| One of the major requirements of product critical surfaces of components is specific surface finish in order to perform the intended functions as part of an assembly. Surface finish is one of the essential quality control parameter to ensure that manufactured parts conform to specified standards. Surface finish of parts can significantly affect their friction, wear, fatigue, corrosion, tightness of contact joints, positioning accuracy, etc. The average surface roughness is the most widely used surface finish parameter by researchers and in industry as well. Measurement of surface roughness is carried out with the help of stylus technique which is the widely used in industry. Stylus measurement technique has disadvantages like squashed or degraded surface finish, limited line sampling, limitation of radius of stylus tip, offline inspection, etc. With the advent of automation, surface characterization needs to be totally computerized so that the task of inspection of surface is greatly simplified. In recent years, with the advent of high - speed general - purpose computers, image analysis has become easier, faster and more flexible. Unlike the conventional stylus instrument, digital image processing is non-physical contact type and capable of area sampling and online automated inspection. Hence, the application for surface roughness evaluation of non-contact approaches has received a considerable attention. Literature survey shows that researchers used the digital image process technique to quantify the surface finish of conventional material and for conventional machining processes. Researchers have used histogram analysis and Fourier transform analysis to extract the surface (textural) features of metallic material specimens of steel, brass and copper, etc., produced by conventional machining operations turning, grinding, milling, shaping, etc. This work is pertaining to the estimation of surface roughness of machined surfaces of high - tech materials like carbon-carbon composites using digital image processing. Carbon - carbon composites, which are first introduced in the late 1950s, were only used as an ablative material until the late 1960. Thereafter, these materials use found in space shuttle programs. Carbon - carbon composites are used in a variety of sectors requiring high mechanical properties at elevated temperatures, good frictional properties for brake pads in high speed vehicles or high thermal conductivity for thermal management applications. However, for extended life applications, these composites need to be protected against oxidation either through matrix modifications with Si, Zr, Hf, etc. or by multilayer oxidation protection coatings consisting of SiC, silica, zircon, etc. It has been observed, through literature survey, that little work has been reported in the proposed area. No work has been reported on the estimation of the surface finish of carbon-carbon composites, machined by EDM (Electrical Discharged Machine), with the non-contact type digital image processing. Apart from this, extraction of surface roughness value from optical parameter has been obtained from which online inspection strategy could be developed. This work is focused on the quantification of the surface finish of machined carbon-carbon composites with the help of digital image processing. This investigation includes estimation of surface finish of OHNS (Oil Hardened Non-Shrinkage Steel), graphite and carbon-carbon composite specimens respectively, using image processing. Preliminary investigation to establish the process as well as the required correlation between the measured roughness and optical surface roughness is carried out using OHNS specimens machined on VMC (Vertical Machining Center). Later on, to establish the methodology for surface finish measurement of carbon-carbon composite class of materials, machined surface of high density graphite specimens are analyzed for surface roughness using image processing as well as by the conventional method. Finally, the same approach and methodology are extended to EDM (Electro Discharge Machine) machined surfaces of carbon-carbon composite (a high - tech material uses widely in aerospace and other applications) specimens. OE1NS and Graphite specimens having different roughness values are prepared on VMC machine using specific values of the machining parameters. Carbon - carbon composite specimens are machined on EDM at different values of the process variables. The images of machined OHNS, graphite and carbon-carbon composites having different roughness are grabbed by CCD camera. After preprocessing it, gray scale analysis is done. Power spectrum of gray image is also found out. Optical parameters, 'Ga' and 'Gaf, arithmetic average of gray level, before and after applying the filter, are calculated for all captured surfaces of images. The measurement of surface roughness 'Ra' is to be carried out with the help of conventional stylus instrument for OHNS specimens and graphite specimens. The correlation is established between optical parameter, arithmetic average of gray level (of filtered image), 'Gaf and stylus roughness 'Ra' for OHNS and graphite specimens. Results obtained indicates that the calculation of 'Gaf, optical roughness value, from the preprocessed (improved or filtered) images has a better correlation with the average surface roughness, 'Ra', measured for the specimens of OHNS and graphite specimens, as preprocessing of images removes the noise. It indicates the effectiveness of image processing as a tool for the measurement of surface roughness. It can also be inferred that this scheme of optical roughness estimation seems more promising compared to traditional stylus approach. Database generated for high density graphite (which is the same class of material of carbon - carbon composite) has been successfully utilized to evaluate the average surface roughness parameter of carbon - carbon composites from the optical surface roughness parameter of images of carbon - carbon composite. Thus, correlation between optical parameter 'Gaf and stylus roughness 'Ra' of high density graphite is applied to the preprocessed images of Carbon - Carbon Composite to find out the surface roughness value 'Ra' without moving the stylus on the surface with the help of digital image processing tool of MATLAB. Complete analysis of these results has been successfully carried out in the proposed work. |
