Main technical issue
In milling of impellers and blisks, critical workpiece vibrations of thin-walled blade structures occur due to the excitation by the process forces and the dynamic compliance of the sensitive elements of the parts. Workpiece vibrations lead to an unacceptable waviness and chatter marks on the blade surfaces and thus to the production of defective parts. Also, these vibrations provoke an increased tool wear progress. The machining processes are characterized by the use of long and slender milling tools which allow machining of the blade surfaces and the geometries between the blades. Typically, tool rotational speeds above 10,000 min-1 and tools with 3 (and more) cutting edges are applied. Process vibrations occur at both the tool and the workpiece side. The impeller regarded here has a diameter of 200 mm and a height of 65 mm. The workpiece material is Al7075. The weight of the workpiece amounts to approx. 3.5 kg. The dynamic behavior of the impeller blades can be characterized by the natural frequencies of the bending modes. A first natural frequency was identified at 3.9 kHz by FE analysis. During the milling processes, the dynamic response of the blades changes continuously because of the moving contact point between the tool and workpiece and the change of the modal parameters due to the material removal.
Proposed technical solution
The first objective of the newly developed intelligent fixture is to allow the detection of critical process vibrations during the milling process by integrated sensor systems. The second objective concerns the mitigation of regenerative chatter vibrations. This is realized by the introduction of counter excitations of the workpiece by the fixture, which disturb the regenerative effect. In order to implement counter excitations of the impeller during milling processes, an intelligent rotational chuck was developed. The approach of rotational counter excitation exploits the fact that a rotational vibration of the workpiece provokes bending vibrations of the blades. A light weight CFRP element with integrated piezo patch transducers is mounted at the inner rotating core of the actuated structure which carries a clamping mandrel in order to fixate the workpiece. The CFRP provides ‘sensory arms’ which are pre-stressed against the bottom surface of the workpiece during clamping. By this pre-stress, the contact of the sensory element with the workpiece can be guaranteed and the sensor sensitivity is improved. Several sensory test structures (provided by INVENT) were analyzed in experiments.
Main advantages of the solution
The rotational chuck was proven in milling tests (tool diameter: 12 mm, tool rotational speed: 10,600 min-1, z = 2, ae = 1 mm, ap = 0.6 mm, vf = 3,710 mm/min) without and with counter excitations (fex = 350 Hz) using straight test workpieces. The resulting workpiece surfaces were measured by an ALICONA InfiniteFocus device. A smoother and more regular surface can be achieved applying the counter excitation.
Main advantages of the solution are:
- Detection of critical process vibrations during milling of thin walled workpiece elements;
- Mitigation of regenerative chatter effects by introduction of active counter excitation;
- Reduction of defective parts production.