Michael N. Morgan

Experimental investigation of heat transfer in grinding

Abstract New findings are presented for temperatures, heat flux distribution and the implications for workpiece damage and partition ratio. Workpiece temperatures were measured using a 25 u.m single pole thermocouple assembly. It was found that the critical temperature for the onset of temper colours for ferrous materials lies within the range 450 to 500 deg.C. Measured temperature distributions in the contact zone compared best with theory assuming a square law heat flux. The effective contact length for vitrified CBN and alumina wheels was confirmed to be greater than the geometric value. Substantially lower partition ratios were found with CBN compared to alumina.

The Effect of Deformation on the Contact Area in Grinding

Grinding efficiency and workpiece surface integrity are greatly affected by deflections that occur within the grinding contact zone. This paper is concerned with the effect deflections have on the real length of contact. A new relationship for the contact between the grinding wheel and the surface of the workpiece is introduced based on contact mechanics. The real contact length between the grinding wheel and the workpiece has been modelled based on the theory for cylinders in contact including the effect of the surface roughness of the contact faces. A second formulation is presented which takes account of the contact area at the micro level of the grains. The new model more accurately describes the mechanics of grinding contact than previous contact models. Application of the new model to published experimental data for plunge surface grinding operations explains why measured contact Application length can be 50% - 200% greater than the geometric contact length.

A Simplified Approach to Control of Thermal Damage in Grinding

The critical factors for the control of thermal damage in grinding at conventional workspeeds have been established with reference to experimental and previously published work. For ferrous materials, significant damage occurs above a maximum workpiece background temperature of 475°C. It is also known that the energy entering the workpiece is reduced due to conduction into the grinding wheel. It has been found that the partitioning of energy between the grinding wheel and the workpiece remains approximately constant. However, the overall partition ratio to the workpiece, which takes account of energy transfer to the chips as well as energy transfer to the wheel, is variable. The effective thermal properties of the grinding wheel may be established by correlating theory with grinding experiments. An effective coefficient for the temperature equation can be obtained corresponding to the use of the geometric contact length in the equation. Using these conclusions, a simplified approach has been developed for control of thermal damage.

An Advance in the Modelling of Thermal Effects in the Grinding Process

The proportion of the grinding energy entering the workpiece may be analysed either for the whole grinding wheel-workpiece contact zone or for the average grain contact zone which is two orders of magnitude smaller. An analysis which does not clearly apply to one zone or the other introduces conceptual difficulties since the relative speed seen by the workpiece is very different for the two cases. At the grinding zone level the workpiece sees a relative speed of vw whereas at the grain level the workpiece sees a relative speed of vs +/- vw. An analysis has been presented for partitioning at the grain level which overcomes these conceptual problems. Results from the new model are compared with results from other models.

Validation of Thermal Properties in Grinding.

Abstract Thermal properties of the abrasive material are required for energy partitioning and prediction of temperatures in grinding. Alumina and cubic boron nitride wheels are investigated by several methods. A novel sensor was designed to measure bulk thermal property. It is shown that the effective thermal properties exhibited in the grinding process are lower than the values measured directly. It is therefore concluded that a grain model is more appropriate than a bulk property model. A case study based on grinding AISI 52100 with cbn and alumina is used to illustrate the sensitivity of the most significant parameters for fine grinding.

Effective Thermal Properties of Grinding Wheels and Grains

Abstract The thermal properties of the grinding wheel are required for energy partitioning in grinding. This paper describes an investigation of the effective thermal properties of alumina and cubic boron nitride (CBN) grinding wheels. Results are presented for a novel sensor that was designed to measure the bulk thermal properties of grinding wheel samples. The effective bulk thermal properties of the grinding wheel and the effective thermal properties of the abrasive grains were also investigated. It was found that the bulk thermal property is dominated by the properties of the bond and does not account for the improved thermal performance of CBN compared with alumina. Values of the effective thermal conductivities for alumina and CBN abrasive grains are therefore proposed. It is concluded that the effective thermal conductivity of the grains is best obtained inversely from grinding experiments.

Process Requirements for Cost-Effective Precision Grinding

Costs in precision cylindrical grinding are compared for different abrasives, machines and grinding conditions. The analysis is for repeated batch production. Account is taken of machine cost and abrasive cost. Cost comparisons were based on extensive trials to assess re-dress life against workpiece quality requirements. Experiments show that different workpiece materials require different strategies to reduce costs. Easy-to-grind AISI 52100 and difficult-to-grind Inconel 718 materials were ground at conventional speeds and at high speeds. It is shown that wheel speed affects production rate through acceptable values of re-dress life, removal rate and dwell time. Advantages were gained using vitrified CBN at conventional speed and at high speed. For both materials, vitrified CBN wheels used at high speed, gave better quality at lower cost than conventional abrasives. Wheel costs became negligible and labour costs greatly reduced. Re-dress life trials, usually neglected, are shown to be essential to reduce costs and maintain quality [1].

The Effect of Porosity on the Grinding Performance of Vitrified CBN Wheels

In grinding with vitrified cubic boron nitride (CBN) wheels, frequent dressing should be avoided. It is expensive in time and also in consumption of an expensive a brasive. Wheel loading is a main cause of wheel redressing especially for difficult-to-g rind materials such as Inconel 718, and must therefore be prevented. One solution to prevent loading is to empl y an open wheel structure in order to help coolant delivery to the grinding zone and provide more space for hip-flow and swarf removal. The grinding performance of high-porosity CBN wheels i s compared with that of a medium-porosity CBN grinding wheel. Analysis and discussion of perform ance is presented in terms of observations and measurements of the changes in wheel topography. Introduction CBN is two orders of magnitude more expensive than conventional abrasive s. The justification for CBN therefore relies not only on the benefits of improved quality and a ccur cy, but also on reduced costs due to the potential for extended redress life and increased removal rates which must more than offset the increased cost of the wheels, the increased dr essing time and the need for initial wheel conditioning to open up the surface of the wheel. Too frequent re-dr essing and large dressing depth negates the cost benefits and may even close up the wheel. A pr oblem is, that wheel loading especially for ductile workpiece materials creates the need f or large dressing depth to clean up the wheel surface and more frequent wheel dressing. Therefore, it is important that wheel loading should be prevented in order to lengthen the wheel redress life. Westkämpfer[1] reported problems in grinding due to insufficient pores i n plated CBN wheels. Onchi [2] proposed vitrified CBN wheels with giant pores to overcome the problem of wheel loading in super-finishing. Initial experiments with a B91 CBN wheel for internal grindi ng to achieve surface roughness of 0.25 micron Ra led to problems with wheel loading and completely unsatis factory grinding results. It was only after the fluid delivery system was replaced wi th a higher pressure, 4 bar multi-jet system that satisfactory results were achieved. The fluid delivery sys tem i illustrated in Fig. 1[3]. Experiments were performed with an easy-to-grind material, M2 tool s ee , with hardness of HRC 60-62 and with a difficult-to-grind material, Inconel 718, with hardne ss of HRC 42-43 to determine whether the conclusions reached might be valid for both situ ations. Comparisons of grinding performance were made according to grinding power, whee l redress life, G-ratio and workpiece roughness. Based on microscopic observation and measurements of the grinding wheel surface, the grinding performance with the different wheel structures is dis cussed. Key Engineering Materials Online: 2003-04-15 ISSN: 1662-9795, Vols. 238-239, pp 295-300 doi:10.4028/www.scientific.net/KEM.238-239.295

Measurement of the air boundary layer on the periphery of a rotating grinding wheel using LDA

In this paper, the velocity profile of the air boundary layer around a rotating grinding wheel was measured using the Laser Doppler Anemometry technique. Experimental results show that the tangential velocity of the air decreases greatly with increasing distance from the wheel surface. The distribution of the tangential velocity is also found to be almost uniform near to the centre of the wheel width, and decreases greatly as the wheel edge is approached. Generally, the radial velocity of air in the area close to the wheel surface is small, and then increases with the increasing distance from wheel surface.

The Real Contact Length in Grinding Based on Depth of Cut and Contact Deflections

Workpiece surface integrity is greatly affected by deflections within the grinding contact zone. A new relationship for the contact length between the grinding wheel and the surface of the workpiece is introduced based on contact mechanics. The model is based on the geometric contact length and also on the theory for cylinders in contact and the plastic deformation of the surface layer of the workpiece at the micro level of the grain contact. The new model suggests that the real contact length is typically 60% — 150% greater than the geometric contact length due to contact deflections, compared with 50% — 200% from measured values.