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。     

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.     

The influence of cryogenic cooling on milling stability

Cryogenic cooling is emerging as an effective process for high performance machining. However, the influence of cryogenic cooling on milling stability is seldom reported. This paper involves experimental study on the effect of cryogenic cooling on milling stability, using a dedicated cryogenic cooling system to applying liquid nitrogen (LN2) jet to the cutting zone. We observe that cryogenic cooling leads to higher stability limit compared with conventional milling operations, which indicates that the cutting efficiency can be improved greatly in LN2 environment as opposed to the conventional one. The stability improvement is explained from the perspective of machining dynamics parameters variation between the two conditions. Cutting force coefficients and modal parameters of spindle-tool system are identified during cryogenic machining, then milling stability lobe diagrams are predicted by time domain and frequency domain methods. On the basis of milling stability analysis, the enhancement of stability boundary is attributed to the significant reduction of cutting force coefficients during cryogenic cooling. Additionally, the experiment result indicates that cryogenic cooling decreases the dominant modal frequency of the spindle-tool system, which shifts the milling stability boundary slightly to lower spindle speed range. The explanations are verified by a plenty of cutting tests.     

Investigation on cooling efficiency of grinding fluids in deep grinding

The cooling efficiency of grinding fluids in deep grinding, at different material removal rates and grinding speeds, has been investigated. Two ‘inverse’ methods have been proposed to determine the level of convective heat transfer coefficients of grinding fluids, by matching the theoretical and experimental grinding fluid burn-out thresholds or matching the theoretical and measured grinding temperatures. Instead of using a constant chip melting temperature to estimate the energy partition to the grinding chips, the chip temperature and chip energy were calculated using the newly developed approach considering the variation of chip size, deformation and heat transfer at the abrasive/work interface. The variation of grinding heat taken away by the process fluids and grinding chips under different process parameters has been calculated, which shows the importance of cooling effects by the grinding fluids and the transition of thermal characteristics of deep grinding from cooling dominant to ‘dry’ grinding regime, where a large percentage of grinding heat is taken away by the grinding chips.     

Effect of grinding forces on the vibration of grinding machine spindle system

In this paper the effects of dynamic changes in the grinding force components due to changes in the grinding wheel wear flat area and the workpiece material on the vibration behavior of the grinding spindle are studied. The steady-state dynamics and vibration behavior of the grinding machine spindle is simulated by a five degree of freedom model. The results indicate that when grinding different materials using a grinding wheel with fixed wear flat area, different vibration behavior is noticed. As the grinding wheel flat area increases, the level of vibration increases for all the degrees of freedom, which indicates that there is an upper limit for the level of wear on the grinding wheel that can be tolerated, and after that level dressing operation must be conducted on the grinding wheel.     

The influence of cryogenic cooling on process stability in turning operations

This paper presents results of the influence of cryogenic machining on the process stability. The stability diagrams are obtained experimentally using the coarse-grained entropy rate estimator for chatter detection from measured cutting forces. In comparison with conventional machining, enlarged stability windows are observed for the case of cryogenic machining. Based on the defined specific force models in turning operations, it is shown that a higher machining stability is achievable in cryogenic machining due to the reduction of specific cutting force components, in comparison with dry machining.     

The effect of wheel eccentricity and run-out on grinding forces,waviness, wheel wear and chatter

The effect of grinding-wheel eccentricity on grinding forces, wheel wear and final waviness height was studied. Eccentricity was evident in force oscillations and acceleration and audio measurements. A model was developed to predict final scallop-profile shape from grinding parameters and eccentricity. Recommendations are given on detecting eccentricity and determining when eccentricity is tolerable.     

The effect of cryogenic CO2 cooling on corrosion behaviour of friction stir welded AA2024-T351

Abstract

Cryogenic cooling with CO₂ was applied during friction stir welding of AA2024-T351 in order to reduce the temperature increase during welding, and thus improve the corrosion resistance of the weld. The effect of cryogenic cooling on corrosion susceptibility was investigated with gel visualisation, immersion tests and local electrochemical measurements. The most susceptible area for both uncooled and cooled welds was in the heat-affected zone (HAZ) region, which showed intergranular attack. Cryogenic cooling had no detectable influence on the degree of anodic reactivity in the weld region. However, it did decrease the width of the reactive HAZ.     

超高速点磨削力数学模型的建立与仿真

结合超高速点磨削的特点,将磨粒简化为圆锥形,建立了超高速点磨削力数学模型.通过对磨削力的Matlab仿真,分析了磨削参数和点磨削变量角α和β对磨削力的影响.结果表明:点磨削力随着砂轮线速度的增加而减小,随工件速度、磨削深度、纵向进给速度的增加而增大.点磨削力随磨削变量角α和β的增大而降低,其中,β对降低磨削力的贡献要大...     

Modeling and experimental study of grinding forces in surface grinding

Grinding forces are composed of chip formation force and sliding force. A new mathematical model of grinding forces in surface grinding is developed in this paper. Effectiveness of this model is proved by comparison of the experimental results and the model calculation results. Chip formation energy can be divided into static chip formation energy and dynamic chip formation energy which is mainly influenced by shear strain, shear strain rate and heat in the metal removal process. A formula for calculating the chip formation force is proposed by analyzing the relationship between specific chip formation energy and chip formation force. Combined with the achievements of other researchers, a new formula for calculating sliding force considering the influence of processing parameters on friction coefficient is obtained.     

Mist jet cooling of grinding processes

One of the problems in modern grinding processes is the use and disposal of coolants, which are expensive and which can lead to ecological problems. This paper describes some experiments with a novel method of cooling the grinding process which relies on nothing but air and water for its effect, thus removing ecological hazards in a most economical way. By injecting a small amount of water into air jets which strike the grinding wheel at speeds near mach 1, it is possible to maintain a sharper wheel and better cooling than with conventional coolants and delivery systems. It is relatively simple and cheap to set up the delivery system proposed, provided the correct nozzle designs are used.     

钎焊金刚石砂轮磨削石材中金刚石粒度对磨削力的影响研究

本文通过测量不同金刚石粒度的高频感应钎焊金刚石砂轮磨削花岗石过程中的磨削力，对砂轮所受的法向力和切向力进行了研究。对不同粒度条件下磨削深度、进给速度和砂轮线速度对磨削力的影响进行了分析。研究发现磨削力随砂轮线速度的增大而减小，随磨削深度和进给速度的增大而增大，磨削深度对磨削力的影响程度比进给速度大。小粒度金刚石磨削时，磨削三要素对磨削力的影响比大粒度金刚石磨削时大。     

The measurement of forces in grinding in the presence of vibration

Most investigations of chatter have made the assumption that torsional vibration is not a significant factor. Some recent work has shown that chatter in grinding is affected by a change in the torsional stiffness of the workpiece drive. Also, a theoretical model of grinding chatter has been developed that confirmed the significance of torsional effects. However, the model for the grinding force was assumed to be a dynamic equivalent of a published steady-state model. This paper describes tests conducted to measure the variation in force caused by an oscillation in workpiece speed. The oscillating test results indicate that the torsional vibration of the workpiece may well be a significant effect on chatter in grinding. Moreover, as the grinding force changes with workpiece speed, it may be possible to use variation of workpiece speed at high frequency to reduce chatter.     

Mist cooling in neurosurgical bone grinding

This research investigates using cryogenic saline mist, in comparison to the conventional saline irrigation, in neurosurgical bone grinding to prevent thermal injury to surrounding nerves and to increase the visibility of surgical area for endoscopic operations. Delivery of cryogenic mist directly towards the grinding zone along with a backward grinding motion was found important to maximize and localize the cooling effect, thus reducing bone temperature rise. Experimental results also showed a pre-cooling effect of the cryogenic mist that helps reduce the initial temperature of bone prior to the occurrence of grinding.     

A review of cryogenic cooling in machining processes

The cooling applications in machining operations play a very important role and many operations cannot be carried out efficiently without cooling. Application of a coolant in a cutting process can increase tool life and dimensional accuracy, decrease cutting temperatures, surface roughness and the amount of power consumed in a metal cutting process and thus improve the productivity. In this review, liquid nitrogen, as a cryogenic coolant, was investigated in detail in terms of application methods in material removal operations and its effects on cutting tool and workpiece material properties, cutting temperature, tool wear/life, surface roughness and dimensional deviation, friction and cutting forces. As a result, cryogenic cooling has been determined as one of the most favourable method for material cutting operations due to being capable of considerable improvement in tool life and surface finish through reduction in tool wear through control of machining temperature desirably at the cutting zone.     

电镀金刚石砂轮磨削火成岩质水晶的磨削力研究

采取顺磨和逆磨两种磨削方式,采用电镀金刚石砂轮对火成岩质水晶进行了平面磨削实验。通过测量磨削过程中的水平磨削力和垂直磨削力,得出了磨削深度、进给速度对磨削力的影响规律。结果表明,随着磨削深度的增大磨削力增大,随着进给速度的增大磨削力也增大。同时分析并讨论了火成岩质水晶的磨削力比,磨削力比Ft/Fn为0.33～0.36。     

细粒度钎焊金刚石砂轮磨削花岗石的磨削力特征分析

通过测量磨削力,研究细粒度钎焊金刚石砂轮磨削花岗岩过程磨削力随加工参数的变化特征。结果表明:磨削力是随着砂轮线速度的增大而减小,随着工件进给速度的变大而增大,随着磨削深度的增大而增大。回归分析表明,磨削力受磨削深度的影响程度最大。不同加工条件下,法向磨削力与切向磨削力之间存在良好的线性关系,比值约为7.6。磨削过程中,金刚石与花岗石之间的运动符合Coulomb定律描述的滑动摩擦方式。     

The effect of machine stiffness on grinding of silicon nitride

This study investigates the effect of machine stiffness on normal forces, actual depth of cut, and workpiece strength in grinding of silicon nitride. To obtain a grinding system with an adjustable stiffness, a compliant workholder is added to a precision grinder. Single-pass and multi-pass grinding experiments are conducted to evaluate the effect of machine stiffness. Cup-type diamond wheels of two different bond types and three grit sizes are used in the grinding experiments. Static and dynamic simulation is carried out to correlate grinding forces and actual depth of cut with machine stiffness. Since the simulation uses a time-domain model, it can accommodate non-linearities caused by the effect of machine stiffness on grinding forces and actual wheel depth of cut, workpiece regeneration, wheel wear, as well as wheel bond type and grit size effects, etc. Particularly, the model allows simulating grinding instability and the interference phenomenon due to residual material removal in multi-pass grinding. The study concludes that both simulation and experimental results have a good agreement.     

单颗金刚石磨粒磨削SiC的磨削力实验研究

选取两种不同锥度角的金刚石磨粒,分别用其对SiC陶瓷和单晶SiC进行了单颗磨粒磨削实验,检测了磨削过程中的磨削力大小,分析了磨削参数对磨削力的影响。实验结果表明:磨削力随着切削深度的增大而增大,随磨削速度的减小而增大;随着磨粒顶锥角的增大,磨削力明显增大;磨削单晶SiC(6H)的磨削力大于磨削SiC陶瓷的磨削力。