Unified Threads
Gauges for testing Unified threads to BS 1580: Parts 1&2:2007, BS 1580: Part 3: 2007, and BS 919: Part 1: 2007
BS 1580: Pt1: 2007 specifies details of product threads. The standard includes nominal sizes and limits of size for standard threads for 1A, 2A and 3A for external threads, and 1B, 2B and 3B for internal threads in the following sizes:
UNC
(coarse pitch) from ¼" to 4" dia.
UNF
(fine pitch) from ¼" to 1½" dia.
UNEF
(extra fine pitch) from ¼" to 1 11/16" dia.
UN
(constant pitch) for 4, 6, 8, 12, 16, 20, 28 and 32 tpi for various diameters as specified.
Allowances
(deviation from basic size) 1A and 2A provide allowances of 30% of the effective diameter tolerances of 2A external threads. 3A is basic (no allowance). 2AG is 0.001" smaller on diameter to provide for plating to finish at 2A . (0.001” on diameter will allow for an average deposit of 0.00025”). 1B, 2B and 3B for internal threads are all basic. Before plating, threads are not designated. They are referred to as before plating and are 0.001” larger on diameter than basic size.
Classes of fit
1A/1B are used where easy assembly is required. 2A/2B are the classes used for the majority of general engineering purposes. 3A/3B apply to threads requiring a closer fit, and are only used when a close accuracy of form and pitch are required.
BS 1580: Pt 3: 2007 specifies nominal sizes and limits of size for standard threads for 2A and 3A for external threads, and 2B and 3B for internal threads for UNC, UNF and UNEF for threads below 1/4” diameter. The allowances and classes of fit are the same as above.
BS 919: Pt 1: 2007 specifies the limits for gauges for testing the product thread. The basis for determining the gauge limits is the diameter/pitch. Generally, the larger the diameter and coarser the pitch, the larger the gauge limits within defined diameter/pitch bands.
The types of gauges used are the same as for ISO threads,
Gauges can be supplied to comply with the requirements of the American National Standards Institution specification ANSI/ASME B1.1 1989 for product threads, and B1.2 1983 for gauges.
The designated diameter/pitch combinations are similar to those in BS 1580, with similar allowances and classes of fit. The gauging philosophy, however, in B1.2 differs from the British system. In the ANSI system, generally the gauging limits are contained within the product tolerance whereas, in the British system, the gauging limits may transgress the minimum material limit.Further, the ANSI standard specifies that the major diameter of the No Go plug gauge is truncated to high addendum, whilst the British standard specifies low addendum truncation.