I want my leg vise to open smoothly when clamping large objects. I will already have to stoop to adjust the pin in the parallel guide, so a smooth action, especially when the the vise is open to its extents is important. I don't want to have to fiddle with a parallel guide that gets stuck.
One idea (see above sketch) was presented to me by Rob of Evenfall Woodworks. His idea was to build a rigid frame around the leg vise chop, thus taking weight off the screw itself and allowing a smoother in-out action. I like Rob's idea, but I wanted to minimize the friction between the parallel guide (and Rob's guide bar) and the leg. The chop is a thick and wide piece of ash, and is quite heavy. One of Rob's ideas was to incorporate a wheel at the bottom of the chop so it can roll along the floor when opening the vise. The chop on my leg vise is quite a ways off the floor, and obviously the sliding leg vise will be too. I thought, why not put the wheel under the parallel guide, and place an opposing wheel above the guide, but behind the leg? The bottom of the vise chop is flush with the bottom of the parallel guide, so the wheel will not interfere with the vise closing completely. And the wheel behind the leg is equally unobtrusive.
To test my idea I made a couple prototype wheel assemblies with some scrap metal I had lying around the shop. I've placed the assemblies so the parallel guide is not touching the mortise in the leg, but simply contacting both wheels only. The weight of the chop keeps the parallel guide in contact with the wheels at all times.
This is the vise in the open position. The screw is not installed.
The outer guide wheel.
The inner, and opposing guide wheel. The action of the chop was surprising. My wheel assemblies were not rolling properly (these are just stop collars on a steel shaft--not true bearings), but when they did, the vise rolled open and closed very smoothly and easily. I pulled and pushed on the chop directly in the area where the screw will pass through, for an accurate representation of how the vise will work. I think this rather crude prototype has proven that this idea is going to work, and work quite well I think, especially after I design some proper roller bearings or wheels into the mechanism. I realize that most of the time the parallel guide won't be moving much, but when it comes time to open up the vise for some thick work, I know that fiddling with a sticky parallel guide and heavy chop will drive me nuts. I think this solution will solve that.