A barrel nut is a female fastener used to bolt individual pieces of an assembly together without having an exposed nut on one of the work-piece faces. The barrel nut consists of a short, straight length of round bar with a perpendicular, threaded hole through its center. To install the nut, a hole is drilled, typically from a hidden surface, on a work piece into which the nut fits. Another hole is then drilled into the work piece to correspond with the position of the nuts' threaded opening. This allows a second work piece to be bolted to the first without any nut being visible on its faces, a useful feature in aesthetically-sensitive applications such as furniture making.
Bolts and nuts are strong, simple, and cheap, making them ideal fastener solutions for a wealth of applications. Exposed nuts and bolt heads, however, tend to spoil the sleek lines of certain items designed for aesthetic as well as functional appeal. They may not look out of place on the boiler of a steam locomotive, but will certainly lend a designer chaise lounge a distinctly tacky air. This does not necessarily mean that no bolts and nuts may be used in the manufacture of the up-market chair, or any other piece for that matter, which could benefit from their inherent strength. The solution is to use a barrel nut instead of conventional types, thus avoiding having hordes of ugly, utilitarian fasteners dotting the glossy burl walnut.
These fasteners consist of a simple length of round bar with a perpendicular hole drilled through their centers. This hole is threaded with a standard pitch thread to facilitate the joint. Installation is achieved by drilling a hole to the level of the intended joint into one of the two pieces to be joined, usually from a face which will be hidden later. A second hole is then drilled to correspond with the hole through the barrel nut. A second work piece may then be bolted to the first using the hidden nut.
This joint creates a face on the first work piece that can be presented, unblemished by any unsightly fasteners, at the front of the unit, while the bolt head is hidden on the inside of the second work piece. The use of a barrel nut rather than conventional types is particularly useful in furniture making, where sleek, decorative facing finishes are essential. Some barrel nuts may be cut with a screwdriver slot on their one end so that they can be turned in tight holes to match the threaded hole to the hole in the work piece. In some cases, they may even be equipped with a second or third threaded holes in their ends running parallel with the nut for the attachment of additional hidden bolts.