Class of fit 2AG for UN-series External Screw Threads
The 2AG class-of-fit was a puzzle to me for a long time. I find it specified on part drawings and listed in the Machinery's Handbook. Several other web pages refer to the 2AG class-of-fit, but I could not find it in ANSI/ASME B1.1; which is the official document which defines the UN-series screw thread and all of its designations. Today I got sick of not knowing so I went looking and found the class-of-fit 2AG defined in BS 1580-1:2007.
BS 1580-1:2007; Page 9; Paragraph 6; Note 1 states: Class 2AG external threads are Class 2A coated external threads for which there is a requirement for the allowance shown in Figure 4 to be retained after coating. This is necessary to allow, for example, for the use of a lubricant in high-temperature applications.
BS 1580-1:2007; Page 11; Paragraph 9c Class 2AG, external threads states: Both the maximum and minimum diameter limits before coating shall be 0.001 in less than those given in Table 2 for Class 2A. After coating the maximum tabulated limits shall not be exceeded.
BS 1580-1:2007; Page 13; Figure 5 gives a graphical presentation of the tolerance zones for 2AG compared with the other authorized classes-of-fit.
BS 1580-1:2007; Page 18; Paragraph 126.96.36.199 Designation of Coated Class 2AG threads states: For coated Class 2AG external threads, the maximum major diameter and maximum effective diameter of the corresponding Class 2A thread shall be given followed by AFTER COATING thereby indicating that the allowance has to be maintained. The major diameter and effective diameter limits of size before coating [calculated in accordance with Clause 9a)] shall also be given followed by SPL (to stand for special) and by BEFORE COATING.
a. The 2AG class-of-fit appears to be specific to the British Standard BS 1580. It may have been listed in some previous version of B1.1 or H28, but my searching of those documents and the historical copies available to me did not return any instances of the 2AG class-of-fit being defined.
b. The 2AG is a specific class-of-fit which indicates that the external product may be coated up to the standard 2A class-of-fit requirements.
c. Use of the 2AG class-of-fit must always be in conjunction with words indicating if the thread is BEFORE COATING or AFTER COATING. In addition to indicating the coating status the thread minimum and maximum major and pitch diameters should be clearly delineated.
The 2AG class-of-fit was someone's good idea for a shortcut designation which turned out to be more work than it was worth and thus not broadly accepted by the manufacturing industry as a whole. Because it is not given in ANSI/ASME B1.1 it should not be used. In its place ANSI/ASME B1.1 authorized methods of indicating before and after coating threads should be utilized.
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Original Posting: 1/2/2015
Last Revision: 9/24/2015
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