Introduction
There is no end to the development and improvement of an active standard. This CSA edition of B1.1 marks an important step forward in that it embodies the Unified Thread standards which have been agreed upon by standardization committees of Canada, the United Kingdon and the United Sates and signalized by the signing of an Accord at Washington, D.C. on November 18, 1948. These United standards constitute the basic American standards. The earlier American standards are, in part, continued herein. In order to distinguish clearly between the Unified standards and those which are American standards, both of which appear together in many of the numerical tables, different type faces are employed.
This standard incorporates a number of changes resulting from practical experience in manufacture, assembly and use. World War II saw many of them tried and proved. Of particular interest are:
a) Maximum metal thread contours which provide for greater fatigue strength, root clearance, easier assembly and longer wear of cutting tools.
b) Class 2A tolerances and allowances for external threads, computed by formula, computed by formula, which provide a clearance under all conditions.
c) Class 2B tolerances for internal threads, computed by formula.
d) New classes of tolerances designated 1A for external threads and 1B for internal threads, intended to replace the old Class 1 for new work, which are formulated according to the principles of the Class 2a and Class 2B tolerances and allowances. The tolerances of Classes 1A and 1B are 1.5 times those on Classes 2A and 2B, respectively, and the allowance on 1A is the same as that on 2A.
e) New classes of tolerances, 3A for external and 3B for internal threads, which also are in accord with the principles and formula of the 2A and 2B classes of tolerances, the tolerances on 3A and 3B, being 0.75 times those allowed on 2A and 2B, respectively, but no allowance is provided on 3A. These classes of tolerance are less liberal than 2A and 2B.
f) Classes 2 and 3 have been retained without change but they are not among the Unified classes. The thread forms, however, are identical.
g) Class 4 has been dropped. It has been used rather seldom and almost invariably to meet particular requirements.
h) Interference fits are now the subject of active study and will be contained in a separate standard as soon as details are developed.
i) The 1/2 in. 12 UNC size has been listed as an alternate to the 1/2 in. 13 NC size.
j) Twelve (12) threads have been made standard for 1 in. UNF thread, but the former 1 in. 14 NS thread is included in the tables of selected combinations of special diameter and pitch.
k) Classes of threads are distinguished by the amount of manufacturing tolerances and allowances (if any) permissible for externally of internally threaded components. Various fits of thread assemblies can be obtained by combining suitable classes of threads.
