Speckle metrology based study on the effect of chattering on machined surfaces

Techniques of Speckle Metrology are used for the study of the onset of chatter during machining process. In this method, it is found that with an increase of cutting depth, the cutting regime was changed from chatter- free cutting to one with chatter. This is detected by the offline surface texture analysis of the work-piece using speckle effect. Laser speckle is an interference pattern produced by light reflected or scattered from different parts of the illuminated surface. It is the superposition of many wavefronts with random phases, scattered from different parts of the rough surface. Hence, the information of surface texture is encoded in the speckle pattern even though it is a random phenomenon. The statistical study of speckle contrast and speckle size distribution can be used to extract the information about the roughness of the surface from which the light is getting diffusively reflected. Given that machining processes are very complex due to their non-linear and non-stationary characteristics, and since many process- variables cannot be directly measured, process- monitoring is a challenging problem. Chatter is one of such problems, which is essentially the self-excited vibration during machining that will adversely affect the surface finish, tool life and thereby affecting the quality of the machined surface. In the present study a tapered work piece is machined to form a cylindrical piece, by continuously varying the depth of cut. As the depth of cut increases the surface finish is expected to deteriorate, mainly due to the onset of chatter vibrations. To analyze the surface texture characteristics, the speckle pattern obtained by illuminating this curved surface using a collimated laser beam (5mW Diode Laser at 676nm wavelength.). The laser beam was made to incident obliquely to the curved surface of the work piece, and the speckle pattern was recorded using a Charge Couple Device (CCD) camera. The beam was scanned along the axis of the work-piece and recorded the speckle pattern at different regions at constant intervals The speckle contrasts of the patterns are evaluated as the ratio of the normalized standard deviation to the normalized RMS height of the histogram. It is found that the speckle contrast is decreasing as moving from lower depth of cut region to higher depth of cut region. An abrupt decrease of the speckle contrast is observed at the beginning of the onset of chatter vibrations. This method can be extended to online detection of chatter, which is not possible with the conventional methods, like stylus methods. This can also be used for online Tool Condition Monitoring (TCM).