Types of Taps

When internal thread cutting is done by hand, the tool used is called a tap. There are many styles of taps, the

Fig. 81. Types of Hand Taps: Left-Taper Tap; Center-Plug Tap; Right-Bottoming Tap.

Courtesy of Wiley and Russell Manufacturing Company, Greenfield, Massachusetts names in some cases being suggested from the shape, but more often from the use. In most machine shops are found the following forms: hand, machine screw, pipe, pulley, stay-bolt, boiler, and tapper; of these the hand and machine screw are the most common. The object of all is to make helical grooves, called threads, in holes, so that they may receive and hold screws, bolts, studs, etc.

Size of Drill for Tapped Hole

As the size of a tap is the outside diameter of its threads, it is evident that the hole drilled for tapping must be smaller than the tap by nearly, if not quite, twice the depth of the thread., The shape of the thread partly determines the amount to be subtracted from a tap diameter. There are now recognized as standard, five different threads-sharp or V; Franklin Institute or United States standard; Whitworth; International or metric; and the 29 degrees or Acme. These shapes will be described and compared under "Screw Cutting". Table II shows the diameters of the holes that are to be drilled for cutting the various sizes of the threads according to the United States standard and the ordinary V-thread.

Table II. Taps And Corresponding Drills

Hand taps are most commonly used in shop practice, and a description of their operation will answer for all styles. They are usually sold in sets of three-taper, plug, and bottoming-Fig. 81.