GrindBall: an advanced micro-grinding tool

Small machine tools and inherent miniaturized components are persistent development topics in scientific research. Miniaturization usually requires not only reproducing existing systems at a smaller scale, but also a complex integration of various functions into one single element. This concept is presented here by means of a miniature spherical grinding module (GrindBall). It combines a specifically developed magnetic bearing with fluid dynamic propulsion, thus enabling novel grinding kinematics and the possibility of integration in small machine tools. In this paper, the requirements of micro-grinding processes are introduced and the manufacture as well as performance of grinding spheres are discussed; the design of the magnetic bearing is presented and its functionality validated in experiments. Finally, results from numerical simulations leading up to the development of the propulsion system as well as its geometric layout are reported.     

基于环境模型优化的机器人磨抛阻抗控制

对比传统加工机床,机器人的刚度与定位精度较低,为提高机器人辅助磨削的加工质量,需要控制其加工过程中的接触压力。基于球形弹性磨具磨削时的接触特性,以磨具的下压量为自变量,磨具与工件的接触压力为因变量进行有限元仿真,对下压量-接触压力仿真结果进行曲线拟合,并通过接触实验验证其准确性,以此建立基于环境模型修正的阻抗力学控制模型,从而进行自由曲面力控实验。结果表明:建立的基于环境模型修正的阻抗控制方法能有效改善球形弹性磨具的力控性能,自由曲面力控精度达到±0.5 N,且可为球形弹性磨具复杂曲面的磨削加工提供稳定的磨削压力。      

New tool concept for grinding a plateau-like surface for tribological applications

One of the most important factors for energy efficiency is the reduction of friction in machine elements. The grinding process is often the final machining process in machining hardened steel parts and the resulting surface finish influences the tribological behavior. The combination of grinding with a honing process can generate a plateau-like surface to reduce friction and create an oil reservoir to decrease abrasive wear and improve the fluid film stability. Additional processes like laser machining, micro milling or etching are able to generate micro dimples to improve the reduction of friction. Today, grinding processes are limited to machine plateau-like surfaces. Within this paper, a new tool concept will be presented, composed of a grinding tool with two different grain sizes and a metallic bonding. The use of small abrasive grains generates a smooth surface with low roughness values. A few additional larger grains induce stochastic scratches and create the plateau-like surface. Grinding experiments are conducted to analyze the effect of feed rate, feed angle and ratio between small and large grains on the resulting surface.     

Five-axis grinding of wear-resistant,thermally sprayed coatings on free-formed surfaces

The abrasive wear resistance of tribologically stressed free-formed surfaces can be increased with thermally sprayed tungsten carbide coatings. In order to improve the surface topographies and shape accuracies, the workpieces must be finished prior to industrial application. A suitable machining process is NC grinding on five-axis machining centres using abrasive mounted points. However, the high hardness of the applied coatings and the small diameter of the utilized tools pose a great challenge for the process design. In this paper both, the results of fundamental investigations on the grinding of tungsten carbide coatings as well as a process optimization for the finishing of a coated forming tool are presented. This includes the heat transfer into the coating and the tool wear during the grinding process as well as the wear behaviour of the coating in dependence of the generated surface topography. In order to achieve a smooth surface, elastic-bonded diamond tools were used during polishing in a multi-stage machining process.     

钎焊金刚石铣磨刀具磨粒粒度尺寸与排布间距对CFRP磨削质量的影响

碳纤维增强复合材料（carbon fiber reinforced plastics,CFRP）由于其低层间结合力和各向异性导致其加工过程中易出现分层、毛刺、撕裂等加工缺陷和刀具耐用度低等问题。采用有序排布钎焊金刚石磨削刀具及"以磨代切"加工工艺能够有效减轻分层缺陷。为制备出适合CFRP磨边加工的钎焊金刚石刀具,本试验制备了不同磨粒排布间距与不同磨粒粒度尺寸的5种刀具,对比分析了刀具结构变化对CFRP磨边加工磨削力与加工表面质量的影响。试验结果表明:在一定范围内,在相同磨粒排布间距和加工参数下,随着磨粒粒度尺寸变大,磨削力变化很小,加工表面质量变差;在相同磨粒粒度尺寸与加工参数下,随着磨粒排布间距减小,磨削力先增大后减小,加工表面质量变好。      

Machining of micro rotational parts by wire electrical discharge grinding

Micro rotational parts are used in several industrial sectors. Well-known applications are micro shafts of gears, ejector pins in forming tools, pin electrodes for micro electrical discharge drilling or micro stamping dies. Depending on the geometrical complexity of micro rotational parts different process variants of micro electrical discharge machining characterized by a rotating work piece can be used: wire electrical discharge grinding (WEDG) with fine wire electrodes, electrical discharge turning (EDT) with micro structured tool electrode, cylindrical electrical discharge grinding (CEDG) with micro profiled disk electrode. Characteristic to these process variants is the superimposed relative motion between the rotating electrodes and the feed. This relative motion can be varied in a wide circumferential velocity range to improve the material removal process. The paper gives an overview of kinematic and technological restrictions and requirements of the WEDG process influencing the process behavior with respect to the technological requirements of micromachining.     

Tool wear monitoring for micro-end grinding of ceramic materials

As grinding process usually is a final step of a machining procedure, excessive grinding tool wear could deteriorate both workpiece surface quality and its dimensional accuracy. This becomes more severe in the case of microgrinding than in conventional grinding because microgrinding wheels are more sensitive to tool wear. An effective tool wear monitoring technique is, therefore, crucial for maintaining consistent machining quality and high efficiency in microgrinding. In this paper, the influence of tool wear and tool stiffness on microgrinding process signals such as grinding force, grinding system vibration, acoustic emission signal and spindle load, are analyzed during end grinding of ceramic materials. To indicate the actual wear status of a microgrinding wheel, this study proposed a new monitoring parameter by fusing grinding force and system vibration signals, based on the concept of varying cutting stiffness. This new monitoring parameter is then experimentally tested in microgrinding a series of ceramic miniature features with consistent and inconsistent geometry.     

Precision and micro CVD diamond-coated grinding tools

Both for ultra-precision and for micro-machining diamond is used very often as tool material. The reason is the very high dimensional stability of diamond due to its extreme hardness. Diamond is used for two kinds of machining processes: for cutting, like turning, drilling or milling, as well as for abrasive processes, like grinding. Diamond cutting tools can be made with massive diamond (monocrystal, CVD diamond, PCD) or with diamond coatings. Standard diamond abrasive tools are made by bonding diamond monocrystals onto a base body. A new grinding layer technology is presented: chemical vapour-deposited microcrystalline diamond layers have crystallite tips with very sharp edges that can act for grinding processes. Base body materials and coating technology is presented. Application results of grinding experiments show that very high workpiece quality can be reached, e.g. a roughness Ra of 5 nm with glass workpieces. Truing and recoating techniques are discussed for reuse of worn CVD diamond grinding wheels. Micro grinding tools (abrasive pencils, burrs) can be manufactured with the same coating technology. Very small tools with diameters of 50 μm have been made and successfully tested.     

Tribological behavior of micro structured surfaces for micro forming tools

Mechanical micro machining processes, like milling and grinding are appropriate technologies for the flexible production of precise molds with complex shapes for metal forming processes. In most cases machining strategies are orientated towards form accuracy of the desired forming tool only. Thus, the generation of tribologically advantageous surfaces is often carried out in subsequent machining steps like honing. In micro scale the subsequent treatment of complex surfaces is very difficult. For that reason it is desirable to create the shape and a suitable surface texture with one tool in one step.This paper is focusing on the comparison of the tribological behavior of polished surfaces with structured surfaces machined by micro milling and micro grinding processes. Micro milling tools and grinding pins with ballend shape are used to create micro structured surfaces. The machining strategy (tool path and line pitch) was varied for both tool types in the same manner. The experiments were carried out on hardened cold working steel using tungsten carbide micro cutters with TiAlN coating and micro grinding pins with an abrasive diamond layer. White light interferometry was used to characterize the machined surfaces and determine the surface parameters. Moreover, a strip drawing test was set up to investigate the tribological behavior of the system consisting of the machined surfaces and thin sheet metals. The results of the strip drawing test suggest a relationship between micro structure and tribological behavior. Finally, the dependencies between machining technology, surface parameters and tribological behavior will be discussed.     

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.     

一种球面零件法向跟踪磨削加工运动学模型

针对当前航空航天、高铁等工业领域对难加工材料球面零件的高效精密加工的需求,简要介绍球面零件的加工原理与方法,重点讨论分析传统球面零件磨削加工的三种方法及其优缺点。提出一种新的基于砂轮法向跟踪原理的球面磨削加工方法,改善球面零件精密加工的情况,充分利用多轴联动磨床的加工优势。根据几何运动分解处理划分为类外圆磨削和类平面点磨削,并从砂轮-工件几何接触长度、砂轮磨削路径等角度进行了几何学运动分析,为该方法的深入研究提供指导。      

Fabrication of optical freeform molds using slow tool servo with wheel normal grinding

Precision molding is recognized as the most appropriate technology for the mass production of glass freeform optics. However, difficulty arises when fabricating freeform molds with sub-micron form accuracy and nanometre-scale surface finish. A novel wheel normal grinding approach with fillet-end grinding wheel based on slow tool servo is proposed to ensure that the wheel errors are not transferred to mold surface and, thus, that the required machining accuracy is achieved. The wheel path generation strategy and the corresponding wheel truing method are presented. A toric tungsten carbide mold is ground to experimentally verify the validity of the proposed method.     

Micro grinding tool for manufacture of complex structures in brittle materials

This paper presents a novel micro shaft grinding tool with cylindrical tool tip diameter between 13 μm and 100 μm. The manufacturing of the tool itself is carried out on a prototype desktop machine. The shaping of the tool is accomplished by grinding the tool tip directly onto the cylindrical carbide tool shank. During tool grinding, the tool shank rotates in a micro spindle with air-lubricated bearings, leading to high concentricity and low run-out. The tool tip is electrically nickel plated with diamond grains. During first experimental tests of the tool, very low surface roughness values and sharp edges without burrs were achieved.     

NURBS在微机控制球面轴承内套双沟一次磨削加工中的应用

球面内套双沟道一次磨削加工技术对保证对称球面加工精度、提高粗糙度水平和磨不镁粗利，是提高球面轴承等机械产品质量和生产效率的先进技术手段。本文介绍应用ＮＵＲＢＳ技术实现在对砂轮修整中的数字插补控制方法。     

Dressing of filigree fine-grained metal bonded grinding wheels

Metal bonded grinding tools offer a remarkable potential for micro grinding because of their favorable wear behavior. However, metal bonded grinding tools, especially dicing blades, are hard to dress by conventional dressing methods. Electro contact discharge dressing, which is presented in this paper, is a numerically controlled dressing process offering the possibility to create the geometry and topography of the grinding wheel simultaneously at negligible dressing forces. The power of an electric circuit thermally removes the metal bond of the grinding wheel during the dressing process. The quality of the created profiles and the influence of the dressing parameters on the specific material removal rate of the grinding tool as well as the wear of the electrode are investigated at fine-grained grinding wheels.     

基于旋转超声振动的氧化锆陶瓷小孔磨削加工质量研究

针对陶瓷材料小孔加工质量较差以及加工成本较高等问题,设计一种基于旋转超声辅助的氧化锆陶瓷小孔磨削加工工艺。首先分析旋转超声加工原理,然后在超声振动条件下利用金刚石刀具对氧化锆陶瓷小孔进行单因素磨削加工试验,并对小孔的内壁进行形貌分析和粗糙度检测,最后研究主轴转速、超声功率以及进给速度对小孔表面粗糙度的影响规律。研究结果表明:与普通磨削方式相比,在旋转超声辅助加工条件下,小孔表面质量和残余应力都得到较大改善,当超声功率达到300 W时,加工后的小孔表面粗糙度下降了52%,加工精度明显提高。      

An integrated model of tool grinding: challenges,chances and limits of predicting process dynamics

The objective of manufacturing technology, to reduce machining costs, increase product quality and optimize process setups, is only achievable by considering the entire manufacturing process and its interrelations. Especially in tool grinding, strong interactions exist between workpiece dynamics, grinding wheel engagement, structure deformations and cutting process conditions, which makes an integrated model necessary. This paper introduces a tool grinding model that combines time-dependent dynamical aspects of the grinding wheel and workpiece with local varying contact conditions to predict the final workpiece geometry and cutting forces with a high resolution in space and time. The model is capable to reproduce systemic effects on cutting forces and ground geometries which only occurs in the interplay of all system components. The model is also positively tested in representing influences of process parameters and in optimizing the machining. The excellent agreement of simulations and experimental data shows the model’s potential in manufacturing technology, but also in investigating grinding aspects, which are not fully understood yet.     

Evaluation of superabrasive grinding points for the machining of hardened steel

Small diameter grinding points offer greater flexibility for machining free-form contours compared to traditional grinding wheels, despite fewer effective cutting edges. The paper evaluates the influence of grit size (B32, B46, B76), feed rate (125, 250 mm/min) and depth of cut (20, 40 μm) when machining D2 tool steel using electroplated CBN grinding points. Highest G-ratios (～2441) were obtained using B32 tools with corresponding workpiece surface roughness (Ra) of ～0.8 μm after ～6000 mm³ material removed, due to the greater number of effective cutting edges. Attritious wear was the primary wear mechanism although material loading was observed with B76 tools.     

High-frequency tool-spindle for multifunctional, replaceable rotor-modules

The increasing demand for micro-parts and -structures requires micro-tools, precision machines and components. For machining the microparts, tool-spindles with very high rotational speeds of the spindle rotor are needed. In this paper a new developed small high-frequency tool-spindle capable for use of multifunctional micro machining and measuring tools is introduced.     

Tool grinding of end mill cutting tools made from high performance ceramics and cemented carbides

The paper presents different approaches to improve the process knowledge in tool grinding with a focus on ceramic shank-type end mills. The flute grinding operation was analyzed using a kinematical simulation to acquire an insight into the local distribution of the material removal rate or the microscopic chip parameters. Further investigations cover the cutting edge quality emerging in characteristic tool grinding operations on end mills with helical flutes made from advanced ceramics. Final machining test prove a reliable cutting behaviour without catastrophic failure and a gentle abrasive and adhesive wear observed on the ground cutting tools.