An active system of reduction of vibrations in a centerless grinding machine using piezoelectric actuators

Centerless grinding has been extensively used in production engineering to produce accurate cylindrical parts together with high productivities. On the other hand, regenerative chatter vibrations are one of the major problems that limit the ability to produce round workpieces. This constraint can be solved selecting proper machine setup conditions, which still largely relies on a trial and error method, and sometimes this approach is not optimum in a productivity sense. This paper shows a novel method to reduce chatter vibrations in a centerless grinding machine using actively controlled piezoelectric actuators. A simplified model of the machine is used to simulate the behavior of several commercially available piezoelectric actuators in two different locations of the machine. Based on these simulations, a selection of proper actuators and their optimal location is obtained and the control system is implemented experimentally. Experimental results show that the control strategy provides a stabilizing effect on chatter. Thus, the viability of using piezoelectric actuators as active components is demonstrated, providing an important advance in the knowledge of chatter control in centerless grinding machines.     

A new perspective on the stability study of centerless grinding process

Regenerative chatter is one of the most complex dynamic processes in machine tools. It is characterized by the presence of self-excited vibrations during machining, limiting the achievable tolerances in the workpieces. In order to predict the set-up conditions that produce these vibrations, it is necessary to model the regenerative mechanism responsible of their appearance accurately, so that the system stability can be studied solving the characteristic equation of the chatter loop. Although the dynamic behavior of machining processes like milling, turning or drilling is governed by a time delayed differential equation with one time delay term, a very particular problem is presented in centerless grinding. In this process, in addition to the dynamic instabilities, geometric instabilities must be analyzed, which are another important factors limiting the workpiece tolerances and lead to three time delay terms in the modeling procedure. This fact complicates its study remarkably, and the resolution of the characteristic roots of the dynamic process of these kinds of machines has not been tackled in the specialized literature as extensively as in other machining processes, being this field a challenging research line. According to this, in this paper an original and efficient method is presented to solve the roots of the characteristic equation of the centerless grinding process, based on the application of the root locus method. The main features of the proposed procedure are its ability to obtain the solutions accurately and that it is capable of determining the origin of the instabilities, so it constitutes a powerful tool to predict machine response for different set-up conditions. These interesting properties are demonstrated through the simulation results presented in this paper.     

Electrohydraulic Active Damping System

A major characteristic of machine tools is the relative dynamic flexibility at the tool centre point. Poorly damped resonance frequencies often cause self-excited vibrations, so called chatter vibrations, which derogate the machined surface and may cause tool breakage. In practice, typically the metal removal rate and therefore the productivity of the machine are reduced, in order to avoid such vibrations. This paper deals with an active damping system for the improvement of the dynamic flexibility of machine tools. The damping system is based on an electrohydraulic actuator, which combines comparatively large forces and a compact design. The control input for the actuator is determined from the acceleration measurement based on the concept of a so-called velocity feedback control. In experimental investigations, the depth of cut could almost be tripled.     

Chatter suppression with an active workpiece holder

The productivity of machine tools is often limited due to chatter vibrations caused by relative displacements between the tool and the workpiece. The following article presents the systematic approach of the integration of an active workpiece holder with two high dynamic axes controlled by piezoelectric actuators onto a milling machine. With these additional highly dynamic axes near the tool center point, the active workpiece holder offers possibilities to prevent chatter vibrations.     

高精度复杂型线刀具制造工艺与装备研究

以高精度复杂型线刀具制造工艺与装备为研究对象，以菌形叶片叶根型线加工专用刀具为切入点，通过分析叶根型线的特点及加工工艺的优劣，确认以单边两次成型加工作为刀具加工工艺方案；并在此基础上确定刀具的轮廓度精度、前后刀面表面粗糙度等各项参数及精度要求。通过分析专用刀具的加工要求，确定专用磨床的整体布局及结构方案，并对机床的关键部件磨削装置的结构做了具体设计与阐述。分析磨削点在B轴回转轴线延伸线上的结构优点可知，B轴回转的角度误差未使磨削点位置发生偏差，可有效提升刀具的加工精度。为验证设计的合理性，通过制造与装配机床，利用雷尼绍XK10激光校准仪检测可知，几何精度均达到设计要求。最后，采用设计的刀具加工工艺方案及专用磨床对某菌形型线刀具进行加工，Zoller检测仪检测结果表明：刀具轮廓公差为工作面±0.005 mm,其余面±0.015 mm。     

Modeling and measurement of active parameters and workpiece home position of a multi-axis machine tool

The complex structures of a multi-axis machine tool may produce inaccuracies at the tool tip caused by dimensional errors in the machine's link parameters. This paper addresses two important issues for precision machining: (1) which link parameters (denoted as active parameters) of a machine tool can affect the machining accuracy of a workpiece and (2) how to measure the active parameters by using a grinding wheel as a measuring probe. To achieve this, a modified Denavit–Hartenberg (D–H) notation is introduced to model a multi-axis machine tool. The NC data equations are then derived in terms of the machine's link parameters. It is found that the link parameters of a machine tool can be divided into two types: active and nonactive parameters. The prerequisite for obtaining an accurately machined workpiece is to have correct values of the active parameters and the workpiece home position. Based on the developed NC data equations of a multi-axis machine tool, this paper also addresses the technique of using a grinding wheel as a measuring probe to determine the active parameters and the workpiece home position. Experimental results are also given with illustrative examples.     

Design process for adaptronic machine tools

A consistent goal in machine tool construction is to enhance the accuracy while increasing productivity. These two design parameters are negatively coupled. Increasing the machine dynamic is realized by increasing the engine power or with a lightweight design. As the engine power grows, so do the forces generated, which then lead to static deformations and excite the structure, leading to vibrations. Within the scope of this paper, the adaptronic framework has been adapted to machine tools. The development of such an adaptronic machine tool is no trivial task, as it requires an extensive knowledge overhead across a number of different scientific fields. Though these fields are interrelated, a methodical approach which takes advantage of a standard component is presented. This approach allows the interdisciplinary knowledge barriers to be overcome.     

An adaptronic approach to active vibration control of machine tools with parallel kinematics

In recent years, more and more lightweight components are used in machine tools in order to provide advantages in highly dynamic applications because of their reduced mass. One major drawback of these lightweight systems is their sensitivity to structural vibrations which are induced by high jerks during fast positioning and the machining process itself. Moreover, in case of parallel kinematic structures, the dynamic response depends nonlinearly on the actual pose of the mechanism in the workspace. In this paper, a concept for active vibration control for a machine tool with planar parallel kinematics is presented. For this purpose, an adaptronic rod has been developed and integrated into the existing machine tool. The unit consists of a high-power lead–zirconate–titanate piezo stack actuator, which is able to induce forces in longitudinal direction of the rod, and various sensors so that different control concepts from the field of active vibration control can be implemented and tested. Based on a flexible multibody system model of the machine tool, a hierarchical controller consisting of an integrated force feedback as low authority Control combined with a frequency-shaped linear quadratic regulator has been designed. The experimental results presented here demonstrate the ability of the chosen approach to clearly suppress the first dominating resonance without loss of performance in the low frequency range, which is often a serious problem of active damping concepts.     

Modelling and simulation of process: machine interaction in grinding

This article presents an overview of current simulation methods describing the interaction of grinding process and grinding machine structure, e.g., vibrations, deflections, or thermal deformations. Innovative process models which describe the effects of the grinding wheel–workpiece interaction inside the contact zone are shown in detail. Furthermore, simulation models representing the static and dynamic behaviour of a grinding machine and its components are discussed. Machine tool components with a high influence on the process results are modelled more detailed than those with low influence. The key issue of the paper is the coupling of process and machine tool models for predicting the interactions of process and machine. Several coupling methods are introduced and the improvements of the simulation results are documented. On the basis of the presented simulation approaches, grinding processes and machines can be designed more effectively resulting in higher workpiece quality and process stability.     

Simulation and development of an active damper with robust <Emphasis Type="Italic">μ</Emphasis>-control for a machine tool with a gantry portal

Machine tools are generally used with process parameters that are as productive as possible yet stable. One way to raise productivity is to increase the process parameters like cutting speed or depth of cut (DOC). However, this approach will lead to process instabilities sooner or later. An increased rotational speed of the spindle will excite higher eigenfrequencies depending on the tools teeth count. In combination with higher cutting forces resulting from a deeper DOC, the process can become instable because of chatter or other oscillations and vibrations of the machine tool. This paper describes the identification of a critical eigenfrequency and corresponding eigenmode. An active damper was then developed to mitigate the negative effect this critical eigenfrequency has including a robust controller which protects the process from instabilities through changing eigenfrequencies caused by changing machine positions. It will also enable increased process parameters for a higher productivity of the machine tool. A simulation environment of the active damping system with a classic control and a robust \(\mu\)-control was developed. The damper was applied to the machine tool and tested.     

The static and dynamic stiffness of interference shrink-fitted joints

The main objective of this work is to investigate the effect of size, surface roughness and the interference value on the static and dynamic stiffness of shrink-fitted joints.Most of the previous research has been concerned with the joint holding load under static conditions without consideration of many of the applications in which the joints are exposed to dynamic loading. The dynamic performance of a machine tool is greatly affected by the collected response of its joints. In many applications in which shrink-fitted joints are used, such as engines and machine tools, their dynamic characteristics can be an important factor in deciding the overall performance of the machine. Hence, studying the joints' stiffness characteristics under static and dynamic conditions is of importance.     

Mechatronic Systems for Machine Tools

This paper reviews current developments in mechatronic systems for metal cutting and forming machine tools. The integration of mechatronic modules to the machine tool and their interaction with manufacturing processes are presented. Sample mechatronic components for precision positioning and compensation of static, dynamic and thermal errors are presented as examples. The effect of modular integration of mechatronic system on the reconfigurability and reliability of the machine tools is discussed along with intervention strategies during machine tool operation. The performance and functionality aspects are discussed through active and passive intervention methods. A special emphasis was placed on active and passive damping of vibrations through piezo, magnetic and electro-hydraulic actuators. The modular integration of mechatronic components to the machine tool structure, electronic unit and CNC software system is presented. The paper concludes with the current research challenges required to expand the application of mechatronics in machine tools and manufacturing systems.     

PVA/PF复合凝胶磨具精密磨削碳化硅陶瓷工艺实验研究

目的 针对传统粉末热压成形细粒度金刚石磨具存在颗粒团聚、磨削碳化硅陶瓷容易在表面产生较深划痕的问题,提出一种基于冷冻-解冻凝胶成形的细粒度金刚石磨具,用于精密磨削碳化硅陶瓷,并研究其加工工艺.方法 制备聚乙烯醇-酚醛树脂复合凝胶胶水,将金刚石和填料在凝胶胶水中剪切分散,得到的浆料浇筑在模具中,在–20℃低温条件下反复冷...     

Mechanisms in the generation of grinding wheel topography by dressing

For the process of dressing vitrified bonded grinding wheels with diamond tools it has been unknown how the wheel topography is generated. Moreover, the influence of the kinematical dressing parameters on the wheel wear behavior has not been quantified. In the course of this article the grinding wheel was dealt with as a porous ceramic composite. In FEM simulations common dressing forces and usual dressing tool geometries were applied. The results were verified by dressing tests and grinding wheel scratch tests which show the wheel wear mechanisms. The common practice of decreasing the grinding wheel surface roughness by a finishing dressing stroke has to be reconsidered, because previous dressing strokes with higher depths of cut can weaken the grinding wheel structure and lead to an unsteady phase with high grinding wheel wear after dressing.     

Injection molded spherical grinding tools: manufacture and application of a novel tool concept for micro grinding

Micro grinding offers the possibility of machining micro structures in hard and brittle materials producing small-scaled parts. Novel micro grinding systems and machines require miniaturized tools and spindles to meet the demands of small or desktop machines providing a small working space. This paper introduces a novel grinding module called ’GrindBall’, with highly integrated tool drive and bearing functions as well as a shaft-free, spherical grinding tool for micro machining applications in small-scaled machine tools. One of the challenges within the development of the GrindBall module is the manufacture of spherical grinding tools, which are not commercially available. A promising method to produce such grinding tools, the injection molding of micro particle filled polymers, is demonstrated in this paper. Injection molding and grinding experiments show that such spherical grinding tools meet the main requirements of this novel grinding technique and show significant material removal rates.     

Konzipierung und Optimierung des Werkzeugantriebs für das Verfahren Fräsbohren und seine Lagerung im Bohrstangenschaft

A new machining process, called mill-drilling has been developed, made up by a combination of deep hole drilling and Milling. The origin, the main characteristica of the process, the tools and the machine tools are being explained. A typical advantage of the mill-drilling process is the constant production of short chips even with tough workpiece materials. A special Problem of the mill-drilling tool is the dynamic behavior of the highly elastic miller drive. Some main points of influence on the dynamic tool load are shown and the results of the mill-drilling tool with and without vibrations. Advantages and disadvantages of the mill-drilling in comparison to conventional drilling processes are discussed.     

砂轮刚性对磨削性能及产品加工质量的影响

通过测量不同砂轮磨削时机床功率、磨削区温度、刀具的表面粗糙度和刃口质量,分析砂轮刚性改变对砂轮磨削性能和刀具质量的影响。研究发现:在一定范围内降低砂轮的刚性可以提高砂轮的磨削性能,改善刀具的加工质量。其中,加入体积分数10%的尼龙制备的砂轮对其刚性改善比较明显,在相同的磨削条件下,其磨削区的平均温度较普通砂轮磨削区的平均温度低50℃左右;累计磨削相同工件后,机床负载约为使用普通砂轮时的30%~50%;磨削得到的刀具表面粗糙度降低,可以达到R_a0.02μm以下,且磨削纹路规则;在×500倍显微镜下,观察不到刀具崩刃等缺陷,刃口质量得到明显改善。      

Compact design for high precision machine tools

Due to raising functional integration in micro fluidic, micro mechanic, micro electronic and micro optical systems the trend to scaling down the work piece sizes while increasing its complexity requires high precise machine accuracy. With respect to the process and geometrical parameters, most of the finishing manufacturing processes can be covered by milling and grinding operations with three or five machine axes. But whereas the available machine tools hardly achieve the required process dynamic and accuracy in all degrees of freedom, the requirements still increase. For this reason the Fraunhofer IPT has developed high precise machine tools following a compact design strategy by reducing the overall machine dimensions as far as conventional machine components such as measuring or drive systems were available. The developments of two compact machine tools exemplify the dynamic and accuracy enhancement by compact design and are described in the following.     

Machinability of hardened steel using alumina based ceramic cutting tools

Alumina based ceramic cutting tool is an attractive alternative for carbide tools in the machining of steel in its hardened condition. These ceramic cutting tools can machine with high cutting speed and produce good surface finish. The wear mechanism of these ceramic cutting tools should be properly understood for greater utilization. Two types of ceramic cutting tools namely Ti[C,N] mixed alumina ceramic cutting tool and zirconia toughened alumina ceramic cutting tool are used for our investigation. The machinability of hardened steel was evaluated by measurements of tool wear, cutting forces and surface finish of the work piece. These alumina based ceramic cutting tool materials produce good surface finish in the machining of hardened steel. In this paper an attempt is made to analyse the important wear mechanisms like abrasive wear, adhesive wear and diffusion wear of these ceramic cutting tool materials and the performance of these ceramic cutting tools related to the surface finish is also discussed here.     

Abrasive points for drill grinding of carbon fibre reinforced thermoset

In this study, an approach for using diamond grinding tools to machine holes in epoxy carbon laminates is described. The development of a suitable tool design and the influence of the cutting parameters are presented. With these tools, large total drilling lengths can be achieved without significant grain wear. Analysis of the results shows that the coolant supply can be reduced without exceeding the resin's glass transition temperature. In addition to the experimental results, a process simulation is applied to improve the tool layout and hence to avoid material clogging at the grinding layer.