Advances in the INTEFIX Case Study 1.2 : Turning of Low Pressure Turbine Casing

This case study covers one experiment within the INTEFIX project (FP7 Grant Agreement 609306). This application is proposed by ITP (Industria de Turbopro-pulsores, S.A.) and it deals with the reduction of vibrations in the manufacturing of a component of an aircraft engine: the case of the Low Pressure Turbine (LPT). The participants in this case study include the following companies: ITP, IK4-Tekni-ker, Invent GmbH, Cedrat Technologies, Compotech s.r.o., Alava Ingenieros and Adaptronics International GmbH.

The objective is the improvement of the turning process performance of the turbine case made of low machinability Inconel 718 alloy with 1900 mm in di-ameter, 600 mm in height and common thickness of 3-6 mm. The performance is limited by the presence of vibrations that lead to limited cutting conditions and reduced the tool life. At the same time, the deterioration of the cutting tool feeds the vibrations, and in some cases self-excited vibrations appear. And finally the vibrations can lead to poor surface quality and integrity, leading to a reject of the manufactured component.

The size of this component, together with its low stiffness and difficult fixturing process, drives to the vibrations problems, which are enhanced because of the variable dynamic behaviour of the component due to the relevant quantity of material removed during the machining process

Solution proposed
The solution to the limitations in the manufacturing of the cited components passes through the development of intelligent fixtures able to detect vibrations and modify the behaviour of the machining system (machine-fixture-workpiece) to counteract those vibrations. The use of sensors will provide the fixture with the capability for the detection of vibrations, while the use of advanced fixturing elements and vibration absorbers will allow the modification of the component behaviour under dynamic loads. The demonstrator will consist of several modifications to the current fixture in order to change the structural dynamic behaviour of the fixture-workpiece system to reduce the vibrations during the machining process; these modifications include the following options:

  • Integration of active and semi-active vibration dampers
  • Introduction of advanced fixture elements to achieve tuneable workpiece damping and stiffness by modifying the clamping position and pressure in a controlled way through the whole process (changing stiffness due to material removal, variable cutting forces due to tool wear, etc)
  • Introduction of composite materials to reduce inertial/centrifugal loads and to increase fixture stiffness and damping properties.
  • Automation of the clamping process substituting the mechanical clamping elements with other solutions (hydraulic, pneumatic…)
  • The demonstrator will detect the vibrations and will change the configuration to modify the behaviour of the workpiece by changing the boundary conditions (force, pressure, damping). This will be developed controlling the different active fixture elements attending to the characteristics of the workpiece and the process conditions (position, speed).

Current advance
The experiment started in January 2014 and the current work is related to the theoretical and experimental characterization of the dynamic and static behavior of the workpiece in free and clamped situations, this information is being used to evaluate the needs regarding
sensors and actuators. The theoretical analysis is done using FEM models reproducing the clamping conditions in the current fixtures. After the correlation of the model, this could be used to evaluate the improvements introduced by the new intelligent systems added to the fixture. The experimental measurements involve the development of experimental modal analysis and signal recording of the machining process; the available information will be used to adjust and correlate the FEM model with the experimental measurements.
The main data to be obtained from the current phase is the information related to the measurement and actuation frequency wideband, the requirements of precision for the sensors, and the required actuation capacity of the actuators, as well as the size and geometrical limitations for the new fixturing systems.

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Intelligent fixtures