立柱移动式MK7125精密数控平面磨床横向进给和纵向进给机构及床身设计【含CAD图纸】
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Grinding-Some observations For the production of finished components of desired shape,size and accuracy,machining is the commonly used manufacturing process.Machining process involvesthe usage of single or multiple point cutting tools to remove the unwanted materials form the stock in the form of chips (Komandurai,1993). Among the various metal cutting process available,Grinding is one of the important metal cutting process usedextensively in the finishing operation of discrete components. It is a versatile and also finish machining process in the production of components requiring close dimensional tolerances, geometrical accuracies and required surface finish (Rajmohan et al.,1994).Most of the production processes are incomplete without grinding process.According to Subramanian (1999), it is a major manufacturing process,which accounts for about 25% of the total expenditure on machining operations in industrialized countries. Almost all the engineering components are processed in grinding machining machines at some stages of its production. Grinding is a slow process in terms of unit removal of the stock. Hence,other methods are used first to bing the work close to its required dimensions and then it is ground to achieve the desired finish. In some applications,grinding is also employed for higher metal removal rate. In such heavy duty grinding operations more abrasive is consumed. In these cases,the main objective is to remove more amount of material that too as quickly and effectively as possible. Thus,the grinding process can be applied successfully to almost any component requiring precision or hard machining and it is also one of the widely used methods of removing material from the work piece after hardening. In order to decrease the cost and increase the production rate, the grinding machine must be set to operate within the shortest possible grinding cycle time. Hence, it is often important to set the correct grinding machine parameters so as to produce parts of required quality. The selection of grinding parameters if it is done on hit and miss technique not only wastes time but also leads to an inefficient process.To over come this difficulty, Gupta et al. (2001) in their work optimized the grinding process parameters using the enumeration method. The parameters should be selected so as to result in an optimal solution. Selection of grinding process parameters is made easy employing the “Expert system”. Shaji and Radhakrishnan (2002) analyzed the process parameters such as speed, feed, unfeed and mode of dressing as influential factors on the force components and surface finish developed based on Taguchis Experimental design methods. Fengguo Cao et al.(2003) developed the concept of integrating neural network, grey relational analysis and genetic algorithm for the optimization of process parameters in increased. Explosive Electrical Discharge Grinding Process lies in the proper selection and introduction of suitable design of experiment at the earliest stage of the process and product development cycles so as obtain quality and productivity improvement. Among the existing types of grinding processes, cylindrical grinding process is the one , which is very widely used in the finish machining of number of automobile components with surfaces of revolution. In cylindrical grinding process, the frictional resistance encountered between the work material and the tool, chip tool interface and the resistance to deformation during shearing of the chips contributes to rise in temperature at the contact zone (Trigger et al. 1951). The temperature generated is not only very high but the temperature gradients are also severe. Such temperatures of sufficient magnitude can cause adverse changes in workpiece metallurgical structure, loss in dimensional accuracy and accelerated wear or dulling of the tool (Des Ruisseaux and Zerkle, 1970; Takashi Ueda et al., 1985). In addition to causing surface damage,grinding heat may cause thermal expansion/distortion in the component ground and thus adversely affect the attainable accuracy.Masuda and Shiozaki(1974) demonstrated how grinding heat in plunge surface grinding results in out-of-flatness of the finished part. Better flatness was obtained with smaller depths of cut and higher workpiece velocities. Both of them cause lesser grinding heat and with increased coolant flow rate the cooling of the workpiece is enhanced and the thermal distortion is minimized. Chandrsekar et al. (1996) studied the thermal aspects of surface finishing process. In grinding, the localized abrasive workpiece contact pressures and high sliding speed produce high temperatures at the interface between an abrasive particle and the work surface, as well as in the work sub-surfaces due to frictional heating. High temperatures are the important source of damage on the machined surface. First, the transient temperature and the temperature gradient are the principle sources for residual stresses and micro cracking on ground surfaces. Secondly, the localized temperatures can cause warping of the components being machined, especially, when it is of small size and has a relatively large surface area to volume ratio. This is a serious problem in the finishing of small electronic devices such as recording heads. Thirdly, this high temperature can also lead to phase transformations in the materials being machined. The nature of grinding damage was surveyed by Tarasov (1950), who identified three main kinds of grinding damage, namely cracking, rehardening burn and tempering burn. During grinding of hardened steel, if the surface temperature of the work piece is sufficiently high, the surface reaustenizes and is rapidly quenched. Consequently, there is a formation of brittle, untempered martensite at the surface. This type of thermal damage is also commonly referred to as workpiece burn and is highly undesirable (Tarasov,1950; Torrance,1978). A martensitic type of phase transformation also occurs during the grinding of toughened zirconia. Here, the transient mechanical and thermal stresses generated during grinding drives the transformation. These forms of thermal damage change the mechanical, magnetic and electrical properties of the work materials. The local temperatures play an important role in the degradation of the abrasive particles and the bonding property of the material. The heat generated during grinding is characterized by, i) Instantaneous concentrated source, ii) High rate of liberation,and iii) Very small contact period. Heat associated with the energy expended by grinding is transported away from the grinding zone by the work piece, grinding fluid, grinding chips and grinding wheel. Of particular interest is the fraction of the total grinding energy transported to the work piece at the grinding zone, which causes the rise in workpiece temperature and possible thermal damage. For regular grinding with conventional Aluminum oxide wheels, the energy partition to the work piece typically ranges from 60-80% depending on the actual grinding situation (Malkin and Anderson,1974; Rowe et al., 1995 and 1997). Only a few isolated attempts have been reported so far on experimental analysis of the temperature developed at the wheelwork contact zone, energy partition ratio, grain contact time and thermal damages. At this point, it appears that practical optimization strategy and reliable mathematical models are still required to analyze the thermal damage in grinding. Field and Kahles (1971) investigated the dissipation of heat in grinding and the resulting influence on the surface integrity of the work piece. Guo and Malkin (1992) described that depending on the grinding condition the heat flux takes part mainly via the work piece and leads to a large thermal loading in the surface. This thermal load is superimposed by mechanical load causing a high temperature in the surface. This thermo-mechanical load causes some undesired alterations in the surface layer, like cracks,tempered zone or white etching areas (WEA). Shaw and Vyas (1994) gave an impressive theoretical description of metallurgical changes in ground surfaces. Under abusive grinding conditions, the formation of heat-affected zone was observed. Des Ruisseaux and Zerkle (1970) analyzed that the heat-affected zone under abusive grinding conditions damages the ground surface of the hardened steel very frequently. A thermally damaged component may therefore incur a significant cost to the manufacturer in failing quality standard. Thus, the thermal phenomena play a key role in the economics and mechanics of abrasive machining processes. An estimation of the amount of energy generated ,work surface temperature and an understanding of their role in metallurgical changes on ground surfaces are still challenging to the production engineers (Soyes and Maris 1978). Malkin and Fedoseev (1991) analyzed the method to predict the undesired alterations to avoid thermal damages in grinding gardened steel. In any case, the generated heat quantities in grinding are considered as a restricting factor. The invention of advanced grinding processes, which enabled the surface hardening of steel parts, was described for the first time in 1994. In such operations,named grind hardening, the dissipated heat in grinding is utilized to induced martensitic phase transformation in the surface layer of components (Brinksmeier and Brockhoff, 1997). Better surface finish with increased hardness at the surface by utilizing the heat generated during grinding is possible under optimum operating conditions. Thus, one of the area for the researchers to concern about the unique optimal settings of grinding process parameters - Depth of cut ,Number of passes, Wheel speed and work speed for maximizing the surface hardness and minimizing the surface roughness while grinding AISI steel materials with Al2O3 grinding wheels. “Ishikawa cause effect diagram”of machining is studied to identify the influential process parameters that may affect the surface integrity of grounded parts by Ramamoorthy et al., 2001 and; Harisingh et al., 2004. Taguchis parameter design approach has been used to accomplish the objective. A special mathematical tool known as grey relational analysis can be used with response graph approach and signal to noise ratio approach for the optimization. It is well known that physical surface properties can determine the lifetime and the function of highly loaded workpiece and components. For this reason, manufacturing industries require information about the techniques to influence the surface state of workpiece and achieve consistent properties (Kegg, 1982). This interest has its importance due to the fact that magnitude of the residual stress interferes on the fatigue strength of the materials (Novasaki et al., 1996). Residual srtess is the most representative parameter to describe the quality of the surface (Brinksmeier et al., 1982) among various surface alterations like phase transformations, hardness variations, micro cracks,grinding burn etc. Banerjee and Chattopadhyay (1987) investigated the control of residual stress in grinding by cryogenic cooling which results in much less tensile residual stresses. Kruszynski et al. (1991) made an attempt to predict residual stresses in grinding of metals with the aid of a new grinding parameter. Hucker (1994) showed that there was a quantitative relation between the effective work-surface temperature and the residual stress produced on ground surfaces of hardened steels. X-ray diffraction techniques were used to measure the residual stresses. It was reported that CBN grinding is found to produce compressive stress at the surface in contrast to Al2O3 grinding. However, many of the researches proved that under the conditions of martensitic formation (rough grinding) compressive residual stresses are formed when ground with Al2O3 wheel. Brockhoff and Brinksmeier (1997) in their comprehensive view on grind hardening fund out that compressive residual stresses are existing in the White Etching Areas, which continue into the area of etchable martensite and which are compensated by low tensile residual stresses in a greater distance from the surface. Litmann and Wulff (1955) found that for hardened steels, which have been burned during grinding, the workpiece sub-surface consists of a rehardened zone near the surface and a softened tempered zone beneath it. This would suggest that the onset of burning is characterized by the formation of austenite over some portion of the workpiece sub-surface. Rehardening at the surface occurs by acicular martensite ( that appears in the form of parallel needles within former austenite grains ) formation as the cooler material in the bulk of the workpiece quenches the surface. This refers to phase transformation in grinding. After grinding under ideal conditions, the ground surface will be crack free and will exhibit compressive residual stresses favorable for corrosion resistance and long life under cyclic loading conditions. In contrast, many grinding conditions are such that the surface produced suffers tensile stresses, sub-surface cracking and oxidation leading to failure in surface. In order to strike a balance between quality and strength in grounded parts it is desirable to have a control over the residual stress. This necessitates a detailed study of the free work-surface temperature, amount of heat generated and the magnitude of residual stress formed. 对磨削的一些观察为了使在零部件的生产中达到预期的形状、尺寸和精度，机械加工被广泛运用于生产加工工艺中。机械加工过程中会运用到一个或多个切削工具，来去除工件上不需要的部分，使之成为切屑。在众多已应用的金属切削工艺中，磨削加工是金属加工工艺常用于零件最终加工的重要加工工艺之一。它用途广泛，也经常用于尺寸公差、几何精度和表面精度要求高的零件的机械加工工艺中。绝大多数产品的生产工艺中都少不了磨削加工。根据Subramanian的统计数据，在工业国家的生产支出中，磨削加工占了25%，处于主要地位。几乎所有的工程零件在其生产的某些阶段会在磨削机床上加工。在工件的单元切削中，磨削加工是一个缓慢的过程。因此，在工件开始加工时，一般采用其他的加工方式使工件达到与要求相近的精度，然后采用磨削完成加工。在某些应用中，磨削也具备更高的金属切削效率。在如重载磨削中，更多的磨料会被消耗，在这些情况下，尽可能快而有效去除更多的金属材料是主要的目标。因而，磨削加工能成功地用于任何高精度或难加工零件的加工过程中，并且它也是可广泛应用于硬化表面材料去除的加工工艺之一。Shaw曾报告称，磨削加工是存在很多相关变量的复杂工艺，而这些相互作用的变量是同磨削方式所决定的。在平面磨削中所产生的几何形状会受到如下因素的影响：1. 砂轮因素：砂轮直径、磨粒类型和尺寸、砂轮等级、砂轮构造、粘结剂、敷 料工艺、砂轮的平衡等级等。2. 工件因素：加工表面硬度、构造、化学特征等。3. 机床因素：主轴和工作台刚度、阻尼、动力特性等。4. 加工参数：砂轮转速、进给量、背吃刀量、磨削液等。为了减少消耗，提高生产效率，磨削机床必须设定加工时间处于最短的可能磨削周期内。因此，设置正确磨削机床参数对获得需要的精度往往非常重要。如果磨削参数选择不符合技术要求，就会导致时间浪费效率低下。为了解决这个问题，Gupta在他们的研究中，采用列表的方法来使磨削参数最优化。参数的选取应使工作方案最优化，当采用“专家系统”时，磨削工艺参数的选取就变得容易了。Shaji和Radhakrishnan在Tagudhi的实验设计方法基础上分析了砂轮转速、进给量、背吃刀量、敷料的方式对磨削力的构成、表面加工的影响。Fengguo Cao提出了一体化神经网络、灰色相关分性分析、遗传算法的概念，来对工艺参数进行优化提高。爆炸式电火花磨削工艺正是立足于对最早工艺阶段和产品开发周期的合理实验设计的选择推广上，来获得品质和效率的提升。在已有的磨削工艺形式中，外加磨削广泛应用于汽车回转零件的表面加工中。在外圆，产生于工件材料与刀具之间的摩擦阻力，刀具表面的剪切变形抗力会使得接触区域的温度上升。产生的温度很高并且分布很不均匀，这样剧烈的高温会使工件的金相结构发生不利的改变，使其尺寸精度丧失，并且加速刀具钝化。除了导致表面损坏，磨削热也会使工件在磨削过程产生热膨胀或热变形，从而对工件精度产生不利的影响。Masuda和Shiozaki阐述了磨削热如何寻到工件表面变得不平整。当采用较小的切深和更高的切削速度时，会获得更高的平面度。同时，也能减少磨削热的产生。再加快冷却液的流动速率，使工件冷却效果加强，能使热变形减小。Chandrsekar研究了表面加工过程的热效应。在磨削过程中，局部的接触应力和高的滑动速度会在工件和磨削刃接触面产生高温，同时在次层面产生摩擦热。高温是造成已加工表面损坏的重要因素。首先，分布不均匀的瞬态高温是工件残余应力和表面微裂纹的主要来源。第二，局部高温会使已加工的部分发生形变。尤其是对尺寸较小却具有较大体积比率的工件，变形尤为严重，这对于某些小型电子设备如电磁记录头的加工，是一个很严峻的问题。第三，高温会导致已加工材料的物相发生改变。Tarasov对磨削操作的性质做了调查研究，确定了三种主要的损伤类型，分别是开裂，二次淬火烧伤和高温烧伤。在磨削硬质的钢材时，如果表面温度过高，就会发生表面再次奥氏体化，并急速冷却，从而在工件表面会形成具有的回火马氏体。这种形式的热损伤也是觉的工件烧伤形式，需要避免。在更质氧化锆的磨削过程中，也会发生类似马氏体类型的物相变化。这种变化是同磨削过程中产生的瞬态机械应力和热应力所导致的。这些形式的热损伤会改变加工材
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