UNC: the American coarse thread, cut by hand
UNC is the standard series for screws and nuts in the USA and Canada, and therefore the thread you meet most often on American machines, vehicles and equipment. VÖLKEL makes 220 hand taps for it to DIN 352 in HSS-G, as three-piece sets and as individual taper, second and bottoming taps, from UNC No. 1 x 64 to UNC 3" x 4.
Three pieces, because it is a coarse thread
As with the metric coarse thread, the set consists of three tools: the taper tap with a six to eight thread lead, the second tap with four to five, the bottoming tap with two to three. They are marked on the shank: one ring, two rings, no ring. For the finer UNF and UNEF series a two-piece set is enough, because the thread depth is smaller.
Reading the designation
In 1/4-20 UNC, 1/4 is the outside diameter in inches and 20 is the number of threads in one inch. The pitch in millimetres is 25.4 divided by the thread count, here 1.27 mm. Diameters below 1/4 inch carry numbers instead of fractions, for example No. 10 for 0.190 inch.
Not the same as Whitworth
UNC has a flank angle of 60°, British BSW 55° with rounded crests. On some sizes the thread count matches, but they still will not screw together. When in doubt, a thread gauge with 55° and 60° blades settles it.
Tap drill sizes are in the table in the article calculate tap drill size, machine versions among the machine taps, complete imperial sets under UNC and UNF, matching gauges among the thread plug gauges.
