ELID grinding characteristics of glass-ceramic materials

Zerodur glass-ceramic materials have been widely used in optical, opto-electronic and precision engineering industries; their efficient ultraprecision machining, with extremely low surface roughness and high form accuracy, is in great demand in those fields. The authors have been conducting studies on realizing high-quality surface and form accuracy of zerodur glass-ceramic materials efficiently by use of electrolytic in-process dressing (ELID) grinding process. This paper proposes a new grinding mode in which top surface and sides of zerodur block were ground by cylindrical surface and side surface of grinding wheel. Grinding experiments were carried out using #1200, #2000 and #4000 diamond cast-iron bond wheels, and grinding characteristics such as grinding performance, ground surface roughness, surface topographies and perpendicularity between ground surfaces were investigated. Experiments on grinding using #4000 wheel successfully produced smooth top surface and side surfaces that were about 10 nmRa in surface roughness, 1.5 μm/□400×400 mm² in flatness and 90°±6″ in perpendicularity. AFM observation of the ground surface also showed that material removal in the ductile mode occurs for fine abrasive wheels. The results showed that grinding was stable without severe clogging for wheels by choosing suitable ELID parameters and grinding conditions.     

Ultra precision grinding of spherical convex surfaces on combination brittle materials using resin and metal bond cup wheels

This paper reports on a parametric investigation on ultra precision grinding of combination materials of zirconia and silica. Rotational grinding was adopted for the generation of spherical convex surfaces using metal and resin bonded cup wheels. The wheels have grit sizes ranging from 4 to 20 μm. A simplified geometrical model was used to analyse the effects of wheel wear on ground dimensional accuracy. Besides, a new truing and dressing technique of the cup wheel using loose abrasives was introduced. This new truing and dressing technique enables the precision profile truing of cup wheels. With the well prepared cup wheel, high form accuracy (average PV 0.25 μm) with mirror/near-mirror surface finish for silica and zirconia was achieved.     

On-machine dry electric discharge truing of diamond wheels for micro-structured surfaces grinding

Precision grinding with diamond wheels gives a promising alternative to achieve high quality micro-structured surfaces on optical molds. However, it is difficult to true these diamond wheels efficiently, because of the remarkable resistance property and the geometrical limitation of small wheel profile. In this paper, an on-machine dry-EDT method to precision shape and prepare diamond wheels with various profiles was proposed for micro-structured surface grinding. Firstly, the fundamental truing errors were analyzed based on the dry-EDT kinematics. And then the capabilities of dry-EDT truing for high abrasive concentration metal bonded diamond wheels were presented. Next, the effects of kinematic parameters variables on trued wheel profile accuracy were investigated. Finally, the micro-structured surfaces on SiC ceramic and tungsten carbide WC were ground by these trued diamond wheels. The experiments results showed that the arc-shaped diamond wheel (diameter of 200 mm) with 4 μm profile error (PV) and 1.0023 mm profile radius, and the V-shaped diamond wheel with 22.5 μm V-tip radius and 120.03° profile angle could be obtained by on-machine dry EDT. The kinematic parameters of dry-EDT have an important influence on truing profile accuracy of diamond wheels, especially for the tip of V-shaped wheel. The subsequent grinding show that the edge radius of V groove array on SiC is less than 2 μm, while the radius of included corner is around 55 μm. The PV error of ground arc groove array on WC is less than 5 μm. The surface roughness of ground micro-structured surface R_a is 142 nm and 97 nm for SiC and WC, respectively.     

Temperatures and wear mechanisms in dressing of vitrified bonded grinding wheels

For the first time dressing temperatures were measured directly in the contact zone of grinding and diamond dressing tool. With average temperatures of 400 °C and maximum temperatures of 810 °C without cooling lubricant, the need to cool the dressing tool sufficiently is affirmed. For a diamond dressing tile and form rollers in contact with vitrified bonded corundum and CBN wheels, the main grit wear mechanisms were adhesions instead of breakage or high volumetric splintering. Extensive analyses proved that components of the grinding wheel bonding and abrasives were molten during the dressing process and solidified amorphously. This verifies high peak surface pressures and flash temperatures.     

Finishing characteristics of brittle materials by ELID-lap grinding using metal-resin bonded wheels

ELID-lap grinding is a method of constant pressure grinding which utilizes an electrically conductive wheel and the electrolytic in-process dressing (ELID) method. This method has the advantage of using micro grain-wheels above #10 000 and also, through simple modification, can be used on existing lap machines. To find the characteristics of metal-resin bonded wheels developed for ELID-lap grinding, experiments on the grinding of brittle materials were performed using wheels with a variety of grain diameters. The wheels used in the experiments were #8000, #120 000 and #3 000 000 metal-resin bonded diamond wheels (#8000 MRB-D, #120 000 MRB-D and #3 000 000 MRB-D wheels). The workpieces were silicon and glass. The results of the experiments showed that stable grinding can be achieved with the #8000 to #3 000 000 MRB-D wheels. With the #3 000 000 MRB-D wheel, very smooth surface finishes were obtained for both silicon (PV 2.8 nm) and glass (PV 2.5 nm). Atomic Force Microscope (AFM) and Scanning Electron Microscope (SEM) observations indicated these surfaces to be very smooth in the order of several nanometers, obtained by mechanical removal using an ultrafine wheel.     

Electrochemical oxidation analysis for dressing bronze-bonded diamond grinding wheels

Electrochemical dressing of fine-grained metal-bonded diamond grinding wheels enables to grind hard and brittle materials in the ductile mode. Optical surfaces can be manufactured by grinding, which reduces the need for subsequent, time-consuming polishing work. When using metal-bonded grinding wheels, the emerging oxides regulate the electrochemical dissolution. Bronze-bonded grinding wheels are more suitable for grinding cemented carbides and ceramics than iron-bonded grinding wheels, as it is easier to modify their chemical composition to suit a specific grinding task. They can also be sintered at lower temperatures, which reduces the risk of thermal damage to the diamond. In this paper, the dissolution and the oxidation of different bronze alloys are characterized for the electrochemical dressing process. The relevant evaluation criteria are the oxide layer thickness, the electrical behavior and the different emerging bronze alloy oxides. This work is funded by the German Research Association DFG within the Transregional Collaborative Research Center SFB/TR4 “Process Chains for the Replication of Complex Optical Elements”.     

熔石英磨削砂轮截面轮廓分析与优化

为提高熔石英光学元件磨削加工质量,降低亚表面缺陷层深度,通过实验和理论分析发现,具有平直截面轮廓的平行金刚石砂轮,在加工熔石英光学元件时砂轮边缘位置处会产生加工应力集中,该应力集中直接导致了元件表面产生"连续白线"状深缺陷,其深度达22 μm。为抑制砂轮边缘位置加工应力集中现象,设计了"中央平直线+两侧边缘圆弧过渡"的复合式截面轮廓,并提出了相应的修整方法。通过砂轮修整实验,验证了该方法的可行性,磨削元件"连续白线"状深缺陷得到明显改善,亚表面缺陷深度小于2.5 μm,实现熔石英元件的低缺陷磨削加工。      

Application potential of coarse-grained diamond grinding wheels for precision grinding of optical materials

Fine-grained resin bonded diamond tools are often used for ultra-precision machining of brittle materials to achieve optical surfaces. A well-known drawback is the high tool wear. Therefore, grinding processes need to be developed exhibiting less wear and higher profitability. Consequently, the presented work focuses on conditioning a mono-layered, coarse-grained diamond grinding wheel with a spherical profile and an average grain size of 301 µm by combining a thermo-chemical and a mechanical-abrasive dressing technique. This processing leads to a run-out error of the grinding wheel in a low-micrometer range. Additionally, the thermo-chemical dressing leads to flattened grains, which supports the generation of hydrostatic pressure in the cutting zone and enables ductile-mode grinding of hard and brittle materials. After dressing, the application characteristics of coarse-grained diamond grinding wheels were examined by grinding optical glasses, fused silica and glass–ceramics in two different kinematics, plunge-cut surface grinding and cross grinding. For plunge-cut surface grinding, a critical depth of cut and surface roughness were determined and for cross-grinding experiments the subsurface damage was analyzed additionally. Finally, the identified parameters for ductile-machining with coarse-grained diamond grinding wheels were used for grinding a surface of 2000 mm² in glass–ceramics.     

Internal mirror grinding with a metal/metal–resin bonded abrasive wheel

Precision internal cylindrical surfaces are increasingly utilized in industries where highly efficient methods to fabricate such surfaces are of great importance. Presently, several processes usually performed on different machines are used successively to achieve a mirror finish. In this paper, a novel method is presented to accomplish roughing and finishing processes on one machine, using super abrasive cast–iron–bond CBN (CIB–CBN) wheels and metal–resin–bond CBN (MRB–CBN) wheels. Two dressing methods, electrolytic interval dressing and electrolytic in-process dressing (ELID), are also developed for these wheels respectively. The CIB–CBN wheel is trued by an on-machine electrical discharge truer (EDT) and dressed at intervals with a pipe-shape electrode; the MRB–CBN wheel is dressed in process, in which no specific dressing electrode is utilized, instead the conductive workpiece acts as a dressing electrode. Precision grinding has been carried out on an ordinary grinder with an attachment for internal cylindrical grinding. Wheels of #325, #1200, #2000, and #4000 grit sizes have been applied in the process to obtain mirror surfaces.     

Application of shallow circumferential grooved wheels to creep-feed grinding

A single-point diamond dressing tool was used to cut shallow circumferential groove on aluminum oxide grinding wheels. Creep-feed grinding experiments were then carried out to compare the performance of these grooved wheels with a non-grooved wheel. The results showed that, for the conditions used in this research, a grooved wheel could remove twice as much material as a non-grooved wheel before workpiece burn occurred. The results also showed that a grooved wheel can improve grinding efficiency by reducing the consumed power by up to 61%. Although the use of grooved grinding wheels caused the workpiece surface roughness to increase slightly when compared to a non-grooved wheel, the grooved wheel enabled up to 37% more material to be removed while still maintaining workpiece surface roughness values below 0.3 μm (“fine quality” surface finish), and up to 120% more material to be removed while still maintaining workpiece surface roughness values below 1.6 μm (“average quality” surface finish).     

Improving the Wear Behavior of WC-CoCr-based HVOF Coating by Surface Grinding

WC-CoCr-based high velocity oxy fuel (HVOF) coatings are being used for several components which are prone to severe erosion or abrasion. In this study, the HVOF coating was applied by liquid fuel-based equipment. These coated samples were subjected to surface grinding of various depths (100, 200, and 300 μm). Hardness test after surface grinding showed that the coating hardness increased by 33% after grinding to a depth of 200 μm (1472 Hv). The residual stress after different depths of grinding was measured using x-ray diffraction. It showed that the compressive residual stress of coating increased with grinding. Increase in hardness of the coating (after grinding) is believed to be due to the increase in compressive residual stress. The abrasive wear resistance increased after grinding to a depth of 100 μm thickness and remained constant during successive grinding. In contrast, the erosive wear resistance increased the most when the grinding thickness was 200 μm. It is concluded that the surface grinding of coatings helps in increasing abrasive and erosive wear resistance. The increase in microhardness of the coating is believed to be the reason for high wear resistance. SEM studies of worn out surface show carbide grain pull out due to removal of softer phase, i.e. cobalt and chromium, and is followed by tungsten carbide grain pull out.     

Dressing and truing of hybrid bonded CBN grinding tools using a short-pulsed fibre laser

This paper presents the results of an investigation into the dressing and truing of hybrid bonded (metal-vitrified) CBN grinding wheels using a short-pulsed fibre laser. Truing of complex contours on CBN grinding tools with sharp edges (edge radii of less than 20 μm) could be successfully applied, whereas other dressing methods have been neither technically nor economically successful. Sharpening by laser can provide the same wheel surface topography which is conventionally produced by SiC and/or Al₂O₃ sharpening tools. Grinding characteristics and long-term performance of the laser-profiled tools are discussed.     

Form crush dressing of diamond grinding wheels

Form crush dressing is a method to profile diamond grinding wheels. Especially for complex profiles with intricate details and high accuracy an improved form crush profiling system has been developed. The system consists of a hydrostatically supported form crushing disc embedded in a swiveling axis. Form disc wear, which forms a major bottleneck in reaching high accuracy, has been minimized through the development of an advanced wheel synchronization control strategy. Various vitrified grinding wheels and a crushable metal bonded wheel have been tested to show the general applicability of the method to grinding wheels with brittle bond systems.     

A study on the analysis of grinding mechanism and development of dressing system by using optimum in-process electrolytic dressing

In recent years, grinding techniques for precision machining of brittle materials used in electric, optical and die parts have been improved by using superabrasive wheels and precision grinding machines. The completion of optimum dressing of superabrasive wheels makes possible the effective precision grinding of brittle materials, but the present dressing system cannot provide sufficient control for optimum dressing of the wheels. In this study, a new and grinding technique of optimum in-process electrolytic dressing is proposed. This system can carry out optimum in-process dressing of superabrasive wheels, and give very effective control according to unstable current and gap increase. Therefore, the optimum in-process electrolytic dressing is a good method to efficiently obtain and mirror-like grind brittle materials.     

Abrasive waterjet turning—An efficient method to profile and dress grinding wheels

Abrasive waterjet (AWJ) turning is a technology that still tries to find its niche field of application where it can be economically viable. The paper reports on a particular application of AWJ turning that proved its technological and economical capability, i.e. profiling and dressing of grinding wheels. Starting from the theoretical considerations, the key operating parameters of AWJ turning are identified and included in a methodology to generate various profiles of grinding wheels by means of tangential movement of the jet plume. Roughing in single pass to concave/convex geometries (experimented depth of cuts <30 mm), generation of thin walls/slots (thickness <2 mm, depth >30 mm) and intricate profiles (e.g. succession of tight radii) on a variety of grinding wheels show the capability of AWJ turning to fulfil the requirements of this niche application. With or without employing abrasive grits, a summary of the parameters of AWJ turning is presented along with an assessment of geometrical accuracy of profiled grinding wheels; additionally, evaluations of operational times to support the efficiency of the methods are presented. Finally, the paper discusses the limitations of the method and other possible applications of the AWJ in profiling of grinding wheels to help the definition of the “new applicative portfolio” of this technology.     

A study on the mirror-like grinding of die steel with optimum in-process electrolytic dressing

In recent years, grinding techniques for the precision machining of brittle materials used in die, mold and optical parts have been improved by using super-abrasive wheels and precision grinding machines. The completion of the optimum dressing of a super-abrasive wheel makes possible the effective precision grinding of die steel STD-11. However, the present dressing system cannot have control of the optimum dressing of the super-abrasive wheel.In this study, a new system and the grinding mechanism of optimum in-process electrolytic dressing are proposed. This system can carry out the optimum in-process dressing of a super-abrasive wheel. Therefore, optimum in-process electrolytic dressing is a good method to for obtaining the efficient and mirror-like grinding of STD-11.     

一种新型磨胶辊砂轮制造技术

在专利ZL201510030023.1《一种新型磨胶辊砂轮及其制备方法》的基础上,对新型磨胶辊砂轮中的气孔作用机理进行理论探讨。新型磨胶辊砂轮与传统的磨胶辊砂轮相比,不易发生气孔堵塞问题,具有良好的工件加工能力,在磨削工件时不会产生振纹和划伤痕迹,有利于提升工件的加工品质、提高劳动效率。通过试验样品磨削应用反馈:该新型磨胶辊砂轮气孔率达到70%~80%,比一般大气孔碳化硅砂轮高16%以上;回转破裂速度达到120 m/s,比一般大气孔碳化硅砂轮强度提高20%以上;磨削后工件表面光洁度有所提高,修复周期延长2个月以上。并且,通过针对性地选择刚玉空心球粒度,能够制造出具有设计的性能的砂轮,能有效地避免传统磨胶辊多孔磨具的一些问题,比如大气孔碳化硅砂轮常见的烧成黑心、使用强度偏低、成孔剂不环保等问题,同时该新型磨胶辊砂轮的生产工艺性能稳定可靠,易于推广应用。      

微圆弧金刚石刀具修磨技术研究

微圆弧金刚石刀具是超精密加工中的重要应用工具,基于机械研磨法开展微圆弧金刚石刀具修磨技术研究。通过分析微圆弧金刚石刀具制备技术现状和制备难点,设计一种微圆弧金刚石刀具的修磨技术路线。通过修磨实验解决难磨、易磨方向定位,刀尖容易崩刃及精修刀尖圆弧等技术难点,制备刀尖半径R 8.2μm的微圆弧金刚石刀具。利用原子力显微镜和高倍光学显微镜进行观测,结果表明:前刀面粗糙度R_a=1.50nm,刀尖轮廓波纹度优于0.1μm,与进口高精度刀具指标相当。将其应用在微结构的超精密切削中,取得良好效果。      

Minimization of effect of CBN wheel wear on ground gear errors

Resistance to wear of CBN grinding wheels is much higher than that of conventional (Al₂O₃) wheels. However, CBN grinding wheels are seldom used in gear grinding, when production scale is small, due to the difficulty in truing. Moreover, wear of the grinding wheel greatly affects the accuracy of tooth trace, in most cases. Considering this problem, the authors proposed new processes in which the accuracy of the tooth trace and tooth profile was hardly affected by wear of the wheel. While grinding was performed using a conventional method, a concave tooth-trace error of about 2 μm occurred after grinding only 30 teeth. In one of the proposed methods, the concave tooth trace error never occurred even when 2000 teeth were ground by the same wheel. In the third method, the CBN layer is effectively used for gear grinding by giving an additional motion which is independent of the tooth profile generating motion.