Investigations on near-net shape manufacturing of elliptical helical gears by wire spark erosion machining process

Abstract

Non-circular gears (NCG) gears have teeth placed on circumference of a non-circular shape. NCG have different characteristics than the circular gears. In addition to power transmission, they can reduce unwanted torque, enhance the required torque, produce nonuniform rotation, generate the special motion required in control and automation mechanisms, convert constant input speed to variable speed, and generate start-stop-dwell type motion. Different types of NCG are used for different applications such as flow meters, chain transmission systems, transmissions of bike plates, oil pumps, chain belts of combustion engines, agriculture machineries such as trans-planters and fertilizer distributors, step-less shift components for instruments or vehicles, slow speed and heavy load fields, replacement of cam and linkages, and for generation of nonlinear relation between the pinion and the driven gear. Application areas of NCG are still being explored because existing ways of their manufacturing are limited, very difficult, inaccurate, time consuming, require high technical expertise, and are not suitable for their mass production. Many more practical application areas can be generated by developing or exploring an effective manufacturing process of NCG manufacturing. Helical teeth on a gear provide silent, efficient and accurate operation to gears. Provision of helix angle to the teeth of gears enables them to engage gradually making their performance quieter and smoother as compared to straight teeth gears and therefore, they can be used for higher speed power transmission. To provide these properties in NCG, their teeth have to be made inclined. NCG due to their complicated shapes already difficult to manufacture and provision of teeth angle, makes their manufacturing more complex. Consequently, this research explores WSEM process for manufacturing of NCG having straight and inclined teeth on an elliptical blank, non-circular bevel gears and non-circular bevel-helical gears. Elliptical helical gears (EHG) are NGC which have inclined teeth on an elliptical gear blank. They have comparatively more practical applications particularly in the mechanisms which require irregular rotational motion i.e. pumps, bicycle chain drives, steering mechanism, and packaging and conveyor applications. Therefore, prime focus the present research is detailed investigations on manufacturing of EHG from 7.5 mm thick stainless steel 304 plate by WSEM process. Experimental investigation was designed and conducted in four stages. Twenty exploratory experiments were conducted during the 1st stage of experimentation to assess feasibility of WSEM process to manufacture macro-sized and meso-sized non-circular gears of different types namely elliptical spur gear (ESG), elliptical helical gear (EHG), and elliptical bevel gear (EBG). Eighteen experiments were conducted in stage-2 using Taguchi method of design of experiments to study effects of non-electrical parameters (parameters related to wire, dielectric and cutting speed such as wire tension, wire feed rate, dielectric pressure, and cutting speed overwrite) of WSEM process on the considered responses i.e. maximum and average surface roughness (Rmax and Ra) of gear teeth flank surfaces, gear cutting time (Tc) and material loss (Lm) so as to identify their optimum values. Thirty experiments were conducted in Stage-3 using central composite design of response surface methodology to study influence of electrical parameters (i.e. pulse-on time, pulse-off time, servo voltage, peak current) of WSEM process on the considered responses parameters and to identify their optimum values. Grey relational analysis (GRA) and analysis of variance (ANOVA) of the experimental results were used to optimize the considered parameters of WSEM process Finally, confirmation experiments were conducted which confirmed the optimized results. AutoCAD and ELCAM software were used for modelling and partprogramming of all the elliptical helical gears manufactured by WSEM during all the experiments. Topography of flank surface, recast layer formation, heat affected zone and meshing were studies for the best quality EHG (having minimum values of the considered responses) manufactured by WSEM process. This research work proves that WSEM is a suitable process for near-net shape manufacturing of EHG. The process is non-toxic, user-friendly, flexible, and easy-tooperate process which can be easily adopted by industries for mass production of EHG.