Machinery's Handbook, 31st Edition
ACME SCREW THREADS 2055 ANSI General Purpose Acme Thread Form ANSI/ASME B1.5-1997 (R2014) , and Stub Acme Screw Thread Form ANSI/ASME B1.8-1988 (R2016)
Internal Thread
0.25 P (For General Purpose Acme Threads) 0.15 P (For Stub Acme Threads)
One-half allowance on major diameter
29° 14 1/2° 14 1/2°
F rn
Basic Pitch Line
F cs
t = 0.5 P
t = 0.5 P F cn F
h
One-half allowance on pitch diameter
F rs
One-half allowance on minor diameter
External Thread
p
Fig. 1. General Purpose and Stub Acme Thread Forms Formulas for Basic Dimensions of General Purpose and Stub Acme Screw Threads General Purpose Stub Acme Threads Pitch = P = 1 ÷ No. threads per inch, n Pitch = P = 1 ÷ No. threads per inch, n Basic thread height h = 0.5 P Basic thread height h = 0.3 P Basic thread thickness t = 0.5 P Basic thread thickness t = 0.5 P Basic flat at crest F cn = 0.3707 P (internal thread) Basic flat at crest F cn = 0.4224 P (internal thread) Basic flat at crest F cs = 0.3707 P − 0.259 × (pitch dia. allowance on ext. thd.) Basic flat at crest F cs = 0.4224 P − 0.259 × (pitch dia. allowance on ext. thread) F rn = 0.3707 P − 0.259 × (major dia. allowance on internal thread) F rn = 0.4224 P − 0.259 × (major dia. allowance on internal thread) F rs = 0.3707 P − 0.259 × (minor dia. allowance F rs = 0.4224 P − 0.259 × (minor dia. allowance on ext. thread − pitch dia. allowance on ext. thread) Pitch Diameter Allowances and Tolerances: Allowances on the pitch diameter of Gen eral Purpose Acme threads are given in Table 4. Pitch diameter tolerances are given in Table 5 . The ratios of the pitch diameter tolerances of Classes 2G, 3G, and 4G, General Purpose threads are 3.0, 1.4, and 1, respectively. An increase of 10 percent in the allowance is recommended for each inch, or fraction thereof, that the length of engagement exceeds two diameters. Application of Tolerances: The tolerances specified are designed to ensure interchange ability and maintain a high grade of product. The tolerances on diameters of the internal thread are plus, being applied from minimum sizes to above the minimum sizes. The toler ances on diameters of the external thread are minus, being applied from the maximum sizes to below the maximum sizes. The pitch diameter (or thread thickness) tolerances for an external or internal thread of a given class are the same. The thread thickness tolerance is 0.259 times the pitch diameter tolerance. on ext. thread − pitch dia. allowance on ext. thread)
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