Selecting Standard Taps:
When selecting standard taps for your tapping application, there are three basic factors for you to consider:
1. The Material to be threaded. For optimum results, select taps that are recommended for the material to be threaded; two flute taps are recommended for some materials in certain sizes and pitches and three, four or six flute taps for other materials, sizes and pitches.
2. The type of hole to be threaded.
An open or through hole - passes through the workpiece. Plug chamfered taps are usually recommended for this type of hole. When the material to be threaded produces a continuous chip such as steel, aluminum, magnesium, or a die casting, plug chamfered spiral pointed taps are highly recommended for open or through holes for diameters through 3/4".
A blind hole - does not pass through the workpiece. If this type of hole is required to be threaded close to the bottom, a bottoming chamfered tap is necessary. If the blind hole is sufficiently deep to allow a longer chamfer, plug is recommended. In either case, tapping problems are minimized if space is provided at the bottom of the hole for chip collection.
Select a chamfer according to the type of hole you are going to tap.
3. The class of thread required. The Unified Thread Form, designated UNC and UNF for coarse and fine pitch series respectively, was adopted as standard by Canada, Great Britain and the United States. It is mechanically interchangeable with the American National Thread Form, NC and NF, but there is a difference in the pitch diameter tolerances. In the America National, pitch diameter tolerances on both the nut and screw are equal with the nut above and the screw below basic, but in the United Series, the tapped hole has 1.3 times the tolerance of the screw for the corresponding class. The United Thread classifications 2B and 3B, signify internal thread and 2A and 3A, external thread.
NOTE: The above recommendation table is to be used for general recommendations. A wide selection of optional standard taps is available with variations in flutes and chamfers to meet specific requirements.