U nail making machines are automatic or double-stroke devices that produce U-shaped nails, also known as fence staples, from wire. Specifications vary, but common features include a 3 HP motor, production rates from 100 to 400 nails per minute, and the ability to process wires from 6 to 14 gauge. Other specifications to consider are the nail length and the wire thickness the machine can handle, which depend on the specific model.