Case Study 2.4. Machining of aircraft turbine support structures

Main technical issue
The main objective is the control and minimization of the deformation of large,  complex geometry and low stiffness workpieces during the setup and clamping  process in order to achieve improved precision during the subsequent operations:  machining, welding, assembly.  The workpiece in study is the Tail Bearing House (TBH) of a turbine (1900 mm  in diameter, 350 mm in height and common thickness of 6-10 mm), and the  machining operation to control is the turning of the flanges. This structural  component is made by welding different forged and cast components that  are machined afterwards. The welding produces deformations and distortions,  so the fixture has to adapt the clamping of the workpiece to avoid additional  deformations and hyperstatic load states in order to assure suitable precision and  to fulfil the requirements for subsequent assembly process.  The precision and tolerances required in the workpiece are ±0.2 mm for general  diameters; ±0.05 mm for reference dimensions within turned features, and  ±0.015 mm for reference holes. The fixture tolerances must be related to the part  tolerances in the contact areas.

Proposed technical solution
In order to solve the problems and limitations explained above, it has been  decided to develop a system with integrated sensors and actuators to correct  and control the deformations of the workpiece while improving the precision and  performance of the machining process. The development of the clamping unit  has been done based on an existing flexible clamping element developed by  Roemheld, adding different subsystems.  The solution comprises a series of intelligent clamping units able to detect and  correct the position of the workpiece, with the following characteristics:

  • Integration of sensors (displacement and force) in the clamping unit to detect  undesirable deformations;
  • Integration of actuators in the clamping unit to counteract the deformations  by modifying and adapting the position of the yaws and the clamping force;
  • Achieve an automatic position correction based on the measurement of the  position and force sensors.

Main advantages of the solution
The developed intelligent clamping unit aims at substituting the current manual  clamping elements in order to obtain the following improvements in the set up of  the workpiece:

  • Achievement of an automatic clamping and position correction based on the  measurement of the displacement and reaction force in the clamping unit;
  • The positioning precision of the clamping unit depends on the stiffness of the  workpiece and in the case of the TBH it is below 0.01 mm;
  • Using the force control in the clamping unit, it can achieve a reaction force  precision of ±8N with a positioning repeatability within 0.01 mm;  -Using the position control in the clamping unit, it can achieve a repositioning  precision of 0.007 mm;
  • Reduction of workpiece exchange and set-up time by the introduction of  the clamping units, being this independent of the number of units. With the  developed clamping unit, it takes just 4 seconds to set up, compared to up to  15-20 minutes using the mechanical clamping elements.

The clamping unit allows keeping the same clamping and holding forces of the  mechanical units.