Multi-tasking Machine Tools Research Articles

Chatter Stability of Orthogonal Turn-Milling Process in Frequency and Discrete-Time Domains

Abstract As the industry seeks better quality and efficiency, multitasking machine tools are becoming increasingly popular owing to their ability to create complex parts in one setup. Turn-milling, a type of multi-axis machining, combines milling and turning processes to remove material through simultaneous rotations of the cutter and workpiece with the translational feed of the tool. While turn-milling can be advantageous for large parts made of hard-to-cut materials, it also offers challenges in terms of surface form errors and process stability. Because tool eccentricity and workpiece rotation lead to more complexity in process mechanics and dynamics, traditional milling stability models cannot predict the stability of turn-milling processes. This study presents a mathematical model based on process mechanics and dynamics by incorporating the unique characteristics of the orthogonal turn-milling process to avoid self-excited chatter vibrations. A novel approach was employed to model time-varying delays considering the simultaneous rotation of the tool and workpiece. Stability analysis of the system was performed in both the discrete-time and frequency domains. The effects of eccentricity and workpiece speed on stability diagrams were demonstrated and validated through experiments. The results show that the tool eccentricity and workpiece speed alter the engagement geometry and delay in the regeneration mechanism, respectively, leading to significant stability diagram alterations. The proposed approach offers a comprehensive framework for the stability of orthogonal turn-milling and guidance for the selection of process conditions to achieve stable cuts with enhanced productivity.

Study on Process Design Based on Language Analysis and Image Discrimination Using CNN Deep Learning

At present, machining with numerically controlled (NC) machine tools is mostly performed by NC programs generated by computer-aided design and computer-aided manufacturing (CAD/CAM) systems. However, even if the machining shape to be machined is the same, there are numerous machining processes involving a series of operations such as determining the machining area, machining order, and machining conditions. These are entrusted to the user, and automation is difficult. In addition, these tasks depend on the experience and know-how of skilled engineers, and it is very difficult to convert them into algorithms and reflect them in the creation of NC programs. Therefore, in this study, artificial intelligence (AI) was used for the process design of multi-tasking machine tools, with the goal of determining and automating the process design using shape examples. We propose a shape recognition method that includes image analysis by AI. This image analysis makes it possible to determine the characteristics of the machining shape, and the machining operator can easily judge the machining process based on the CAD model. Furthermore, because there are shapes that cannot be determined from image data alone, shape features are also extracted from the STEP file of the CAD model. A language analysis of the STEP file can find the characteristic components and their numerical information to determine the coordinates of the shape features. By combining image analysis and language analysis, the method can easily judge the process based on the information in the CAD model. Finally, using the generated learning model and analysis program, we conducted a test to determine whether a multitasking machine tool is necessary for machining.

Machining process analysis of multi-tasking machine tools based on form-shaping motions

With the increase in machine tool types, it becomes difficult to select an appropriate machine tool to achieve high-efficiency and high-quality machining. This study aims to automate the selection of machine tools by evaluating and comparing machining processes for the machining of targeted parts. A fundamental method for analyzing machining processes is proposed by introducing the functional description of machine tools in the previous study. However, a simple machine tool structure is supposed as a first step in the proposed method. Therefore, in this study, the machining process analysis method is reconstructed to handle complex machine tool structure such as that of a multi-tasking machine tool. The functional description of machine tools is expanded to represent form-shaping motions of a multi-tasking machine tool by adding new suffixes that indicate the orientation of plural spindles and the posture of non-rotational specific functions. Additionally, evaluation indexes of rationality and similarity of machining operations to exclude unsuitable machining operation candidates are proposed. Case studies are conducted and confirmed that the system developed based on the novel proposed method can derive accountable machining processes according to a multi-tasking machine tool. The results of the machining process analysis may thus support the machine tool selections.

Position error reduction of tool center point in multi-tasking machine tools through compensating influence of geometric deviations identified by ball bar measurements

A method to compensate the influence of geometric deviations on tool center point (TCP) for a multi-tasking machine tool is proposed in this paper. Some methods to compensate geometric deviations of a rotary axis in five-axis machining centers have been proposed. However, due to the special topological structure of multi-tasking machine tools, the identification and compensation methods for geometric deviations are different from those of the five-axis machining centers, which have been seldom researched until now. In this paper, the main attention is paid to analyze the eccentricities of the trajectories measured by a ball bar under simultaneous three-axis motions and to reduce the influence of the identified geometric deviations on the position error of TCP by the compensation method. It is divided into two sequential subtasks. At first, the geometric deviations are identified by using the eccentricities of measured trajectories. A simple and practical measuring procedure is proposed to identify geometric deviations of rotary axes existing in a multi-tasking machine tool. For the second step, a method is proposed by modifying the original NC code according to the kinematic chain model of the targeted machine tool to compensate the influence of the existing geometric deviations on TCP. An experiment is conducted on a multi-tasking machine tool with a swivel tool spindle head in the horizontal position. The repeatability of the measured eccentricities based on three experimental results is also investigated to reduce the influence of measuring error on the identified results. As a result, the corresponding values of geometric deviations after the compensation are less than 2.2 arcseconds or 2.4 μm. It is concluded that the influence of geometric deviations on TCP is compensated effectively, and the position error of TCP is reduced significantly.

In-House Investment

Multi-tasking machine tool helps hydropower specialist bring manufacturing back home

Identification Method of Geometric Deviations for Multi-Tasking Machine Tools Considering the Squareness of Translational Axes

Some methods to identify geometric deviations of five-axis machining centers have been proposed until now. However, they are not suitable for multi-tasking machine tools because of the different configuration and the mutual motion of the axes. Therefore, in this paper, an identification method for multi-tasking machine tools with a swivel tool spindle head in a horizontal position is described. Firstly, geometric deviations are illustrated and the mathematical model considering the squareness of translational axes is established according to the simultaneous three-axis control movements. The influences of mounting errors of the measuring instrument on circular trajectories are investigated and the measurements for the B axis in the Cartesian coordinate system and the measurements for the C axis in a cylindrical coordinate system are proposed. Then, based on the simulation results, formulae are derived from the eccentricities of the circular trajectories. It is found that six measurements are required to identify geometric deviations, which should be performed separately in the B axis X-direction, in B axis Y-direction, in C axis axial direction, and three times in C axis radial direction. Finally, a numerical experiment is conducted and identified results successfully match the geometric deviations. Therefore, the proposed method is proved to identify geometric deviations effectively for multi-tasking machine tools.

Proposal of a machining features recognition method for 5-axis index milling on multi-tasking machine tools

Proposal of a machining features recognition method for 5-axis index milling on multi-tasking machine tools

Development of a CAPP system for multi-tasking machine tools to deal with complicated machining operations

Development of a CAPP system for multi-tasking machine tools to deal with complicated machining operations

Chatter Suppression in Parallel Turning Assisted with Tool Swing Motion Provided by Feed System

Parallel turning technology has been attracting attention as an important technology to enhance the productivity of multitasking machine tools. To maximize the productivity advantage of parallel turning, chatter avoidance or suppression is one of the most noteworthy concerns. In this study, a novel chatter suppression technique using tool swing motion is provided by a feed drive system. The optimal design methodology of the tool swing motion for effective chatter suppression is also introduced based on its analogy with the spindle speed variation technique under the shared-surface parallel turning and rigid-tool and flexible-workpiece assumptions. The proposed method was evaluated with regard to the chatter stabilizing performance and workpiece runout as compared to conventional equal pitch turning and unequal pitch turning for chatter suppression. As a result, the proposed tool swing parallel turning exhibited a high chatter stabilizing performance without eccentricity of the workpiece and enhanced surface quality, although particular swing marks were left on the machined surface.

Chatter Avoidance in Parallel Turning With Unequal Pitch Angle Using Observer-Based Cutting Force Estimation

Parallel turning is garnering attention as one of the most important technologies for multitasking machine tools. This is because a potential exists to enhance the stability limits compared to the turning operation using a single tool when cutting conditions are properly selected. Although stability prediction models for parallel turning have been developed in recent years, in-process monitoring and in-process chatter techniques are almost not discussed. In this study, to suppress chatter vibration, an unequal pitch turning method was proposed. In this method, the upper tool was controlled based on the optimum pitch angle calculated from spindle speed and chatter frequency. Chatter frequency was identified from estimated cutting force by a disturbance observer (DOB). From the result of the parallel turning test, it is clear that chatter vibration can be avoided by controlling the upper tool based on optimum pitch angle. Meanwhile, the pitch angle difference that can suppress chatter had a certain range. Subsequently, the robustness of the optimum pitch angle difference is experimentally evaluated by both the continuous moving test and the stepwise moving test of the pitch angle.

Development of thermal deformation analysis model of multitasking machine tool with turrets considering bed

Development of thermal deformation analysis model of multitasking machine tool with turrets considering bed

Special Issue on Multiaxis Control and Multitasking Machining

Machine tools using numerical control (NC) devices are typical mechatronics products, and introducing them is a powerful way to automate plant production. NC machine tools in workshops meet the requirements of high accuracy and efficiency in the machining of a variety of parts and mold dies. Turning centers and machining centers are typical examples of such machine tools. Various cutting processes have been integrated in them to cope with the increase in machine parts that not only have complicated geometries but also must be made with high accuracy, in small quantities, and in a short machining time. In addition, turning and machining centers have been given multitasking capabilities, and the number of control axes has been increased so that complex products may be manufactured efficiently. Given that the strong attention and interest in multiaxis control and multitasking machine tools are rapidly increasing, it is fitting that the current state of the art of these tools and their practical and applicable technologies be presented. This special issue features 16 research articles – one review and 15 papers – related to the latest research results and practical case studies in multiaxis control and multitasking machining. Their subjects cover various advances in machine control, motion accuracy evaluation, machining error analysis, chatter vibration monitoring or suppression, trouble-free tool path generation, process planning, and new applications of the machine tools. We thank the authors for their contributions to this special issue, and we are sure that both non-specialists and specialists alike will find the information the authors provide both interesting and informative. Moreover, we deeply appreciate the reviewers for their incisive efforts. Without these contributions, this special issue could not have been realized. We truly hope that this special issue will trigger further research on multiaxis control and multitasking machining.

Machining Process Evaluation Indices for Developing a Computer Aided Process Planning System

In the manufacturing industry, there is an urgent need to shorten the manufacturing lead time of products. Therefore, optimizing process planning is essential to realize high efficiency machining. In this study, in order to develop a computer aided process planning (CAPP) system using previously proposed machining features, a prediction method for some process evaluation indices is proposed. Many candidates for the machining process exist, depending on the recognized machining features in a previous study. Therefore, by using these indices, operators can select a suitable process from among these candidates according to their ideas. Case studies of process planning are conducted to confirm that the operator’s strategy affects the selection of the machining process candidates. From the case study results, it is found that the proposed process evaluation indices have potential use in determining the machining process utilized, and are suitable for a flexible CAPP system of multi-tasking machine tools.

Integrated Chatter Monitoring Based on Sensorless Cutting Force/Torque Estimation in Parallel Turning

Parallel turning technology is considerably important in future multi-tasking machine tool because it has the potential to enhance the stability limits, compared to turning operations using a single tool. Although stability prediction models for parallel turning have been developed recently, the technique of in-process monitoring of chatter is almost out of focus. In this study, the monitoring of chatter based on the sensorless cutting force/torque technique was evaluated in the parallel turning and cutting of the same surface of an elongated workpiece. Two cutting force/torque estimation methods were evaluated: a conventional disturbance observer (DOB) using internal information from a servomotor and a multi-encoder-based disturbance observer (MEDOB) using load-side position/angular information as well. In the DOB-based monitoring, chatter frequency components were observable regardless of the guideway type and drive system. However, chatter monitoring may be difficult when the angle of the servomotor is changed slightly because of the damping properties of the sliding guideway. In the MEDOB-based monitoring, the waveform of the estimated cutting force reflected the vibrational state at the cutting point well, and the extraction of chatter frequency components became easier regardless of the guideway type.

Decision Method of Target Shape Position and Orientation Corresponding to Actual Objects

Multi-tasking machine tools are popular owing to properties such as high flexibility and productivity. It is essential for an operator to avoid collisions between the machine structure and workpieces as machine tools realize complicated motions. Thus, a machine simulator is widely used prior to machining operations to solve this problem. However, unexpected collisions often occur in a commercial machine simulator when the setup of a workpiece or a jig differs from the 3D models created in advance. Therefore, this study proposes a machine simulator that utilizes 3D models created by measuring the shape and position of the workpiece and jig on the machine tool. In a case in which the workpiece differs from the desired workpiece, it is necessary to determine a suitable position and orientation of the target shape based on the obtained objects to modify NC (Numerical Control) programs. In this study, a decision method of the position and orientation of target shape is devised such that it corresponds to actual objects obtained by on-machine measurement.

複雑形状に対応した工程設計支援システムの加エフィーチャ認識に関する研究

Manufacturing industry tends to high-mix low-volume production in recent years. Therefore, it is strongly required to develop a computer aided process planning (CAPP) system to shorten the preparation time and to generate NC program. Feature recognition has been treated as a key technology to develop a CAPP system. In authors’ previous study, novel machining features for multi-tasking machine tools have been proposed. The machining features can correspond to several alternative machining methods. However, complex target shapes of practical mechanical parts have not been considered. In order to solve this problem, special shapes such as chamfer part and freeform surface are firstly approximated and machining features such as cylinder or cuboid are recognized from the converted simple shapes in this study. From the results of conducted case study, it is found that the proposed feature recognition method has a potential to deal with complex target shapes of practical mechanical parts.

Polishing of Large-Sized Spiral Bevel Gears Using Elastic Grindstone

A manufacturing method of large-sized spiral bevel gears has been developed using a multi-axis control and multi-tasking machine tool in recent years. As a result of trial manufacturing of large-sized spiral bevel gears in a cyclo-palloid system using a multi-axis control and multitasking machine tool, and the surfaces roughness and tool mark of the machined gears were compared with those using a special generator, the surfaces were rough and leprous tool mark occurred. In this paper, the tooth surfaces are polished using a PVA elastic grindstone in order to improve the surface roughness and leprous tool mark. As a result, the surface roughness was improved after polishing although the wear and clogging of the elastic grindstone occurred.

Development of A Hybrid Multi-tasking Machine Tool: Integration of Additive Manufacturing Technology with CNC Machining

Reduction in manufacturing cost and total in-process time is critical for industries in developed countries in order to maintain competitive advantage. At the same time, high added value manufacturing is also required. If not addressed, it will become increasingly difficult for the manufacturing industries in high-wage countries who are at a competitive disadvantage to survive in the future. In order to meet these requirements by creating a new possibility for machine tools, a Hybrid Multi-tasking machine tool has been developed by equipping Laser Metal Deposition functionality in addition to existing integrated turning and milling capabilities. The concept of a Hybrid Multi-tasking platform enables a further evolution of Done-in-One processes enabling building near-net shape components to be produced by additive manufacturing and then quickly generating the net shape through high-precision finish machining operations. It is particularly well suited to small-lot production of difficult-to-cut materials, such as aerospace alloys, high-hardness materials used in the energy sector, production tools and components and high-precision, specialty alloy designs common in medical device manufacturing. This paper introduces the features of the Mazak Hybrid Multi-tasking machine tool with application examples and demonstrates possibilities and tasks for the next generation manufacturing.

把持替えと同時加工を可能にする複合加工機用工程設計支援システムの開発

把持替えと同時加工を可能にする複合加工機用工程設計支援システムの開発

Effects of tool axis offset in turn-milling process

The orthogonal turn-milling process draws increasing attention parallel to the developments in multi-tasking machine tools. However, the effects of process parameters used in orthogonal turn-milling are not fully understood. The basic objective of this study is to describe one of the most important and unique cutting parameters which is the tool axis offset, and determine its effects on process outputs such as cutting force, surface quality and tool wear. In order to achieve this objective, the effect of the tool axis offset is defined based upon geometrical relationships which are used to develop a mechanistic force model. Surface quality including the tool axis offset effect is examined by both analytical and experimental methods. Additionally, tool wear experiments were conducted to obtain the tool life under different settings for the tool axis offset. It is found that the process outcomes such as cutting forces, surface quality and tool life can be improved by taking into account the tool axis offset in orthogonal turn-milling operation. As a valuable conclusion, the proposed models and experimental results can be used to select a proper tool axis offset value while performing orthogonal turn-milling process in order to improve the process outcome.