Case study 2.2: Clamping of thin-walled curved workpieces

Main technical issue
The case study is focused on production of thin-walled curved workpieces typical  for structural parts of airplanes. The current issue in the workpiece clamping and  subsequent machining is changeable workpiece stiffness. The existing clamping  strategy and machining strategy cause large local deformation of the part. As a  result, the workpiece wall thickness is out of tolerance and subsequently the part’s  weight was also out of tolerance. This causes part weight changes which is also  undesirable.

Proposed technical solution
Workpiece clamping: New fixtures with integrated support and clamping function  were developed. The workpiece is clamped using a vacuum. Concurrently,  suitable control for automatic position setting of the fixtures was developed. A  suitable thickness measurement sensor was selected and integrated into the  machine tool. The new fixture elements are autonomous and plug-and-produce  ready, with integrated safety to monitor the minimal workpiece clamping force.
Machining planning: Suitable tools and cutting conditions for roughing and also  finishing operations were selected and optimized. The main criterion for tool  selection is minimization of the cutting force in the surface normal direction. The  main criterion for cutting condition settings was chatter avoidance during thin  wall machining. The method was developed for adaptive modification of finish  tool path with respect to current workpiece thickness and deformation.
Process automation: The integration of thickness measurement sensors in order to  control the precision of the workpiece and subsequent modification of the tool  path for finishing operations; development of a unique software for quick and  easy workpiece inspection and thickness measurement.

Main advantages of the solution
The proposed manufacturing process leads to shortening of the production time  by about 15% with requested quality of Ra=0.8. The proposed manufacturing  procedure is beneficial from the productivity point of view. Although the wall  thickness is over the upper limit at some points, the part total weight is on the  requested upper limit. It means that the requested key parameters were fulfilled  using the described fixture system and manufacturing technology. The proposed  manufacturing procedure is acceptable from the accuracy and weight point  of view. A group of fixtures, including the necessary harness and control, offers  a universal possibility how to replace a set of six specific fixtures designed as a  mould with part negative shape. Moreover, the new fixtures can be used for  clamping during machining of both workpiece sides. Comparing the total price  of the whole existing fixture set with the proposed universal solution, it can be seen  that the proposed fixture system is cost-effective.

The main advantages for endusers are summarized in the table below.