Roughness due to workpiece wear generated in surface grinding of metals

Assuming the grinding wheel surface to be fractal in nature the maximum envelope profile of the wheel and contact deflections are estimated over a range of length scales. This gives an estimate of the ‘no wear' roughness of a surface ground material. Four test materials, aluminium, copper, titanium and steel are surface ground and their surface power spectra estimated. The departure of this power spectra from the ‘no wear' estimates is studied in terms of the traction induced wear damage of the surfaces.     

Influence of surface roughness effects on the performance of non-recessed hybrid journal bearings

The effect of journal and bearing surface roughness on the performance of a capillary compensated hole-entry hybrid journal bearing system has been theoretically studied. The analysis considers the average Reynold’s equation for the solution of lubricant flow field in the clearance space of a rough surface journal bearing system. The finite element method and Galarkin’s technique has been used to derive the system equation for the lubricant flow field. The non-dimensional parameters Λ (surface roughness parameter) and γ (surface pattern parameter) have been defined to represent the magnitude of height distribution of surface irregularities and their orientation, respectively. The influence of surface roughness on the bearing performance has been studied for the transverse, isotropic and longitudinal surface patterns. The bearing performance characteristics have been computed for both symmetric and asymmetric capillary compensated hole-entry journal bearing configurations for the various values of surface roughness parameter (Λ), surface pattern parameter (γ) and restrictor design parameter ( ). The computed results indicate that the inclusion of surface roughness effects in the analysis affects the performance of a bearing quite significantly vis-à-vis smooth surface bearing. The study indicates that for generation of accurate bearing characteristic data, the inclusion of surface roughness effects in the analysis is essential.     

Studies on friction and transfer layer using inclined scratch

Friction influences the nature of transfer layer formed at the interface between die and sheet during forming. In the present investigation, basic studies were conducted using ‘Inclined Scratch Test’ to understand the mechanism of transfer layer formation during sliding of pins made of an Al–Mg alloy on EN8 steel flats of different surface roughness under dry and lubricated conditions. The surfaces produced can be categorized into three different types: (a) uni-directional (b) 8-ground and (c) random. Rubbing the EN8 flat in a uni-directional manner and a criss-cross manner on emery sheets produced the uni-directional and 8 ground surfaces. The random surfaces were produced by polishing the EN8 flats using various abrasive powders. The influence of the ‘nature of surface roughness’ on material transfer and coefficient of friction were investigated. Scanning Electron Microscopy studies were performed on the contact surfaces of the Al–Mg alloy pins and EN8 steel flats to reveal the morphology of the transfer layer obtained. It was seen that the transfer layer is dependant on the coefficient of friction. The coefficient of friction, which has two components—the adhesion component and the plowing component, is controlled by the ‘nature of surface’. A surface that promotes plane strain conditions near the surfaces increases the plowing component of friction.     

Friction variation in the cold-rolling process

Friction variation along the rolling contact interface was measured by a ‘sensor roll’ embedded with sensors on an experimental rolling mill. The measured results show that the friction coefficient is not constant along the contact length. Empirical formulae describing the friction variation for certain rolling condition were obtained. Experiments were also carried out to study the surface roughness of the strip. It was found that the roll and strip surface roughness can be transferred to each other along the rolling direction. Only slight change of roughness in the transverse direction was observed.     

A new chip-thickness model for performance assessment of silicon carbide grinding

The chip-thickness models, used to assess the performance of grinding processes, play a major role in predicting the surface quality. In the present paper, an attempt has been made to develop a new chip-thickness model for the performance assessment of silicon carbide grinding by incorporating the modulus of elasticities of the grinding wheel and the workpiece in the existing basic chip-thickness model to account for elastic deformation. The new model has been validated by conducting experiments, taking the surface roughness as a parameter of evaluation .     

Preliminary investigation of the effect of roughness in Dynarat simulation

Surface roughness has a significant effect on how loads are transmitted at the contact interface between solid bodies. It causes high local pressures in the contacting roughness peaks, i.e., asperities. Even for a low friction coefficient the surface roughness will still play an important role in the early surface wear. A dynamic ratcheting model (Dynarat) for studying the wear rate of ductile materials in rolling/sliding contact is presented. The material is divided into equal-sized rectangular elements (or ‘bricks’). Each material brick accumulates plastic shear strain when the orthogonal shear stress exceeds the brick's shear yield stress. When the accumulated plastic strain exceeds a critical value, the ductility is exhausted and the material is deemed to have failed. Inclusion of surface roughness and refinement of the near-surface brick size cause earlier failure of bricks very close to the surface. In order to model surface roughness, brick size needs to be reduced to, at most, a few microns. The purpose of this investigation is to study the effect of surface roughness in the Dynarat model by comparing the model prediction with the results from two different rolling sliding wear testing machines. Further development of the model is needed, such as more inclusive representations of microstructural behaviour. In addition to that, the ratcheting equation, which drives the Dynarat model, needs to be improved to cover other rail materials and more loading configurations.     

Development of a hybrid particle swarm optimization algorithm for multi-pass roller grinding process optimization

In the field of metal rolling, the quality of steel roller’s surface is significant for the final rolling products, e.g., metal sheets or foils. The surface roughness of steel rollers must fall into a stringent range to guarantee the proper rolling force between the sheet and the roller. To achieve the surface roughness requirement, multiple grinding passes have to be implemented. The current process parameter design for multi-pass roller grinding mainly relies on the knowledge of the experienced engineers. This always requires time tedious “trial and error” and is insufficient to work out cases: (1) multi-pass with complex interaction for one pass with its neighboring passes; (2) large number of process parameters setup; (3) multiple process objectives and constrains. In this paper, a process planning method for multi-objective optimization is proposed with a hybrid particle swarm optimization while incorporating the response surface model of the surface roughness evolution. The hybrid particle swarm optimization regards the entire grinding process parameters (from rough grinding, semi-finish grinding, finish grinding to spark-out grinding) as a whole, and realizes the parameter optimization by considering multiple objectives and constrains. The establishment of the response surface model of surface roughness evolution is capable to incorporate the inter-correlation of neighboring passes into the multi-pass parameter optimization. Finally, the experimental verification was implemented to verify the effectiveness of the proposed method. The error between predicted roughness and experimental roughness is less than 16.53%, and the grinding efficiency is improved by 17.00% compared with the empirical optimal process parameters.     

Physical nature and properties of dynamic surface layers in friction

Simulations of the dynamic processes in micro contacts with the Method of Movable Cellular Automata (MCA) show that their common feature is formation of a boundary layer where intensive plastic deformation and mixing processes occur. The boundary layer is well localized and does not spread to deeper layers. We call this layer a ‘quasi-fluid layer’. The thickness of the boundary layer is roughly proportional to the viscosity of solid. This parameter thus should play an important role in determining the wear rate of materials in friction.To better understand the physical nature of the dynamic surface layers, we consider a simplified model of a solid consisting of many thin sheets, interacting with each other according to a ‘friction law’ of Coulomb type. A quasi-fluid layer is always developing if the ‘friction law’ does allow a bi-stability in some range of stresses with one static and one dynamic state at the same stress.The existence of the boundary layer motivates us to change the existing approach to calculating wear in frictional contacts. The wear should be understood not as ‘fracture’ but as ‘mass transport out of friction zone’. The process of stochastic transport of wear particles in the closed friction zone is at the same time the main mechanism of development of surface topography.A very important fact is that the conditions for appearance of a quasi-fluid layer depend on the minimal size of structural elements of the medium, which means that this effect cannot be principally described in the frame of a continuum model.     

Demonstration of topography modification by friction processes and vice versa

Contact formation and development are the basis of friction and wear modelling and understanding. Unanimously topography formation and development in friction contacts are regarded of highest importance for understanding and modelling friction processes. The frequently found running in behaviour of sliding contacts is—aside from the build up of reaction and transfer layers-at least partly caused by the topography development due to friction processes until a stable equilibrium state is reached.Experimental results of friction and topography measurements are presented which demonstrate the mutual modification of friction and contact topography.A special experimental set up with an AFM allowed to correlate the measured friction forces with the contact position and the topography at this point. In this way, friction force transitions and changes can be assigned to topography changes due to abrasion, adhesion and wear particle agglomeration.Contact surfaces with artificial regular structures have been prepared to avoid problems with topography and friction correlation due to the statistical nature of roughness on technical contact surfaces. The friction effects of roughness were simulated by etched ditches of defined width, depth and distance on silicon or metal surfaces. This allowed to explain the mutual influences of topography and friction. The effect of a single ‘asperity’ and of the ‘roughness structures’ could be demonstrated.Topography measurements with an AFM correlated with the friction force could help to understand friction changes without changing any parameter.     

Producing gear teeth with high form accuracy and fine surface finish using water-lubricated chemical reactions

A new method was investigated for high-accuracy fine finishing of gear teeth surfaces using a water-lubricated tribo-chemical technique. A pair of shaved gears with rather low surface roughness was rotated in water lubricant for 30 min so that the gear tooth surface contacting the mating tooth was ‘worn’ to a mirror surface and ideal tooth profile, due to the mechano-chemical mild erosion of the contact area. The wear rate was 2.0 μm per 20,000 meshings, corresponding to a wear of one atomic radius thickness per meshing. Oxidation of the steel surface by water molecules is proposed as the dominant wear process. Operation noise from the gear pair rotation was drastically reduced to lower than about 10–15 dB compared to conventionally machined gear surfaces (30 dB in average), as a result of the wear of the tooth surface to form a best-fit profile. The noise increased with further processing of the gear pair. Thus, there is an appropriate number of rotations for suitable surface wear treatment. This new and simple procedure for surface treatment assures saving in energy, and does not require expensive honing techniques or high-accuracy grinding tools.The wear mechanisms used in this process are discussed along with the application of the technique to other processes for precision finishing.     

快速点磨削变量角度对表面粗糙度影响机理研究

基于点磨削周边接触宽度和点磨削表面粗糙度的数学模型,分析了磨削变量角度的变化对工件表面粗糙度的影响机理,完成了磨削变量角度影响工件表面微观几何特性的快速点磨削加工实验,并将基于点磨削表面粗糙度理论模型的计算值与实验结果进行了比较.结果表明:随着垂直点磨削变量角度的增大,接触区周边接触宽度沿工件轴向的分量减小,工件表面粗糙度增大.实验和计算结果基本吻合,并具有相同的变化趋势.     

A rolling ball test for establishing contact fatique behaviour of bulk materials, surface layers and coatings

This paper describes a new method for characterising contact fatique behaviour of bulk materials, surface layers and coatings. It is based on contact between a tungsten-carbide ball and a cylinder. In principle most ‘pin-on-ring’ type apparatus can be adapted to meet the test requirements. The method seems particularly attractive for charaterising the performance of surface layers and coatings under dynamic loading conditions. Examples relate to carburised steel surfaces and to a series of six hard chromium coatings with different bond strengths, obtained by applying different plating conditions.The test with the chromium coatings were performed under conditions of substantial initial plastic deformation of the steel substrate material. They were supposed to yield information on the ‘dynamic bond strength’ of the coatings.It was found to be possible to distinguish four quality classes, in agreement with expectations expressed by a plating expert.     

An investigation into parallel and cross grinding of BK7 glass

Conventional grinding of BK7 glass will normally result in brittle fracture at the surface, generating severe sub-surface damage and poor surface finish. The precision grinding of BK7 glass in parallel and cross grinding modes has been investigated. Grinding process, maximum chip thickness, ductile/brittle regime, surface roughness and sub-surface damage have been addressed. Special attention has been given to the condition for generating a ductile mode response on the ground surface. A polishing–etching method has been used to obtain the depth of sub-surface damage. Experiments reveal that the level of surface roughness and depth of sub-surface damage vary differently for different grinding modes. This study gives an indication of the strategy to follow to achieve high quality ground surfaces on brittle materials.     

磨削参数对GH4169高温合金磨削表面特征影响研究

采用单因素试验法,使用不同特性的砂轮进行GH4169高温合金的外圆磨削试验,研究了单晶刚玉砂轮和CBN砂轮对GH4169高温合金磨削表面特征中表面粗糙度和表面形貌的影响,分析了各磨削工艺参数对表面粗糙度的影响规律,并分析了单晶刚玉砂轮和CBN砂轮切屑的形态,还检测了磨削加工的表面形貌。结果表明：采用粒度为80、中软级、陶瓷结合剂的单晶刚玉砂轮磨削GH4169高温合金时,其磨削表面粗糙度较小,表面特征较稳定;磨削进给运动轨迹构成了试件已加工表面形貌轮廓的主要特征。在工件速度为8~21.66m/min、砂轮速度为15~30m/s、径向进给量为0.005~0.02mm、纵向进给量为1.3~3.6mm/r范围内,可以保证表面粗糙度Ra在0.14μm以内。     

Tribology based research at UCT: some recent case studies

Cape Town University has not only the important task of teaching, but to carry out research that is of benefit to industry and at the same time to educate aspiring and promising graduates in various aspects of technology. The philosophy and approach in the Centre for Materials Engineering is to develop a solid basis in designing methodologies or routes of producing the optimum material components in the major industries viz. the mining, agricultural, chemical and power industries. The South African Industrial sector has undergone dramatic changes over the last decade and the University will have to follow suit to produce graduates that ‘slot in quickly’ once they qualify. The phrase ‘hit the road running’ is often quoted in this context. The style and type of teaching and research in the faculty and more specifically the department can often determine the number of students choosing that particular field of study. So too are the awareness of industrial collaboration and the tailoring of innovative programmes critical. The importance of understanding the relationship between micro-structure and macro-properties in metallic, ceramic, polymeric and composite engineering materials can therefore not be emphasised enough.The Centre (formerly the Department of) for Materials Engineering has established relationships and long-term research programmes with producers and users of steels, polymers, and hard ceramic materials. The research focus in tribology in particular has been built around the academic strength of the late Professor Tony Ball. In this paper, selected examples and case studies in simulated tribo-testing are described, in which he was the principal researcher. The development and proper selection of material couples are taken from metals, ceramics and polymers to illustrate how a good and thorough understanding of the material and the environment has led to successful applications. Studies are selected from:

• Chromium containing corrosion and abrasion resistant steels, having adequate mechanical properties;

• High molecular weight polyethylenes and filled polymer composites for use as bearings and seals;

• Hard ceramic and surface-coated and surface-treated materials with good abrasion and erosion resistance in applications in the power industry. Examples are also given where cavitation erosion resistance is important in situations where particles are entrained in fluids and airstreams.

Predictive Modeling of Surface Roughness in Grinding of Ceramics

The surface roughness represents the quality of ground surface since irregularities on the surface may form nucleation for cracks or corrosion and thus degrade the mechanical properties of the component. The surface generation mechanism in grinding of ceramic materials could behave as a mixture of plastic flow and brittle fracture, while the extent of the mixture hinges upon certain process parameters and material properties. The resulting surface profile can be distinctively different from these two mechanisms. In this article, a physics-based model is proposed to predict the surface roughness in grinding of ceramic materials considering the combined effect of brittle and ductile material removal. The random distribution of cutting edges is first described by a Rayleigh probability function. Afterwards, surface profile generated by brittle mode grinding is characterized via indentation mechanics approach. Last, the surface roughness is modeled through a probabilistic analysis of ductile and brittle generated surface profile. The model expresses the surface finish as a function of the wheel microstructure, the process conditions, and the material properties. The predictions are compared with experimental results from grinding of silicon carbide and silicon nitride workpieces (SiC and Si₃N₄, respectively) using a diamond wheel.     

K9玻璃磨削亚表面损伤深度预测模型及实验研究

基于压痕实验原理,建立了K9玻璃亚表面损伤深度预测模型。为获得预测模型中的未知参量,开展了K9玻璃磨削实验。利用激光扫描共聚焦显微镜检测工件磨削后的表面粗糙度值,利用扫描电镜检测磨削后的工件表面微观形貌和亚表面损伤层形貌,分析了工艺参数对表面粗糙度值和亚表面损伤深度的影响规律。考查了工艺参数对法向磨削力的影响规律,并根据实验数据,采用多元线性回归拟合法得到法向磨削力的经验公式,进而确定了亚表面损伤深度预测模型的参数。模型预测值与实验值具有较好的一致性,表明预测模型具有一定的可靠性。     

航空发动机叶片数控智能磨削加工技术研究

现有磨削加工技术已经无法满足发动机的需求,故提出航空发动机叶片数控智能磨削加工技术。应用参数线法规划叶片磨削加工轨迹,以此为基础,提取磨削加工余量,模拟与计算对应数值,适当处理获取的叶片磨削加工轨迹与加工余量数据,推出叶片数控智能磨削算法(数控车床转轴、直线轴与压力轴运动控制模型),以此控制数控车床运动姿态,并通过刀位点偏移补偿叶片的反变形误差,实现了航发叶片的数控智能磨削。实验数据表明:应用该技术后叶片型面加工前后粗糙度变化明显;叶片边缘加工误差保持在标准误差范围内;叶片根部粗糙度得到了大幅降低,充分证实了该技术具有可行性。     

评定硬脆材料磨削表面质量的一种新参数

用表面粗糙度参数Ｒａ等来评定易于产生加工破碎的硬脆材料的表面质量是不全面的。本文提出以表面破碎率这一新参数作为评定指标。实验结果证明，表面破碎率能较完整地反映陶瓷等硬脆材料磨削表面的微观几何特性。