Simulation and verification of parametric numerical control programs using a virtual machine tool

A parametric computer numerical control (CNC) program was developed to make a test complex surface on a vertical milling machine equipped with an external fourth rotational axis. Parametric programming was used in order to get higher flexibility of the manufacturing process. The ball-end milling process was simulated and verified in a virtual model of the machine tool developed with the module integrated simulation and verification of the product lifecycle management software of Siemens NX7^®. After that, the real process was carried out on the shop floor to machine the test surface. This demonstrates that the virtual machine tools are an effective resource to simulate and verify the performance of machining processes controlled by CNC parametric programs. The possibility of accurately simulating the parametric CNC program reduces the risk of its implementation and enables a more frequent use of this efficient feature of contemporary CNC machine tools.     

Controller-integrated predictive oscillation compensation for machine tools with parallel kinematics

Since computerised numerical control (CNC) systems were introduced into the field of mechanical engineering, the manufacturing technology made significant progress. In spite of drastic improvements, mainly in terms of productivity and precision (e.g. finishing surface quality), opportunities for further advancements for the manufacturing processes still exist. One possible method for increasing precision is to adopt a machine structure with parallel kinematics. This kinematic structure can enhance the system dynamics, because the moving mass is divided among all actuators, contrary to the conventional, cartesian machine structures. The problem, which still exists even with this new structural solution, is the machine oscillation that appears during machining. This oscillation may decrease the machining accuracy. This paper highlights a new possible method to compensate the machine oscillation by using a controller-integrated compensation principle. Because of this principle, additional mechanical components (e.g. piezo stacks, actuators) are not necessary. This compensation concept was developed and verified under Matlab/SIMULINK and then installed into the machining centre Dyna-M. The kinematic structure of Dyna-M is defined as a hybrid-kinematic and also one of the typical parallel structures. The obtained results show up to 60% reduction of the machine oscillation and prove the practical usability of this compensation system.     

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.     

Friction in feed drives of machine tools: investigation,modeling and validation

Virtual simulation and optimization of the dynamic behavior of machine tools in the development phase is required to satisfy the increasing demands on machine tool performance. While mass and stiffness properties can be simulated with sufficient accuracy, often no suitable damping models are available for the components of machine tools. The commonly used linear damping models are predominantly linear hysteretic or viscous models. However, the linear damping models are often not appropriate to reflect the occurring nonlinear effects in machine tools with the required accuracy. The reason for these nonlinearities are predominantly the friction forces in feed drive components. To resolve these deficits, the friction in feed drive components is comprehensively investigated in this paper, models for friction forces are identified and coupled with a reduced, flexible multi-body system. With the identified friction models the measured friction curves can be reproduced very precisely. The coupled, reduced, flexible multi-body model allows to simulate the nonlinear effects and to predict the dynamic behavior of machine tools with high accuracy. Consequently, a further important step towards accurate virtual simulation of machine tools is made.     

NURBS-based fast geometric error compensation for CNC machine tools

In this paper, a novel method for the compensation of geometric errors of CNC machine tools is presented. The key idea is to use the basis functions of the setting NURBS path to approximate its error compensation function and to generate a new compensated NURBS path. In this way, both the setting and the compensated NURBS path have the same NURBS form. More importantly, the control points of the error compensation function can be obtained by simply calculating the positioning deviations of the control points of the setting NURBS path using the error model. A high compensation accuracy can be achieved through the systematic insertion of new knots, which creates new control points and raises the flexibility of NURBS in representing the error compensation function. The real-time interpolation of the compensated NURBS path completes the error compensation automatically. Simulations and experiments have shown that the new method delivers the same positioning accuracy as a model-based real-time geometric error compensation method does, but without additional real-time CPU loading. The proposed method can also be implemented in the post-processor of a CAM system for off-line compensation.     

Design method for machine tools with bionic inspired kinematics

Complete processing in one machine calls for a configuration targeting major process stability as well as a structure geared towards accuracy and dynamism. The compromise between potential dynamism and machine size can be improved by using redundant drives. The advantageous use requires the specialization on partitioning of the complete motion. With the presented algorithm the reactive sectioning of a multidimensional motion into two components with different dynamic characteristics succeeds. This provides a useful basis for the application of the principle of dynamic sectioning for a suitable design of drive redundant machine structures. This paper describes models, simulation tools and the method of the development process.     

A general approach for error modeling of machine tools

This paper presents a general and systematic approach for geometric error modeling of machine tools due to the geometric errors arising from manufacturing and assembly. The approach can be implemented in three steps: (1) development of a linear map between the pose error twist and source errors within machine tool kinematic chains using homogeneous transformation matrix method; (2) formulation of a linear map between the pose error twist and the error intensities of a machine tool; (3) combination of these two models for error separation. The merit of this approach lies in that it enables the source errors affecting the compensatable and uncompensatable pose accuracy of the machine tool to be explicitly separated, thereby providing designers and/or field engineers with an informative guideline for the accuracy improvement by suitable measures, i.e. component tolerancing in design, manufacturing and assembly processes, and error compensation. Two typical multi-axis machine tools are taken as examples to illustrate the generality and effectiveness of this approach.     

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.     

Modularity in small machine tools

Up to now, machine tools in micro production hardly feature modularity and hence offer only few opportunities to reconfigure or individualize a manufacturing process. This article gives a summary of concepts, how modularity in small micro machine tools can be designed and implemented. A size-adaptable machine frame is the basis for various possible process layouts. Adapters, a feed module with replaceable drive and kinematics, equipped with appropriate interfaces as well as a supply concept are introduced to allow further configuration. The combination of those concepts characterizes a small modular machine tool system. Technical solutions and fully functional prototypes will be shown and discussed in the following paper.     

Virtual Machine Tool

This paper presents current state of Virtual Machine Tool Technology and related ongoing research challenges. The structural analysis of machine tools using Finite Element models and their experimental calibration techniques are presented. The kinematic analysis and optimisation of machine tool elements are discussed with sample examples. The interaction between the control of the feed drives, cutting conditions and machine tool structure is presented. Multi-body dynamic models of the machine, which allow integrated simulation of machine kinematics, structural dynamics and control techniques, are discussed. The interaction between the machine tool, controller and cutting process disturbances are discussed with sample examples. The simulation of machining operation and its impact on the dynamics of the machine tool and CNC are elaborated. The paper presents both the summary of current and past research, as well as research challenges in order to realise a fully digitised model of the machine tool.     

A holistic integrated dynamic design and modelling approach applied to the development of ultraprecision micro-milling machines

Ultraprecision machines with small footprints or micro-machines are highly desirable for micro-manufacturing high-precision micro-mechanical components. However, the development of the machines is still at the nascent stage by working on an individual machine basis and hence lacks generic scientific approach and design guidelines. Using computer models to predict the dynamic performance of ultraprecision machine tools can help manufacturers substantially reduce the lead time and cost of developing new machines. Furthermore, the machine dynamic performance depends not only upon the mechanical structure and components but also the control system and electronic drives. This paper proposed a holistic integrated dynamic design and modelling approach, which supports analysis and optimization of the overall machine dynamic performance at the early design stage. Based on the proposed approach the modelling and simulation process on a novel 5-axis bench-top ultraprecision micro-milling machine tool – UltraMill – is presented. The modelling and simulation cover the dynamics of the machine structure, moving components, control system and the machining process, and are used to predict the overall machine performance of two typical configurations. Preliminary machining trials have been carried out and provided the evidence of the approach being helpful to assure the machine performing right at the first setup.     

Computation of thermo-elastic deformations on machine tools a study of numerical methods

Modern machine tools are highly optimized with respect to their design and the production processes they are capable to. Now for further advances, especially a detailed knowledge about the thermo-elastic behavior is needed, because the nowadays still existing deficits are mainly related to this. That is why, endeavors in improvement, like the optimization of the design, the evaluation of new materials and the regulation of the production process, particularly rely on accurate computed thermal deformations. One possible approach to increase their quality is to also include the relevant structural variabilities of the machine tools as well as the resulting interactions between the coupled parts within the calculations. In this article, three different numerical methods are presented, which include structural motions in thermo-elastic analyses. Thereby, several conflicting criteria, like real-time capability, memory saving issues and accuracy are fulfilled each time in a different manner. Those methods are afterwards compared with respect to their runtime and accuracy. Finally, the paper concludes with a classification of the usability of the methods in real-time control and optimization tasks.     

Predictive simulation of damping effects in machine tools

A predictive simulation of the different damping effects in machine tools is required to optimize the dynamic behavior and thus increase their performance and working accuracy. Previously, holistic optimization based on damping was not possible due to non-predictive damping models and the lack of adequate modeling approaches. This paper presents a modeling approach, which allows the efficient simulation of the dynamic behavior. By applying this procedure and suitable damping and friction models, the dynamic behavior of a four-axes machining center was simulated with high accuracy – FRAC values above 95% were achieved.     

Mechanical module interfaces for reconfigurable machine tools

Reconfigurable manufacturing systems (RMS) enable industrial companies to adapt to frequent and unpredictable changes of production requirements in a cost-efficient way. RMS are constituted by modular machine tools that provide variable overall functions with the ability to add, remove, rearrange and replace functional sub-units. The performance of these machine tools as regards the quick and flexible arrangement of modules and high work piece quality strongly depends on the properties of the mechanical module interfaces. In this paper, performance parameters for mechanical module interfaces were defined and their influence on the machine tool’s performance discussed. Then flexibly arrange-able quick-coupling interfaces as a promising solution for module assembly were analyzed. Finally, tools for the determination for those interface performance parameters are presented, which require technical testing.     

三轴数控机床加工精度可靠性分析

机床作为机械制造业的基础,几何误差、热误差、装配误差等都会影响数控机床的加工精度,数控机床加工精度的高低直接决定产品的生产质量。为保证数控机床对产品的加工质量,需要对数控机床的加工误差数据处理,求得数控机床加工精度可靠性,而一次二阶矩法和蒙特卡罗法是常用的可靠性分析方法。以三轴数控机床为研究对象,针对给定的误差数据,运用一次二阶矩法和蒙特卡罗法分析出数控机床加工精度可靠性。此分析对提高数控机床加工精度及保证使用寿命具有重要指导意义和参考价值。     

Semi-Double-loop machine learning based CPS approach for predictive maintenance in manufacturing system based on machine status indications

The paper presents two original and innovative contributions: 1) the model of machine learning (ML) based approach for predictive maintenance in manufacturing system based on machine status indications only, and 2) semi-Double-loop machine learning based intelligent Cyber-Physical System (I-CPS) architecture as a higher-level environment for ML based predictive maintenance execution. Considering only the machine status information provides rapid and very low investment-based implementation of an advanced predictive maintenance paradigm, especially important for SMEs. The model is validated in real-life situations, exploring different learning algorithms and strategies for learning maintenance predictive models. The findings show very high level of prediction accuracy.     

Form-accuracy analysis and prediction in computer-integrated manufacturing

The manufacturing of high-quality products at low costs is one of the greatest challenges of every company today. Form accuracy is among the quality parameters of machined parts and is directly related to functional performance. Control and improvement of form accuracy is to be performed under the computer-integrated manufacturing (CIM) concept. This paper investigates the form accuracy of mechanical machining and studies the essential aspects and procedures of form-accuracy simulation. The emphases are on the integration of form-accuracy analysis and the simulation into the CIM database construction and virtual manufacturing. Form-accuracy analysis in this paper reveals that the inherent drawbacks in design will directly affect the geometric quality of a workpiece, and proper process planning can enhance the manufacturability of parts with the required geometric quality. The simulation procedure implemented in this paper can be used at the design stage to predict the form accuracy of a machined part and functional performance. The procedure can also be used at the process planning stage to predict and control form accuracy during the machining process.     

Flexibility in metal forming

Flexibility in metal forming is needed more than ever before due to rapidly changing customer demands. It paves the way for a better control of uncertainties in development and application of metal forming processes. Although flexibility has been pursued from various viewpoints in terms of machines, material, process, working environment and properties, etc., a thorough study of the concept was undertaken in order to with problems of manufacturing competiveness and tackle new challenges of manufacturing surroundings. Therefore, in this paper, flexibility in forming is reviewed from the viewpoints of process, material, manufacturing environment, new process combinations and machine–system–software interactions.     

Modelling machine tool dynamics using a distributed parameter tool–holder joint interface

Increasing productivity in machining process demands high material removal rate in stable cutting conditions and depends strongly on dynamic properties of machine tool structure. Combined analytical–experimental procedures based on receptance coupling substructure analysis (RCSA) are employed to determine the stability of machine operating conditions at different tool configurations. The RCSA employs holder–spindle experimental mobility measurements in conjunction with an analytical model for the tool to predict the dynamics of different sets of tool and holder–spindle combinations without the need for repeated mobility measurements. In this paper an alternative approach using the concept of tool on resilient support is adopted to predict the machine tool dynamics in various tool configurations. In the proposed model the tool, represented by an analytical model, is partly resting on a resilient support provided by the holder–spindle assembly. The support dynamic flexibility is measured by performing vibration tests on the holder–spindle assembly. Tool–holder joint interface characteristics are included in the model by considering a distributed elastic interface layer between the holder–spindle and the tool shank part. The distributed interface layer takes into account the change in normal contact pressure along the joint interface and comparing with the lumped joint model used in RCSA it allows more detailed representation of the joint interface flexibility and damping which have crucial roles in machine dynamics. Experiments are conducted to demonstrate the efficiency of proposed model in prediction of milling operation dynamics and it is shown that the model is capable of accurately predicting the dynamic absorber effect of spindle in a tool tuning practice.     

Manufacturing of WC–Co moulds using SLS machine

Tungsten carbide–cobalt composite is widely used as a wear-resistant material for cutting tools, molds and other applications. Its production by Selective Laser Sintering (SLS) technique promises to combine the material properties of a composite with the flexibility of a production process. The present paper deals with SLS of a powder mixture of tungsten carbide and cobalt with an aim to make functional parts using a modified Rapid Prototyping (RP) machine (100 W DTM Sinterstation 2000). The associated manufacturing problems and their solutions are discussed. Various compositions of powders have been tried before being settled to a particular composition of WC–9 wt.% Co for making final parts. Bronze infiltration is done on laser sintered parts to enhance their mechanical properties. Fretting wear test is taken as a main characterizing test which is used for estimating the wear resistance of samples.