FALL 2020

12 THE DISTRIBUTOR’S

12 THE DISTRIBUTOR’S LINK Bruno Marbacher Bruno Marbacher earned his mechanical engineering degree in Switzerland, he also holds a business degree. He started out as a tool and die maker (poly-mechanic) and over the years he has held various management positions in quality and engineering. During his time in America he has developed and given numerous seminars on topics related to the proper use of mechanical fasteners and machine elements, and assists engineers in solving fastening/assembly issues. His has groomed and directed many young engineers in fastening/assembly technology. He now offers his 40 years of experience through writing and lecturing. ISO SYSTEM FOR LIMITS & FITS - INTERCHANGEABILITY OF METRIC MECHANICAL FASTENERS WORLDWIDE Making Things Fit Together Metric fasteners and machine elements can be fitted together worldwide. To assure this they must be manufactured to international recognized standards. However, there is another reason, why interchangeability should be achieved. The critical features of parts have their tolerances derived from the international system for limits and fits. Thus, mechanical machine elements made in a certain country of the world can be assembled with a part made in a different country of the world. In this article we provide some insight why this is the case. some electronic/computer parts). With the introduction of mass production, it became evident that efficient mass assembly is only possible, if parts can be fitted together without a great deal of modifying and adjusting. This led to the creation of the ISO system for limits and fits Negotiations to introduce a system for international use started in the 1920’s. However, it took years to develop a practical system. The current system is based on the ISA system for limits and fits that was first introduced in the 1940’s. Since the German standards association (DIN) was entrusted with the creation of the system, it obviously was based on the metric system. FIGURE 1 The ISO system for limits and fits was original designed for precision pins and/or shafts going into a precision hole. But now it is also widely used for metric fasteners etc. For metric threads, a similar system has been designed to assure interchangeability, it will be addressed in a future article. For thousends of years, parts were fitted together individually, which meant they were not interchangeable. They possibly could be fitted together if they were made by the same manufacturer (today, still the case with TECHNICAL ARTICLE FIGURE 2 The System Is Used Worldwide Today, the tolerance system is used by all industrial nations including the U.S. Although the system was created in metric, there is also an inch version available. ANSI B 4 .1 is based on this system. The system is intended to provide a systematic approach to fit two parts together, namely a pin/shaft fitted into a part with a hole, regardless of whether they are made in-house or at the other side of the world. CONTINUED ON PAGE 106