Here we have the bottom of an over engineered, under performing wagon vise. On the left, the dog strip with some dogs. On the right, a hand wheel. Turn the wheel and a big (1 1/4”) stainless steel square thread screw turns. In a big brass nut. Which is bolted in a custom machined aluminum housing, which in turn is bolted to an aluminum sled. That sled has two grooves that ride on two 1” x 3/16” steel rails, which are screwed to the bottom of the bench on 1/4” oak spacers. As the screw turns, it moves a wooden shuttle that is screwed to the sled. The shuttle rides in a slot in the bench that is in line with the dog strip. A dog resides in the shuttle. An object that is between fixed & shuttle dogs can thus be pinched. The act of pinching makes the screw want to move to the right and out of the bench. To prevent this from occurring, a collar presses against a plastic bushing mortised into the bench end cap. The bushing also acts as the [guide] bearing for the screw. You can also hold objects in the gap between the bench and shuttle (eg for dovetailing). The shuttle moves 6 3/4”.
The holes hold the nuts for the bolts that hold bench end cap on. I cut a pipe section in half to make a curved washer.
It works pretty well and cost around $35 (gotta love salvage yards). Alignment is a total and complete pain and very finicky. If I do this again I want a complete stand alone unit that I can just bolt into a bench. The garter/bearing is pretty Mickey Mouse and should be replaced with a ball bearing unit as it isn’t the most smooth turning unit. Ideally, bearings at both ends of the screw so the sled just keeps the nut from turning and holds the shuttle but that is probably over kill. As it is, the sled constrains the screw vertically and the slot constrains the screw horizontally. Both have to be adjusted so the screw doesn’t bind (by moving the bench end cap around).