Lightweight construction moves into the fast lane

Fierce competition has broken out in the automotive industry. Who can build the lightest car? The less a vehicle weighs, the more likely it is to meet maximum energy efficiency requirements. Cars of the future will be measured by their fuel consumption and CO2 emissions. Regardless of whether the car is powered by an electric drive system, a hybrid drive, or has a conventional combustion engine - the lightweights will come out top! However, car bodies must lose none of the necessary rigidity. Modern car body engineers are experimenting with suitable compound materials, combining different metals such as steel and aluminium. This meant a rethink of the assembly processes. With these material pairings, conventional fastening techniques such as resistance welding are extremely impractical and have to be achieved by other joining processes. Fasteners such as rivets are experiencing a revival, where access to the fastening point is possible from both sides.

If access from both sides is not possible, an alternative fastening process is required. In such situations screw fastenings made with flowform screws come to the fore as the solution to the problem. Flowform screws create a very high-strength fastening and form a process-reliable joint for different types of sheet metal. In industrial robotic manufacturing processes, these modern fasteners are placed by automated screwdriving systems. This is a market where DEPRAG, the well-known supplier of fastening technology, is currently providing some up-to-date developments.

The new DEPRAG system is recommended principally due its ability to adjust flexibly in places where space is very tight. Sales Manager Jürgen Hierold points out: "Robot screwdrivers are specially designed to deal with situations where space is limited. By using a 1:1 ratio offset gear to offset the drive motor and screwdriving axis to the side the screwdriving system can address screw positions located as little as 14 millimetres away from an obstacle!" The screw module can turn in any direction, so can be easily deployed in any position. DEPRAG SCHULZ GMBH u. CO. is a byword in the automation industry. Established product lines such as the proven MINIMAT-EC and MICROMAT-EC hand-held screwdrivers, DEPRAG measurement technology, DEPRAG feeder technology as well as its screwdriving stations and assembly systems have proven themselves a hundred-fold. These well-established technologies form the basis of the new flowform screwdriving system.

What happens when a flowform fastening is made? There are six steps involved in inserting these special screws, which are supplied by several well-known manufacturers. A flowform screw is positioned onto the aluminium or steel sheet, which has not been pre-drilled, and the screwdriving process begins. The screw is applied with a high down-force, and rotates at high speed, generating a high level of friction heat, which first of all forms a "crater". As it goes through the sheet metal the screw creates a "through-funnel", which allows the forming of a multiple thread at reduced pressure. This produces a machine thread which can also accept a "normal" screw in the event that a repair is required. Once the thread has been formed, the speed reduces and the screw is tightened to preliminary torque values. The final fastening is then tightened to the pre-configured torque and angle parameters. The entire process generally takes less than 2 seconds. Additional fastener elements such as nuts or bolts are not required, because the screw's through-hole and thread are optimally adapted at assembly. The screw "sits" correctly. There is no need to pre-drill or punch the component.

Flowform fasteners have now become an established technique used by car body engineers in the automotive industry. In automotive plants, robot-supported screwdriving systems process component after component. DELETED BY CLIENTDepending on the design, several hundred screws will be inserted into some larger vehicles. DELETED BY CLIENT On long robot assembly lines the routes from the screw feeder system to the screwdriver can often be a hindrance to fast cycle times. Sales Manager Jürgen Hierold points out another advantage offered by DEPRAG's latest innovation: "there are always two flowform screws in the screwdriver system. While the first is being fastened, the next one is fed through and ready in the waiting position behind the nosepiece. This reduces the time taken to refill the system and it can only contribute to improved cycle times." The fastener to be inserted is held in position by a holding and positioning device, which opens under pneumatic control. A sensor monitors the "finding" process, in order to ensure connection between the driving tool and the screw's drive. The holding device opens as soon as the screw has penetrated through the sheet metal. This allows the DEPRAG screwdriving system to turn flexibly and to be deployed optimally in any position.

The new DEPRAG screwdriving system can be adapted to flowform screws from all well-known suppliers. For the spindle drive motor, the designers used the tried and tested EC motor from the MINIMAT and MICROMAT range with a rev speed of 6,000 rpm and torque of 15 Nm. An air cylinder provides the forward feed for the screwdriver's connection with the fastener. A proportional valve controls the air cylinder. The down-force for the screw process (max 2,500 N) is also regulated by an air cylinder with a proportional valve. The 1:1 ratio lateral offset of the EC motor gear keeps the screwdriver structure slim, and is why it can be deployed even where space is very limited. A strain gauge transducer at the motor spindle controls the torque.

A digital position sensor monitors each step of the joining process. As the fastener approaches and is positioned to the car body, the sensor system in the mouthpiece and the DEPRAG screwdriver stroke compares the "is" status against a configured reference value. If the parts geometry does not match the "should" value, for example because the car body has flexed, the screwdriver process and the torque parameters selected automatically adjust to the relevant screw environment. An enable signal is emitted to start the screwing process. Finally, the last step is to check the condition of the joining process. The parameters associated with a screw fastening - speed, down-force, torque and angle - are separately analysed, compared against the specified values and, provided they are within the tolerances, confirmed with an "OK message".

Parameterising the various process steps for flowform fastenings is particularly tricky. While during the heating and flow phases, the system works at maximum down-force and a high revolution rate, when it comes to the thread-forming process, the speed and down-force reduce. The speed and down-force reduce once again for the steps involving preliminary tightening and final torque. The AST32 process control software used to control these delicate steps of the process is based on the proven standards used in the sensor-controlled DEPRAG EC-Servo screwdriver. Jürgen Hierold points out: "This software is trusted and well-established. It is recognised as being particularly user-friendly in operation, and it is easy to configure the complex flowform screwing process." The automation specialist is offering an additional component in its high-quality DPU system control unit (DEPRAG PROCESS UNIT) with operator-friendly standard software, as well as appropriate screw feeders.