How-to-Guide / Screwdriving and Riveting Assembly
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How To Reduce Vehicle Weight with Aluminum and Flow Drilling

March 5, 2014
Trans

Weight reduction has become the focus of attention for most if not all major automotive OEM’s such as Audi, Ferrari, Lamborghini, Jaguar, Porsche, BMW, VW, GM, and Ford. This involves attaching aluminum sheet metal, body, and chassis assembly components to lightweight aluminum chassis producing weight savings of several hundred pounds per vehicle yielding much improved performance and fuel efficiency. In vehicle production, the ability to connect aluminum parts securely with a perfect fit can be particularly difficult for engineers. The task becomes even more complicated when the material has complex curves, or the aluminum has to be joined to other materials like steel in a composite structure. The most common methods for coupling two sections of sheet, or extruded metal together requires either self-piercing rivets, through hole rivets, welding, or standard collapsible inserts with screws or clips. Conventional rivets require clearance holes to be drilled, and the panels aligned before insertion. They also do not offer a convenient way to disassemble the joints after assembly is completed. Welding these components circumvents the need for pilot holes or clearance holes; however completely hinders the ability to disassemble the join after the process is completed.

Flow Drilling screws

Unlike traditional rivet joints, Flow Drilling screws use a completely different method of joining two materials together. The system utilizes a specially designed flow drilling screw inserted directly onto the surface of the assembly. The system increases the speed of rotation to around 5,000 rev/min and the force applied to 1.5kN. This effectively causes localized heating via friction between the flow drilling tip and application parts. When this screw tip and application mate becomes warm enough, the screw can now penetrate and form a rear extruded ‘boss’ through the plasticized metal surface. During this “flow” segment of the installation, the material migrates with the tip of the screw creating an elongated surface for the thread rolling process to tap into. This creates a substantially stronger joint than typical fasteners. The RPM and thrust are reduced, and carefully controlled via proprietary controls to about 800 rpm to produce a high quality thread formed hole without using any pilot hole or generating debris inside the panel. In the third phase, the spindle speed and thrust is further reduced to drive the screw to the seated position with full torque, angle and depth verification. The end result produces a reusable air/water tight engagement creating a tough, long lasting joint. The entire operation is typically completed within two to four seconds. Once the fastening process is completed, and the joint temperature normalizes, the material contracts around the fastener to create an even stronger structural bond with high shear, pull out, and stripping resistance.

Hard to reach places

 Fastener positions used in vehicle body production are often accessible only from one side. This process can become increasingly complex and extremely costly using conventional methods of installation. Using flow drilling techniques to assemble these units can reduce the complexity of such projects, and increase part reliability with stronger bonds. Weber’s RSF units require only single sided installation without the need to monitor the overlapping of clearance and pilot holes. Further benefits include no material contamination due to the forming process. Weber’s RSF units incorporate several advanced process sensors and control algorithms to ensure that each screw is inserted with consistent force, torque, and depth regardless of surface conditions. The system also uses a linear transducer to measure the initial compression as the unit comes into contact with the surface of the vehicle or application part. This means that the active datum system takes part surface changes into account so they have no net effect on the correct installation depth of the fastener. 

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