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.     

面向绿色制造的快速点磨削磨削液供给参数计算

高速／超高速磨削条件下,砂轮边缘的高速空气带会阻碍磨削液注入磨削区。空气带压力与砂轮速度的平方成正比。快速点磨削是一种新型高速／超高速磨削技术,接触区很小,实际磨削功率低,冷却及散热效果好。在分析了高速／超高速磨削砂轮周围旋转空气带动压力及速度分布特点的基础上,根据热力学原理及快速点磨削特点,分析并建立了磨削液的供给流量和供液速度的理论模型。在此基础上,建立了面向绿色制造的快速点磨削的磨削液喷嘴直径及供液压力的工程计算公式。     

磨削区内气流场速度和压力分布规律的研究进展

在高速旋转的砂轮表面形成一层空气附面层，即气流场。气流场的存在，不仅影响工件的加工精度和加速砂轮的磨损，而且还阻止磨削液有效地注入磨削区，使加工条件恶化，磨削力上升，磨削温度升高，严重影响了工件的加工质量和表面完整性。本文综合分析了在磨削区速度和压力的分布规律及影响因素，为今后工业生产中进一步控制、利用气流场进行磨料流光整加工奠定基础。     

Experimental simulation of the efficiency of high speed grinding wheel cleaning

The grinding of certain materials such as ductile material which are hard to grind implies particular conditions of work. Maintaining the cutting ability of the wheel is necessary and wheel cleaning is one of these conditions. In this paper, the parameters which are influential in maintaining a clean wheel are identified. A cleaning criterion is proposed to estimate the efficiency of the cleaning process. Using an experimental setup, the significant of the influence of the nozzle position, the flow rate and pressure, the boundary layer of air around the rotating wheel and the particle rate contained in the fluid are assessed. It is observed too that the fluid temperature has no significant effect. Lastly, the best method to clean a wheel when high speed grinding is discussed.     

Cooling action of grinding fluid in shallow grinding

A new method is proposed for measuring the heat transfer coefficient in the vicinity of the wheel-workpiece contact zone. The accuracy of measurement is estimated by using a finite element method and the factor for correcting the measured results is derived. The experiments are performed in a number of conditions where grinding fluid is supplied and the following measures are consequently recommended for increasing the cooling efficiency: (1) set the velocity of coolant to more than the critical value to penetrate the air flow layer formed around the wheel periphery; (2) use a nozzle with a thin throat, about 1 mm in height, and attach a scraper plate above the nozzle outlet; (3) choose a wheel of large grain size and dress roughly, or form shallow grooves on the wheel periphery; and (4) set a higher wheel speed.     

The effects of cutting fluid application methods on the grinding process

It is well known that a boundary layer of air is entrained around a rotating grinding wheel. The effects of the boundary layer have been under some scrutiny in recent years with most research being based on trying to overcome the boundary layer. The current investigation aims to show through experiment and modelling, the effects of the boundary layer on cutting fluid application and how it can be used to aid delivery by increasing flow rate beneath the wheel. Results from three experiments with different quantities of cutting fluid passing through the grinding zone are presented.     

Influence of oil mist parameters on minimum quantity lubrication – MQL grinding process

Promising alternatives to conventional dry and fluid coolant applications are minimum quantity lubricant (MQL) or near dry grinding. Despite several researches, there have been a few investigations about the influence of MQL parameters on the process results, such as oil flow rate, air pressure, MQL nozzle position and distance from the wheel–workpiece contact zone. The current study aims to show through experiment and modeling, the effects of the above parameters on grinding performance such as grinding forces and surface roughness. The results show that the setting location of the nozzle is an important factor regarding the effective application of MQL oil mist. It has been shown that optimal grinding results can be obtained when the MQL nozzle is positioned angularly toward the wheel (at approximately 10–20° to the workpiece surface). In addition, it is found that the efficient transportation of oil droplets to the contact zone requires higher mass flow rate of the oil mist towards the grains flat area and longer deposition distance of an oil droplet. Applying the new setup, considerable reduction in the grinding forces and surface roughness has been achieved.     

An investigation on surface grinding using graphite as lubricant

Grinding requires high specific energy and the consequent development of high temperature impairs workpiece quality by inducing tensile residual stress, burn, micro cracks etc. Control of grinding temperature is achieved by providing effective cooling and lubrication. Conventional flood cooling is often ineffective due to the relative inaccessibility of the fluid to the actual grinding zone, film boiling etc. Further these fluids are also a source of health hazards. Minimization and possibly the elimination of fluid coolants by substituting their functions by some other means is of current research interest. This paper deals with an investigation on using graphite as a lubricating medium to reduce the heat generated at the grinding zone. An experimental set-up has been developed for this and a detailed comparison has been done with dry and coolant flooded grinding in terms of forces, specific energy, temperature and surface finish. Results show that grinding force, energy and temperature are reduced and resultant surface finish depends on workpiece material.     

Improving minimum quantity lubrication in CBN grinding using compressed air wheel cleaning

The application of minimum quantity lubrication (MQL) in grinding has emerged as an alternative for reducing the abundant flow of cutting fluids, thus achieving cleaner production. Although considered an innovative technique in grinding operations, its widespread application is hindered due primarily to the high heat generation and wheel pore clogging caused by machined chips, harming the final product quality and increasing tool wear on the machine. This study sought to improve MQL use in grinding. In addition to the conventional MQL injected at the wheel/workpiece interface, a compressed air jet was used to clean the mixture of MQL oil and machined chips from clogged wheel pores. Experiments were conducted using external cylindrical plunge grinding on AISI 4340 quenched and tempered steel, and a vitrified cubic boron nitrite (CBN) wheel. The cooling-lubrication methods employed were the conventional flood coolant application, MQL (without cleaning), and MQL with a cleaning jet directed at the wheel surface at different angles of incidence. The main goal of these experiments was to verify the viability of replacing the traditional abundant flow of cutting fluid with MQL and wheel cleaning. The analyses were conducted by measuring the following output variables of the process: workpiece surface roughness and roundness errors, diametrical wheel wear, acoustic emission generated by the process, and metallographic images of the ground surface and subsurface. Results show the positive effects of implementing the cleaning jet technique as a technological improvement of minimum quantity lubrication in grinding in order to reduce the usage of cutting fluids. The MQL technique with cleaning compressed air jet, for a specific angle of incidence (30°), proved to be extremely efficient in the improvement of the surface quality and accurate workpiece shape; it also reduced wheel wear when compared to the other cooling-lubrication methods that were tested (without a cleaning jet).     

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.     

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.     

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。     

镍基单晶高温合金磨削润滑方式对表面完整性的影响

为提高镍基单晶高温合金DD5的磨削表面质量,采用单因素试验探究3种磨削加工方法（干磨削、传统浇注式、微量润滑（minimum quantity lubrication, MQL））对其表面完整性的影响。结果表明：在不同冷却条件下,DD5磨削表面粗糙度从低到高依次为MQL、传统浇注式、干磨削方式下的。当砂轮线速度较小时,磨削表面质量较差,存在较深划痕和沟槽;当砂轮线速度较大时,磨削表面质量较好,磨痕较小,且分布均匀。在距磨削表面为5～15μm时,DD5亚表面显微硬度随着深度的增加而急剧下降;在距磨削表面为20～150μm时,DD5亚表面显微硬度趋于平衡,其在540 HV附近波动。     

Hydraulic design of a grinding wheel with an internal cooling lubricant supply

This paper presents a grinding wheel with an internal cooling lubricant supply (GIC). The cooling lubricant is supplied through channels inside the grinding wheel instead of conventional supply using external nozzles. With the GIC, direct coolant supply into the contact zone is possible. The hydraulic design of the GIC is similar to that of a centrifugal pump impeller. The design process is supported by simulation of fluid dynamics. First tests show that the GIC is able to supply the coolant at different flow rates with cutting speeds up to 60 m/s.     

基于有限差分模型的磨削弧区流场动压力数值分析

磨削弧区动压力对通过磨削区磨削液的有效流量、润滑和冷却作用有重要影响。本研究基于流体动压理论,建立了磨削弧区的动压力分布数学模型,将微分方程简化至近似泊松方程形式后,采用有限差分法将连续方程离散化,得出了磨削区动压力的数值解,并提出了迭代优化算法,提高了计算效率。将砂轮特性参数纳入数学模型之中,可根据砂轮材质、砂轮与工件间隙、砂轮转速等参数预报磨削弧区的磨削液动压力分布。在此理论模型基础上,进行了验证实验,证明模型的科学性。结果表明:通过输入砂轮各项参数,该模型可以快速、准确地预报动压力的分布,为磨削加工提供参考。      

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.     

Effects of minimizing hydrodynamic pressure in ultra-precision mirror grinding

This paper describes an investigation about the fluid delivery method that minimizes the generation of hydrodynamic pressure and that improves grinding accuracy. Traditionally, grinding fluid is delivered for the purpose of cooling, chip flushing and lubrication. Hence, numbers of conventional investigations are focused on the delivery method to maximize fluid flux into the contact arc between grinding wheel and workpiece. It is already known that hydrodynamic pressure generates due to this fluid flux, and that it affects overall grinding resistance and machining accuracy. Especially in the ultra-precision mirror grinding process that requires extremely small amount of cut per each pass, its influence on the machining accuracy becomes more significant. Therefore, in this paper, a new delivery method of grinding fluid is proposed on the point of minimizing hydrodynamic pressure effect. Experimental data indicate that the proposed method is effective not only to minimize the hydrodynamic pressure but also to improve machining accuracy.     

Useful coolant flowrate in grinding

A model has been developed for flowrate between a rotating grinding wheel and a workpiece. It was found that the useful flow that passes through the contact zone is a function of the spindle power for fluid acceleration, wheel speed and delivery-nozzle jet velocity. Two loss coefficients having values less than 1 are required to be calibrated for the particular grinding wheel and fluid delivery type. The model is then valid for a range of nozzle flowrates for the particular wheel and nozzle conditions. The flowrate delivered is related to unit width of the delivery nozzle assumed to be unit width of grinding contact. The model makes it possible to determine a suitable value of nozzle outlet gap to achieve a required fluid film thickness in the grinding zone. A guide is given to optimisation of the jet velocity in relation to the power required to accelerate the fluid and the particular velocity of the wheel. The model has been validated experimentally. Its simplicity and accuracy allow application to a wide range of grinding situations.     

Experimental study on grinding force and grinding temperature of Aermet 100 steel in surface grinding

This paper investigates grinding force and grinding temperature of ultra-high strength steel Aermet 100 in conventional surface grinding using a single alumina wheel, a white alumina wheel and a cubic boron nitride wheel. First, mathematical models of grinding force and grinding temperature for three wheels were established. Then, the role of chip formation force and friction force in grinding force was investigated and thermal distribution in contact zone between workpiece and wheel was analyzed based on the mathematical model. The experimental result indicated that the minimum grinding force and the maximum grinding force ratio under the same grinding parameters can be achieved when using a CBN wheel and a single alumina wheel, respectively. When the phenomenon of large grinding force and high grinding temperature appeared, the workpiece material would adhere locally to the single alumina wheel. Grinding temperature was in a high state under the effect of two main aspects: poor thermal properties of grinding wheel and low coolant efficiency. The predicted value of grinding force and grinding temperature were compared with those experimentally obtained and the results show a reasonable agreement.     

Determination and control of grinding zone temperature under cryogenic cooling

Grinding processes, though employed widely as a finishing process, have their own share of problems, like high grinding zone temperature which may lead to thermal damage to the work surface, like induction of tensile residual stresses, development of microcracks, enhanced risks of wheel loading and excessive wheel wear. Grinding fluids are applied in different forms to control such high temperature, but they are partially effective within a narrow working range; recent studies also indicate their polluting nature. cryogenic cooling, if employed properly, could control the grinding zone temperature more effectively by intensive removal of heat from the grinding zone. The present study deals with the effect of cryogenic cooling on grinding zone temperature for five commonly used steels both experimentally and computationally. Results indicate that the effectiveness of cryogenic cooling is substantial throughout the experimental domain.