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Lightweight construction moves into the fast lane

Lightweight construction moves into the fast lane
Mixed materials for car bodies require alternative fastening techniques. That's where the Deprag flowform screw fastening system comes in. It also copes with tight spaces.

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. 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 has meant a rethink of the assembly processes. With these material pairings, conventional fastening techniques such as resistance welding are extremely impractical. 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 are coming to the fore. 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. 

For increased flexibility in tight spaces, Deprag has developed a new automated screwdriving system. 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. 

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.

Improved cycle times
On long robot assembly lines the routes from the screw feeder system to the screwdriver can often be a hindrance to fast cycle times, but 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 contributes 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 6,000rpm and torque of 15Nm. 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 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.

As the fastener approaches and is positioned to the car body, a 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, the screwdriver process and the torque parameters selected automatically adjust to the relevant screw environment.
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