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.