ASME B1.11-1958 (2006) — Microscope Objective Thread

🔬 ASME B1.11-1958 (2006) is a dedicated industry standard for the screw thread used to mount microscope objective assemblies to the body or lens turret (revolver) of a microscope. The thread is based on, and intended to be interchangeable with, the thread introduced and adopted many years ago by the Royal Microscopical Society (RMS) of Great Britain, which used the British Standard Whitworth form with a 55° included angle and rounded crests and roots. Over the decades this thread has become the de facto world standard for microscope objective mountings. Initially approved as an American Standard by ASA on January 7, 1958, the document was reaffirmed in 1989, 1994, and 2006.

1. Background and Historical Development 📌

Standardization of the objective thread was one of the projects toward unification of screw thread standards among the inch-using countries. The Royal Microscopical Society established its standards for microscope objectives in 1858 based on the Whitworth system, and they were subsequently used throughout the world; the history is recorded in the Society’s Transactions for 1858, 1859, 1896, 1911, 1915, 1924, and 1936. In practice, American manufacturers always employed modifications of the Whitworth form because of their preference for flat crests, and such modified threads are completely interchangeable with the RMS threads.

At the Conference on Unification of Engineering Standards held in Ottawa in 1945, the American Delegation presented ASA Paper B1/57 and Drawing ED-95 giving limits of size for a truncated Whitworth thread. Because a rounded-crest form was preferred in Great Britain for optical instruments, the title was amended to “Proposed Permitted Truncation and Tolerances for RMS Thread.” A draft of April 1948 was circulated for comment, a revised draft of October 1954 was approved by Subcommittee No. 4 on Instrument Screw Threads, and final approval as an American Standard was granted on January 7, 1958.

2. Scope and Thread Form 🔧

The Standard covers the thread used for mounting the objective assembly to the body or lens turret of microscopes. It is based on, and intended to be interchangeable with, the “RMS thread.” Beyond objective mountings, it is also recommended for other optical assemblies of microscopes and associated apparatus such as photomicrographic equipment. The basic form of the thread is the British Standard Whitworth form; the design, or maximum-material, form conforms to the truncated Whitworth form established in ASA B1.6-1944, American War Standard for Truncated Whitworth Threads.

The Standard explicitly identifies the three principal attributes of a good fit:

• Adequate clearance to protect against binding caused by 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 manufacturing errors.

3. Basic and Design Dimensions 👓

The thread is single-start (single lead) and has only one nominal size: a basic major diameter of 0.800 inch and a pitch of 0.027778 inch (36 threads per inch). Only one class of thread is established, which experience has proved adequate for the applications. Key parameters from Table 1 of the Standard (all in inches) are:

For the truncated design form: height of truncated thread k = 0.566410P = 0.0157 in.; width of flat at crest F = 0.243624P = 0.0068 in.; width of flat at root F_r = 0.166667P = 0.0046 in.; and basic truncation of the crest from the basic Whitworth form U = 0.073917P = 0.00205 in.

4. Allowances, Tolerances, and Interchangeability 📏

The Standard departs from the war-standard truncated Whitworth threads (ASA B1.6-1944, withdrawn 1951) in three significant ways:

• A larger allowance on the pitch diameter of the external thread, giving more clearance and better centralizing.
• Smaller tolerances on the external major diameter and the internal minor diameter, tightening fit precision.
• Allowances on the external major and minor diameters, deliberately removing material from the maximum-material side.

Where interchangeability with product having full-form Whitworth threads is not required, the allowances on the external major and minor diameters are not necessary, since the forms at the root and crest of the truncated internal thread provide the desired clearances. In such cases, either both limits or only the maximum limit of the major and minor diameters may be increased by the amount of the allowance; increasing both limits improves engagement depth, while increasing only the maximum limit grants a larger manufacturing tolerance. Unless such deviations are specifically covered in purchase negotiations, however, the threads are assumed to be supplied in accordance with the Standard.

5. Terminology and Referenced Nomenclature 📖

Section 7 of the Standard states that its nomenclature, definitions, and letter symbols conform to ASA B1.7-1949, Nomenclature, Definitions, and Letter Symbols for Screw Threads. A July 1972 Errata corrected the Section 7 reference to “American National Standard ANSI B1.7-1965 (R1972)” and changed the Section 14 heading to “Angle Deviation.” Anyone working with older printings should be aware of these two corrections.

6. Engineering Practice Essentials 🛠️

• 🔬 Engagement length: objective interfaces have short engagement; check effective engaged turns to control pull-out and loosening risk.
• 🔧 Cleanliness and protection: objective threads jam easily from particles; clean threads and protect crests before assembly, and cap unused objectives.
• 📏 Functional gaging: for production, use functional plug/ring gages keyed to the 36-TPI effective-diameter limits.
• 🔄 Traceable interchangeability: when mixing objectives and bodies from different vendors, obtain conformance declarations and verify centering by test.

7. Frequently Asked Questions ❓