Effects of Abrasive Grit Shape on Grinding Performance

The quality of grit in grinding wheel has predominate influence on the grinding wheel performance, such as wheel sharpness and wheel wear. This paper presents an investigation on the effect of difference grit shapes on grinding force and grit holding capacity. Some critical grinding behaviours are analysed in relation to grit shapes to establish a foundation for grit quality assessment. A desirable grit shape is identified for better cutting efficiency and grinding wheel life

An experimental study of the effects of dressing parameters on the topography of grinding wheels during roller dressing

Vitreous-bonded grinding wheels are widely used for machining features on aerospace components achieving high material removal rates under high pressure coolant. Dressing is a vital stage in the grinding process to ensure a consistent wheel topography and performance. However, the effects of roller dressing on functional performance of vitreous grinding wheels as well as its influence on different abrasive grit morphologies have not been fully characterised. This paper studies the influence of dressing parameters on the topography, morphology and characteristics of the surface of different vitrified abrasive wheels in order to better understand the process and therefore optimise the preparation of grinding wheels for industrial machining. Alumina grinding wheels with conventional and engineered grit shapes were dressed at two different infeed rates over a range of seven different speed ratios (from −0.8 to +1). An experimental methodology has been developed incorporating a range of known techniques to define the abrasive wheel condition including measured power consumption and ground graphite coupons as well as using optical microscopes to measure grain fracture flats, peak density and abrasive grain shape. It has been found that power consumption of the grinding wheel spindle increases at higher infeed rates and speed ratios. This leads to increased fracturing of the grains and whole-grain pull out. According to the results the infeed rate has a more substantial effect on wheel topography than speed ratio and the response of engineered grit morphologies to dressing is dependent on grit orientation

Single Grit Grinding Simulation by Using Finite Element Analysis

In this research, basic material removal characteristics in a single grit grinding have been investigated by using Finite Element Analysis (FEA). ABAQUS/Standard is used as a computational environment. The influences of both friction and undeformed chip thickness are considered in the analyses of the grit ploughing, stress distribution and total force variation. Remeshing strategy is performed in the simulation to produce very fine meshes in the contact area to mitigate the material distortion due to large plastic deformation. The results show that the increase of undeformed chip thickness and frictional coefficient would increase ploughing action and grinding stress magnitude. Moreover, friction would cause the stress distribution circle on grit inclined backwards. Finally, FEM analysis can be considered as a strong tool for the single grit simulation of grinding process. ©2010 American Institute of Physic

On modelling and experimentation of planar grinding using abrasive pads with grits of defined geometries and distributions for enabling controlled surface texture

The surface of diamond grinding tool manufactured by current production method exhibits stochastic nature in terms of different grit size, irregular geometry, random crystallinegraphic orientation, non-uniform spacing, and varying protrusion heights. These topography characteristics has been found having various negative effects for the abrasive performance, especially in high-precision applications, causing less controllable surface topography, inefficient chip flow, high working temperature and cutting force. The thesis covers the design and manufacture of laser generated novel abrasive pad with ordered abrasive grits from CVD polycrystalline diamond films, providing repeatable patterns and shapes for abrasive tool used in load controlled plane grinding process, which is regarded as engineered abrasive pad. Two major design parameters for engineered abrasive pad, including abrasive grit geometry and planar contact area between grits and workpiece surface, were investigated. An inverted test setup was developed to enable the evaluation for the performance of ordered abrasive pad under plane grinding operation. The analysis of material removal and surface topography was made and compared between conventional abrasive pad (randomly shaped grits) and two engineered abrasive pad (saw tooth and square frustum) to evaluate the influence of grit geometry on the grinding performance. Furthermore, a geometrical model was developed to simulate the surface roughness generated by engineered abrasive pad. Inverted tests were conducted allowing the validation of the model. Besides grit geometry, three abrasive pads (one conventional, two engineered) with different planar contact were also studied through inverted grinding test to evaluate the influence of planar contact area on grinding performance. An improvement of design for better material removal (31.5% in Chapter 7) was achieved by adjusting the planar contact area of engineered abrasive pad. It was found that laser generated abrasive pad can provide not the only superior surface finish, but also introduce less damage to the workpiece surface. More efficient chip flow and very little tool wear leading to a longer tool life was also observed. In particular, the engineered grits with symmetric shape (square frustum) exhibits superior performance over conventional grits (randomly shaped) and asymmetric grits (saw tooth). A good agreement between simulation and experimental results was found for surface roughness prediction, therefore provide good initial results for numerically studying engineered abrasives under planetary abrasive machining processes. Moreover, the evaluation of planar contact area also shows laser generated abrasives can provide significant advantages in material removal when designed with comparable planar contact area as conventional abrasive pad under specific applications, particularly when the abrasive contact geometry is designed to provide clearance in the cutting directions. Combining the findings above, a preliminary benchmark methodology was proposed for design of engineered abrasive pad, which enables the future optimization of engineered abrasive tool

Development of Multi-grit cBN Grinding Wheel for Crankshaft Grinding

A crankpin, part of a crankshaft, has a complex profile that is difficult to grind. The process often causes challenges such as excessive heat on the crankpin sidewall and wheel wear on the radius, causing reduced dressing interval. Different solutions were proposed to overcome these challenges, mainly focusing on the process, i.e. grinding strategies. However, the work presented in this thesis is concerned with optimising the superabrasive grinding wheel.A novel analytical assessment framework was developed for evaluating grinding wheel performance that can account for the effects of grit properties and dressing conditions on the wheel topography and, in turn, grinding performance. Based on the model of cutting and sliding grinding force components, a set of performance indicators were derived and then used to evaluate the effect of the wheel topography on the grinding process. Results showed that grit toughness, thermal stability, size and concentration affect the intrinsic specific grinding energy via grit protrusion and sliding component via wear flat area. On the other hand, the grit shape only affects the wear flat area but maintains the intrinsic specific grinding energy regardless if the grit has a higher or lower aspect ratio (blockier or elongated). To complement grinding performance information, wear was evaluated via grinding and lapping tests. The analyses revealed that wheels containing grits with a higher aspect ratio (elongated grits), lower toughness, lower concentration, or smaller size generate lower grinding forces; however, they wore faster. On the other hand, wheels featuring grits with a lower aspect ratio (blocky grits), higher toughness, higher concentration or coarser grit had the opposite effect. They generated higher forces and wore slower, exhibiting longer tool life. Findings from laboratory-based trials resulted in two crankpin wheel designs. One aimed to reduce heat generation, while the other targeted less wheel wear. Industrial tests at the end user demonstrated that the favourable design contained elongated and smaller grits at a lower concentration, because it reduced heat generation despite the higher wheel wear. This was confirmed via the Barkhausen noise measurements, which showed a 20% reduction in intensity compared to the reference wheel and a 30% reduction in intensity compared to the wheel design containing blockier and larger grit at higher concentration

A lapping-based test method to investigate wear behaviour of bonded-abrasive tools

Grinding-wheel wear is a critical factor affecting grinding performance and tool cost. Unfortunately, wear tests – particularly with superabrasives – can be notoriously time-consuming. Therefore, a novel lapping-based method is proposed for investigating wear behaviour of the grit-bond system. Wear tests were performed in (i) lapping, (ii) surface grinding, and (iii) cylindrical grinding for a range of grit-shape aspect ratios and grit-toughness values for the same grit-bond systems. Results showed that all three methods yielded similar trends. This indicates that the lapping tests could be a viable substitute for lengthy grinding tests, resulting in shorter testing times and smaller specimen sizes

Study of Micro Rotary Ultrasonic Machining

Product miniaturization for applications in fields such as biotechnology, medical devices, aerospace, optics and communications has made the advancement of micromachining techniques essential. Machining of hard and brittle materials such as ceramics, glass and silicon is a formidable task. Rotary ultrasonic machining (RUM) is capable of machining these materials. RUM is a hybrid machining process which combines the mechanism of material removal of conventional grinding and ultrasonic machining. Downscaling of RUM for micro scale machining is essential to generate miniature features or parts from hard and brittle materials. The goal of this thesis is to conduct a feasibility study and to develop a knowledge base for micro rotary ultrasonic machining (MRUM). Positive outcome of the feasibility study led to a comprehensive investigation on the effect of process parameters. The effect of spindle speed, grit size, vibration amplitude, tool geometry, static load and coolant on the material removal rate (MRR) of MRUM was studied. In general, MRR was found to increase with increase in spindle speed, vibration amplitude and static load. MRR was also noted to depend upon the abrasive grit size and tool geometry. The behavior of the cutting forces was modeled using time series analysis. Being a vibration assisted machining process, heat generation in MRUM is low which is essential for bone machining. Capability of MRUM process for machining bone tissue was investigated. Finally, to estimate the MRR a predictive model was proposed. The experimental and the theoretical results exhibited a matching trend

Characterisation of the relationship between surface texture and surface integrity of superalloy components machined by grinding

The surface texture of a machined component is influenced largely by the processing parameters used during machining and hence, there is a relationship between both the formation of the surface texture and surface integrity of the machined component. In the study to be reported in this paper, GH4169, a hard-to-cut superalloy, widely used in aero-engines, was selected for a detailed investigation into the relationship between the surface texture and the component-performance (surface integrity) of the machined components for which a series of grinding experiments with different grinding-wheels and grinding parameter-values was carried out in order to quantitatively analyze variations of the surface roughness with processing parameters. Further, considering that the features of the ground-surfaces measured are of a random nature, statistic properties of the produced surfaces were revealed and characterised with power spectral density function (PSD) and auto-covariance function(ACV) method respectively

Direct Laser Fabrication of a Gas Turbine Engine Component - Microstructure and Properties - Part I

This paper presents the development of a new technique for the production of abrasive turbine blade tips by direct laser processing. This superalloy cermet component is an integral part of the low pressure turbine sealing system in a demonstrator engine. Direct laser fabrication of this component fiom a bed a loose powder results in significant cost savings and improved performance over the currently employed production technique. The technology has been demonstrated by fabricating a prototype lot of 100 blade tips, which will be subjected to an engine test. This is the first instance of a direct fabrication method applied to the production of functional engine hardware. This research was funded by the United States Air Force contract F33615-94- C-2424 titled "Affordable Turbine Blade Tips".Mechanical Engineerin