Machinery's Handbook, 31st Edition
Alternate Stub Acme Screw Threads
2075
P
Basic thickness of thread, P /2
Internal Thread (Nut)
External Thread (Screw)
Limits of Size, Allowances, Tolerances, and Crest Clearances for American National Standard Stub Acme Threads
Alternate Stub Acme Threads.— Since one Stub Acme thread form may not meet the re - quirements of all applications, basic data for two of the other commonly used forms are included in the appendix of the American Standard for Stub Acme Threads. These so- called Modified Form 1 and Modified Form 2 threads utilize the same tolerances and allowances as Standard Stub Acme threads and have the same major diameter and basic thread thickness at the pitchline (0.5 P ). The basic height of Form 1 threads, h , is 0.375 P ; for Form 2 it is 0.250 P . The basic width of flat at the crest of the internal thread is 0.4030 P for Form 1 and 0.4353 P for Form 2. The pitch diameter and minor diameter for Form 1 threads will be smaller than similar values for the Standard Stub Acme Form and for Form 2 they will be larger owing to the differences in basic thread height h . Therefore, in calculating the dimensions of Form 1 and Form 2 threads using Formulas 1 through 12 in Table 14a, it is only necessary to substitute the following values in applying the formulas: For Form 1, D 2 = D − 0.375 P , D 1 = D − 0.75 P ; for Form 2, D 2 = D − 0.25 P , D 1 = D − 0.5 P . Thread Designation: These threads are designated in the same manner as Standard Stub Acme threads except for the insertion of either M1 or M2 after “Acme.” Thus, 0.500-20 Stub Acme M1 for a Form 1 thread; and 0.500-20 Stub Acme M2 for a Form 2 thread. Former 60-Degree Stub Thread.— Former American Standard B1.3-1941 included a 60-degree stub thread for use where design or operating conditions could be better satis - fied by the use of this thread, or other modified threads, than by Acme threads. Data for 60-Degree Stub thread form are given in the accompanying diagram.
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