D = work piece diameter. U = axial feed rate α = thermal diffusivity = thermal conductivity / volumetric specific heat. Since Pi 3 is dimensionless, either the SI or ...
EFFECT OF PROCESS PARAMETERS ON THE GRINDING OF POLYMERIC COMPOSITES Sherif D. El Wakil1, and Nahid Abd El-Salam Azab2 1
Department of Mechanical Engineering, the University of Massachusetts, MA 02747, U.S.A. 2 Department of Design and Production Engineering, Ain Shams University, Cairo, Egypt
Keywords: grinding of polymeric composites, surface roughness of ground composites, nondimensional analysis of composite grinding.
INTRODUCTION The present paper discusses and compares the results of two similar studies that were carried out in the U.S. and Egypt simultaneously. The objective in both cases was to validate a previously published mathematical model (1-3), which describes the process of grinding polymeric composites by dimensionless numbers. Cylindrical grinding was carried out by means of a precision lathe and a grinding attachment in the U.S., while an industrial cylindrical grinding machine was employed in the experiments performed in Egypt. This allowed investigating a wide range of values for each process parameter, thus enabling the optimal conditions of the grinding of polymeric composites to be determined. Expressing the grinding process quantitatively by a non-dimensional number provides the flexibility of selecting a value for one of the variable and determining the values of the other variables to yield best ground surface quality and closest tolerances.
THE DIMENSIONLESS NUMERS By employing the Buckingham PI theorem and using the parameters affecting the process of grinding of polymeric composites, four dimensionless numbers which quantitatively describe that process could be obtained. After excluding the ratio between the mechanical and thermal strains which is one of those numbers, the most influential among the remaining the three numbers was found to be
π3 =
UD
α
(1)
where
D = work piece diameter U = axial feed rate α = thermal diffusivity = thermal conductivity / volumetric specific heat Since Pi 3 is dimensionless, either the SI or the English system of units can be used, without affecting the value of Pi 3.This is another advantage of employing dimensionless numbers.
RESULTS AND DISCUSSION Experiments were carried out on graphite-epoxy composite tubes each was about 0.15m in length, 0.1m in diameter, and 0.05m in thickness. The fiber-to-epoxy ratio was 70 to 30 by volume and the fibers of the outer layers made an angle of 80-110 degrees with the axis of the tube. The properties of the fibers and the epoxy were provided by the supplier, and the properties of the composite used could accordingly be determined. The surface roughness of the composite tube was measured by a profilometer after each grinding operation. The results of the experiments carried out in the U.S. indicated that the values of the surface roughness first go down with increasing Pi 3, because the period of the tool contact with the work piece becomes shorter. But with further increase in Pi 3 (i.e. U), the amount of material removed markedly increases and so does the heat generated due to grinding. That heat would result in large expansion of the work piece; build up of friction, and possibly clogging. The values of surface roughness, therefore, markedly increase. This trend was always the same for the five values of depth of cut used, namely 0.025, 0.05 , 0.1, 0.125 , and 0.15 mm. Because an industrial cylindrical grinding machine was employed for the experiments carried out in Egypt, the values of the axial feed rate and, therefore, Pi 3 were extremely high. Unexpectedly, the values of surface roughness did not increase and clogging did not occur. This is because the high axial feed rate resulted in a shorter contact time between the grinding wheel and the work piece, thus reducing the amount of heat dissipated into it. This is added to the lower friction between the wheel and the work piece at higher cutting speeds and axial feed rates. Moreover, the depths of cut were very small resulting in less generated heat during grinding. In fact, the other two dimensionless numbers cover that. It can be concluded that employing higher cutting speeds and lower depths of cut when carrying out cylindrical grinding of composites is recommended, when the objective is to obtain smoother surfaces and close tolerances.
REFERENCES 1. Laoulache, R. N. and El Wakil, S.D. Proceedings of the Sixth International Conference on Production Engineering &Design, Cairo ,Egypt , Vol.1, pp134142,2002. 2. Fares, G.F.”Grinding of fiber-reinforced polymeric composites “Master’s Thesis, University of Massachusetts Dartmouth, 2006. 3. El Wakil, S.D. Proceedings of 9International AVK Conference, Essen, Germany, Vol.1, A10,pp 1-6,2006.
