Machinery's Handbook, 31st Edition
972 Taps number of half-thousandths (0.0005 in.) represented by the limit number. The maximum tap PD equals the minimum PD plus the PD tolerance given in Table 20. Tap Marking with H or L Limit Numbers Example 1: 3 ⁄ 8 -16 NC HS H1 Maximumtap PD Basic PD 0.0005 8 3 0.64951904 16 1 0.0005 0.3344 0.0005 0.3349 Minimumtap PD Maximum tap PD 0.0005 0.3349 0.0005 0.3344 # = + = − + = + = = = − − = a k
8 -16 NC HS G L2 Minimumtap PD Basic PD 0.0010 8 3 0.64951904 16 0.3344 0.0010 0.3334 Maximumtap PD Minimum tap PD 0.0005 0.3334 0.0005 0.3339 # = − = − − = − = = = + + = a k
Example 2: 3 ⁄
1 0.0010
Oversize or Undersize: When the maximum tap PD over basic PD or the minimum tap PD under basic PD is not an even multiple of 0.0005, the tap PD is usually designated as an amount oversize or undersize. The amount oversize is added to the basic PD to establish the minimum tap PD. The amount undersize is subtracted from the basic PD to establish the minimum tap PD. The PD tolerance from Table 2 is added to the minimum tap PD to establish the maximum tap PD for both. Example: 7 ⁄ 16 -14 NC plus 0.0017 HS G Min. tap PD = Basic PD + 0.0017 in. Max. tap PD = Min. tap PD + 0.0005 in. Whenever possible for oversize or other special tap PD requirements, the maximum and minimum tap PD requirements should be specified. Special Tap Pitch Diameter: Taps not made to H or L limit numbers, to the specifications in, or to the formula for oversize or undersize taps may be marked with the letter “S” enclosed by a circle or by some other special identifier. Example: 1 ⁄ 2 -16 NC HS G. Left-Hand Taps: Taps with left-hand threads are marked “LEFT HAND” or “LH.” Example: 3 ⁄ 8 -16 NC LH HS G H3.
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