Machinery's Handbook, 31st Edition
Taps
971
Blank Design 1
Blank Design 2
Blank Design 3 Pulley Taps: These taps were originally designed for tapping line-shaft pulleys by hand. Today, these taps have shanks that are extended in length by a standard amount for use where added reach is required. The shank is the same nominal diameter as the thread. This standard applies to fractional size and ground thread with plug and bottom chamfer.
Pipe Taps: These taps are used to produce standard straight or tapered pipe threads. This standard applies to fractional size in high-speed steel, ground thread, to high-speed steel and carbon steel in cut thread, and to straight pipe taps having plug chamfers and taper pipe taps.
Standard System of Tap Marking.— Ground thread taps specified in the US customary system are marked with the nominal size, number of threads per inch, the proper sym bol to identify the thread form, “HS” for high-speed steel, “G” for ground thread, and designators for tap pitch diameter and special features, such as left-hand and multi-start threads. Cut thread taps specified in the US customary system are marked with the nominal size, number of threads per inch, and the proper symbol to identify the thread form. High-speed steel taps are marked “HS,” but carbon steel taps need not be marked. Ground thread taps made with metric screw threads (M profile) are marked with “M,” followed by the nominal size and pitch in millimeters, separated by “X.” Marking also includes “HS” for high-speed steel, “G” for ground thread, designators for tap pitch diam eter and special features, such as left-hand and multi-start threads. Thread symbol designators are listed in the accompanying table. Tap pitch diameter designators, systems of limits, special features, and examples for ground threads are given in the following section. Standard System of Tap Thread Limits and Identification for Unified Inch Screw Threads, Ground Thread.— H or L Limits: For Unified inch screw threads, when the maximum tap pitch diameter is over basic pitch diameter by an even multiple of 0.0005 inches, or the minimum tap pitch diameter limit is under basic pitch diameter by an even multiple of 0.0005 inches, the taps are marked “H” or “L,” respectively, followed by a limit number, determined as follows:
0.0005 Basic PD Tap PD − 0.0005 Tap PD Basic PD −
H Limit number L Limit number
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The tap PD tolerances for ground threads are given in Table 2, column D; PD tolerances for cut threads are given in Table 3, column D. For standard taps, the PD limits for various H limit numbers are given in Table 20. The minimum tap PD equals the basic PD minus the
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