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TAP DRILL SIZES AND MINOR DIAMETER CONSTANTS, Continued . . . American National and Unified thread specifications provide for a maximum of 83 1/3% thread. These specifications also provide a minimum value that varies from approximately 53% to 75%, depending upon the diameter and pitch of thread. In actual practice, however, the minimum percentage of thread required is generally determined by the design requirements of the thread involved. If the length of thread in the tapped hole is very short, it may be necessary to produce a minimum of 75% or 80% thread to develop the required strength. However, if the length of thread produced is several times the diameter, a 50% or 60% thread will usually provide a greater strength than that of the mating screw. Tapping costs increase very rapidly as the percentage of thread increases and it is poor economy to specify a greater percentage of thread than is actually necessary.To provide for practical tapping, the percentage of thread must be kept to a minimum in the cases of long |
