ON CUMULATIVE DEPTH OF TOUCH-DRESSING OF SINGLE LAYER BRAZED cBN WHEELS WITH REGULAR GRIT DISTRIBUTION PATTERN

The present work deals with a single layer brazed type cBN grinding wheels, which have been developed in-house. Grits were actively brazed on the working surface of the wheels in a regularly distributed pattern so that it could perform grinding without loading. In general, such a wheel produces substantially high transverse roughness. This happens because of its low active grain density. A touch-dressing technique, developed in-house, has been successfully applied on these brazed wheels so that most of the grits did participate and the average roughness could be brought down to an acceptable magnitude. This work investigated the co-relation between the grit size and cumulative depth of dressing required to achieve an acceptable magnitude of transverse roughness. It was found that the required cumulative depth of dressing was dependent on the size of cBN grits. This paper also shows the effect of gradual touch-dressing on improvement of roughness of the surface ground by such new class of recently developed wheels. Experiments were conducted with microcrystalline cBN grits of three different sizes.     

Investigation and development of a molding process for the production of micro-hairs

Cubic boron nitride (cBN) is a unique synthetic material on account of its high hardness, high wear resistance, excellent cutting edge stability and relative chemical inertness compared to diamond. The introduction of monolayer electroplated cBN wheels replaced the complex pre-grinding wheel preparation work (truing and dressing) of composite cBN wheels and thereby extensively facilitating the application in high-efficiency deep grinding, creep feed grinding, etc. The present work has aimed at developing a precisely controlled brazing technique suitable for bonding the cBN grits to a steel substrate in monolayer form with higher bond strength, larger grit protrusion and more uniform grit distribution compared to that in the currently used galvanically bonded wheels. Experimental investigation have clearly demonstrated the potential of the newly developed brazed wheels under varying grinding conditions for processing materials like bearing steel. Improved capability of these wheels over galvanically bonded wheels could be better recognised during dry grinding at high material removal rate and for large stock removal when galvanically bonded wheels were found to suffer from severe wheel loading in grinding bearing steel and from unusual increase in grinding forces due to grit pullout. Creation of wider inter-grit spaces with strong bonding and uniform grit spacing happened to be the essence of the present brazed cBN wheel.     

On the grindability of low-carbon steel under dry, cryogenic and neat oil environments with monolayer brazed cBN and alumina wheels

Grinding of low-carbon steel often exhibits severe wheel loading due to the formation of long chips and high adhering tendency of the work material with the grits. Conventional composite-type alumina wheels are commercially utilised for grinding low-carbon steel. However, the actual nature of grit wear cannot be truly understood in a composite wheel. The truing and dressing conditions also have some influences on the wear mechanism. Therefore, in order to explore the wear pattern on a single layer of grits, monolayer brazed cBN, white and grey Al₂O₃ wheels were used in the present study. The grindability of AISI 1020 steel was evaluated under dry, liquid nitrogen and neat oil environments. The surface profile of the workpiece after being ground in each environmental condition was traced with a surface profilometer to reveal the mechanism of grit wear. The post-grinding conditions of the wheels were observed using scanning electron microscopy. The cBN wheel was found to outperform the alumina wheels in terms of grinding forces and grit wear. The wear of the cBN wheel was remarkably arrested with the application of neat oil. On the other hand, large-scale adhesion and breakage of grits in white alumina wheel were observed under cryogenic environment. In fact, the beneficial role of liquid nitrogen could not be realised in reducing grinding forces and grit wear with all the three types of wheel. A lubricating agent like neat oil appeared to be more suitable than cryogenic cooling when grinding low-carbon steel.     

Creep feed profile grinding of Ni-based superalloys with ultrafine-polycrystalline cBN abrasive grits

This paper presents the grinding characteristics of a newly developed ultrafine-polycrystalline cBN abrasive grit in creep feed profile grinding of Nickel-based superalloys. Experiments for producing a rectangular-shaped groove on a flat surface in one pass by creep feed grinding have been carried out using the new polycrystalline cBN (PcBN-U) grit and a representative conventional monocrystalline cBN (McBN-B1) grit. The grinding forces in grinding with PcBN-U grits are reduced by 20 30% compared with McBN-B1 grits. When grinding with PcBN-U grits, both radial wear and profile wear are less, and hence the grinding ratio is around 4 times higher than that with McBN-B1 grits. The size of grit fracture during the grinding process with PcBN-U grits is smaller than that with McBN-B1 grits. It gives lower wheel wear rate and longer wheel life in grinding with PcBN-U than with McBN-B1.     

MODELLING OF DISC DRESSING FORCES

A new mathematical model of the dressing of grinding wheels by diamond disc dresser is developed. It predicts the dressing forces between the diamond grits of a rotating disc dresser and the grits of an alumina grinding wheel. This model is based on the fracture of grits, the fracture of the bond and the contact forces between the dresser and grits. It considers the kinematical influences and in particular speed ratio of the dressing process. Also a Weibull distribution was used to predict the probability of bond fracture. Dressing forces are measured experimentally and are compared with those calculated from the new model. Theoretical results closely match with the experimental findings. The dressing forces generated from this model provide a basis for further prediction of wheel topography and the grinding process.     

MODELLING OF DISC DRESSING FORCES

A new mathematical model of the dressing of grinding wheels by diamond disc dresser is developed. It predicts the dressing forces between the diamond grits of a rotating disc dresser and the grits of an alumina grinding wheel. This model is based on the fracture of grits, the fracture of the bond and the contact forces between the dresser and grits. It considers the kinematical influences and in particular speed ratio of the dressing process. Also a Weibull distribution was used to predict the probability of bond fracture. Dressing forces are measured experimentally and are compared with those calculated from the new model. Theoretical results closely match with the experimental findings. The dressing forces generated from this model provide a basis for further prediction of wheel topography and the grinding process.     

砂轮表面形貌定量评价及修整效果研究

基于砂轮表面磨粒出刃面积,提出砂轮表面磨粒出刃面积率Sr的概念,在此基础上提出磨粒出刃度βd和磨粒出刃面积分散度δs两个指标来评价砂轮表面形貌特征。用滚轮修整器对树脂结合剂金刚石砂轮进行修整实验,分析了不同修整方式、修整深度和修整速比对βd和δs的影响。采用修整后的砂轮进行磨削实验,分析了不同特征参数的砂轮对磨削力和工件表面粗糙度的影响。研究结果表明,采用较小的修整深度、较小的修整速比,多次进给修整砂轮时,磨粒出刃度高、磨粒出刃面积分散度小,修整效果好。当βd为69.35%、δs为1 000 μm2时,磨削力、表面粗糙度最小,表明砂轮磨削性能最好,证明修整质量最高。因此所提出的βd、δs两个指标能对修整后的砂轮表面形貌进行有效评价,且磨粒出刃面积的检测方法简单,检测效率较高。     

MODELING OF DRESSING FORCES DURING ROTARY DIAMOND CUP DRESSING OF VITRIFIED CBN GRINDING WHEELS

This paper presents a mathematical model of dressing of vitrified CBN grinding wheels by a diamond cup dresser. It predicts the dressing forces during rotary diamond cup dressing of vitrified CBN grinding wheels. This model is based on the fracture of abrasive grits, the fracture of the bond and the contact forces between dresser and grinding wheel. It considers the kinematical influences and in particular speed ratio and overlap factor during the dressing process. A Weibull distribution is used to predict the probability of bond fracture and also the collision number between the diamond grits of a rotating dresser and the CBN grits. This model is validated by experimental results. The theoretical modeling values agree reasonably well with the experimental results. On the basis of this model the effect of different cup dressing parameters on dressing forces is theoretically discussed with the aim of establishing appropriate dressing process configurations. Furthermore the presented model provides a basis for further prediction of wheel topography and the grinding process.     

磨削参数对超细硬质合金磨削表面粗糙度的影响

在使用金刚石砂轮的平面磨床上对超细硬质合金进行了磨削试验研究。通过扫描电子显微镜观察磨削表面形貌和用表面粗糙度测定仪测量磨削表面粗糙度,分析了磨削参数对超细硬质合金磨削表面粗糙度的影响。研究结果表明,同一切深下,超细硬质合金磨削表面粗糙度随砂轮粒度的增大而增大。采用相同粒度砂轮磨削,切深较小时,超细硬质合金磨削表面粗糙度随切深的增加而增大,当切深增大到一定值后,磨削表面粗糙度值逐渐降低。     

应用环形磁场控制的微粉砂轮制备及其磨削性能

在超精密磨削中,金刚石微粉砂轮的磨粒分布均匀性对提高磨削表面质量至关重要,为了使微粉磨粒规则排布,提出了一种采用环形磁场控制磨粒规则排布的砂轮制备方法,制备了多种金刚石微粉砂轮,使用磁场控制制备的微粉砂轮对硬质合金YG8进行了平面及非球面磨削试验。结果表明:应用环形磁场控制可使金刚石微粉砂轮的磨粒实现规则排布,极大改善砂轮加工性能,利用环形磁控方法制备的砂轮可获得最佳表面粗糙度Ra3 nm、最佳面形精度PV318 nm的光滑镜面。     

Experimental study of time-dependent performance in superalloy high-speed grinding with cBN wheels

The time-dependent performance of grinding is expressed as the change of process output measures as a function of time during grinding. Although the wheel capability will be restored by dressing, the time-dependent performance of grinding during one dressing skip is the determinant on the grinding quality variation in terms of surface integrity and workpiece geometric accuracy. Therefore, understanding of grinding time-dependent performance in relation with the wheel–workpiece microscopic interaction is critical for wheel and process development to achieve stable grinding processes. In this article, the grinding of superalloy with cubic boron nitride (cBN) grinding wheels is performed. The time-dependent performance is recorded to represent the characteristic features, and the microscopic wheel topography is measured under scanning electron microscope (SEM) throughout the grinding process, so as to reveal the root cause for the time-dependent performance and its impact on the workpiece quality variation. The experiment results indicate that during the grinding process, there exist three characteristic stages, namely, initial wheel wear stage, severe wheel wear stage, and wheel resharpening stage. Moreover, the change trend of spindle power consumption, workpiece quality on surface hardness and roughness, wheel wear condition, and G ratio are consistent with the wheel topography evolution reflected by SEM photos, which can be used to present the three grinding stages. The wear and replacement of the efficient grain cutting edges result in the time-dependent performance during superalloy high-speed grinding with cBN wheels.     

单层钎焊金刚石砂轮的精密修整及效果评价

单层钎焊金刚石砂轮作为一种新型的磨削工具,具有磨粒固结强度高、磨粒出露大、容屑空间大等优点,比较适合高效率大切深的强力磨削,然而这种工具对高性能的脆性材料的精密磨削却比较困难。本文通过两种精密的修整工艺,使得加工表面质量大大提高。通过观察砂轮磨粒形态的变化可知,磨粒在修整过程中存在有磨损钝化、破碎、表面粘附等现象;通过对砂轮轮廓的激光测量可知,砂轮的磨粒等高性在修整过程中是明显改善的。通过修整磨粒粒径300μm的钎焊砂轮磨削氧化锆的表面粗糙度达到了Ra0.2μm。     

单层钎焊金刚石砂轮的修整实验研究

为满足单层钎焊金刚石砂轮高效精密加工的性能要求,对磨粒有序排布单层钎焊金刚石砂轮的修整进行了实验研究。采用磨粒有序排布的钎焊金刚石修整工具对钎焊砂轮进行了修整,该修整方法通过修整工具粒度的变化以及修整速比的调整来控制砂轮形貌,从而使氧化锆的磨削表面粗糙度达到了精密加工的水平。对砂轮形貌进行了观测统计,数据表明,砂轮磨粒的等高性得到明显改善且避免了磨粒端部的严重钝化。     

A novel technique for dressing metal-bonded diamond grinding wheel with abrasive waterjet and touch truing

The dressing of metal-bonded diamond grinding wheels is difficult despite their availabilities on hard and brittle materials. In this paper, a novel compound technology that combines abrasive waterjet (AWJ) and touch truing is proposed for dressing metal-bonded diamond grinding wheel precisely and efficiently. The dressing experiments of a coarse-grained and a fine-grained bronze-bonded diamond grinding wheel were carried out on a surface grinder with a developed AWJ system. The feasibility of this method was verified by analyzing the wheel runout, the truing forces, and the wheel surface topography. The variations of 3D surface roughness of wheel surface topography during the compound dressing process were quantitatively analyzed. The mechanism of AWJ and touch compound dressing is also discussed. Further, a reaction-bonded silicon carbide block was ground to validate the dressing quality. The experiment results indicate that the grinding wheels that were well dressed by the proposed technique leads to a smaller grinding force and a smaller surface roughness than that of undressed wheels.     

陶瓷结合剂金刚石砂轮磨削超细晶粒硬质合金的表面粗糙度研究

本文结合单因素实验和正交实验,研究了从低速到高速磨削条件下,砂轮速度、进给速度、磨削深度、最大未变形磨削厚度以及磨削方式(顺磨或逆磨)对陶瓷结合剂金刚石砂轮磨削超细晶粒硬质合金表面粗糙度的影响规律,分析了影响超细晶粒硬质合金表面加工质量的原因。研究表明,总体来说磨削参数的变化对超细晶粒硬质合金表面粗糙度的影响程度不大。高速磨削时的表面粗糙度相比低速磨削得到了比较明显改善。逆磨时的粗糙度比顺磨大,随砂轮速度增加下降更快。相比传统硬质合金,磨削WC颗粒更细、强度更高的超细晶粒硬质合金的表面粗糙度更低。磨削参数对表面粗糙度的影响程度从小到大依次是磨削深度、砂轮速度和进给速度,实际加工时为同时获得较高的磨除率和表面质量,宜采用高砂轮速度、低进给和大切深的磨削组合。     

Mist-jetting electrical discharge dressing (MEDD) of nonmetal bond diamond grinding wheels using conductive coating

Diamond wheels are widely used in high-precision grinding of hard and brittle materials; unfortunately, they are difficult to true and dress. This paper addresses that problem in that it proposes an effective dressing technique—mist-jetting electrical discharge dressing (MEDD) of nonmetal bond diamond grinding wheels using conductive coating. A conductive phase is coated on the wheel surface to increase the conductivity of the nonmetal bond. Electrical discharge model was built to analyze feasibility and select optimized parameters of MEDD. Experiments were conducted to evaluate the dressing performance of MEDD in terms of surface morphology of the wheel surface, grinding force, and surface roughness of the workpiece. Experimental results show that abrasive grains on the wheel protrude are satisfied. The discharge parameters have an important influence on the dressing result. The grinding force and the surface roughness of the workpiece significantly reduced after dressing.     

Cylindrical grinding of bearing steel with electrolytic in-process dressing

Cylindrical surfaces are increasingly utilized in various areas, and related high-efficiency and high-quality fabricating technologies are of great importance and significant benefit to modern industry. To provide fundamental knowledge for the bearing industry, studies have been conducted on the use of cast-iron-bonded cubic boron nitride (cBN) superabrasive wheels, based on electrolytic in-process dressing (ELID) technique for realizing high-efficiency grinding of steel cylindrical workpieces. Therefore, in this investigation, experiments were carried out on an ordinary cylindrical grinder with a hydrodynamic spindle, and straight type grinding wheels of different grit sizes were used. The effects of grit sizes on surface roughness as well as waviness, roundness, and surface stress were evaluated in both the traverse and plunge grinding modes. Comparison of ELID grinding with other processes was also carried out. Mirror surface grinding of different materials was achieved with the #4000 CIB-cBN wheel. ELID grinding was confirmed to induce compressive stress and to be more cost effective for small batch production of larger components when it works in the traverse mode.     

A study on wear mechanism and wear reduction strategies in grinding wheels used for ELID grinding

Metal-bonded superabrasive diamond grinding wheels have superior qualities such as high bond strength, high stability and high grindability. The major problems encountered are wheel loading and glazing, which impedes the effectiveness of the grinding wheel. Electrolytic in-process dressing (ELID) is an effective method to dress the grinding wheel during grinding. The wear mechanism of metal-bonded grinding wheels dressed using ELID is different form the conventional grinding methods because the bond strength of the wheel-working surface is reduced by electrolysis. The reduction of bond strength reduces the grit-depth-of-cut and hence the surface finish is improved. The oxide layer formed on the surface of the grinding wheel experiences macrofracture at the end of wheel life while machining hard and brittle workpieces. When the wheel wear is dominated by macrofracture, the wheel-working surface is free from loaded chips and worn diamond grits. When the oxide layer is removed from the wheel surface, the electrical conductivity of the grinding wheel increases, and that stimulates electrolytic dressing. The conditions applied to the pulse current influence the amount of layer oxidizing from the grinding wheel surface. Longer pulse ‘on’ time increases the wheel wear. Shorter pulse ‘on’ time can be selected for a courser grit size wheel since that type of wheel needs high grinding efficiency. Equal pulse ‘on’ and ‘off’ time is desired for finer grit size wheels to obtain stable and ultraprecision surface finish.     

金刚石表面的膜层沉积处理及其感应钎焊的研究

将热丝化学气相沉积(HFCVD)处理的金刚石作为磨料感应钎焊制作金刚石工具。HFCVD处理试验中,混合气为H2和CH4(体积流量比为100∶1.5),炉内压力为2.0kPa,700℃下处理45min后,在金刚石表面沉积了一层非晶碳膜。感应钎焊HFCVD处理的金刚石显示,出露部分的金刚石棱边能保持良好的锋利性;浸没在钎料层下面的金刚石表面形成了有均匀孔隙且形状不规则的铬碳化合物,液态钎料充填这些化合物孔隙之间,能够增强钎料对金刚石的把持强度。3种金刚石磨料感应钎焊制作的金刚石磨盘的高效重负荷石材磨削试验显示,HFCVD处理的金刚石的整体破碎率和脱落率最低。