Machinery's Handbook, 31st Edition
2116 Microscope Objective Threads of a microscope. This screw thread is also recommended for other microscope optical assembles as well as related applications such as photomicrographic equipment. It is based on, and intended to be interchangeable with, the screw thread produced and adopted many years ago by the Royal Microscopical Society of Great Britain, generally known as the RMS thread. While the standard is almost universally accepted as the basic standard for microscope objective mountings, formal recognition has been extremely limited. The basic thread possesses the overall British Standard Whitworth form. (See Whitworth Standard Thread Form starting on page 2086 ). However, the actual design thread form implementation is based on the WWII era ASA B1.6-1944 “Truncated Whitworth Form” in which the rounded crests and roots are removed. ASA B1.6-1944 was withdrawn in 1951. However, ANSI/ASME B1.11-1958 (R2016) is still active for new design. Design Requirements of Microscope Objective Threads: Due to the inherent longevity of optical equipment and the repeated use to which the objective threads are subjected, the following factors should be considered when designing microscope objective threads: Adequate clearance to afford protection against binding due to the presence of foreign particles or minor crest damage. Sufficient depth of thread engagement to assure security in the short lengths of engagement commonly encountered. Allowances for limited eccentricities so that centralization and squareness of the objective are not influenced by such errors in manufacture. Deviation from the Truncated Whitworth Thread Form: Although ANSI/ASME B1.11- 1958 (R2016) is based on the withdrawn ASA B1.6-1944 truncated Whitworth standard, the previously described design requirements necessitate a deviation from the truncated Whitworth thread form. Some of the more significant modifications are: A larger allowance on the pitch diameter of the external thread. Smaller tolerances on the major diameter of the external thread and minor diameter of the internal thread. The provision of allowances on the major and minor diameters of the external thread. Thread Overview: The thread is a single start type. There is only one class of thread based on a basic major diameter of 0.800 in. and a pitch, p , of 0.027778 inch (36 threads per inch). The AMO thread shall be designated on drawings, tools and gages as “0.800-36 AMO.” Thread nomenclature, definitions and terminology are based on ANSI B1.7-1965 (R1972), “Nomenclature, Threads, and Letter Symbols for Screw Threads.” It should also be noted that ISO 8038-1:1997 “Screw threads for objectives and related nosepieces” is also based on the 0.800 inch, 36 tpi RMS thread form. Tolerances and Allowances: Tolerances are given in Table 2. A positive allowance (minimum clearance) of 0.0018 in. is provided for the pitch diamter E , major diameter D , and minor diameter, K If interchangeability with full-form Whitworth threads is not required, the allowances for the major and minor diameters are not necessary, because the forms at the root and crest are truncated. In these cases, either both limits or only the maximum limit of the major and minor diameters may be increased by the amount of the allowance, 0.0018 inch. Lengths of Engagement: The tolerances specified in Table 2 are applicable to lengths of engagement ranging from 1 ∕ 8 in. to 3 ∕ 8 inch, approximately 15 to 50 percent of the basic diameter. Microscope objective assembles generally have a length of engagement of 1 ∕ 8 inch. Lengths exceeding these limits are seldom employed and not covered in this standard. Gage testing: Recommended ring and plug testing gage dimensions for the 0.800-36 AMO thread size can be found in ANSI/ASME B1.11-1958 (R2016), Appendix. Dimensional Terminology: Because the active standard ANSI/ASME B1.11-1958 (R2016) is based on the withdrawn ASA Truncated Whitworth standard, dimensional nomenclature is described below.
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