The kind of hole to be tapped has much to do with the style of tap that’s best suited. Some holes go all the way through. Others, while not through holes, still are relatively deep. Some are quite shallow, little deeper than diameter. Each of these three kinds of holes through, deep-bottoming blind, and shallow bottoming, has a tap or group of taps best suited to requirements.
Taper Taps (starter taps) have 7 to 10 thread chamfers to distribute cutting action over many teeth and the taper also acts as a guide in starting.
Intermediate Taps (middle taps), with a chamfer over four threads, is most widely used in through holes and where there is sufficient room at the bottom in blind holes.
Bottoming Taps (finishing taps) are made with just enough chamfer for starting in the hole, only 1 to 2 half threads. As the name implies, it is designed to thread blind holes to the bottom.
Tap Sizes have been standardised to conform with those of standard screws, bolts, and studs. Machine Screw tap sizes range from No. 0 through No. 14; No. 0 being .0600″ outside diameter; No. 1, .0730″; No. 2, .0860, etc.–all in .0130 increments. Hand Taps, more commonly designated as Fractional Taps and used today on all production machines, are designated in fractional and integral inch sizes from 1/4″ upwards.
Threads per Inch are shown for various tooth forms: the Unified series adopted by Great Britain and the United States during the war, and the corresponding American National Standard. NC and UNC mean coarse thread. NF and UNF mean fine thread. NS means special thread.
Pitch Diameter is the basic dimension of a screw, threaded hole, or a tap the diameter of an imaginary cylinder, the surface of which passes through the thread where width of thread and space between threads are identical. This cylinder, of course, would be a cone for tapered taps. It is upon Pitch Diameter that tolerance limits are based to establish Class of Thread.
Please click to view Goliath’s Thread Identification Chart to identify thread pitch/TPI, correct drilling size etc.