Fox: from animal to bolt-tightening wedge

To most people, a fox is a cunning animal, reddish-furred and swift. In mechanical engineering, it is something altogether different: a tapered metal wedge driven into the split end of a bolt to expand it and lock the joint securely. Once the bolt end has been split axially, the fox is hammered home between the split leaves, forcing them outward against the hole or mating surface and creating friction that prevents loosening.

The fox serves where conventional nuts and washers cannot fit or where a permanent, tamper-resistant fastening is needed. It is most common in heavy machinery, boilers, and structural assemblies where vibration or thermal cycling might otherwise work a standard bolt loose. The wedge itself is typically made from steel and hardened to withstand repeated hammering. Its depth and angle are calibrated so that full seating occurs well before the bolt shank is overstressed.

The term appears nowhere else in engineering with quite the same meaning, though related shop vocabulary abounds. In industrial printing, foxing denotes the rust-brown stains that appear on aged paper, possibly named for their color rather than any mechanical action. In metallurgy, foxtail

The shift from animal name to tool is characteristic of shop language: craftspeople have long borrowed vivid, memorable words for objects whose shape or behavior suggested a comparison. A fox wedge, driven into a split and spreading it apart, may have evoked the animal's sharp, pointed nose; or the metaphor may have come simply from the word's brevity and force, apt for naming a small, aggressive tool. Once adopted, the term stuck in the mechanical trades and is still recorded in fastening specifications and engineering handbooks, even as the technique itself has become less common with the rise of lock washers and threaded inserts.