Investigations on manufacturing of high quality meso-sized bevel and helical gears by wire spark erosion machining process

Abstract

Miniaturization of products, equipment and devices has become an essential requirement globally due to increasing costs of the materials and manufacturing processes and emphasis on making the products smaller, lightweight, and compact. Gears having addendum or tip diameter in a range from 1 to 10 mm are referred as meso-gears. Different types of mesogears such as meso-spur gear (MSG), meso-helical gear (MHG), and meso-bevel gear (MBG) are primarily used for the purpose of actuation, positioning, and motion transmission at very high speed in the miniaturized products, equipment and devices, and micro electro-mechanical systems (MEMS). MSG and MHG are used between the two parallel shafts and MBG between two intersecting shafts. They offer many worthmentioning advantages such as compactness due to smaller size, lightweight, higher dimensional accuracy, zero backlash, lower energy consumption, efficient transmission, superior operating performance and ability to perform under extreme environmental conditions. Their typical applications include meso-sized gearboxes, pumps and motors,actuating devices, scientific instruments, precision tools, smart watches, digital camera, tail and main rotors of meso-sized unmanned aerial vehicles (UAV), medical and dental instruments, precision instruments, prototype models, and domestic appliances (Gupta and Jain, 2014a). Most of these applications require quieter and smoother operating performance, more efficient and accurate transmission of motion, higher wear and corrosion resistance, increased fatigue strength, and longer service life. These characteristics are governed by overall quality of the meso-gears which is determined by their microgeometry, macrogeometry, surface roughness, surface integrity, and noise, vibration and wear related characteristics. These aspects in turn depend on material, manufacturing, finishing, and surface treatment of the meso-gears. Material of the meso-gears should have higher yield and fatigue strength, higher resistance to friction, wear and corrosion, better manufacturability (i.e. machinability, formability, and liquidity), less cost and easier availability. Brass, bronze, copper, aluminum, stainless steel, low alloy steel, and polymers are commonly used materials. Stainless steel has higher resistance to wear and corrosionand performs satisfactorily for longer period in a corrosive environment (Gupta and Jain 2016). Traditional manufacturing processes of meso-gears can be classified into three categories: (i) subtractive processes namely hobbing and milling; (ii) formative processes such as injection molding die casting, powder metallurgy, and lithography; and (iii) deformative processes which include extrusion, cold rolling, forging, stamping, and hotembossing. Unfortunately, meso-gear manufactured by these processes have poor quality ranging from 9-12 in Deutsches Institut für Normung (DIN) standard, tool marks on their flank surfaces, sharp edges, burrs, poor edge definition and high geometrical and dimensional inaccuracy. These processes also have certain manufacturing limitations with regards to shape, size and materials of the meso-gears. This necessitates subsequent finishing process such as grinding, lapping, honing, shaving, skiving and burnishing to achieve the desired quality of the meso-gears and surface hardening, work hardening and appropriate coating processes to enhance their wear resistance, corrosion resistance, and fatigue strength (Gupta et al., 2017).All these factors have motivated the researchers to explore technically superior, economically viable, material and energy efficient manufacturing process for the mesogears. Wire spark erosion machining process (WSEM) has potential to overcome the limitations of traditional manufacturing processes of the meso-gears. It is an electro-thermal type advanced machining process which uses a very thin wire as tool, deionized or distilled water as dielectric and very high pulse frequency (in the order of MHz). Despite of many advantages of WSEM process for manufacturing of meso-gears, very limited work has been reported in this area. Moreover, most of the past work only focused on manufacturing of external MSG analysing their surface finish (Ali and Mohammad, 2008), microgeometry, macrogeometry, and cutting rate (Gupta and Jain, 2014a and 2014b). Very limited work has been reported on using μ-WSEM process to manufacture meso-ratchet wheel (Benavides et al., 2002) and internal dies and micro-gears (Di et al., 2006), and multi-response optimization for manufacturing meso-gears by WSEM process (Gupta and Jain, 2014a and2014b). Bouquet et al. (2014) have concluded that manufacturing of helical gears and correction in their profile are very difficult by WSEM process. Consequently, present research work was undertaken with following objectives to bridge the identified research gaps using experimental investigation depicted in Fig. 1:  To establish WSEM process to manufacture high quality MBG and MHG.  To study the effects of WSEM process parameters on microgeometry parameters, average and maximum surface roughness and volumetric gear cutting rate of MBG and MHG so as to identify optimum values of WSEM process parameters.  To study microstructure and microhardness of the best quality MBG and MHG manufactured using the identified optimum parameters of WSEM process.  To develop models of the considered responses for the MBG and MHG. Multi-response optimization of the WSEM parameters to simultaneously optimize the conflicting responses of MBG and MHG.  Experimental validation of the optimized results.  Comparative evaluation of WSEM with milling process for manufacturing MBG and with hobbing process for manufacturing MHG to prove technical superiority and economic viability of WSEM process over them.