PRECISION FINISHING OF HELICAL GEARS BY ELECTROCHEMICAL HONING (ECH) PROCESS

Abstract

This thesis reports about the experimental investigations on precision finishing of helical gears by electrochemical honing (ECH) process. Helical gears are used to transmit motion between parallel, non-intersecting shafts by meshing the teeth lying along a helix at an angle to the axis of shafts in such a way that two or more teeth of each gear are always in contact permitting more smoother and quite transmission of power and/or motion. Therefore, helical gears are used in various high speed applications. But, helical gears running at high speeds and transmitting large forces are subjected to additional dynamic forces because of the errors in tooth profiles. These profile errors may be eliminated largely by gear finishing methods such as gear grinding, gear shaving, gear honing, gear lapping etc. Unfortunately, productivity of these conventional gear finishing methods is low and applicability is limited. ECH can be a very good alternative to these conventional gear finishing processes because of its high productivity and high accuracy. ECH is a hybrid electrolytic micro finishing technology characterized by a distinct coupling of electrochemical machining (ECM) and conventional honing processes to provide controlled functional surface generation capability of honing and fast material removal capability of ECM in a single operation. It can produce surface roughness as low as 0.05 gm while material removal rate 2-8 times higher than conventional processes. In the present work, existing experimental setup for ECH of cylinders has been modified for the ECH of helical gears by designing and fabricating a tooling system and machining chamber. The experimental investigations involved studying the effects of three key input parameters namely voltage, electrolyte concentration, and rotational speed of workpiece gear, on the measure of ECH process performance namely average surface roughness, maximum surface roughness and micro-hardness of the helical gear teeth. Experimental plan involved three-phase experiments: pilot, main, and confirmation experiments. Pilot experiments were conducted to study the time dependent behaviour of ECH to fix the finishing time and to fix the electrolyte composition and electrolyte temperature by varying one parameter at a time. Based upon the pilot experiment results, 7.5 minutes as finishing time, a mixture of NaCI and NaNO3 in a ratio of 3:1, and 32°C as electrolyte temperature were fixed for the main experiments. Main experiment were designed using Box-Behnken approach of response surface methodology (RSM) in which DC voltage, rotating speed of the workpiece gear and volumetric concentration of the electrolyte were varied in the ranges of 26 — 34 Volts, 50 — 80 rpm, and 5% - 10% respectively. Development of regression models and their statistical analysis including ANOVA, for the two responses namely average surface roughness (Ra) value and maximum surface roughness (R,) value have been done with the help of Design-Expert 7.1.6 software. Study of the main effects and interaction effects of the variable process parameters on the responses has been done. Optimum combination of the process parameters has been found using iii the desirability analysis. Confirmation experiments have been conducted to validate the regression models and optimum values of the process parameters. Microstructure has been studied for the confirmation experiments using scanning electron microscopy (SEM). It is observed that both average and maximum surface roughness values are improving with the concentration of electrolyte. For better surface finish, there exists an optimum value of the voltage supplied should be at lower level while rotating speed shows better result at its middle level. No significant interaction effects have been observed. At the optimum setting of parameters, the present study shows an improvement of 94% in average surface roughness value and 85% in maximum surface roughness value. Microhardness tests showed that the ECH process has no significant effect on it. SEM photographs were taken before and after the experiments to identify the improvement of work surface after ECH process. SEM images have revealed that ECH produces a surface having uniform structure and free of scratches and microcracks. ry