What a corner clamp does
A corner clamp has two jaws set at a fixed 90° and a screw pressing each part against them. Bessey’s WS-3 holds and aligns parts at 90° up to 4 in thick, adjusts itself to parts of different thickness, and leaves room at the corner for a dowel, screw or staple. That makes it a third hand for butt and mitre joints that are glued, screwed or nailed: the joint is held square while it is fixed (see the plate above).
What it holds is one joint. The angle it holds is the angle between its own jaws, at the point where the parts touch them — not the angle of the whole frame, and not whether the parts are straight, the right length, or flat. A frame with a corner clamp at every corner can still come out of square if the parts are different lengths, the clamps are knocked, or the joints move as they are tightened.
Ways to clamp a corner
| Method | Joints | Keeps square? | Limitations |
|---|---|---|---|
| Corner clamp (Bessey WS-3) | Butt and mitre joints, one corner at a time; parts up to 4 in thick | Holds one joint at 90° | Does not check the whole frame; Bessey sells its simpler WS-1 for light-duty clamping |
| Band or strap clamp (Bessey VAS-23; Pony) | Frames, boxes, polygons and round work, all corners at once | Pressure is even; squareness depends on the parts | Presses at the corners; long, thin sides can bow |
| Bar or parallel clamps across the frame | Frames, boxes and cases | Parallel jaws help; check the diagonals | Clamps set off-line pull the frame out of square |
| Glued-on clamping blocks | Mitres, where there is nothing square to clamp to | Only as well as the clamps are aligned | Blocks must be removed and cleaned up afterwards |
Bessey says its VAS-23 strap pulls from both ends so pressure is equal on every corner without skewing the work; most strap clamps pull one way.
Holding square versus checking square
A frame is square when all four corners are 90°, and the practical test is its diagonals. Measure from corner to corner both ways: if the two diagonals are equal, the frame is square; if one is longer, the frame has racked into a parallelogram, and the long diagonal joins the two corners that have closed to less than 90°. The test works whatever held the joints, and it catches what a corner clamp cannot — a frame whose joints are each held tight but whose shape has shifted. A square checks each corner; the diagonals check them all at once.
Diagonals do not check flatness. A frame can be square and still twisted, with one corner lifted. Lay it on a flat surface, or sight across it, before the glue sets.
Gluing a frame or box square
Gluing a frame square
Check the parts
Opposite parts must be the same length and their ends square; no clamp can make a frame square from parts that are not.
Dry-fit and measure
Assemble without glue, clamp lightly, and measure both diagonals. Find and fix any joint that will not close.
Glue and assemble
Glue the joints, assemble on a flat surface, and set corner clamps or a band clamp in place without tightening them fully.
Tighten in turn
Tighten a little at each corner in turn, or draw a band clamp up evenly, so no one joint is pulled closed before the others.
Measure the diagonals
If they differ, shorten the long diagonal: ease the clamps and press its two corners toward each other, or clamp lightly across it, and measure again until the diagonals match.
Check flat, then leave it
Check the frame lies flat without rocking, then leave it undisturbed until the glue has set.
Checking a frame for square
- Both diagonals are measured from the same points at each corner — the outside corners, or marks you have made.
- The two measurements agree.
- Each corner is checked with a square as well, when the parts might not be straight.
- The frame lies flat on a flat surface with no corner lifting.
- The check is repeated after the last clamp is tightened, not only before.
For clamps across a frame or case, see bar, pipe, parallel and F-clamps; the rest of the family is in the clamps guide.
Sources
Clamp constructions and capacities are the makers’ own. Checking and correcting an assembly for square is established bench practice explained in BenchRoot’s own words.
- 1.
Clamping angles and miters. BESSEY Tools North America.
Used for: The WS-1 corner clamp, a fixed 90° for mitre or butt joints up to 3 in per side, with two spindles and mounting holes; the WS-3 angle clamp, which holds and aligns parts at 90° up to 4 in thick, adjusts to parts of different thickness with a free-floating jaw, and leaves clearance for dowels, screws or staples; a table clamp to fix it to the bench.
- 2.
Strap clamps. BESSEY Tools North America.
Used for: A strap that draws in from both ends so pressure is equal on all corners without skewing the work, where most strap clamps pull one way; corner pieces that adjust from 60° to 180°; a 23 ft strap.
- 3.
Product Catalog, Spring 2024. Pony Jorgensen.
Used for: Band clamps for round, rectangular and irregular work, from a 15 ft band up to a 30 ft canvas band with a 2,200 lb band limit; the Cabinet Master parallel clamp, whose jaws stay parallel to keep corners square.
About this guide
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