This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
It is frequently the case that the nut on a bench-vise screw breaks from being subjected to a too violent strain. If one is working in a place where a new nut cannot be obtained, the broken part may be replaced by the substitute shown in the sketch. Any piece of strap iron may be used, and with a round file and a drill the two pieces can soon be made and attached to the bench with screws or bolts. A slight twist of the shaped ends is necessary to make them fit the angle of the thread. - Contributed by Oscar M. Waddell, Lamedeer, Mont.
Ill: Pieces of Strap Iron Shaped to Fit the Square Thread Make a Good Substitute Nut