Several years ago I made a twin screw chain drive vise for my old work bench and now I have a new bench so I decided to make a new one. This vise is different than any other ones I have seen before. It operates like a mechanics vise. The screws are towards the bottom of the jaws and there are slides that keep the jaws from wracking. I will try to go through the steps to make one.
First you need some steel tube to make the slides, you need 2 different sizes and the smaller one needs to slide inside the larger one without much play. I happened to have some laying around. The larger piece was 1.25" outside and the small piece was 1.125" outside.
I made the jaws out of beech and drilled holes for the slides to go in. To get the holes accurately spaced I clamped both jaws together then clamped them the the drill press. I drilled the 1.25" in the stationary jaw then changed the bit and drilled the 1.125" hole in the moving jaw, I did that without moving the the jaws on the drill press. This way the slides will line up.
For the screws I used 5/8" all thread. You could use whatever you want but for me the 5/8 was cheap and easy to get. At the end of the large slides you need to weld on some nuts. Then put the slides in the jaws. You can press them in with a clamp or gently hammer them in using a wood block to protect the slides.
At this time you should be able to slide the jaws together
The next step is to cut all thread. To measure it just pull out the movable jaw to what you want the max opening to be and measure from the outside of the jaw to the nut at the end of the slide and maybe add and inch or so.
To allow the movable jaw to follow when opening the vise I double nutted inside small slide. A washer goes on the outside and is screwed on the jaw. Then the sprocket goes on. I welded a nut on the sprocket so I could screw the sprocket on then tighten the set screws on the sprocket.
Then screw on the handle which I tapped to screw right on the all thread and tighten against the nut. and put on the chain. It may be a good idea to plan the distance between screws so the length of the chain works out well. Somehow I got lucky both times I built the vises and the chain is snug but not tight.
Right now its getting late and some of this may not make sense so if you have questions let me know.