EXPERIMENT INVESTIGATION ON PRECISION FINISHING OF SPUR GEARS BY PULSE•EIECTROCHEMICAL HONING (PECH) PROCESS

Abstract

This thesis reports the experimental investigations on precision finishing of spur gears by pulse-electrochemical honing (PECH) process. Gears which run at high speed and transmit large forces may be subjected to additional dynamic forces due to errors in tooth profile. Therefore, the gear teeth must be smooth and error free. These errors may be reduced significantly by finishing the tooth profiles. Several types of conventional gear finishing processes are available to improve the quality of gears. But the finishing that can be obtained and productivity of these conventional processes is low which led to the evolution of advanced finishing process of gears like electrochemical honing (ECH). 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 μm while material removal rate is 2-8 times higher than conventional processes. Based upon the literature review, it can be concluded that using pulse power in ECM-based processes gives (i) better dimensional accuracy because inter-electrode gap (IEG) can be reduced below 0.1 mm, (ii) better process control through a change in the distribution of over potential and current efficiency over machining area, (iii) improved electrolyte flow in the IEG, and (iv) offers added opportunity for control of electrolyte conductivity distribution in the machining gap. It is proposed to utilize pulse DC supply further improve the finishing capabilities and process performances of electrochemical honing (ECH) of external gears. In the present work, existing experimental setup for ECH of helical gears has been modified for the PECH of spur gears by designing and fabricating a tooling system and machining chamber. The experimental investigations involved studying the effects of four key input parameters namely current, pulse-on time, pulse-off time, and electrolyte concentration, on the measure of PECH process performances namely percentage improvement in average surface roughness (PIRa) and percentage improvement in maximum surface roughness (PIR,,n). The Experimentation consists of three-phases: pilot experimentation, main experimentation, and confirmation experimentation. Pilot experiments were conducted to study the time dependent behaviour of PECH to fix the finishing time, to fix the electrolyte composition and electrolyte temperature by varying one parameter at a time and also some experiments were conducted to find out the range of pulse-on and pulse-off time with respect to different duty cycles. Based upon the pilot experiment results, 24 minutes as finishing time, a mixture of NaCl and NaNO3 in a ratio of 3:1, and 30°C as electrolyte temperature were fixed for the main experimentation. Based on pilot experiments, range of 1 ms to 2 ms for pulse-on time and 3 ms to 7 ms for pulse-off time has been selected. Main experimentation was designed using Box-Behnken approach of Response Surface iii Methodology (RSM) in _ which DC pulsed current, pulse-on time, pulse-off time and volumetric concentration of the electrolyte were varied in the ranges of 10-30 A, 1-2 ms, 3-7 ms and 5% - 10% respectively. Development of regression models and their statistical analysis including ANOVA, for the responses have been done with the help of Design-Expert 7.1.6 software to study the main and interaction effects of the process parameters on measures of process performances. Optimum combination of the process parameters has been found using the desirability analysis. Confirmation experiments have been conducted to validate the regression models. Micrographs are taken using Wyko NT 1100 interfaced with Vision®32 software to study the surface roughness values before and after the process. It is observed that both average and maximum surface roughness values are improving with the concentration of electrolyte. At the optimum setting of parameters, the present study shows an improvement of 79% in average surface roughness value and 84% in maximum surface roughness value. Micro-hardness tests showed that the PECH process has no significant effect on it. SEM photographs were taken before and after the experiments to identify the improvement of work surface after PECH process. SEM images have revealed that PECH produces a surface having uniform structure and free of scratches and micro-cracks.