orient the counterbored bolt slot on the jaw blocks facing down, against the base — mounting it facing up is an easy mistake that a previous builder ran into, and it keeps the bolt heads from interfering with the clamping action.
| Item | Qty |
|---|---|
| 1/2" plywood, partial sheetThe flexing base. | 1 sheet |
| Hardwood scrap (jaw blocks)≈ about 3 ft of 3/4" × 6" board | 1 board ft |
| Carriage bolts, 1/4" x 2" | 4 each |
| Wing nuts | 4 each |
| Washers | 8 each |
A board foot is a volume, not a length: 144 cubic inches, or a piece 1″ thick by 12″ wide by 12″ long. The “≈” note beside each row shows roughly how much 3/4″ × 6″ board that works out to.
Overall cost: $. Lumber prices vary by region, species, and season, so we give a band rather than a figure we would only be pretending to know.
| Part | Qty | T × W × L |
|---|---|---|
| Base1/2" plywoodSlotted to let the jaw sections flex inward. | 1 | 1/2" × 6" × 10" |
| Jaw blocksHardwood scrapAngled face; counterbored bolt slot faces DOWN against the base. | 4 | 3/4" × 1 1/2" × 3" |
Buy 2 boards at 8 ft.
1 board
1 board
Assumes 8 ft stock, a 1/8" saw kerf, and buying each part at its own width. Change the board size or kerf on the full board plan →
Cut the plywood base with slots that let the fixed jaw sections flex inward under pressure.
Glue the angled hardwood jaw blocks to the base, making sure the counterbored bolt slot on each block faces down against the base, not up.
Drill through the base and jaw blocks for the carriage bolts.
Assemble the clamp with the carriage bolts, washers, and wing nuts.
Clamp a mitered corner by tightening the wing nuts, which draws the flexing jaw sections together across the miter.
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