A knurled nut is a fastener that uses a knurled finish in place of hexagonal sides. This design allows the knurled nut to be tightened and loosened by hand without the aid of a wrench. Often made of brass or steel, the knurled nut has been used on electrical components, musical instruments and automobiles as well as a vast assortment of other items. Sometimes referred to as thumb nuts, these fasteners are easily turned by gripping the rough knurled surface around their perimeter between the thumb and forefinger. Typically, an entire length of bar stock is knurled in a lathe prior to cutting and machining knobs from the piece.
Knurling is the art of machining small diamond-like patterns into the side of a round metal bar. The knurling tool is comprised of two or more steel rollers, with the knurling pattern machined in their surface. This tool is placed into the tool holder of a metal lathe and then pressed into the side of a rotating bar stock that has been affixed into the lathe's jaws. As the tool travels along the side of the bar stock, the knurled pattern is imprinted into the stock. Once the bar stock has been knurled, the task of machining knurled nut-size pieces out of it begins.
It is common practice to use a knurled nut on a small-diameter threaded rod or bolt. This is due, in part, to the use of hands to tighten and remove the nut. Larger bolts would require the strength of a wrench to be used to adequately tighten or loosen a nut that would be of the proper size to fit the larger fastener. By using the knurled fasteners on primarily smaller bolts, the proper torque required to tighten the nut can be applied with an individual's bare hands. For stubborn nuts, a pair of leather gloves is usually all that is required to break the knurled nut free.
The unique pattern found in the knurling works well when wet or even when oil-covered. For this reason, a knurled nut is often used on adjustable mechanisms on fishing reels and carburetors where water and petroleum products frequently come into contact with the components. By machining a knurled nut out of brass, the nut will commonly retain the sharp edges of the knurling even after many years of use. A steel knurled nut will often become smoothed off over time and become very slippery.