Patented technique for reliable assembly of thin metals

Thread-tapping screw saves work steps and removes need for additional supporting fastening elements.

In many industries joining thin metals plays an important role – whether in the automotive industry and its suppliers, or the manufacture of white goods such as washing machines. Until now extra fastening elements and manufacturing steps have been needed to ensure secure screw fastenings in thin materials that are 0.25 times thinner, or less, than the nominal diameter. But the recently patented SHEETITE®  from ARNOLD UMFORMTECHNIK puts an end to that. It integrates easily into existing production lines and because it is a fastener that can be used flexibly, it saves time and money in production.
The new thin metal screw is a thread-tapping screw used to fasten pre-drilled sheet metals. How it works: During the fastening procedure, the tapping section of the SHEETITE® fastener chiplessly penetrates the screw-in part and forms a nut thread. Depending on the type of hole drilled beforehand, this pushes material in the direction of the join, so that the penetration depth is greater than the original metal thickness. At the same time there are more bearing thread turns inside the tapped threaded hole. Jan Büchle, who works in the Research and Development Department at ARNOLD UMFORMTECHNIK, says that, depending on sheet thickness, the screw-in depth can increase by up to 60 percent."As the joining process progresses, once the screw fastening has been completed and the head setting reached, a greater proportion of material is available to counter the overload from the screw fastening," said Büchle, explaining the positive effects of the technique. He continued by explaining that the joining process was a way of reducing throughput times and optimizing processes: "New fasteners must ensure continued process reliability. And innovative joining processes must reduce processing times or save on upstream preparatory processes. With SHEETITE® we achieve both these aims. All that is needed is to pre-drill the sheet metal. There is no need at all for nuts or other ancillary fastening elements." With the large number of joining points, this drastically reduces throughput times.

The determining factors for using thin metal screws
SHEETITE® is designed to connect two or more plate-shaped components placed one on top of the other, with a total composite of 0.5 times the nominal diameter or less. This composite consists of a minimum of an upper clamping part with a through-hole and a lower pre-bored screwed part with or without a through-hole. In his development work, Jan Büchle conducted a detailed examination of the advantages of thin metal screws. "In direct comparison with conventional sheet metal screws, we can see that due to the large thread pitch conventional screws increasingly skew to one side during the screw-in process and cannot be inserted vertically to the surface of the workpiece. So the head does not sit evenly on the surface, and thus more load occurs on one side. In addition, the sheet metal screws used to be overturned manually because the material did not offer sufficient security for a reliable assembly when the screw-in depth was small. However, with increasingly stronger materials, and at the same time, greater screw-in depths, design engineers now have access to conventional fasteners that they can apply directly into the metal.

Special screw geometry provides advantages in application
Several patents are witness of the technological progress achieved by ARNOLD UMFORMTECHNIK with its new fastener. A minimum of two different cross-section formations occur along the shaft of the screw. The different cross-sections ensure load-relief during the screw-in process, while at the same time increasing safety during tightening. In the thread-forming area and during the transfer to the bearing thread section the screw's shaft has a polygonal cross-section. As it gets closer to the thread runout or screw head, the shaft takes on a round cross-section in the bearing thread. Büchle, an expert on the subject, explained further. "The thread runout forms the end of the bearing thread section to the screw head. With thin metals, this is a function-critical characteristic and so has to be designed as short as possible, due to the lower screw-in depths and clamping thicknesses."
"We have developed SHEETITE® for customers who produce in large quantities, such as large domestic equipment," explained design engineer Jan Büchle. We have developed SHEETITE® for customers who produce in large quantities, such as large domestic equipment." Design Engineer Jan Büchle. If a repair is ever needed the same or a different metric screw can be used, because the SHEETITE® forms a metric nut thread.The patent application has been completed, and various certifications are still running."The fasteners can be processed using existing screwdriving systems, giving a clear process and investment advantage. According to forecasts, they are currently principally used in sheet steels, however they can also be used to fasten other metallic materials such as magnesium and aluminum. This is principally of interest for manufacturers of component assemblies destined for the automotive industry.