Investigations on finishing of spur and straight bevel gears by abrasive flow finishing process

Abstract

Abrasive flow finishing (AFF) is an advanced high-quality finishing process that involves flow of a finishing medium in the form an abrasive laden viscoelastic self-deformable putty though the internal surfaces or over the external surfaces of a component to be finished. This putty is filled in two oppositely placed cylinders and is extruded back and forth through a passage formed by combination of the workpiece and its suitably designed fixture (for finishing the external surfaces i.e. external gears, impellers, turbine blades, biomedical implants, etc.) or by the workpiece itself (for finishing internal surfaces i.e. internal gears, cylinder, bushes, sleeves, etc.). It causes shearing off the roughness peaks from the workpiece surfaces. More aggressive finishing is done where restriction to flow of the putty is more. Hydraulic unit or mechanical means can be used to provide the required extrusion pressure between the cylinders containing the AFF medium. Such working principle enables the AFF process to smoothen sharp corners, remove burrs and recast layers, and impart mirror-like finish (surface roughness less than 50 μm) to an inaccessible or difficult-to-reach surfaces of a complicated component made of any difficult-to-machine material. Gear is a modified form of a wheel containing teeth on its outer or inner periphery. It is the most commonly used mechanical component for positive and accurate transmission of power and/or motion in the machines and/or equipment used in avionics, automobiles, machine tools, electrical machines, defense, construction, agricultural, food processing, and other industries. Gear quality defines the level of errors in microgeometry of a gear. Deutsches Institut für Normung (DIN) standard is the most commonly used international standard to express gear quality in a range from 1-12 with lower value indicating better gear quality. More than 10 billion gears are manufactured and consumed annually worldwide. Such a huge demand forces the gear manufacturers to focus on producing light-weight, highquality gears economically which gives low running noise, possess higher wear and fatigue resistance, higher transmission efficiency, and enhanced service life (Karpushewski, 2008). Fulfillment of these expectations require reducing surface roughness, improving surface integrity (i.e. surface morphology, microhardness, wear resistance, fatigue resistance), improving gear quality by reducing errors in microgeometry (i.e. error related to profile, lead, pitch, radial runout, and flank topography), functional performance parameters (i.e. total composite error, tooth-to-tooth composite error, radial runout, transmission error, profile conjugacy), and levels of noise and vibrations of a gear by an appropriate finishing process. In this context, AFF process has great potential to fulfill expectations of good gear finishing process but only three references are available in the literature. Xu et al. (2014) used AFF for finishing the helical gears and obtained 84%; 80%; and 35% improvement in the average surface roughness values measured along the profile, and along the lead on left hand and right-hand flank surfaces. Venkatesh et al. (2014, 2015) explored AFF and ultrasonic assisted AFF (UA-AFF) for improving surface finish of a straight bevel gear and achieved reduction in average surface roughness as 55% by AFF and 73.1% by UA-AFF respectively. Following are the identified research gaps from review of the relevant past work on gear finishing by AFF process:  Past work focused using AFF to reduce only surface roughness of helical and straight bevel gears made of EN8 steel.  No work reported on reducing microgeometry errors of straight bevel gears and on reducing maximum and average surface roughness and microgeometry errors of spur gears by AFF process.  No work attempted for finishing of 20MnCr5 alloy steel gears by AFF process which is most commonly used for commercial manufacturing of spur and bevel gears.  No work done to identify optimum parameters of AFF process i.e. extrusion pressure, abrasive particle size, concentration of abrasive particle, concentration of the oil and finishing time.  No work reported on improvement of surface morphology, microhardness and wear characteristics of spur and straight bevel gear finished by AFF.  No work reported on reducing errors in functional performance parameters and noise and vibrations of spur and straight bevel gear by AFF.  No work has been done on using laser texturing to improve performance and productivity of AFF process for gear finishing. Consequently, following research objectives were identified to bridge the identified research gaps using the research methodology depicted in Fig. 1:  To develop a robust experimental apparatus and fixtures for finishing spur sand straight bevel gear by AFF. The developed fixtures should sustain high extrusion pressure and guide the AFF medium to reduce surface roughness, errors in microgeometry and functional performance parameters of spur and straight bevel gears.  To study effect of AFF processes parameters on reduction of microgeometry error and surface roughness of spur and straight bevel gears and identify optimum parameters. To study surface morphology, microhardness and wear resistance of the spur and straight bevel gears using the identified optimum parameters.  Multi-response optimization of AFF parameters to simultaneously optimize considered responses of spur gear and straight bevel gear and their experimental validation.  Studying role of laser texturing in improving finishing performance and productivity of AFF process by a comparative study of AFF of laser textured and untextured spur and straight bevel gears in terms of microgeometry errors, surface roughness, surface morphology, wear resistance, microhardness, and MRR.  Study on reducing errors in functional performance parameters, and noise and vibrations of spur and straight bevel gears by finishing them by AFF process.