Case study 2.1: Detection and compensation of workpiece distortions during machining of slender and thin-walled aerospace parts

Main technical issue
Thin-walled workpieces are particularly affected by distortions due to residual  stresses which are induced or delivered during the machining processes. In this  case study, the reference workpiece is a structural aluminum part of the following  dimensions: length: 1,970 mm, width: 100 mm and height: 48 mm. The final  workpiece is machined out of an aluminum block by several milling steps. After  the machining, removal of the clamps and demounting the workpiece off the  fixture are completed, distortions occur which lead to a bended shape with a  height difference of up to 10 mm between the center and the edges of the part.  These distortions are not always distributed evenly over the entire length of the  workpiece. In order to reduce the resulting shape deviations, several re-clamping  steps are currently conducted in which the workpiece is dismounted off the  conventional fixture, turned and clamped again in order to allow machining from  both the front and the rear sides. The aim of this procedure is to achieve a uniform  distribution of residual stresses and, thus, minimize distortions. However, this manual  re-clamping is a costly and time consuming solution to overcome the given  problem. Furthermore, the re-clamping not always leads to acceptable results in  terms of workpiece shapes which satisfy the tolerances defined by the customers.

Proposed technical solution
Within the INTEFIX project, an investigated approach is based on the application  of a fixture frame that integrates several adjustable clamping elements. The  fixture frame holds the workpiece in an upright position, so that accessibility of the  clamped part from the front and the rear side is provided. This allows automated  both-sided milling operations inside a horizontal machining center with a rotary  table. The adjustable clamping elements allow an adaptation of the locations  of the clamping points to distorted shapes of the clamped workpiece by floating  degrees of freedom (DoF). By this, a relaxation of the workpiece at intermediate  states of the total machining process becomes possible. Thus, residual stresses  can be compensated and the final distortions can be reduced. During the milling  operations, the floating DoFs of the clamping elements are blocked by hydraulics.  In order to align the locations of the clamping points actively, additional hydraulic  actuators are integrated into the fixture system which move the floating clamps  separately. For an adaptation of the clamping point locations to distorted  intermediate workpiece shapes, a controlled positioning of the floating DoFs is  necessary. To obtain measurement signals for position and force control, sensors  are integrated into the actuation sub-systems. Consequently, with its sensor,  actuator and control integration, the fixture provides the additional ‘intelligent’  functionality to adapt itself to occurring distorted part shapes

Main advantages of the solution

Main advantages of the solution are:

  • Avoidance of re-clamping by automated both-sided machining;
  • Reduction of workpiece distortions by relaxation of intermediate residual stress distributions;
  • Adaptive configuration and adjustment of clamping points based on sensor and actuator integration.