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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 involves
the 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 used
extensively 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 Taguchi’s 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. Taguchi’s 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.
對(duì)磨削的一些觀察
為了使在零部件的生產(chǎn)中達(dá)到預(yù)期的形狀、尺寸和精度，機(jī)械加工被廣泛運(yùn)用于生產(chǎn)加工工藝中。機(jī)械加工過(guò)程中會(huì)運(yùn)用到一個(gè)或多個(gè)切削工具，來(lái)去除工件上不需要的部分，使之成為切屑。在眾多已應(yīng)用的金屬切削工藝中，磨削加工是金屬加工工藝常用于零件最終加工的重要加工工藝之一。它用途廣泛，也經(jīng)常用于尺寸公差、幾何精度和表面精度要求高的零件的機(jī)械加工工藝中。絕大多數(shù)產(chǎn)品的生產(chǎn)工藝中都少不了磨削加工。根據(jù)Subramanian的統(tǒng)計(jì)數(shù)據(jù)，在工業(yè)國(guó)家的生產(chǎn)支出中，磨削加工占了25%，處于主要地位。幾乎所有的工程零件在其生產(chǎn)的某些階段會(huì)在磨削機(jī)床上加工。
在工件的單元切削中，磨削加工是一個(gè)緩慢的過(guò)程。因此，在工件開始加工時(shí)，一般采用其他的加工方式使工件達(dá)到與要求相近的精度，然后采用磨削完成加工。在某些應(yīng)用中，磨削也具備更高的金屬切削效率。在如重載磨削中，更多的磨料會(huì)被消耗，在這些情況下，盡可能快而有效去除更多的金屬材料是主要的目標(biāo)。因而，磨削加工能成功地用于任何高精度或難加工零件的加工過(guò)程中，并且它也是可廣泛應(yīng)用于硬化表面材料去除的加工工藝之一。
Shaw曾報(bào)告稱，磨削加工是存在很多相關(guān)變量的復(fù)雜工藝，而這些相互作用的變量是同磨削方式所決定的。在平面磨削中所產(chǎn)生的幾何形狀會(huì)受到如下因素的影響：
1. 砂輪因素：砂輪直徑、磨粒類型和尺寸、砂輪等級(jí)、砂輪構(gòu)造、粘結(jié)劑、敷
料工藝、砂輪的平衡等級(jí)等。
2. 工件因素：加工表面硬度、構(gòu)造、化學(xué)特征等。
3. 機(jī)床因素：主軸和工作臺(tái)剛度、阻尼、動(dòng)力特性等。
4. 加工參數(shù)：砂輪轉(zhuǎn)速、進(jìn)給量、背吃刀量、磨削液等。
為了減少消耗，提高生產(chǎn)效率，磨削機(jī)床必須設(shè)定加工時(shí)間處于最短的可能磨削周期內(nèi)。因此，設(shè)置正確磨削機(jī)床參數(shù)對(duì)獲得需要的精度往往非常重要。如果磨削參數(shù)選擇不符合技術(shù)要求，就會(huì)導(dǎo)致時(shí)間浪費(fèi)效率低下。為了解決這個(gè)問(wèn)題，Gupta在他們的研究中，采用列表的方法來(lái)使磨削參數(shù)最優(yōu)化。參數(shù)的選取應(yīng)使工作方案最優(yōu)化，當(dāng)采用“專家系統(tǒng)”時(shí)，磨削工藝參數(shù)的選取就變得容易了。Shaji和Radhakrishnan在Tagudhi的實(shí)驗(yàn)設(shè)計(jì)方法基礎(chǔ)上分析了砂輪轉(zhuǎn)速、進(jìn)給量、背吃刀量、敷料的方式對(duì)磨削力的構(gòu)成、表面加工的影響。Fengguo Cao提出了一體化神經(jīng)網(wǎng)絡(luò)、灰色相關(guān)分性分析、遺傳算法的概念，來(lái)對(duì)工藝參數(shù)進(jìn)行優(yōu)化提高。
爆炸式電火花磨削工藝正是立足于對(duì)最早工藝階段和產(chǎn)品開發(fā)周期的合理實(shí)驗(yàn)設(shè)計(jì)的選擇推廣上，來(lái)獲得品質(zhì)和效率的提升。
在已有的磨削工藝形式中，外加磨削廣泛應(yīng)用于汽車回轉(zhuǎn)零件的表面加工中。在外圓，產(chǎn)生于工件材料與刀具之間的摩擦阻力，刀具表面的剪切變形抗力會(huì)使得接觸區(qū)域的溫度上升。產(chǎn)生的溫度很高并且分布很不均勻，這樣劇烈的高溫會(huì)使工件的金相結(jié)構(gòu)發(fā)生不利的改變，使其尺寸精度喪失，并且加速刀具鈍化。
除了導(dǎo)致表面損壞，磨削熱也會(huì)使工件在磨削過(guò)程產(chǎn)生熱膨脹或熱變形，從而對(duì)工件精度產(chǎn)生不利的影響。Masuda和Shiozaki闡述了磨削熱如何尋到工件表面變得不平整。當(dāng)采用較小的切深和更高的切削速度時(shí)，會(huì)獲得更高的平面度。同時(shí)，也能減少磨削熱的產(chǎn)生。再加快冷卻液的流動(dòng)速率，使工件冷卻效果加強(qiáng)，能使熱變形減小。
Chandrsekar研究了表面加工過(guò)程的熱效應(yīng)。在磨削過(guò)程中，局部的接觸應(yīng)力和高的滑動(dòng)速度會(huì)在工件和磨削刃接觸面產(chǎn)生高溫，同時(shí)在次層面產(chǎn)生摩擦熱。高溫是造成已加工表面損壞的重要因素。首先，分布不均勻的瞬態(tài)高溫是工件殘余應(yīng)力和表面微裂紋的主要來(lái)源。第二，局部高溫會(huì)使已加工的部分發(fā)生形變。尤其是對(duì)尺寸較小卻具有較大體積比率的工件，變形尤為嚴(yán)重，這對(duì)于某些小型電子設(shè)備如電磁記錄頭的加工，是一個(gè)很嚴(yán)峻的問(wèn)題。第三，高溫會(huì)導(dǎo)致已加工材料的物相發(fā)生改變。
Tarasov對(duì)磨削操作的性質(zhì)做了調(diào)查研究，確定了三種主要的損傷類型，分別是開裂，二次淬火燒傷和高溫?zé)齻?。在磨削硬質(zhì)的鋼材時(shí)，如果表面溫度過(guò)高，就會(huì)發(fā)生表面再次奧氏體化，并急速冷卻，從而在工件表面會(huì)形成具有的回火馬氏體。這種形式的熱損傷也是覺的工件燒傷形式，需要避免。在更質(zhì)氧化鋯的磨削過(guò)程中，也會(huì)發(fā)生類似馬氏體類型的物相變化。這種變化是同磨削過(guò)程中產(chǎn)生的瞬態(tài)機(jī)械應(yīng)力和熱應(yīng)力所導(dǎo)致的。這些形式的熱損傷會(huì)改變加工材料的機(jī)械和電磁特性。
局部的高溫在磨粒鈍化和材料粘合性下降的過(guò)程中起主導(dǎo)作用。磨削中產(chǎn)生的熱是由以下因素決定的：
1. 瞬時(shí)的集中源
2. 高速率的釋放
3. 極小的接觸時(shí)間
磨削過(guò)程中產(chǎn)生的磨削熱可以通過(guò)工件、磨削液、切屑及磨削砂輪傳遞出磨削區(qū)域。而其中通過(guò)工件傳遞的磨削熱會(huì)使工件升溫而可能導(dǎo)致熱損傷。對(duì)于采用氧化鋁砂輪的磨削加工，由工件傳遞的熱量大約在60%到80%之間視具體磨削狀況而定。到目前只有一少部分單獨(dú)的關(guān)于齒輪接觸區(qū)溫升、能量分配比、磨粒接觸時(shí)間和熱損傷實(shí)驗(yàn)分析的嘗試。在這個(gè)方面，還是需要切實(shí)可行的優(yōu)化方法和可靠的數(shù)學(xué)模型來(lái)分析磨削中的熱損傷問(wèn)題。
Filed和Kahles研究了磨削熱的散失及其對(duì)工件表面完整性的影響。Guo和Malkin認(rèn)為依據(jù)磨削所處的加工狀態(tài)，磨削熱主要通過(guò)工件傳遞，從而導(dǎo)致工件表面的一個(gè)大的熱應(yīng)力。這個(gè)熱應(yīng)力是由于機(jī)械應(yīng)力在工件表面產(chǎn)生高溫而形成的。這種熱—機(jī)械應(yīng)力會(huì)在工件表層造成不想要的變化，如開裂，回火區(qū)或白蝕區(qū)（WEA）。
Shawl和Vyas給出了一個(gè)令人信服的磨削層金相變化的理論描述。在惡劣的磨削條件下，熱影響區(qū)的結(jié)構(gòu)可以被觀察到。Des Ruisseaux和Zerkle分析，極端磨削條件下熱影響區(qū)會(huì)頻繁地破壞磨削加工表面。一個(gè)熱損傷的零件會(huì)使生產(chǎn)商的品質(zhì)標(biāo)準(zhǔn)失敗，這無(wú)疑是一個(gè)很大的代價(jià)。因而，發(fā)熱現(xiàn)象在經(jīng)濟(jì)學(xué)和磨削加工過(guò)程中發(fā)揮重要作用對(duì)于產(chǎn)生熱量的估計(jì)，工件表面溫度和對(duì)金相變化的影響仍舊是產(chǎn)品工程師所面臨的挑戰(zhàn)。Malkin和Fedoseev分析出了在磨削硬化鋼時(shí)可預(yù)測(cè)不想要的變化的方法，來(lái)避免熱損傷。無(wú)論怎樣，磨削所產(chǎn)生的熱是影響加工的負(fù)面因素。能夠用于硬質(zhì)表面加工的先進(jìn)磨削技術(shù)在1994年第一次出現(xiàn)。在如名為硬質(zhì)磨削的加工中，傳導(dǎo)的熱量被用來(lái)引起工件表層金屬的馬氏體轉(zhuǎn)變。
在最佳的加工條件下，利用好磨削中的熱來(lái)提高表面質(zhì)量和增加表面硬度是可能的。因此，獨(dú)特合理的磨削參數(shù)是研究人員熱衷的領(lǐng)域之一。切削深度、進(jìn)給方式、砂輪轉(zhuǎn)速、工件速度怎么確定，來(lái)增加Al2O3砂輪磨削AISI鋼材時(shí)的表面硬度，減小其表面粗糙度。
“Ishikuwa變量效果圖”可用來(lái)分析工藝參數(shù)對(duì)磨削表面質(zhì)量的影響。“Taguchi參數(shù)設(shè)計(jì)”已被用來(lái)完成上面的目標(biāo)。一個(gè)通過(guò)分析反應(yīng)曲線和聲音比率數(shù)學(xué)工具可以使得工藝參數(shù)最優(yōu)化。
眾所周知，表面物理特性能決定高負(fù)載工件的能力和使用壽命。正是于此，制造業(yè)需要關(guān)于影響表面質(zhì)量的技術(shù)和統(tǒng)計(jì)學(xué)參數(shù)。這方面的重要性是由殘余應(yīng)力的大小對(duì)材料疲勞強(qiáng)度的影響而決定的。殘余是眾多表面變化如物相轉(zhuǎn)變、硬度轉(zhuǎn)變、微觀裂紋、磨削燒傷等中最能描述表面質(zhì)量的代表性參數(shù)。
Banerjee和Chattopadhyay進(jìn)行了可導(dǎo)致更少拉伸殘余應(yīng)力的低溫冷卻磨削的殘余應(yīng)力控制的研究。Kruszyski進(jìn)行了采用新的磨削參數(shù)進(jìn)行磨削，以此評(píng)估殘余應(yīng)力的研究。Hucker發(fā)現(xiàn)高效的工作表面溫度與硬質(zhì)鋼磨削表面的殘余應(yīng)力之間存在數(shù)量關(guān)系。X射線衍射技術(shù)被應(yīng)用于殘余應(yīng)力的測(cè)定。據(jù)報(bào)道，較之于Al2O3磨削，研究人員發(fā)現(xiàn)CBN磨削產(chǎn)生更能產(chǎn)生壓應(yīng)力。但是，很多研究人員證實(shí)，在馬氏體形成的條件下（粗加工）采用Al2O3砂輪磨削時(shí)產(chǎn)生壓應(yīng)力。
Brokhoff和Brinksmeier在他們對(duì)磨削的綜合看法中指出，壓應(yīng)力存在于白蝕區(qū)，并延伸到馬氏體蝕刻區(qū)，從離表面更遠(yuǎn)的距離獲得較小的拉伸殘余應(yīng)力補(bǔ)償。Litman和Wuff發(fā)現(xiàn)已經(jīng)發(fā)生過(guò)磨削燒傷的硬質(zhì)鋼，其次表層是由接近表面的再次硬化區(qū)和其下方的回火軟化區(qū)組成。這將表明，燒傷是工件次表層上形成奧氏體所導(dǎo)致的，針狀馬氏體（出現(xiàn)在平等針面的成形前的奧氏體晶粒）的形成成為工件表面淬火的冷卻體，這涉及到了磨削物相變化。
在理想條件下磨削過(guò)后，磨削表面不會(huì)開裂，并且會(huì)產(chǎn)生壓應(yīng)力阻礙腐蝕，在循環(huán)應(yīng)力條件下的工作壽命延長(zhǎng)。相對(duì)地，很多磨削表面處于拉應(yīng)力狀態(tài)，次層裂紋擴(kuò)展和氧化會(huì)使表面質(zhì)量惡化。為了在質(zhì)量精度和工件強(qiáng)度之間達(dá)到平衡，必須控制殘余應(yīng)力。這需要對(duì)工作表面溫度、產(chǎn)生的熱量和殘余應(yīng)力的構(gòu)成進(jìn)行詳細(xì)的研究。
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