Case study 1.3

Main technical issue  
Auto regenerative vibrations, known also as chatter, constitute a limit for the  milling process productivity since they reduce the allowable material removal  rate. In order to lower the levels of vibration, the manufacturing engineers  are compelled to decrease the depth of cut of the machining cycle or use  bulkier tools that limit the accessibility to some features of the workpiece. This  is an important issue in many industrial sectors, like the die manufacturing one,  that is characterized by very long machining time due to the complexity of  the workpiece and the required surface finish. A system that could reduce the  occurrence of chatter without the modification of the machining set-up (chosen  tooling) would increase the process productivity without creating new constraints  to the production.

Proposed technical solution  
The developed solution is based on an in-depth study of the regenerative  nature of chatter. The system has been created with the objective to “break”  the regenerative effect of the chatter vibrations. This result has been obtained  thanks to the introduction of an actuation of the workpiece in order to reduce  the regeneration of a vibrated surface at each pass of the tool tooth. In order  to be as not intrusive as possible, the actuation of the system using piezo  has been integrated in a new design fixture. The advantage of this solution  is that the manufacturer could fix the workpiece using its usual approach,  avoiding to reduce the accessibility of the features and surfaces that must be  machined. The developed fixture integrates not only the actuation but also  some accelerometers, used to monitor the status of the process and control  the actuation in order to have a positive result. Thanks to these sensors, it has  been possible to create an autonomous system that starts the operation of  the actuators only when the chatter arises and control the actuation using the  information about phase and amplitude of the workpiece vibration. The control  logic is autonomous and could reduce the vibration level of chatter for a large  variety of process parameters, toolpath and tooling. It is not necessary to create  a model of the tooling and process like in other chatter avoidance approaches;  this a very positive issue for the application in a manufacturing plant. The control  logic is installed on an external PLC that process real-time the sensors output and  decide whether and how activate the actuators.

Main advantages of the solution  
The main advantage of the developed active fixture is to increase the process  productivity maintaining the same quality of the surface finish, without creating  new constraints that could limit the accessibility to the workpiece. The solution is  easily acceptable, since it has a very low profile, 5 cm with adaptor plate, and  could be installed on nearly all the actual milling machines. The advantage of  the autonomous control logic is that no advanced knowledge is required to  the operator to use the active fixture, he just needs to turn the system on after  installing it properly. A brief recap of the obtained advantages measured in  Girardini, a company that operates in the die manufacturing sector, are:

  • Decrease the roughing operations by more than 10% of the productivity time, that leads up to €15k savings for a medium sized die;
  • Operate the machine without human presence and better use of the resources improves the OEE factor of +5% generating up to €18k savings for a medium sized die;
  • Extended machinability of the workpiece by the use of slender toolmachining;
  • Equipment working safely during plant closure;
  • Increase competitiveness due to the introduction of innovative solutions.