ABSTRACT
The threads are obviously an important element of the threaded fastener. The coarseness designations refer to the relative number of threads per inch. Literally hundreds of thread forms have been designed, and many are still in common use in a wide variety of applications. Metric threads generally duplicate the inch series threads, but with threads considered to be only coarse or fine. The fit between mating threads is determined by the basic clearance between them, a clearance determined by the way the threads are dimensioned and by the tolerances placed on those dimensions. The basic clearance is established by a "thread allowance," which determines the minimum clearance between threads of a given class. The clearance between male and female metric threads is also determined by a basic allowance and by tolerances in the direction of less material. The male thread dimensions had been reduced rather drastically, to accommodate the coating, and that resulted in a significant loss of tooth strength.
