Case Study 1.2. Turning of low pressure turbine casing

Main technical issue
The objective is the improvement of the turning performance of a turbine case  made of low machinability Inconel 718 alloy with 1800 mm in diameter, 550 mm  in height and common thickness of 2.5-6 mm. The performance of the machining  process is limited by the presence of vibrations and deformations that lead to  limited cutting conditions and reduce the tool lifespan, affecting also the quality  of the workpiece.  The work performed has allowed the identification of the main limitations of the  current fixture and the manufacturing process. The key point is related to the  contact between the workpiece and the fixture: due to the size and flexibility of  the workpiece and to the fact that the contact cannot be sustained all around  the workpiece, the process forces drive to discontinuities in the contact of both  elements. Another point is the lack of apparent stiffness of the workpiece that  finally results in vibrations and deformations during processing.  Up to now, the solutions tested have consisted of the incorporation of flexible  elements to improve the contact stiffness of the wokpiece.

Proposed technical solution
The current fixture has been modified adding several elements able to change  and adapt the behaviour of the workpiece. This modification includes the  integration of sensors, actuators and advanced materials.  The improvement of the workpiece behaviour has been tackled using three  aproaches:
1 – Active modification of mechanical impedance: This solution deals with the  use of active vibration reducers. The proposed system create a counteracting  inertial force with a magnetic actuator controlled in closed loop, minimizing the  vibrations measured by the integrated accelerometer inside the system. The  effect on the workpiece is a reduction of the dynamic amplification factor in a  wide frequency bandwidth.
2 – Controlled deformation: This solution deals with the use of four actuators  integrated in the fixture to apply controlled forces in defined areas of the  workpiece. The objective is the control of the deformation of the workpiece  applying forces that increase the stiffness in the area near the cutting tool,  avoiding shocks and vibrations. The local contact between the workpiece and  fixture is improved and the apparent stiffness of the workpiece is increased.  Thus, the workpiece is being clamped by an active system under a controlled  hyperstatic clamping situation.
3 – Use of CFRP for locators: This solution deals with the introduction of passive  CFRP elements to substitute the metallic rings used as locators in the current  fixture. The use of CFRP increases the damping (10 times higher than steel) of the  fixture without reducing the stiffness. In this way, the effect of the shocks can be  minimized.

Main advantages of the solution
The developed intelligent fixture allows the improvement of the workpiece  behaviour and the cutting conditions can also be increased obtained a better  machining process performance.  This solution is mainly oriented to large components with different behaviours  depending on the zone of the workpiece or components that change their  shape during processing, requiring the modification of the fixture configuration to  adapt the clamping of the workpiece during the process.  This is appicable to many different components that must be analyzed before  proposing a fixture configuration using the proposed systems and approach.