A finished hex machine screw nut is designed to go onto a nut or screw with matching threads. They are smaller in pattern than a Finished Hex Nut and go on machine screws. They are commonly double chamfered and double countersunk . The smallest readily available is a double-aught diameter 00-160 but they certainly are available even smaller. They only go up to 3/8" in diameter. They are available in a wide range of materials including brass, bronze, stainless, aluminum, and steel.