WINTER 2021

14 THE DISTRIBUTOR’S

14 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. THREAD TOLERANCES ASSURE FASTENERS CAN BE ASSEMBLED EASILY Dear Readers, in the previous article, I covered the international tolerance system for limits and fits. The tolerance system for threads has some similarities as it also works with tolerance zones. The main purpose is to assure interchangeability on a worldwide basis. However, the tolerance zones are defined to meet different purposes and conditions as well. DIN (German Institute for Standardization) started developing thread tolerances in the early 1920’s. ISO (International Organization for Standardization) published the first standard, which was the standard for the metric thread, in 1947. The International Standard ISO 965 specifies thread tolerances. Tolerance Classes (Zones) As with “shafts and pins”, the tolerance symbols consist of letters and numbers. The letters indicate the location of the tolerance in relation to the nominal dimensions of a threaded part, for metric threads this is mainly the pitch diameter and the major diameter. The numbers (called tolerance grades) indicate the actual tolerance, the actual limits; a bigger number calls out a bigger spread between the low limit and the high limit. Tolerances for internal threads are expressed with TECHNICAL ARTICLE capital letters; tolerances for external threads are expressed in lower case letters. For the thread tolerance symbol, the number goes first, followed by the letter; that is different from the tolerance system for limits and fits, where the letter goes first. Thread tolerance class examples: 6g, 6h, 6H The tolerance zones are arranged in a certain sequence in relation to the zero line. That zero line represents the nominal dimension of a given diameter. There are fewer tolerance options for threads than we have for Limits and fits. Tolerance Classes of a Screw Thread Depending on what tolerance zone is chosen, it may put the maximum dimension equal to the nominal dimension or a certain amount below. This applies to the nominal pitch diameter and/or major diameter Here are some of commonly used letters h, g. f, e Nominal dimension for pitch diameter as well as major diameter can be found in ISO 724. CONTINUED ON PAGE 104