Wear of the blade diamond tools in truing vitreous bond grinding wheels: Part II. Truing and grinding forces and wear mechanism

Truing and grinding forces and the wear mechanism of particle and rod diamond blade tools used to generate precise and intricate forms on rotating vitreous bond silicon carbide grinding wheels are presented. A Hall effect sensor was used to measure the change of grinding spindle power during truing and grinding. A signal processing procedure was developed to identify individual truing passes and to extract the average, peak-to-valley, and standard deviation of the variation of truing force for each pass. The truing force data and SEM micrographs of worn surfaces on blade tools reveal micro- and macro-fracturing of the diamond. The attritious and erosion wear of the diamond rod and particle, erosion of the metal bond, and pulling-out of the diamond particle are also identified. Grinding force data shows that, for the same truing parameters, a wheel trued by the rod diamond blade tool has higher grinding forces than one trued by a particle diamond blade tool.     

ROTARY TRUING OF VITREOUS BOND DIAMOND GRINDING WHEELS USING METAL BOND DIAMOND DISKS

Experiments on rotary truing of vitreous bond diamond grinding wheels were conducted to investigate the effects of truing speed ratio, type of diamond in the metal bond truing disks (synthetic versus natural), and diamond grit size in the grinding wheel on the wear of truing disk and on the cylindrical grinding of zirconia. Similar to G-ratio, a new parameter called D-ratio is defined to quantify the wear rate of the diamond truing disks. Experimental results show that, under the same truing condition, the truing disk with blocky, low friability synthetic diamond has a higher D-ratio than the truing disk with natural diamond. Diamond wheels trued by the disk with synthetic diamond also generate lower grinding force and rougher surface finish. High truing disk surface speed, 1.8 times higher than the surface speed of the grinding wheel, was tested and did not show any improvement in D-ratio. This study indicates that μm-scale precision form truing of the vitreous bond diamond wheel is difficult due to excess wear of the metal bond diamond truing disk.     

ROTARY TRUING OF VITREOUS BOND DIAMOND GRINDING WHEELS USING METAL BOND DIAMOND DISKS

Abstract

Experiments on rotary truing of vitreous bond diamond grinding wheels were conducted to investigate the effects of truing speed ratio, type of diamond in the metal bond truing disks (synthetic versus natural), and diamond grit size in the grinding wheel on the wear of truing disk and on the cylindrical grinding of zirconia. Similar to G-ratio, a new parameter called D-ratio is defined to quantify the wear rate of the diamond truing disks. Experimental results show that, under the same truing condition, the truing disk with blocky, low friability synthetic diamond has a higher D-ratio than the truing disk with natural diamond. Diamond wheels trued by the disk with synthetic diamond also generate lower grinding force and rougher surface finish. High truing disk surface speed, 1.8 times higher than the surface speed of the grinding wheel, was tested and did not show any improvement in D-ratio. This study indicates that μm-scale precision form truing of the vitreous bond diamond wheel is difficult due to excess wear of the metal bond diamond truing disk.     

Wear mechanism of metal bond diamond wheels trued by wire electrical discharge machining

The stereographic scanning electron microscopy (SEM) imaging was used to investigate the wear mechanism in wire electrical discharge machining (EDM) truing of metal bond diamond wheels for ceramic grinding. A piece of the grinding wheel was removed after truing and grinding to enable the examination of wheel surface and measurement of diamond protrusion heights using a SEM and stereographic imaging software. The stereographic SEM imaging method was calibrated by comparing with the profilometer measurement results. On the wheel surface after wire EDM truing and before grinding, some diamond grain protruding heights were measured in the 32 μm level. Comparing to the 54 μm average size of the diamond grain, this indicated that over half of the diamond was exposed. During the wire EDM process, electrical sparks occur between the metal bond and EDM wire, which leaves the diamond protruded in the gap between the wire electrode and wheel. These protruding diamond grains with weak bond to the wheel were fractured under a light grinding condition. After heavy grinding, the diamond protrusion heights were estimated in the 5–15 μm range above the wear flat. A cavity created by grinding debris erosion wear of the wheel bond could be identified around the diamond grain.     

树脂基圆弧金刚石砂轮的在位精密成形修整技术

针对球面、非球面及自由曲面超精密磨削加工用树脂基圆弧形金刚石砂轮难以精密修整的问题,提出基于旋转绿碳化硅(GC)磨棒的在位精密成形修整技术。在分析GC磨棒和圆弧砂轮几何关系的基础上,确定修整过程中圆弧插补轨迹的补偿方法及GC磨棒运动轨迹的设计方案。采用KEYENCE激光测微仪采集砂轮圆弧特征点,表征圆弧砂轮的修整状况。研究不同粒度的GC磨棒、进给深度和圆弧插补速度对圆弧金刚石砂轮修整率和修整精度的影响规律。研究结果表明,该修整方法可根据加工曲率半径要求实现不同圆弧半径砂轮的精密在位修整,修整后可自动消除砂轮垂直方向的位置偏差;采用400#和800#的GC磨棒对D3和D7砂轮均有较高的修整率(0.7~6.7);与400#和1500#的GC磨棒相比,800#GC磨棒更适合粒度为D3和D7圆弧金刚石砂轮的精密修整;相比圆弧插补速度,进给深度对砂轮的圆弧半径尺寸误差和形状误差影响更大,进给深度越小,圆弧半径尺寸误差和形状误差越小;修整后两种砂轮的圆弧半径误差均可控制在5%以内,D3砂轮的形状误差可控制在3μm/4 mm以内,D7金刚石砂轮可控制在6μm/4 mm以内,修整后比修整前形状误差提高14倍左右。     

SYSTEMS APPROACH FOR THE CERAMIC THRU-FEED CENTERLESS GRINDING

An approach for simulation and production implementation of thru-feed centerless grinding of transformation toughened zirconia using vitreous bond diamond grinding wheels is presented. A set of grinding tests was designed and conducted to identify the effects of abrasive product (diamond grit size), work-material, and operational factors (stock removal, thru-feed rate, number of thru-feed passes and truing and dressing methods) on grinding performance. Evaluation parameters of surface finish, grinding forces and C-ratio were utilized to determine machine tool features. Very fine surface finishes, in the order of 0.05 to 0.1 u.m R_a range, could be achieved using vitreous bond micro-grit diamond wheels, without additional part processing. Such a level of surface finish on ground ceramics has traditionally been generated only by using resinoid bonded diamond wheels. These test results were used to design and manufacture a full-size composite or sandwiched diamond grinding wheel. This grinding wheel has been successfully applied in high-volume ceramic production, which in turn has verified the proposed approach. Lastly, the surface finish data is interpreted based on SEM and AFM observations and materials properties.     

基于气中放电辅助修整金刚石砂轮的试验研究

提出了一种新型的基于气中放电辅助金刚石砂轮在线修整法,它把气中火花放电修整法和金刚石笔机械修整法有机结合在一起,能实现光学曲线磨床上金刚石砂轮的高效、高精度、低成本和不得使用任何冷却液的在线修整。采用自制的试验装置在MK9025数控光学曲线磨床上对1A1／T2 200×6×32×4 MBD6 120／140 M75型号的砂轮进行了在线修整试验,得出了一些影响金刚笔磨损率的主要因素:火花放电时热源的能量分布状况、金刚石笔修整时的修整深度、进给量以及电源电压。相同工况下机械修整法和气中放电辅助修整法中金刚石笔的磨损率对比试验结果表明:气中放电辅助修整和机械修整有着不同的材料去除机理,它比一般机械修整更有利于提高金刚石砂轮的修整效率和降低金刚石笔的磨损量。磨削YG8硬质合金工件的试验表明,采用气中放电辅助法修整的金刚石砂轮和新的金刚石砂轮有着相近的磨削能力。     

IN SITU TRUING/DRESSING OF DIAMOND WHEEL FOR PRECISION GRINDING

An application for achieving on-machine truing/dressing and monitoring of diamond wheel is dealt with in dry grinding. A dry electrical discharge （ED） assisted truing and dressing method is adopted in preparation of diamond grinding wheels. Effective and precise truing/dressing of a diamond wheel is carried out on a CNC curve grinding machine by utilizing an ED assisted diamond dresser. The dressed wheel is monitored online by a CCD vision system. It detects the topography changes of a wheel surface. The wear condition is evaluated by analyzing the edge deviation of a wheel image. The benefits of the proposed methods are confirmed by the grinding experiments. The designed truing/dressing device has high material removal rate, low dresser wear, and hence guarantees a desired wheel surface. Real-time monitoring of the wheel profile facilitates determining the optimum dressing amount, dressing interval, and the compensation error.     

电镀金刚石砂轮高效精密修整及熔融石英磨削试验研究

大尺寸光学玻璃元件主要采用细磨粒金刚石砂轮进行精密/超精密磨削加工,但存在砂轮修整频繁、工件表面面形精度难以保证、加工效率低等缺点。采用大磨粒金刚石砂轮进行加工则具有磨削比大、工件面形精度高等优点,然而高效精密的修整是其实现精密磨削的关键技术。采用Cr12钢对电镀金刚石砂轮(磨粒粒径151 μm)进行粗修整,借助修整区域聚集的热量加快金刚石的磨损,可使砂轮的回转误差快速降至10 μm以内。结合在线电解修锐技术,采用杯形金刚石修整滚轮对粗修整后的电镀砂轮进行精修整,砂轮的回转误差可达6 μm以内,轴向梯度误差由6 μm降至2.5 μm。通过对修整前后的金刚石砂轮表面磨损形貌成像及其拉曼光谱曲线分析了修整的机理。对应于不同的砂轮修整阶段进行熔融石英光学玻璃磨削试验,结果表明,砂轮回转误差较大时,工件材料表面以脆性断裂去除为主;随着砂轮回转误差和轴向梯度误差的减小,工件表面材料以塑性去除为主,磨削表面粗糙度为Ra19.6 nm,亚表层损伤深度低至2 μm。可见,经过精密修整的大磨粒电镀金刚石砂轮可以实现对光学玻璃的精密磨削。     

圆弧形砂轮精密修整及其声发射在线监测技术

针对球面、非球面及自由曲面超精密磨削加工用圆弧形金刚石砂轮难以精密修整的问题,提出基于旋转绿碳化硅（GC）磨棒的端部在位精密修整方法及修整过程的声发射在线监测技术。基于圆弧形金刚石砂轮的结构特性,制订圆弧形金刚石砂轮的在位精密修整与修整过程的声发射在线监测技术方案。依据修整与在线监测方案,对D64圆弧形金刚石砂轮进行修整实验及其声发射信号采集,修整后跳动误差小于10μm,比修整前减小30μm左右,砂轮精度显著提高。利用声发射信号均方根值获取砂轮修整结束的特征预警阈值,实现了旋转GC磨棒端部在位精密修整过程的在线监测以及修整结束时间的准确判断,可以有效提高球面非球面磨削加工过程的效率。     

Effect of nanosized alumina fillers on manufacturing of UV light-curable-resin bond abrasive tool

As one of the rapid prototyping technologies, ultraviolet-curable resin (UV-resin) curing was recently introduced into the manufacturing of resin bond abrasive tools. This research was conducted to evaluate the influence of nanosized alumina filler on the manufacturing of UV-resin bond diamond grinding wheel, and comparatively study the machining performance of filler-loaded and filler-unloaded tools. The UV-resin and diamond abrasive grains were prepared with nanosized alumina filler in a proportion of 0, 2.5, 5.0, 7.5, and 10 wt%. The cure depth, hardness, and tensile strength of the cured mixture was studied, and the interfacial bond between the diamond grain and cured resin matrix was investigated as well. Two UV-resin bond diamond grinding wheels were fabricated to examine the influence of filler loading on wear performance of the tools. Experimental results based on ceramic workpieces showed that the introduction of alumina filler improved not only the material properties of cured resin matrix but also led to a significant improvement on the abrasive machining performance of grinding wheel.     

HIGH SPEED AND HIGH MATERIAL REMOVAL RATE GRINDING OF CERAMICS USING THE VITREOUS BOND CBN WHEEL

High-speed (up to 127 m/s) and high material removal rate grinding experiments were conducted using a vitreous bond cubic boron nitride (CBN) wheel to investigate the effects of material removal rate, wheel speed, dwell time and truing speed ratio on cylindrical grinding of silicon nitride and zirconia. Experimental results show that operating the grinding wheel at a high surface speed can reduce grinding forces, enable high material removal rates, and achieve a higher grinding ratio (G-ratio). The material removal rate was increased to 9.6 and 7.6 mm³/s/mm for zirconia and silicon nitride, respectively, to explore the advantage of using high wheel speeds for cost-effective, high-material-removal-rate grinding of ceramics. Models for specific grinding force vs. the specific material removal rate and G-ratio vs. grinding wheel surface speed were developed based on the experimental results. Overall, this study showed that high grinding wheel surface speed is beneficial to the grinding of ceramics.     

金刚石刀具材料粒度、磨削砂轮粒度与刃口质量的关系

聚晶金刚石(PCD)刀具由于其自身的高硬度、高耐磨性的特征,使得刃磨加工极易出现崩刃——呈锯齿状刃口的典型缺陷,因此其机械磨削加工较其它刀具材料来看难度更大。本文针对金刚石刀具常用的加工方式——金刚石砂轮机械刃磨展开试验,主要针对金刚石刀片自身材料颗粒度与磨削砂轮粒度这两方面进行研究。通过收集数据,分析对比,研究金刚石材料颗粒度、磨削砂轮粒度对刀具最终刃口质量的影响。     

Wear of diamond tools in the dressing of discrete grinding wheels

The wear of a diamond tool in the dressing of continuous and discrete grinding wheels is studied. Formulas are derived for the wear of the diamond grains and the dressing tool when applied to a wheel with a discrete cutting surface.     

陶瓷轴向磨削加工实验分析及砂轮磨损研究

轴向磨削加工是以金刚石小砂轮的端部磨粒作为主切削刃来去除材料,用圆周部分内圆或外圆表面磨粒作为副切削刃对已加工圆柱面进行修磨的一种磨削加工技术。从轴向对工程陶瓷进行外圆或内孔加工时,一次切削的径向磨削深度(即背吃刀量)与进给速度分别可达5～10mm和200mm/min以上,实现了工程陶瓷外圆的高效低成本加工。利用该方法对陶瓷材料制成的发动机精密偶件出油阀套筒进行内孔加工,通过单因素试验,分析了不同参数组合下的砂轮磨损情况及各参数对砂轮磨损的影响,试验表明:砂轮磨损程度随磨削深度的增加而呈非线性增加;为使砂轮磨损最小化,主轴转速和工件转速应匹配,即二者的比值应控制在一定的范围内。     

OPTIMIZATION OF DRESSING CONDITIONS FOR A RESIN-BONDED DIAMOND WHEEL BY TOPOGRAPHY ANALYSIS

The macro-geometry (run-out, shape, profile, etc.) and the micro-geometry (density, distribution, and shape of the grains) of a grinding wheel are measured after both truing and dressing. The effectiveness of the truing is studied by analysing the topography of the wheel to characterize the tool and its effect on workpiece surface quality during the grinding process. Using wheel replicas, the surfaces are analyzed with a non-contact interference microscope and the measurements are processed with MountainsMap Software. For different truing and dressing conditions, the dressing efficiency, the grain density, and its effects on the roughness and wear of the wheel in the grinding process are investigated.     

金刚石微粉烧结棒与杯形砂轮修整碟形金刚石砂轮的对比试验研究

以反映砂轮平面度的端面跳动变化率和径向跳动变化率作为修整效率的评价依据,以插齿刀的齿形精度和齿面精度作为修整效果的评价依据,从试验和实际加工出的插齿刀齿形两方面对金刚石微粉烧结棒和杯形砂轮修整碟形金刚石砂轮进行了对比试验研究。结果表明,杯形砂轮的修整效率高于金刚石微粉烧结棒的修整效率;杯形砂轮修整的碟形金刚石砂轮具有较高的磨削能力。     

Investigation of internal ultrasonically assisted grinding of small holes: effect of ultrasonic vibration in truing and dressing of small CBN grinding wheel

In internal grinding of small holes, it is hard to realize high accuracy truing and dressing for the grinding wheel when a conventional truing/dressing method using single diamond dresser or rotary cup wheel is employed. Because of the cantilever support condition of the spindle shaft the stiffness of shaft is reduced. Thus the truing force leads the shaft to a significant deformation during truing/dressing. In this study, for improving the truing and dressing accuracy, a new truing/dressing method was proposed, in which the grinding wheel is ultrasonically vibrated along its axis during truing/dressing with a GC rotary cup wheel. A series of experiments were carried out to investigate the effects of the wheel ultrasonication on the truing force reduction, the truing accuracy improvement and the wheel surface condition. In addition, the grinding force and work surface roughness experimentally obtained by using the wheels trued with or without ultrasonication were compared. The experimental results indicated that applying ultrasonic vibration to the wheel decreases the normal and tangential grinding forces by more than 20% and 24%, respectively, and the surface roughness by as much as 18%.     

Experimental study on fabrication and evaluation of micro pyramid-structured silicon surface using a V-tip of diamond grinding wheel

A mechanical fabrication of micro pyramid-structured silicon surface is proposed using crossed grooving with a 60° V-tip of diamond grinding wheel. It can obtain high form-accuracy, good surface quality and efficient productivity in contrast to laser machining and etching, and also assure a high aspect ratio in contrast to other mechanical processes. In order to describe its micro-structured topography, a white-light interferometer was employed, and its measured point cloud was matched using an Iterative Closest Point (ICP) algorithm. In micro grinding, a novel CNC mutual-wear truing was first developed to sharpen the wheel V-tip; then, the effects of microscopic wheel topography, silicon crystal-orientation and grinding parameter were investigated on ground micro-topography, truing ratio and material removal ratio; finally, its form-accuracy, pyramid top radius, groove tip radius, surface roughness and aspect ratio were evaluated. It is shown that better microscopic grain protrusion topography on wheel V-tip produces much larger material removal ratio and much better micro-structured topography in micro grinding, but it leads to much less truing ratio in finer GC truing. In micro grinding, silicon crystal-orientation has little effect on micro-structured topography due to diamond crystal-orientations that are randomly distributed on wheel V-tip. Although the micro pyramid-structured form error is only about 3.4 μm, its V-groove bottom and pyramidal top have very large form errors (23.1-47.9 μm) due to the sharpness of wheel V-tip and the frangibility of micro pyramid top. On increasing feed speed, its pyramid top radius decreases and its groove tip radius slightly increases, ultimately leading to an increase in aspect ratio, whereas its surface quality descends. It is concluded that the micro-pyramid arrays may be precisely patterned on silicon surface using a SD600 wheel with crossed tool paths, on-machine V-tip truing and the depth of cut in 1 μm.     

Form-truing error compensation of diamond grinding wheel in CNC envelope grinding of free-form surface

In computer numerical control (CNC) grinding of free-form surface, an ideal arc profile of trued diamond grinding wheel is generally employed to plan 3D tool paths, whereas its form-truing errors greatly influence the ground form accuracy. A form-truing error compensation approach is proposed by using an approached wheel arc profile to replace the previously designed ideal one. The objective is to directly compensate the trued wheel arc-profile errors. It may avoid the time consumption of traditional approach that compensates the measured coordinate point errors of workpiece to an iterative grinding operation. First, the 3D tool path surface was constructed to plan the 3D tool paths. Second, the CNC arc truing of grinding wheel was conducted to analyze the form-truing error distribution relative to the applied wheel arc profile. Then, the form-truing error compensation was carried out in CNC envelope grinding. Finally, the iterative closest point (ICP) algorithm was used to match the measured coordinate points of workpiece to ideal free-form surface. It is shown that the 3D tool path surface constructed is practicable to plan arbitrary 3D tool paths for the form-truing error compensation. The ICP matching may be used to investigate 3D ground form error distribution. It is confirmed that the form-truing error compensation can directly improve the 3D ground form accuracy. It may decrease the 3D ground form error by about 20% when the 2D form-truing error is reduced by about 58% using the same truing conditions for CNC grinding.