低温冷却磨削机理的研究

磨削是各种加工材料获得精确尺寸和表面完整性的主要加工方法,但在加工过程中,由于磨削区温度过高,经常导致工件表面热损伤、微裂纹和产生残余拉应力,严重影响工件表面质量和完整性的提高。本文通过采用低温CO2和液态氮为磨削冷却介质,有效地控制磨削区温度。实验结果表明,与干磨削和油冷却磨削相比,液态氮低温冷却磨削力、比磨削能、磨削区温度明显降低,工件表面质量和完整性显著提高,同时明显提高了砂轮的使用寿命和减少了冷却液对环境地污染。     

Thermal Analysis of Grinding

Thermal damage is one of the main limitations of the grinding process, so it is important to understand the factors which affect grinding temperatures. This paper presents an overview of analytical methods to calculate grinding temperatures and their effect on thermal damage. The general analytical approach consists of modeling the grinding zone as a heat source which moves along the workpiece surface. A critical factor for calculating grinding temperatures is the energy partition, which is the fraction of the grinding energy transported as heat to the workpiece at the grinding zone. For shallow cut grinding with conventional abrasive wheels, the energy partition is typically 60%-85%. However for creep-feed grinding with slow workspeeds and large depths of cut, the energy partition is only about 5%. Such low energy partitions are attributed to cooling by the fluid at the grinding zone. Heat conduction to the grains can also reduce the energy partition especially with CBN abrasives which have high thermal conductivity. For High Efficiency Deep Grinding (HEDG) using CBN wheels with large depths of cut and fast workspeeds, preheated material ahead of the grinding zone is removed together with the chips, thereby lowering the temperature on the finished surface. Analytical models have been developed which take all of these effects into account. Much more research is needed to better understand and quantify how grinding temperatures affect the surface integrity of the finished workpiece.     

Heat flux distributions and convective heat transfer in deep grinding

By using experimental data including the monitored temperature and power signals, combined with detailed theoretical analysis, the relationship between the undeformed grinding chip thickness and specific grinding energy has been studied and used to derive the heat flux distribution along the wheel-work contact zone. The relationship between the grinding chip thickness and specific grinding energy (SGE) has been shown to follow an exponential trend over a wide range of material removal rates. The distribution of the total grinding heat flux, q_t, along the grinding zone does not follow a simple linear form. It increases at the trailing edge with sharp gradients and then varies nearly linearly for the remainder of the contact length. The heat flux entering into the workpiece, q_w, is estimated by matching the measured and theoretical grinding temperatures, and it has been found that the square law heat flux distribution seems to give the best match, although the triangular heat flux is good enough for most cases to generate accurate temperature predictions. With the known heat flux distributions of q_t and q_w, the heat flux to the grinding fluid can then be estimated once the heat partitioning to the grinding wheel is determined by the Hahn model for a grain sliding on a workpiece. The convective heat transfer coefficient of the grinding fluid has been shown to vary along the grinding zone. An understanding of this variation is important in order to optimise the grinding fluid supply strategy, especially under deep grinding conditions when contact lengths are large. It has been demonstrated that the down grinding mode can provide a beneficial fluid supply condition, in which the fluid enters the grinding zone at the position of highest material removal where a high convective cooling function is needed.     

Interactions of grinding tool and supplied fluid

This paper reviews the physical and chemical interactions between the rotating tool and the supplied fluid in grinding. The mechanisms of this tool-fluid interaction are the key for high performance grinding processes due to an efficient fluid supply resulting in a minimal thermomechanical impact on workpiece and tool. Reduced wear, increased surface finish, suitable subsurface properties of the machined material, increased material removal rates, and also energy efficiency can be achieved. In this context, the fluid supply towards the contact zone between tool and workpiece, the tool cleaning with high pressure cleaning nozzles as well as (tribo)-chemical phenomena between the abrasive layer and the supplied fluid are analysed and discussed. Finally, knowledge gaps are revealed which are indicating future research needs.     

Improving grinding fluid delivery using pneumatic barrier and compound nozzle

Grinding fluid is commonly applied to control grinding defects caused by high grinding zone temperature. Delivery of fluid to the grinding zone is obstructed by the formation of a stiff air layer around the grinding wheel. This results in huge wastage of grinding fluid. In the present paper, results of using a pneumatic barrier and a compound nozzle are discussed with respect to delivering fluid deep into the grinding zone. Grinding fluid passing through the grinding wheel contact zone is measured under different modes of fluid delivery using a flood cooling, or a compound, nozzle, with or without the application of a pneumatic barrier. It is found that the system using a pneumatic barrier with flood cooling nozzle, and that employing a compound nozzle perform better than the flood cooling nozzle. A compound nozzle along with a pneumatic barrier renders substantially less wastage of grinding fluid even at a low flow rate of grinding fluid. Above a fluid discharge of 475 ml/min, the compound nozzle alone shows effective penetration of grinding fluid through the grinding zone. Reduction of grinding force, specific energy and roughness of ground surface are obtained after using compound nozzle fluid delivery system. Compound nozzle may be used instead of flood cooling nozzle as it improves grinding performance even using 52.5 % less discharge of grinding fluid.     

An experimental investigation of the effects of workpiece and grinding parameters on minimum quantity lubrication—MQL grinding

Coolant is a term generally used to describe grinding fluids used for cooling and lubricating in grinding process. The main purposes of a grinding fluid can be categorized into lubrication, cooling, transportation of chips, cleaning of the grinding wheel and minimizing the corrosion. On the other hand, grinding fluids have negative influences on the working environment in terms of the health of the machine operator, pollution and the possibility of explosion (for oil). Furthermore, the cost of the grinding fluid, filtering and waste disposal of the metal working fluids is even higher than the tool cost and constitutes a great part of the total cost. Additionally, grinding fluids can not effectively penetrate into the contact zone, are health hazard and their consumption must be restricted. Generally, compared to other machining processes, grinding involves high specific energy. Major fraction of this energy is changed into heat, which makes harmful effect on the surface quality as well as the tool wear. Since there is no coolant lubricant to transfer the heat from the contact zone in dry grinding, surface damages are not preventable. Alternatives to current practices are getting more serious consideration in response to environmental and operational cost pressures. One attractive alternative is the minimum quantity lubrication (MQL) grinding or the near dry grinding (NDG). In near dry grinding an air–oil mixture called an aerosol is fed into the wheel-work contact zone. Compared to dry grinding, MQL grinding substantially enhances cutting performance in terms of increasing wheel life and improving the quality of the ground parts. In this research, the influences of workpiece hardness and grinding parameters including wheel speed, feed rate and depth of cut have been studied on the basis of the grinding forces and surface quality properties to develop optimum grinding performances such as cooling, lubrication, high ecological and environmental safety.     

Improving grinding performance by controlling air flow around a grinding wheel

In grinding, high specific heat is generated, and hence, appropriate control of temperature through effective flow of grinding fluid is necessary to obtain a quality ground surface. It is known that in conventional fluid delivery method, most of fluid is wasted due to presence of a stiff air layer around the grinding wheel. This air layer is generated around the wheel due to the rotation of the porous grinding wheel at a high speed. To improve grinding performance, hence, penetration into this air layer is required.In this work, a pneumatic barrier set-up has been developed for controlling the stiff air layer around the grinding wheel. The formation of stiff air layer has been studied experimentally by measuring the variation of air pressure around grinding wheel periphery at different parametric conditions of pneumatic barrier. This pneumatic barrier tends to break the stiff air layer before the fluid flow area or grinding zone. A remarkable amount of reduction in pressure of the air layer is observed at the fluid flow zone. To observe beneficial effects of suppressing the air layer, grinding experiments are performed under dry, flood cooling and flood cooling with pneumatic barrier setup. Reduction of grinding forces and surface roughness are clearly observed with the use of pneumatic barrier setup, and hence, its applicability.     

Performance evaluation of Ti–6Al–4V grinding using chip formation and coefficient of friction under the influence of nanofluids

Nanofluid, fluid suspensions of nanometer sized particles are revolutionizing the field of heat transfer area. Addition of nano-particles to the base fluid also alters the lubricating properties by reducing the friction. In grinding process, friction between the abrasive grains and the workpiece is a key issue governing the main grinding output. It has a direct influence on grinding force, power, specific energy and wheel wear. Moreover, high friction force increases the heat generation and lead to thermal damage in the surface layer of the ground work. Hence, any effort towards the friction control will enhance the component quality significantly. In this study, nanofluid as metal working fluid (MWF) is made by adding 0.05, 0.1, 0.5 and 1% volume concentration of Al₂O₃ and CuO nano-particles to the water during the surface grinding of Ti–6Al–4V in minimum quantity lubrication (MQL) mode. Surface integrity of ground surface, morphology of the wheel, and chip formation characteristics are studied using surface profilometer, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS) and stereo zoom microscopy (SZM). Coefficient of friction was estimated On-Machine using the measured forces. The results showed that the type of nanoparticle and its concentration in base fluid and the MQL flow rate play a significant role in reducing friction. Application of nanofluid leads to the reduction of tangential forces and grinding zone temperature. The cooling effect is also evident from the short C-type chip formation. MQL application with Al₂O₃ nanofluid helps in effective flushing of chip material from the grinding zone, thereby solving the main problem during the grinding of Ti–6Al–4V.     

Specific energy in grinding of tungsten carbides of various grain sizes

The objective of this study is to investigate specific energy in grinding of tungsten carbides of various grain sizes. Through the construction of a mathematical model, the study demonstrates the correlation of specific energy with the grinding process parameters and the material property parameters for the tungsten carbides. The study also examines material-removal mechanisms and surface finish in grinding of such materials using scanning electron microscopy, X-ray diffractometry and energy dispersive spectrometry techniques, etc. The study concludes that specific energy is related not only to grinding process parameters, but also to the physical–mechanical properties of the workpiece material.     

A study of plane surface grinding under minimum quantity lubrication (MQL) conditions

Abrasive material removal processes can be very challenging due to high power requirements and resulting high temperatures. Effective lubrication and cooling is necessary to ensure temperature levels do not become excessive. Current fluid delivery systems are frequently seen to increase production cost due to fluid purchase and disposal. Moreover, waste fluids have a negative environmental impact. One of the successful fluid reduction methods employed in machining is minimum quantity lubrication (MQL), where a small amount of fluid is directed into the machining area in the form of an aerosol. This study aims to improve understanding of the effectiveness of MQL in the fine grinding plane surface grinding regime. This paper presents a comparative study of three cooling methods: conventional flood cooling, dry grinding and grinding with MQL. Common steels EN8, M2 and EN31 were ground with a general purpose alumina wheel. Results obtained demonstrate that MQL can deliver a comparable performance to flood delivery under the conditions investigated. Performance indicators included: grind power, specific forces (tangential and normal), grind temperature and workpiece surface roughness.     

High Efficiency Deep Grinding of a Low Alloy Steel with Plated CBN Wheels

High efficiency deep grinding (HEDG) of a low alloy steel (51CrV4) has been carried out on an Edgetek 5-axis CNC grinding machine, using electroplated CBN wheels. The initial tests were conducted in a surface grinding mode over a wide range of grinding conditions, to evaluate the levels of specific grinding energy, workpiece surface integrity and wheel wear. The burn threshold conditions for the ground workpiece surface have been proposed in terms of a critical heat flux which is shown to vary with material removal rate. Cylindrical grinding in HEDG mode has also been carried out based on the knowledge obtained from the surface grinding. It has shown that the HEDG technology can be transferred successfully to the field of cylindrical grinding to achieve very high specific material removal rates in excess of 400mm³/mm.s. The successful application of HEDG to cylindrical components depends on the appropriate selection of grinding parameters and also the grinding fluid supply strategy. Thermal modelling of the HEDG process combined with surface integrity studies, has shown that under cylindrical grinding conditions a significant reduction in grinding fluid supply is possible even when operating within the HEDG regime.     

干磨新工艺的试验研究

磨削加工时需要非常高的能量输入,导致磨削区温度升高,因此干磨变得非常困难。介绍一种新的方法来减少热能,即在特殊条件下使用金刚石修整器来修整CBN砂轮,使砂轮表面磨粒面积减少。实验结果表明:与同样的材料磨除率下的普通砂轮磨削过程比较,该砂轮的磨削力大幅下降,没有磨削烧伤和工件表面损伤发生。     

Investigation of the heat partitioning in high efficiency deep grinding

The grinding heat partitioning in the high efficiency deep grinding (HEDG) process has been investigated. The ratios of heat partition to the different heat sinks, i.e. workpiece, chips, fluid and grits, have been calculated, based on both theoretical analysis and experimental data. The heat partitioning ratio to the grinding chips increases with the material removal rate and takes most of the grinding heat away from the grinding zone under HEDG conditions where very high material removal rates apply. The heat partition to workpiece decreases when increasing the material removal rate. Cooling fluid is especially important for the conditions of creep feed grinding when using low feed rates, with over 90% of heat convected away by the grinding fluid. Under HEDG conditions, only 5–10% of the grinding heat is taken away by the grinding fluid. Very high material removal rates can be achieved with good surface integrity, when using an optimised selection of process parameters.     

断续磨削表面烧伤机理与在线监测方法研究

目的 研究断续磨削烧伤机理和声发射在线监测方法,避免产品磨削加工烧伤现象.方法 基于平面磨削温度场理论和镜像热源方法,建立一种断续磨削工件边缘的温度场模型,基于该模型可对断续磨削烧伤机理进行研究.为验证上述模型的有效性,通过正交实验设计不同断续磨削工况实验,利用红外热成像仪和声发射信号对断续磨削区温度进行在线监测,使用酸洗法和巴克豪森噪声检测仪对磨削后工件表面进行烧伤检测验证,通过对声发射信号的小波包能量求解,建立其与磨削区温度之间的关系.结果 该模型可有效反映断续磨削时工件边缘处磨削区温度场分布情况.计算结果表明,断续磨削工件断口边缘比其他位置磨削区温度更高,且更容易引起烧伤.实验表明,声发射信号的小波包变换总能量与磨削区呈一定相关性,基于声发射信号可对断续磨削烧伤实施在线监测.结论 实验结果证明了该模型对断续磨削烧伤机理分析的有效性,以及利用声发射信号对断续磨削烧伤在线监测的可行性.最后针对某一转向螺母产品实际断续磨削加工烧伤进行在线监测应用,实践结果表明,该方法比传统酸洗烧伤检测更加高效环保,对实现磨削加工烧伤检测自动化和智能化具有重要意义.     

An investigation of graphite nanoplatelets as lubricant in grinding

Cooling and lubrication are very critical to ensure workpiece quality in grinding due to the high friction and intense heat generation involved in the process. Liquid lubricants have traditionally been used in flood form or minimum quantity lubrication (MQL), raising however, major environmental and economic concerns. The focus of this study is to evaluate the performance of graphite nanoplatelets as a lubricant in surface grinding. The role of graphite's characteristics such as form, size and concentration; and the effect of the carrying medium and the graphite's application method are determined based on an experimental study. The results indicate that graphite nanoplatelets significantly reduce the grinding forces, specific energy, and improve surface finish during surface grinding of hardened D-2 tool steel. A comparison with results obtained in conventional MQL grinding is also provided. The proper selection of graphite, carrying medium and application method can lead to a low cost, nontoxic and simple alternative to solid lubrication or MQL grinding.     

结构化超硬磨料砂轮设计与制备研究进展

从减摩降力、导屑促排、储液换热的角度出发，探索结构化砂轮在降低磨削力及磨削温度、抑制工件表面热损伤、提高工件加工表面完整性等方面的有效方法。以砂轮表面/基体结构的几何形状、三维尺寸及排布方式等因素对磨削性能的影响为主线，对结构化砂轮设计、制备的基本原理与最新进展进行了全面的论述和总结，重点揭示了结构表征参数-砂轮磨削性能-工件表面质量的内在关联，深入剖析了结构化砂轮在磨削中的优越性，并预测了其未来发展趋势，旨在为推进超硬磨料结构化砂轮的设计及制备技术发展提供理论指导和实践经验。     

Three Dimensional Finite Element Simulation of Transient Heat Transfer in High Efficiency Deep Grinding

3D Finite Element simulations have been carried out to investigate transient heat transfer under high efficiency deep grinding (HEDG) conditions. The results have been compared to those obtained from 2D analytical models and experimental measurements. It has been found that the steady-state heat transfer condition can be readily obtained in HEDG after the maximum contact length is achieved and that side wall convective cooling has little effect on the grinding temperatures for thin steel plates. The temperature distribution on the workpiece across the grinding width in cylindrical grinding shows obvious slopes and film boiling of grinding fluid may occur at the trailing edge of grinding width. Good agreement has been found between the FE results and experimental observations. 3D FE simulation and 2D analytical modelling predict quite similar values for the maximum temperatures on the finished surface of the workpiece.     

Temperatures in deep grinding of finite workpieces

This paper investigates the diverse thermal effects generated in high efficiency deep grinding (HEDG). Using a new thermal model of circular arc contact with transient analysis, the transient behaviour of the maximum contact temperature has been analysed for various grinding conditions. It is found that steady state conditions can be achieved for the conditions of sufficient workpiece length and high workspeeds. The effect is important for the understanding of the deep grinding process and for the prediction of satisfactory grinding conditions. HEDG conditions also have very apparent effects on the depth of heat penetration to the workpiece. The parameters investigated include mean contact angle, Peclet number and the heat source distribution. Experimental results are presented for specific energy, energy partition and mean temperature for high efficiency deep grinding.     

Numerical analysis of grinding temperature measurement by the foil/workpiece thermocouple method

High temperatures generated in grinding are the main factor responsible for thermal damage on the ground surface. The energy partition ratio is often investigated experimentally in order to predict this temperature. The method of the foil/workpiece thermocouple is frequently used. The disruptions of this measurement technique have been studied numerically on a global scale of the wheel by a 2D finite element method taking into account the mica sheets and the constantan foil. The longitudinal thermal inertia of the thermocouple has been determined for various junction thicknesses. The simulations show that the systematic error on the maximum temperature rise (MTR) measurement cannot be neglected and is dependent on grinding conditions such as the workpiece velocity and the arc contact length. Moreover, the junction thickness and the assumed heat flux conducted into the electrode also have an influence on this error. However, the sensor is always accurate during the cooling time outside the foil/wheel contact zone. Consequently, the assessment of the partition ratio is more reliable when made during the cooling phase rather than through the MTR.     

A study of the convection heat transfer coefficients of grinding fluids

By using hydrodynamic and thermal modelling, the variation of the convection heat transfer coefficient (CHTC) of the process fluids within the grinding zone has been investigated. Experimental measurements of CHTC for different grinding fluids have been undertaken and show that the CHTC depends on the grinding wheel speed and the fluid film thickness within the contact zone. The film thickness is determined by grinding wheel speed, porosity, grain size, fluid type, flow rate and nozzle size. The CHTC values are compared for a wide range of grinding regimes, including high efficiency deep grinding (HEDG), creep feed and finish grinding.