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. MANUFACTURING PROCESSES FOR FASTENERS, ETC. Dear reader manufacturers at times do respond to a request for quoting with a “no quote”. There may be various reasons for that, often the quantity is too small to produce them efficiently. However, there are other reasons, the following information will shed some light on this. The tables below provide some ideas on how fasteners are manufactured. Individual features often dictate the type of manufacturing method. In many cases a part can be cold formed, however, because of a certain feature a part may have to be machined or a machining process has to be included in the overall manufacturing process. New manufacturing methods are still being developed so the list is not complete. In the following table, a Yes indicates, which manufacturing methods may be appropriate for a specific group of fasteners. In most cases, machining a part would be possible. However, since there is typically a big amount of waste (shavings and chips), this method is not always the most cost-efficient one. Bolts, screws, nuts, rivets etc. are usually made from wires that have been cold drawn down to a specific diameter with specific tolerances. When cold forming a complex shape it is often TECHNICAL ARTICLE necessary, not only to upset a bolt head, but also to reduce part of the wire diameter to a smaller diameter. Possibilities and Limitations ¤ Cylindrical, concentric parts are the best shapes for cold forming (heading). Eccentric shapes are difficult to form and thus should be avoided. ¤ Typically, bolts and screws can be cold formed up to M24 (~ 1”) and hot forged up to M52 (~ 2”). ¤ Hex nuts are cold formed up to M16 (~5/8”) and hot forged up to M52 (~ 2”) sometimes larger. ¤ Cold forming of screws with shank length of more the than 12 x diameter require special bolt making machines. ¤ If the shank is extruded, the cold formed head diameters can be about 5 times the extruded shank diameter. ¤ Shank cannot be extruded (reduced) by hot forging. ¤ Cross recesses can be cold formed only; they can neither be hot forged nor machined. ¤ Uncommon hexagon socket sizes and TORX recesses may be broached. ¤ Big sizes of hexagon sockets and TORX drives can be hot forged. ¤ Parts with long through holes can only be machined. ¤ Cold headed shapes are not totally cylindrical and do not have sharp edges – this can only be achieved by machining. CONTINUED ON PAGE 108
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