A while back I came across a digital copy of the book, “Every Man His Own Mechanic“, written by Francis Young, and published in 1891 (this was the 9th edition). There was a few pages on designs of parallel devices for leg vises (or vices if your British), which I thought might be of interest to some.
“The most simple means of preserving parallelism between the faces of the bench-front and the cheek of the bench-vice is to be found, perhaps, in the use of a second and smaller screw attached to the bench cheek, and passing through the bench-leg opposite to which the cheek is placed, as shown in fig. 242. In this illustration, A represents the bench-top, B the bench-front, C the bench-leg, opposite to which the bench-vice D is placed, E the bench-screw, and F the block which is attached to the back of the bench-leg, and through which the screw works.
The smaller screw G, by which the parallelism between the bench and bench-vice is preserved, is fastened immovably to the bench-vice at its head H and passes freely through a hole bored through the bench-leg at K. The nuts L and M work up and down the screw at the pleasure of the operator. When it is required to move the vice inwards or outwards, it is clear that the nuts must be turned till one reaches the head H and the other the end G of the screw, and that when the cheek of the bench-vice D has been brought inward or outwards to the extent required, the nuts must be again turned in the contrary direction until they are brought against the bench-leg.
This necessarily involves a great deal of trouble, and causes a great waste of time. Moreover, a hole must be sunk in the face of the bench-leg, large enough to receive the nut M; for if this were not done the face of the bench-cheek could never be brought into close proximity to the bench-leg.”
NOTES: This is obviously an intriguing means of creating parallelism, although it is likely many woodworkers would find it somewhat tiresome to have to bend down and independently adjust the wooden nuts on the lower screw. Both screws, as shown in the figure, have threads which don’t extend all the way to the bench-vice, implying the bench-vice would be well set-apart from the bench-front. A contemporary wooden screw would extend all the way to the bench-vice, however this still leaves the problem of the nut between the bench-leg and the bench-vice. Sinking a cavity into the bench-leg to accommodate it would likely weaken the bench-leg, because it would have to be big enough to allow the nut to be adjusted. It might be possible to replace the wooden screw with a metal screw, or use a thinner metal nut. One novel contemporary solution is that designed by Erik Mortensen (described on Chris Schwarz’s Popular Woodworking blog): the spinning footplate.