FALL 2023

12 THE DISTRIBUTOR’S

12 THE DISTRIBUTOR’S LINK Bruno Marbacher Dear Reader, this concludes the series of “thread shaping screws”. If the material is too hard or too brittle for thread forming, thread-cutting screws must be used. These screws produce chips. We included self-drilling screws as they also produce chips. 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-CUTTING SCREWS AND SELF-DRILLING SCREWS FOR SPECIAL APPLICATIONS Thread Cutting Screws What differentiates thread-cutting screws from thread forming screws is how they produce the mating thread. A thread-cutting screw has a shank slotted point that cuts the material and produces the thread as it is driven in, creating chips that fall out of the hole. The shank slot minimizes chip production. Some screws feature a drill-like fluted point that is designed to penetrate hard substrates. Thread Cutting Screws come with sharp edges at the tip, along with a chip cavity or cutout line for clearing the cut material. One can install them using a power screw driver. . As one drives a thread-cutting screw into an object, it will remove some of the object’s material. Thread cutting screws cannot be used in soft plastics. Their cutting cavity fills up with plastic immediately and makes it ineffective. High radial pressure is formed which deforms or even bursts the work piece. Thread cutting screws should basically only be used if the fastened joints are never loosened again, except for repairs. If the screw is not carefully started, in an already existing thread, it is likely to cross thread. The harder the mating material, the finer the thread profile must be. TECHNICAL ARTICLE The coefficient of friction of the hard thermosets is low. Also, the material surrounding the cut thread does not spring back like a formed thread. Therefore, the resistance to vibration of normal thread-cutting screws is marginal. In automobile manufacturing where there are clamped parts made of temperature resistant thermosets, good resistance to vibration is required. Flank angles that are smaller than 30° are beneficial. However, they can no longer be cost-efficiently manufactured. The Different Types of Thread-Cutting Threads Thread Profile - Points For Metals Type D and G (former Type 1): A thread cutting screw with single flute for general use. Produces a fine standard machine screw thread for field replacement. Type D and G thread-cutting screws have machine screw threads, a blunt point and tapered threads. The primary difference from Type F is that Type D Screws contain only one cutting edge. CONTINUED ON PAGE 118