A vise with two screws clamps chair seats and other large workpieces more effectively than a standard vise. Such pieces must be clamped on one side of a standard vise to avoid its screw and guide rails, and the uneven clamping pressure that results makes them prone to slipping. In a double screw vise, these workpieces simply slide between the screws and are evenly clamped on both sides. I like this type of vise well enough that I’ve installed one on each of the three benches that I regularly use.
You can turn the handles of both bench screws at the same time (using both hands). To reposition a workpiece, simply loosen one screw, adjust the piece, and then re-tighten the screw. It’s true that you'll sometimes butt into one or the other screw head, depending on how you stand while using a tool. But I find the vise’s utility more than compensates for this inconvenience. Note that the adjustable jaw sits 1/2" inside the outside edge of the fixed jaw. For some reason, this tiny inset helps to keep me from whacking my thigh when I walk around the bench!
If you decide to install this vise at the same time you upgrade your bench with a tail vise, plan to complete the first two steps before you install the tail vise's fixed jaw on the bench.
1. Glue a 1-7/16" x 2" x 30" cleat to the bottom edge of the tail vise's fixed jaw, flush with its front face (Fig. A). This cleat is narrower than the jaw's front rail, so it won't interfere when a bench dog is inserted in one of the holes.
2. Drill two 1-1/4" dia. pilot holes in the fixed jaw and thread them using a 1-1/2" thread box.
3. Turn two 3" dia. x 18" wooden screws with 2" dia. x 1-3/8" long collars (Note: these collars are shorter than the collar on the tail vise screw). Center a 1-1/2" dia. x 3/8" wide slot in each collar for the garter.
4. Cut the 1-1/2" x 6" x 36" adjustable jaw to final dimensions (Fig. B). Drill two 2" dia. holes in it to house the screws' collars and chop mortises for the garters. Then rout the rounded profile on the top edge and both sides.
5. Install the screws and garters and assemble the vise.