A Comparison of Low Cost Structure-borne Sound Measurement and Acceleration Measurement for Detection of Workpiece Vibrations in 5-axis Simultaneous Machining

Abstract

In the field of machining technology the vibration of the system machine, tool and workpiece during processing is the limiting factor of productivity. Therefore the process monitoring of vibration today plays an important role for the real time monitoring of machining processes, as well as for the optimization of simulation models.For monitoring workpiece vibrations, different kinds of strategies are in use. As the piezoelectric acceleration sensors were already field-tested at the department of machining technology (ISF), the use of Contact Emission-Sensors, which are customarily used for stringed instrument tuning, could pose an effective alternative strategy. Proving the possibility of using those sensors, could simplify the future choice of the process monitoring strategy for different machining operations and has cost saving potentials, due to an abbreviated measurement chain. Within this work the workpiece vibration during a 5-axis milling process were detected simultaneously by two monitoring strategies. In these experiments, the detection of vibration by a piezoelectric acceleration sensor was chosen as the reference strategy. The second technique deployed was the measurement of the contact-emission with a common contact microphone.On a milling machine for five-axis simultaneous machining, aluminium shafts were fixed in a three-jaw chuck one-sided and processed with an end mill. The geometry of the milling pockets was varied in the different processing sessions, as well as the feed parameters, in order to obtain both stable and unstable processing. The vibration measurements resulting from the two monitoring strategies were compared in form of time signal and frequency spectrum as well as in the combined form of a 3-dimensional waterfall diagram.Monitoring structure borne noise is an easy, cost-efficient alternative to measures with an acceleration sensor. Delivering reliable results concerning the vibration frequencies, which are multiples of the spindle rotation frequency, this method could be applied for process monitoring. The experiments have shown that the location of the Contact Emission-Sensors has major impact on the quality of results. To implement the Contact Emission-Sensors for this kind of measurements, the sensor must be positioned close to the place of vibration origin taking into account both damping characteristics of all components involved and the way of workpiece clamping.