Joining sheet metal directly, quickly and simply

Whereas in the past, steel sheets were easy to join by spot or resistance welding, this is no longer economical due to the modern material mix. The automobile industry is under pressure from legal requirements to reduce CO2 emissions, use lightweight construction and consequently to find alternative joining methods. Where access to both sides of the connection point is available this can be done using rivets. If however only one side is accessible then the well-established direct screw driving flow form principle comes into use. A flow form screw is positioned on an aluminium or steel sheet without pre-drilled holes and the screw driving process begins. The screw is applied with a high pressure down-force and rotates at high speed, generating a frictional heat which forms a crater through which the screw's thread passes under reduced pressure. A machined thread is created, a normal screw can also pass through in the case of repairs. After the flow passage has been formed the speed is reduced and the screw is tightened to the head connecting surface and then the final tightening of the fastener to the pre-determined parameters of torque and angle takes place. The entire procedure usually takes less than two seconds. Additional fastening elements such as nut and bolts are not required as the assembly has been optimally adjusted to the passage and thread of the screw. The screw is fixed. There is no need for preparatory drilling or punching of the component. Deprag engineers have developed a freely programmable gear for a torque range of up to 15 Nm and speed range of up to 8000 RPM from two gears with 15 Nm at 6000 RPM and 12 Nm at 8000 RPM. As well as reduced investment costs, this also means a reduction in processing costs as replacement expenses do not apply. The currently established FFS units usually shoot the flow drill screws tip first through the mouthpiece. Damage to the screw tip geometry is inevitable and can have devastating consequences. The required frictional heat which is necessary to penetrate the material cannot be generated. But it is different with the new flow form screw driving unit where the screw is shot in head first and an integrated revolver separator positions the screw so that its position is secured even for overhead assemblies. The next screw is reloaded already as the first screw is being seated; a decisive benefit in optimising the cycle time. Jürgen Hierold, sales manager at Deprag explained: "Due to the complex geometry of bodywork the highest requirements for mobility of robots and the FFS unit are set. Combined rotation and pivoting movements of the robots are required in order to reach the screw assembly positions. Often the feed hose used can bend or twist so that the screws get stuck during loading. Massive disruptions to the assembly process, high risk to the processing reliability of the complete system and extensive fault clearance are the inevitable consequences. Therefore we were approached by one of the largest internationally active companies in the automobile industry with the demand for a system which presented screws without a feed hose." Deprag has developed a magazine system which is based on three basic elements: the screw magazine, the magazine exchange station and the screw feeder. While one magazine is in operation, docked on the FFS unit systematically presenting 32 fasteners, the second magazine is being filled at the exchange station by the screw feeder. The screw feeder is outside the activity range of the robot, refilling screws without interference in the danger zone. If the first magazine is empty then a robot on the exchange station removes it to be refilled and replaces it with an already filled magazine so that assembly may continue and optimum processing times are achieved. "We now have this exchange magazine available as a standard solution," added Hierold. "Even existing FFS systems can be simply and easily upgraded to include it." Another aim of the developers for the new generation of FFS systems was the parameterisation of the screw driving procedure for the operator so that it is as intuitive and simple as possible to use. System adjustment is automatic. The operator must only enter the screw shaft diameter and length, information such as quantity, material and sheet strength and the tightening parameters. Each screw assembly is then individually analysed and displayed and the software provides comprehensive documentation and archiving options not only for individual screw assemblies but also for different sets of parameters. Another plus point is the mouthpiece exchange device for the application. The adjustment to a different screw type or fault clearance is taken care of at the touch of a button. The lock is loosened and the mouthpiece can simply be exchanged. Usually the holding function of the nosepieces is based on spring-loaded pressure. The disadvantage here is that the screws open the jaws against spring pressure and mechanical load through friction can occur. This is not the case on the new Deprag flow form unit, here the cylinder controlled nosepieces control the release of the screws as soon as the screw begins to form the passage, therefore friction between the screw head and nosepiece is greatly reduced. "There are a number of advantages as a result of the improvements to the FFS unit: as well as the increased user friendliness, reduction of maintenance, varied user options and simplified system integration, above all the processing reliability of the flow form procedure has been decisively increased", concluded Hierold.