Investigation of vibration in micro milling with minimum quantity lubrication

Abstract

Micro milling with minimum quantity lubrication (MQL) have the ability in reducing cutting temperature and prolongs the tool life. However, uncontrolled vibration, known as chatter remains as a critical issue in micro milling process especially with the presence of MQL. Thus, the main aim of this research is to analyse the vibrational signal produced by the various combination of micro milling and MQL parameters during machining in target to control and reduce the undesired chatter. High-precision CNC machine tool was used to machine the microchannels onto a copper workpiece with a 500 μm micro end mill tool which is lubricated using an MQL system. This research consists of two parts which are the preliminary investigation (1st part) and vibration investigation (2nd part). Preliminary experiment (1st part) determines and isolates the significant parameters towards the vibrational signal. During this part, spindle speed, depth of cut, feed rate, nozzle air pressure, and nozzle distance were used as the controlled parameters. The vibrational signal; spindle speed frequency (SSF) amplitudes, and overall amplitudes; were measured using an accelerometer and a data acquisition system. It was found that, the parameters which influence the vibrational signals were the spindle speed (20000 – 50000 rpm), depth of cut (20 – 50 μm), oil flow rate (3.75 – 11.25 ml/hr), nozzle air pressure (0.200 – 0.275 MPa), and nozzle direction (90 - 270°). These parameters were used as the controlled parameters in the vibration investigation (2nd part). In this 2nd part, Taguchi L16 was used to design the experimental run. The accelerometer and the data acquisition system were used to measure the SSF amplitudes and chatter amplitudes. Based on the amplitudes, chatter ratio were calculated and analyzed using signal-to-noise (S/N) ratio and analysis of variance (ANOVA). It was found that the nozzle direction and depth of cut has the highest influence on the chatter ratio followed by nozzle air pressure, spindle speed and oil flow rate. The optimum parameters for minimum chatter ratio (0.0104) were found to be 3.78 ml/hr oil flow rate, 0.275 MPa nozzle air pressure, 270° nozzle direction, 49.89 μm depth of cut, and 49959 rpm spindle speed. The developed model is found to be adequate since the percentage error is relatively small (≈ 7%). As a conclusion, chatter ratio is the best indicator compared to other amplitudes in determining the presence of chatter produced during micro milling process with MQL.