UNF: the American fine thread
UNF is the fine series of the Unified system and is used wherever a bolted joint needs more preload for less tightening travel or must not work loose under vibration: in engines, in hydraulics, on chassis parts. VÖLKEL makes 183 hand taps for it to DIN 2181 in HSS-G, from UNF No. 0 x 80 to UNF 2.1/2 x 12.
When UNF rather than UNC
At the same diameter UNF has more threads, hence a larger stress area and a smaller pitch. In practice that means higher load capacity, finer adjustment and better resistance to self-loosening. The price is a more delicate contour and more turns during assembly. In thin-walled parts UNF is the better choice, in soft materials such as aluminium and cast iron rather UNC.
Two tools are enough
The set is two-piece: a taper tap with a six to eight thread lead and a bottoming tap with two to three. There is no second tap, because a fine pitch removes less material per thread. On the shank you can tell them apart by one ring or no ring.
The designation
In 1/4-28 UNF, 1/4 is the outside diameter in inches and 28 the number of threads per inch. Below 1/4 inch, numbers replace fractions, from No. 0 at 0.060 inch upwards. Only UNEF is finer still.
Left-hand versions under UNF left hand, machine versions under UNF machine taps, complete imperial sets under UNC and UNF, tap drill sizes in the article calculate tap drill size.
