Optimal processing due to automatic adaptation to altering parameters
CO2 reduction and increased load cycles in E-mobility: both are aims which are becoming achievable due to lightweight vehicles. This has been recognised for a long time in modern automobile engineering and it has been relying on lightweight materials instead of conventional steel for chassis design.
From high safety requirements, reliability of materials, simple repairs, easy replacement of bodywork components to recycling and the possibility of reuse - all are considerations which present challenges to the automotive industry. It necessitates a complete rethink, even in assembly: The choice of the correct joining technology is of decisive importance in lightweight construction. Flow drill screwdriving has established a place for itself within chassis design.
The specially formed tip of the so-called flow drill or flow form screw generates frictional heat under high pressure and at high speeds, effecting a flow process in metal sheeting which has not been pre-drilled. At the start of the process a "crater" is formed in which the multiple thread is cut after the screw has penetrated and formed a funnel passage at reduced pressure. A machined thread is created which can accommodate a "normal" screw if repairs are ever carried out. Once the flow hole has been shaped the speed is decreased, the next step is the pre-tightening of the screw until head contact is made and then final tightening of the fastener to the pre-set parameters of torque and angle.
The complete procedure usually takes less than two seconds. Additional securing parts such as nuts or bolts are not required because the assembly optimally penetrates and forms the thread for the screw so that the screw sits securely. There is no need for initial drilling or punching holes in the component.
This process does not only enable the connection of sheets of varying materials but it also offers clear advantages in relation to processing costs and time. But is this screwdriving process also suitable in applications with fluctuating tolerances and varied components?
In order to ensure high screwdriving quality despite component tolerances, such as deviating positions, sheet material thickness and screw length tolerances or structural differences, current solutions on the market are only able to offer screw assembly systems where settings for screwdriving parameters are time-consuming and have to be separately determined for each screw position. Usually the feed movement and force is provided by a compressed air cylinder with proportional valve. The precision needed to alter the status of feed forces and positions can potentially leave a lot to be desired. The behaviour of the compressed air can prevent individual processing steps from being followed with any accuracy. The transition from funnel forming (phase 3) to thread forming (phase 4) is particularly critical. There is a danger that if the screwdriver speed and/or pressure is reduced too early the funnel is not fully formed and the heightened drilling torque could destroy the screw or the component. Any delay could cause damages to the thread as well as compromising the connection joint.
No need for costly analysis of screwdriving parameters
The screwdriving technology experts at DEPRAG have now eliminated these risks: The company offers an adaptive flow form assembly unit with regulated electric drive used for feeding as well as screwdriving processes. This enables a highly dynamic interaction with the processing factors of pressure and screwdriver speed, independent from the constantly determined current status.
Unlike traditional systems, the new adaptive assembly unit Adaptive DFS can prescribe and monitor the feed speed and feeding procedure. The constant data reported by the control modules enables the precise and automatic recognition of all relevant penetration points. Time-critical- and essential parameter changes are autonomously performed by the fastening system.
This system ensures the ideal processing parameter, independently from the tolerances of the product or fastener. It significantly reduces the effort of preliminary analysis and parameterisation. Costly and extensive repair procedures that are caused by imprecisely formed holes, jammed screws or ruined threads are kept to a minimum.
The best possible processing-parameter that have been automatically adapted to suit any situation, guarantee that the parts to be connected (fastener and product) are subjected to the least amount of strain. The additionally captured processing-data allow an increased process documentation.
Separate electronic regulation for the EC-Servo screwdriver and the EC-Servo stroke technology, in combination with the extensive parameterisation possibilities, ensure the highest flexibility during the processing of multiple materials and material combinations. The implementation of special tightening sequences for new fasteners and materials is possible. Especially for the assembly of future materials such as carbon-fiber and CFRP, the controlled feed stroke allows that exact positioning and trigger-points are clearly defined. This is a benefit which should not be underestimated for new materials whose behavioural pattern (e.g. delamination during perforation) is not yet predictable.
The new assembly system works at a speed of up to 8000 1/min at a maximum torque of 15 Nm. The regulated electric feed drive enables any number of precise positioning manoeuvres. This allows secure hold of the screw using a socket which is particularly important for under-floor screw assemblies. In addition, the cycle time is reduced per screw assembly as only a short feed stroke is needed. When thread forming as well as during screw tightening the limits determined by material (such as speed, torque) can be optimally utilised. Whilst the system moves to the next screw position, the next screw is already being presented and the main part of the feed movement is carried out. The screwdriving system is optimised for the shortest cycle time.
A new and important feature of the adaptive assembly unit is the central introduction of pressure in the screw axis. Pressures of up to 3000 N no longer cause transverse exertion which is particularly positive in relation to the wear on bearings and guides. Compact and weight-optimal construction is guaranteed. The low weight means that the unit is particularly suitable for use with robots. The slim design, with width under 170 mm and minimal projecting edges enables accessibility to even the most difficult to reach screw positions. The screwdriving system can be easily adapted to suit the most varied of connection elements by the simple exchange of screw specific components. The modular design in combination with quick change chucks makes the system particularly maintenance-friendly. The system can also be optionally fitted with an automatic tool exchange system.
Screw feeding can be provided by two different versions: screw feeding via hose or presentation by a magazine loader.
In order to satisfy high demands for maintenance and service-friendliness and to minimise downtime, the assembly unit is constructed modularly. Components which are continually in contact with the workpiece or fastening element and thereby more susceptible to wear can be replaced at the touch of a button, without tools or special expertise.
The assembly system has an ergonomic, intuitive operating surface. Comprehensive integrated diagnosis functions comprise the basis for preventative maintenance and maximum efficiency of the system. Complete documentation and evaluation is possible through the recording and readout of processing parameters and screwdriving results.
An addition advantage is that mechanical and electric interfaces are primed for quick installation of a camera or laser pointer for documentation of the screw position geometry, monitoring accessibility of the screw positions or convenient "teaching" of the screwdriving positions.