Methodology to Estimate Grit Density of Abrasives Bonded in Single Layer Configuration This conference paper has to be cited as : Sudhir Kindo, Amitava Ghosh and Saravana Kumar, G., Methodology to estimate grit density of abrasives bonded in single layer configuration, 3rd International and 24th AIMTDR conference, Andhra University, Visakhapatnam, December 13-15, 2010.
Methodology to Estimate Grit Density of Abrasives Bonded in Single Layer Configuration 1
Kindo S.
1
1
Ghosh A.
2
Saravana Kumar G.
Department of Mechanical Engineering, IIT Madras, Chennai-600036, INDIA 2
Email:
[email protected]
Department of Engineering Design, IIT Madras, Chennai-600036, INDIA Email:
[email protected]
Abstract:- In the present investigation, a method for estimating the grit density on working surface of abrasive polishing paper and single layer grinding wheel is presented. Point cloud, containing the surface information, was obtained in the form a triangulated mesh file using laser scanning and post processing. Curvature at vertices of the triangulated surface and then local extremes in curvature above a prescribed threshold value were estimated to determine the grit density. The computed value of grit density was found to be appreciably closer to the actual one as determined using scanning electron microscopy (SEM). Keywords: abrasive tool, single layer, grit density, laser scanning, triangulation, curvature, local extremes
1.
INTRODUCTION
Grinding is commonly practiced as a finishing operation which also imparts desired level of tolerance to the product. Roughness of the machined surface is significantly influenced by operating parameters as well as by the micro and macro geometry of a grinding wheel. Micro geometry includes the features like grit shape, grit size, grit density, grit distribution pattern and participation (apparent and actual participation) of the grit during grinding process. All the grits of a grinding wheel do not participate during a grinding operation [1]. This is also true for a lapping/polishing paper (coated abrasive) on which abrasives are bonded in a single layer configuration. Surface finish of the work piece is significantly affected due to this. The surface finish of the work-piece is significantly influenced by the active grit density, which is a result of apparent grit density and the difference in the grit heights. It is required to know both the information after manufacturing of the abrasive product so that post manufacturing conditioning, like grit shaving can be carried out more precisely. A precise mapping of the surface is also very useful in modeling the surface finish produced by a single layer grinding wheel. There are several methods (contact/non contact type) to map the surface topology. Some of them are stylus probe method [2], optical method [3], laser scanning method [4] etc. Laser scanning process, involves digitization of a surface to give point cloud data (x, y, z values). One of the methods to compute the coordinates is by
triangulation where the triangle formed by the laser source, the laser spot incident on the object surface and the spot location in the camera image is computed. The point cloud thus collected is triangulated to give the first order surface approximation as an STL file. In the present work the laser scanning process is used to map the surface of a single layer grinding wheel and coated abrasive papers. This surface obtained as a point cloud (triangulated) is further analyzed to estimate curvature on the surface. A threshold value of curvature based on grit size is used to segregate regions of high curvature corresponding to the grits. The grit density is computed based on the segregated high curvature points. The method is validated by grit density determined from SEM images. 2.
METHODOLOGY
The overall method followed in the present work is schematically illustrated in Fig. 1.
Fig.1: Schematic of the methodology of grit density determination by laser scanning technique
Surface topography of a strip of coated abrasives and brazed single layer grinding wheel
(shown in Fig. 2a&b) was extracted using laser scanning machine Roland LPX 600™ (shown in Fig. 2c).
laser scanning followed by curvature computation and counted from SEM image.
(b)
(a)
(c) Fig. 2: Pictorial view of (a) single layer brazed wheel, (b) abrasive paper and (c) laser scanning unit
Fig. 3: Grit density determined by the present method and the SEM
The scanning pitch was set at 200m. The point cloud obtained was post processed (cleaning of noise, overlap and then triangulated) using the Roland® software. The curvature values were computed and analyzed to estimate grit density as described in the next section. A Matlab® code was written to import the STL file and compute curvature and estimate grit density.
It has been found out that for 40 grade coated abrasive paper, the error was less than 15% and for the brazed wheel it was less than 10%.
3.
CURVATURE RESULTS
ESTIMATION
AND
The presence of grit was identified by identifying local extremes in curvature of the surface map obtained for the grinding wheel or lapping paper. The principal curvature at the vertices of the triangulated surface was evaluated first and then the local extremes in curvature with a suitable threshold value were counted to estimate the grid density. The algorithm for curvature estimation from point cloud data is based on Chen and Schmitt's method [5, 6]. Initially surface normal on the points was computed based on the normal of the adjacent triangles in the triangulated surface. Then the change in normal direction around the point under consideration was considered to estimate principal curvatures. Once principal curvatures were estimated, the Gaussian and the mean curvature values were computed. Since Gaussian curvature was affected significantly due to presence of noise, mean curvature was used in the present work. Using the value of mean curvature, points with high curvature were estimated. These points indicated presence of grit. Size of the grit was then taken into account to estimate the number of grit particles. Assuming each grit to be spherical, suitable curvature threshold was applied based on the grit radius. The method of estimating the grit density on the abrasive wheel and lapping paper has been validated with values obtained from SEM images. Fig. 3 compares the grit density determined by
4.
CONCLUSION
Surface topography of a strip of an abrasive paper and a single layer grinding wheel (brazed type) was extracted using laser scanning machine. The output obtained as a triangulated mesh file with vertices and face normal was used to estimate the curvature at the vertices of the triangulated surface and then local extremes in curvature with a threshold value were counted to estimate the grit density. Results were found to be appreciably closer to the actual value counted from SEM images. 5. References: 1.
2.
3. 4.
5.
6.
Ghosh, A. and Chattopadhyay, A.K., Influence of process parameters on transverse profile of work surfaces ground by a single layer brazed cBN wheel; Int. J. Abrasive Technology, 2009, 2, No. 1. Bogdan, N. Surface roughness and measurement with new contact methods; International journals of Machine Tool and Manufacturing, 1986, 26, 61-68. Zahide, Y. and Hasmi M., Surface roughness measurement using an optical system; Journal of Material Processing Technology; 1999, 88, 10-22 Lee, C.S. and Kim, S.W., In-Process Measurement Technique Using Laser for NonContact Monitoring of Surface Roughness and Form Accuracy of Ground Surfaces, 1987. Chen, X. and Schmitt, F., Intrinsic Surface properties from Surface Triangulation. Proceedings of the European Conference on Computer Version, 1992, 739-743. Chen-shi, D. and Guo-zhao, W., Curvatures estimation on triangular mesh; Journal of Science, Zhejiang University, 2005.
