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.     

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.     

Interaction of manufacturing process and machine tool

Analysing the machine tool and the machining process individually is necessary in order to tackle the challenges that both have to offer. Nevertheless, to fully understand the manufacturing system, e.g. vibrations, deflections or thermal deformations, the interactions between the manufacturing process and the machine tool also have to be analysed. In cutting, grinding and forming there are important effects that can only be explained through these interaction phenomena. This paper presents the current state of research in process-machine interactions for a wide variety of manufacturing processes. It is based on the findings of the CIRP research group “Process Machine Interaction (PMI)” and on the international publications in this field. Cutting with defined and undefined cutting edges as well as sheet and bulk metal forming are the key processes. The emphasis is on understanding, modelling and simulating all modes of interaction. Additional needs of research in process-machine interaction are identified for future projects.     

Machine tool feed drives

This paper reviews the design and control of feed drive systems used in machine tools. Machine tool guides designed using friction, rolling element, hydrostatic and magnetic levitation principles are reviewed. Mechanical drives based on ball-screw and linear motors are presented along with their compliance models. The electrical motors and sensors used in powering and measuring the motion are discussed. The control of both rigid and flexible drive systems is presented along with active damping strategies. Virtual modeling of feed drives is discussed. The paper presents the engineering principles and current challenges in the design, analysis and control of feed drives.     

A new dynamic model for drilling and reaming processes

This paper presents a new computer simulation model for drilling and reaming processes. The model is made of four parts: the force model for the cutting lips, the force model for the chisel edge, the dynamic model for the machine tool (including the cutter) and the regenerative correlation between the force and machine tool vibration. The models for the forces and the machine tool are similar to the existing models. The key to the model is the regeneration correlation between the cutting forces and the machine tool vibration. It uses a new 3D chip formation model to describe the interaction between the cutter and the workpiece. The model can predict the dynamic forces and chatter limit. It also reveals several interesting phenomena, such as how the feed and the point angle of the drill affect the chatter limit. The model is implemented using C++ language with an interface to I-DEAS™ CAE software system. The simulation results are validated experimentally by both drilling and reaming under various cutting conditions. The experiment results show that the simulation is accurate with average error about 10%. A number of research issues are also proposed for the future work.     

Thermal issues in machine tools

This paper presents a review of the latest research activities and gives an overview of the state of the art in understanding changes in machine tool performance due to changes in thermal conditions (thermal errors of machine tools). The topics are focused on metal cutting machine tools, especially on turning and milling machines as well as machining centres. The topics of the paper thermal issues in machine tools include measurement of temperatures and displacements, especially displacements at the tool centre point, computations of thermal errors of machine tools, and reduction of thermal errors. Computing the thermal errors of machine tools include both, temperature distribution and displacements. Shortly addressed is also to avoid thermal errors with temperature control, the influence of fluids and a short link to energy efficiency of machine tools. The paper presents the summary of research work in the past and current. Research challenges in order to achieve a thermal stable machine tool are discussed. The paper apprehend itself as an update and not a substitution of two published keynote papers of Bryan et al. [28] in 1990 and Weck et al. [199] in 1995.     

Multi-functional machine tool

The functions of metal cutting machine tools have been increasing to meet the demands of high productivity and high accuracy in machining complicated and difficult parts on one machine. This paper presents a comprehensive survey of multi-functional machine tools used for metal cutting, and their kinematic configurations, control and programming technologies. Design principles and assessment of multi-functional machine tools are discussed mainly taking examples of 5-axis machining centers. The paper also presents examples of latest multi-functional machine tools as well as current status of related supporting technologies.     

Feed drive control tuning considering machine dynamics and chatter stability

In large machine tools, where structural dynamics significantly influences the cutting stability, judicious selection of the servo control parameters can increase the damping, thereby improving the chatter stability. This paper presents a new strategy for feed drive controller tuning, which takes this effect into consideration. The proposed strategy generalizes successfully to machine tool structures with high order dynamics, and integrates model-free frequency domain estimation and analysis techniques with model-based root locus design, the latter which is achieved through an efficient and accurate identification approach. The proposed strategy has been tested in machining stability tests, demonstrating up to 30% increase in productivity.     

Dynamic analysis of runout correction in milling

Tool runout and its effects is an important area of research within modelling, simulation, and control of milling forces. Tool runout causes tool cutting edges to experience uneven forces during milling. This fact also affects tool life and deteriorates workpiece surface quality. In this article a procedure, in order to diminish the effects of tool runout, is presented. The procedure is based on chip thickness modification by means of the fast correction of the tool feed rate. Dynamic feed rate modification is provided by superposing our own design of a fast feed system driven by a piezoelectric actuator to the conventional feed drive of the CNC machine tool. Previously, a model of the dynamic behaviour of the system was developed to analyze the influence of fast feed rate modification on cutting forces. The model incorporates the piezoelectric actuator response as well as the structural dynamics of the tool and the designed Fast Feed Drive System (FFDS). Simulated and experimental results presented in this paper show the effectiveness and benefits of this new tool runout correction procedure.     

A virtual machine concept for real-time simulation of machine tool dynamics

When designing CNC machine tools it is important to consider the dynamics of the control, the electrical components and the mechanical structure of the machine simultaneously. This paper describes the structure and implementation of a concept for real-time simulation of such machine tools using a water jet cutting machine as an application. The concept includes a real control system, simulation models of the dynamics of the machine and a virtual reality model for visualisation. The real-time capability of the concept, including the simulation of electrical and rather detailed mechanical component models is proofed. The validation process indicates good agreement between simulation and measurement, but suggests further studies on servo motor, connection and flexibility modelling. However, already from the initial simulation results presented in this paper it can be concluded that the influence of structural flexibility on manufacturing accuracy is of importance at desired feeding rates and accelerations. The fully automated implementation developed in this work is a promising base for dealing with this trade-off between productivity and accuracy of the manufacturing process through multidisciplinary optimisation.     

Implementation of a process and structure model for turning operations

The consideration of the dynamic interaction between the machine tool structure and the cutting process is a prerequisite for the simulative prediction and optimization of machining tasks. However, existing cutting force models are either dedicated to already examined manufacturing operations or require extensive measurements for the determination of cutting coefficients. In this context this paper outlines a modular, analytical cutting force model applicable to common turning processes. It takes into account the dynamic material behavior, nonlinear friction ratios on the rake face as well as heat transfer phenomena in the deformation zones. On the part of the machine tool structure a parametric model based on the Finite Element Method (FEM) is implemented. Both models are coupled for the simulation of process and structure interactions, whereas the influence of the control system is considered as well. The simulation results were verified experimentally on a turning center.     

Virtual process systems for part machining operations

This paper presents an overview of recent developments in simulating machining and grinding processes along the NC tool path in virtual environments. The evaluations of cutter–part-geometry intersection algorithms are reviewed, and are used to predict cutting forces, torque, power, and the possibility of having chatter and other machining process states along the tool path. The trajectory generation of CNC systems is included in predicting the effective feeds. The NC program is automatically optimized by respecting the physical limits of the machine tool and cutting operation. Samples of industrial turning, milling and grinding applications are presented. The paper concludes with the present and future challenges to achieving a more accurate and efficient virtual machining process simulation and optimization system.     

A new receptance coupling substructure analysis methodology to improve chatter free cutting conditions prediction

The cutting process stability depends on machine tool dynamics that is strongly influenced by the tool. Receptance coupling substructure analysis (RCSA) can be used to estimate the tool tip dynamic compliance and consequently the chatter free cutting conditions when the machine is equipped with a tool that has not been previously tested. This methodology can be particularly useful on real shop–floors where a lot of different tool–tool holder configurations are generally used. RCSA typically combines experimental dynamic compliance measurements performed on a machine equipped with a selected tool and the finite element (FE) models of both the already tested tool and the new ones. This paper presents a new receptance coupling substructure analysis (RCSA) approach that overcomes the drawbacks in the estimation of the receptances that contain rotational and moment contributes. This indeed often limits the accuracy of the RCSA techniques presented in other scientific works. The proposed formulation allows to better estimate both the matrices of receptances of the spindle–tool holder assembly and the tool–tool holder connection stiffness. Those quantities are used, together with the FE model of the new tool, to predict the unknown tool tip dynamic compliance. Some useful guidelines to implement the proposed RCSA are also defined: they allow to manage the procedure accuracy considering the experimental methodology typically used to measure dynamic compliances. The proposed innovative RCSA is experimentally tested and validated.     

Prediction of the dynamic behaviour of machine tools during the design process using mechatronic simulation models based on finite element analysis

Increased productivity, higher velocities and acceleration for feed and cutting motions are requirements for innovative machine tools. At the same time the production process must achieve reduced form and position deviations of the work-piece. Therefore knowledge of the dynamic behaviour of machine tools during the design process is essential to develop high-performance machines. Using finite element analysis and mechatronic simulation, taking the mechanical, electrical and control systems into account, is the first step for optimisation. Developing the control parameters using these simulation techniques is one of the major steps in detecting the mechatronic characteristics. This paper presents a method for developing the control parameters concerning tool to work-piece deviations of mechatronic simulation models including disturbance variables. As an example a 2-axis CNC test stand for feed drive axes will be visualised with its simulation and measurement results in the time and the frequency domain.     

Mechanical interfaces in machine tools

Machine tools involve various mechanical interfaces in different forms and styles, which affect performance significantly in terms of rigidity, thermal stability, precision, and accuracy. This paper reviews the state-of-the-art and future trends in machine tool structural interfaces. The main concepts, challenges, and improvements regarding mechanical and thermal characteristics, geometric accuracy and precision, and wear and failure of machine tool interfaces are presented. Advanced methods for modeling static, dynamic, thermal, and geometrical behavior of mechanical interfaces are presented with examples. Furthermore, typical wear and failure mechanisms and available solutions and health monitoring techniques are covered.     

Dynamics design optimization and experimental validation of a miniaturized machine tool for micro-milling

This paper presents a 3-axis miniaturized machine tool for micro-milling in terms of dynamics design and structural optimization. Four different machine tool structure forms are proposed and contrasted in conceptual and fundamental design stage. The best one is selected and optimized to improve the dynamic performance of the machine tool. The measuring errors are also considered in the design stage and the influence from the location of the detecting element on the measuring error is also discussed and optimized. The modal test and the prediction of the milling chatter vibration stability is carried out. An experimental prototype of miniaturized numerical control milling machine was developed according to the design and optimization results. Machining trials have been carried out by the tungsten carbide micro-diameter cutter on the polymethylmethacrylate and the lead brass (HPb63-3) surfaces. The results from the micro-milling validate the theoretical models and analysis very well and provide the evidence of the approach being helpful to design the miniaturized machine tool for micro-machining.     

Analytical model for tool wear monitoring in turning operations using ultrasound waves

This paper presents an analytical model to monitor the gradual wear of cutting tools, on-line, during turning operations using ultrasound waves. Ultrasound waves at a frequency of 10 MHz were pulsed continuously inside several cutting tools, towards their cutting edge. The change in tool geometry, due to gradual wear, has been related, in a mathematical form, to the change in the acoustic behavior of ultrasound waves inside the body of the cutting tools. Physical laws governing the propagation and reflection of ultrasound waves along with geometrical analysis of the wear area were used in deriving the mathematical model. The experimental setup and model evaluation is based on a previously published research work by the author, which presented an empirical model showing a corresponding change in the ultrasound behavior with tool gradual wear. The current work emphasizes the previous findings and presents the relation between the acoustic behavior of ultrasound waves and the progressive tool gradual wear in a mathematical form that can be easily used in machine control operations.     

Machine tool spindle units

This paper presents the state-of-the-art in machine tool main spindle units with focus on motorized spindle units for high speed and high performance cutting. Detailed information is given about the main components of spindle units regarding historical development, recent challenges and future trends. An overview of recent research projects in spindle development is given. Advanced methods of modeling the thermal and dynamical behavior of spindle units are shown in overview with specific results. Furthermore concepts for sensor and actuator integration are presented which all focus on increasing productivity and reliability.     

The form error prediction in side wall machining considering tool deflection

In this research, an effective method for the form error prediction in side wall machining with a flat end mill is suggested. The form error is predicted directly from the tool deflection without surface generation by cutting edge locus with time simulation. The developed model can predict the surface form error accurately about 300 times faster than the previous method. Cutting forces and tool deflection are calculated considering tool geometry, tool setting error and machine tool stiffness. The characteristics and the difference of generated surface shape in up milling and down milling are discussed. The usefulness of the presented method is verified from a set of experiments under various cutting conditions generally used in die and mold manufacturing. This study contributes to real time surface shape estimation and cutting process planning for the improvement of form accuracy.     

Dynamics of boring processes: Part III-time domain modeling

This article presents a mathematical model and a computational algorithm for the time domain solution of boring process dynamics. The model is developed in a modular form; it includes a workpiece geometry and surface topography module, a kinamatics and tool position module, a dynamic chip load module, a dynamic cutting force prediction module and a structural dynamics module. The time domain model takes cutting process parameters, tool and workpiece geometries and modal parameters of the structure as inputs. It predicts instantanous cutting forces and vibrations along the machining time, and machined workpiece topography as outputs. Some of the simulated and experimental results for various cutting conditions are presented and compared for validation purposes.