Automation of machine tools has made them more productive, thereby providing an advantage for sustainability and the welfare of mankind. However, in many cases, the successful automation of machine tools requires the avoidance of self-excited chatter vibrations, resulting in a reliable stable state for cutting. Machine tool operators tend to use the machines close to their power thresholds, thereby unknowingly driving them toward the limits of their stability. Much progress has been made in the last few decades concerning the understanding and prediction of such vibrations, and this has led to improvements such as higher cutting rates and chip thicknesses.
Several countermeasures such as active and passive damping are available for avoiding chatter vibrations in machine tools. However, their industrial use is not common yet. In fact, the industry is somewhat unfamiliar with many of these countermeasures. The hesitant attitude of the machine tool builders to apply such countermeasures is a result of several factors: active and passive damping devices are additional system components that require design, tuning, and maintenance. Furthermore, they are associated with a risk of failure, resulting in additional down times of the machines. Additionally, if a machine requires such devices to achieve the desired specifications, the customer’s opinion regarding it can be negatively affected. This situation is challenging for machine tool builders, users, and academia as well. Therefore, we decided to dedicate a special issue of IJAT to this topic.
This special issue focuses on both active and passive damping measures, particularly the measures that are systematically designed and deliberately implemented to increase the chatter-free depth of cut in machine tools. The papers in this issue identify successful applications or at least a vision for them. Additionally, models demonstrating the effects of the chosen active or passive damping systems are presented. Some of these models can also be used to systematically select the parameters of the system. Some of the systems can be easily applied as low-cost patch-up solutions to improve the behaviors of the machines already in use.
I hope that this special issue delivers a valuable overview of the existing approaches to introduce additional damping in machine tools. I would like to sincerely thank all the authors for their dedication and the well written and illustrated manuscripts. I would also like to thank the reviewers for their efforts to ensure the quality of this issue. Finally, I am very thankful to IJAT for their immense cooperation and support. I wish you all the best and hope that you can benefit from the content of this special issue.
Mechatronic structures deform under static and dynamic loads. These deformations lead to deviations at the tool center point (TCP), affecting the reachable accuracy and/or productivity of the machines. The scope of this work is the comparison of calculations and measurements of different static and dynamic errors on a dynamic test bench. A reduced-order modelling approach is applied for the test bench modelling. It uses a combination of modal condensation and moment-matching methods with Krylov subspaces. The different modelling steps and requirements are presented. The same model is used for all static and dynamic evaluations presented within this paper. Static deformations, leading to roll and pitch deviations at the TCP of the test bench structure, are simulated using the described modelling methodology and validated by inclination measurements. The modal behavior of the system is investigated by calculation and compared to the measurements at a single axes position. The spatial change of the frequency response functions of the modelled system is investigated further, by calculation and measurement of the velocity open-loop FRFs of one axis for different machine configurations. In addition, a transient trajectory simulation is performed and compared to the Heidenhain KGM and encoder measurements. The large variety of comparisons shows the efficient applicability of the modelling environment MORe.
The material removal rates of machine tools are often limited by chatter, which is caused by the machine’s most flexible structural modes. Active vibration control systems mitigate chatter vibrations and increase the chatter free axial depth of cut. However, model-based control strategies reach their limit if the machine tool exhibits highly position-dependent dynamics. In this paper, an adaptive control strategy is presented. This strategy uses online system identification to adapt the controller. The adaption algorithm is mainly automated. However, a few parameters still need to be selected. Therefore, a methodology for the determination of the optimal parameters is proposed. The adaptive controller was implemented on a B&R PLC and its suitability was verified experimentally by the observation of notable increases in the chatter-free material removal rates.
The presence of chatter vibrations is one of the main limitations of machining processes in terms of productivity, as they prevent obtaining the required surface finishes and decrease the life of tools and the mechanical elements of the machine. The use of active dampers governed by a control strategy permits an increase in structural damping without significantly changing the machine design. The main objective of this study is to improve the dynamic capabilities of the machine, and to increase the chatter-free region. This objective is achieved by the addition of an electromagnetic actuator located as close as possible to the cutting point. The electromechanical design of the actuator is described, and a novel double flexure guarantees a constant gap between the moving magnets and the coils. This smart mechatronic system allows the introduction of new additional functions: process and machine monitoring, chatter avoidance by spindle speed modification, and machine dynamics calibration. All of these functions enhance standard milling machines.
It is necessary to increase the damping of a machine support structure (support damping) to reduce the residual vibrations caused by rocking vibration. The stiffness of the machine support system (support stiffness) is also an important parameter that needs to be considered while designing machine tools, to avoid low frequency vibrations. However, conventional passive damper supports decrease the support stiffness while increasing the damping. In our previous study, a passive viscoelastic non-linear damper system for shear vibrations, where the vertical preload determines its damping coefficient, was developed to increase the support damping without decreasing the stiffness by focusing on the horizontal component of rocking vibration. The magnitude dependency of the damping capacity has been modeled. However, this damper system has a tradeoff relationship between natural frequency and damping capacity caused by changes in the preload distribution. Thus, adjustment of the vertical preload applied on the damper is essential for the model-based installation of this damper system. So far, no method has been proposed considering this issue. The vertical preload has been adjusted by trial and error methods. This study proposes a method to determine the damper preload conditions systematically by considering the tradeoff relationship between natural frequency and damping capacity caused by changes in the preload distribution. This method is described based on the case study of a machining center. First, the relationship between preload distribution and support stiffness is investigated using the support stiffness model. Then, the relationship between damping capacity and vertical preloads on the damper is investigated based on material test results. Based on these investigations, the tradeoff relationships are simulated on a machining center by utilizing the damper model. The simulation results are verified with the experimental results. The results show that the proposed method can estimate the tradeoff relationship between natural frequency and damping capacity caused by the changes in the preload distribution. By utilizing this estimated relationship, the preferred preload condition can be decided depending upon the user’s demand.
The material removal rates of machine tools are often limited by chatter, which is caused by the machine’s most flexible structural modes. Active vibration control systems mitigate chatter vibrations and increase the chatter-free depth of cut. The systems can be used for already-in-use machine tools in particular as a retrofit solution. Unfortunately, no dimensioning techniques exist to help in finding the right actuator size required for a specific machine tool. This publication presents a simulation-based dimensioning methodology that determines, based on a stability analysis, the required actuator force and bandwidth. First, the critical machining processes, based on machine tool specific parameters, are identified. Then, the required actuator force and bandwidth are determined with the help of a coupled simulation model that consists of a cutting force model, the machine’s structural dynamics, and a model of the active vibration control system.
This paper presents a novel approach for active chatter reduction using a motor spindle with integrated magnet actuators. Based on the results of previous studies the design of an active damped prototype is described. The system performance as well as the benefits and drawbacks of this solution are discussed. In order to eliminate the known drawbacks a novel actuator design is introduced. The novel approach integrates the actuator windings into the stator core. Next, strategies for electric and magnetic decoupling of the actuator and motor windings are presented. Subsequently the actuator design is discussed. The force generation and distribution of the damping actuator are calculated via analytical and finite elements analysis (FEA). With the novel approach the mechanical integration of the active damping actuators is simplified significantly. Additionally, the maximal attainable spindle power is increased up to 150% in relation to the previous solution.
First, we would like to express our sincere condolences to the victims of the landslides and floods caused by the torrential rain in Japan in July 2018. We were terribly grieved to hear about these disasters during the editing of this special issue of the International Journal of Automation Technology (IJAT), and we sincerely hope for the revival of the disaster-stricken areas.
This special issue focuses on the progress in manufacturing technologies for maximizing product quality and reducing costs, especially in the mechanical industry. Manufacturing technologies have been developing in order to meet the changes in social and economic environments such as progress in informatization, diverse needs, and increasing demands for a sustainable society. At present, engineers and researchers in the field of manufacturing are facing an unprecedented rapid change caused by the fourth industrial revolution. Therefore, research in this field is also expected to develop more than ever before.
This special issue of IJAT contains seven research papers on topics including shearing of metal sheets, machine tools and machining technology, precision dimensional measurement, and nanoimprinting process. Some of the papers, revised and extended at the editors’ request, were presented originally at the 9th International Conference on Leading Edge Manufacturing in 21st Century (LEM21), held in Hiroshima, Japan in 2017.
The editors thank the authors and reviewers for their comprehensive efforts in making this special issue possible, and hope that these articles will encourage further research on manufacturing technologies.
A novel method is proposed for estimating the machining errors on machined surfaces caused by errors of multi-axis machine tools, such as geometric errors, based on a new generating method of tool swept volumes. In the proposed tool swept volume generation method, the boundary surfaces of the tool swept volumes are derived as triangular mesh models satisfying the required approximation accuracy based on the tangency condition. Using the proposed method, tool swept volumes can be derived for various tool paths including the tool self-intersecting motion. A tool path evaluation method based on the error vectors with respect to the start position of a specific tool path is also proposed. In the proposed evaluation method, error vectors on machined surfaces are derived by comparing the points on the nominal tool swept volumes (excluding the machine tool errors) with the triangles on the error tool swept volumes (including the machine tool errors).
Vibration analysis is one method of machining process monitoring. The vibration obtained in machining is often nonlinear and of a nonstationary nature. Therefore, an appropriate signal analysis is needed for signal processing and feature extraction. In this research, vibrations obtained in the milling of thin-walled workpieces were analyzed using the Hilbert-Huang transform (HHT). The features obtained by the HHT served as machining-state indicators for machining process monitoring. Experimental results showed the effectiveness of the HHT method for detecting chatter and tool damage.
The motion trajectories of machine tools directly influence the geometrical shape of machined workpieces. Hence, improvement in their motion accuracy is required. It is known that machined shape errors occurring in numerical control (NC) machine tools can be compensated for by modifying the CL-data, based on the amount of error calculated by the measurement results of the machined shape of the workpiece. However, by using this method the shape errors cannot be compensated accurately in five-axis machining, because the final machining shape may not reflect the motion trajectory of a tool owing to the motion errors of the translational and rotary axes. In this study, a modification method of the cutter location (CL)-data, based on the amount of motion errors of the tool center-point trajectory during the machining motion, is newly proposed. The simulation and experiment of a wing profile machining motion is performed, to confirm the effectiveness of the proposed method. From the results, we confirm that the motion accuracy can be significantly improved by applying the proposed method.
An on-machine measurement method, called the square-layout four-point (SLFP) method with angle compensation, for evaluating two-dimensional (2-D) profiles of flat machined surfaces is proposed. In this method, four displacement sensors are arranged in a square and mounted to the scanning table of a 2-D stage. For measuring the 2-D profile of a target plane, height data corresponding to all measuring points are acquired by means of the raster scanning motion. At the same time, pitching data of the first primary scan line and rolling data of the first subsidiary scan line are monitored by means of two auto-collimators to compensate for major profile errors that arise out of the posture error. Use of the SLFP method facilitates connection of the results of straightness-measurements results obtained for each scanning line by using two additional sensors and rolling data of the first subsidiary scan line. Specifically, the height of a measuring point is calculated by means of a recurrence equation using three predetermined height data for adjacent points in conjunction with data acquired by the four displacement sensors. Results of the numerical simulation performed in this study demonstrate higher efficiency of the SLFP method with angle compensation. During actual measurement, however, it is difficult to perfectly align inline the origin height of each displacement sensor. With regard to the SLFP method, zero-adjustment error is defined as the relative height of a sensor’s origin with respect to the plane comprising origins of the other three sensors. This error accumulates in proportion to number of times the recurrence equation is applied. Simulation results containing the zero-adjustment error demonstrate that accumulation of the said error results in unignorable distortion of measurement results. Therefore, a new self-calibration method for the zero-adjustment error has been proposed. During 2-D profile measurement, two different calculation paths – the raster scan path and orthogonal path – can be used to determine the height of a measurement point. Although heights determined through use of the two paths must ideally be equal, they are observed to be different because accumulated zero-adjustment errors for the two paths are different. In view of this result, the zero-adjustment error can be calculated backwards and calibrated. Validity of the calibration method has been confirmed via simulations and experiments.
This research was carried out to investigate the strain hardening in an aluminum alloy worksheet caused by punch/die shearing by means of microhardness measurement and finite element method (FEM) analysis. To examine the strain-hardened zone at the sheared edge of a worksheet, a 0.36 mm thick AA4047 aluminum alloy cut by punch/die shearing was subjected to microhardness measurements. In addition, a two-dimensional FEM model was developed and used to simulate the shear cutting of the aluminum alloy worksheet. The fundamental shear cutting parameters, punch/die clearance, cutting tool wear, and friction at the worksheet/tool interfaces were numerically varied and simulated. From the investigation results, the strain-hardened zone was observed by hardness measurement. The size of the zone significantly varied under different cutting parameters. From the simulated stresses at the sheared zone, the variation of the width of the strain-hardened zone with respect to cutting parameters was determined by the maximum principal stress on the worksheet being sheared.
Ultraviolet nanoimprint lithography (UV-NIL) can be used to fabricate nanoscale patterns with high throughput. It is expected to serve as a low-cost technique for the production of items in large numbers. However, master molds for UV-NIL are expensive and laborious to produce, and there are problems associated with the deterioration of the master mold and damage to its nanopattern due to adhesion of the UV-curable resin. Consequently, the UV-curable resin has to combine low-viscosity characteristics for coatability with an antisticking property. Coating a master mold with a release layer is important in preventing damage to the master mold or adhesion between the mold and the UV-curable resin. However, the released layer deteriorates as the master mold is repeatedly used to fabricate nanopatterns. By contrast, the use of a replica mold is a valuable technique for preventing the deterioration of the master mold, and there have been several studies on the fabrication of replicas of master molds with the use of UV-curable resins. In many cases, the fabrication of nanopatterns with replica molds requires the use of a release agent. In a previous study, we developed a material for replica molds that does not require a release agent. This material consisted of a UV-curable resin with an antifouling effect that was prepared from cationically polymerizable UV-curable and epoxy-modified fluorinated resins. With the use of this material, replica molds with patterns of pillars or holes were fabricated with UV-NIL. The lifetime of the mold with the nanopattern of pillars was shorter than that with holes. In addition, the replica mold with the pillar-shaped nanopattern had numerous defects and allowed adhesion of the transfer resin after repeated efforts. Herein, we describe an improved release-agent-free hard replica mold. We transferred large numbers of nanopatterns of pillars from the replica mold, and evaluated the error rate and contact angle of our improved release-agent-free hard replica mold. The resulting release-agent-free replica mold with a nanopattern of pillars was capable of transferring up to 1000 sequential imprints. In addition, to improve the release properties of the transfer resin, we included an additive to the transfer resin that contained a reactive fluorinated material. This material improved the release properties of the transfer resin and mitigated the deterioration of the contact angle and increase in the error rate.
In sheet metal processing, nesting and scheduling are important factors affecting the efficiency and agility of manufacturing. The objective of nesting is to minimize the waste of material, while that of scheduling is to optimize the processing sequence. As the relation between them often becomes a trade-off, they should be considered simultaneously for the efficiency of the total manufacturing process. In this study, we propose a co-evolutionary genetic algorithm-based nesting scheduling method. We first define a cost function as a fitness value, and then we propose a grouping method that forms gene groups based on the processing layout and processing time. Finally, we validate the effectiveness of the proposed method through computational experiments.
Structural ceramics components for industrial use are classified under two categories: one that is originally designed for ceramics (Ceramic Origin), and the other that is originally designed for metals and subsequently replaced with ceramics because of their improved hardness and resistance to both heat and corrosion (Metal Origin). Ceramic insulators for power lines and catalytic substrates used to control automotive emissions in gasoline engines are “Ceramic Origin” components. As ceramics are difficult to machine, a precision mold has been used in the forming process to minimize the machining volume in the case of “Ceramic Origin” components. Meanwhile, ceramic turbo charger rotors and valves for automotive engines are “Metal Origin” components, which not only require durability under severe operating conditions but also severe dimensional accuracy, similar to metal parts. These components have been derived from extensive R&D efforts in materials and process technologies for ceramic gas turbines, which have been implemented in the majority of advanced countries since the 1970s. This paper includes some examples of precision forming and machining technologies for both types of ceramic components developed by NGK Insulators, Ltd., and highlight their issues. Finally, the possibility of new types of ceramic-based components will be introduced.
The study deals with an improved method of milling thermo-plastic CFRP with a radius end mill. The authors use inclined planetary milling to carry out a fine CFRP boring technique. The inclined planetary motion milling consists of the two independent spindle motions of tool rotation and revolution. The eccentricity of the tool rotation axis is realized by a few degrees of inclination from the revolution axis. The movement of eccentric mechanism can be reduced by comparing it with that of orbital drilling. The inclined planetary motion milling reduces inertial vibration and decreases cutting force. Owing to the geometrical cutting principle, material delamination and burrs can be decreased. Thermo-plastic CFRP has recently been under development as an alternative structural material for the next generation of automobiles and in response to demands for bored fastening holes. The shape of the cutting edge of the ball end mill is suitable for the inclined planetary milling, as revealed by results of past experiments done on thermo-set CFRP. However, the ball end mill has left burrs and melted matrix on the exit side in the case of thermo-plastic CFRP. The radius end mill has the advantage over the ball end mill in terms of finishing fine boring. Based on the consideration of the schematic model and experiments using the Taguchi method, the improved milling conditions are examined.
Polycrystalline diamond (PCD) tools are widely used for cutting tools because PCD has no crystal orientation and is an isotropic material, it is low in cost, and it is easily machined by electric discharge machining. PCD is sintered from diamond abrasives with an alloy metal, such as cobalt, and it is difficult to reduce the surface roughness and the edge accuracy compared with single crystal diamond. In this study, high efficiency and high precision machining of the PCD wheel were investigated. In the experiments, PCD wheels were ground with a diamond wheel, and the effects of the grinding direction and the load on the tool preciseness and the scribing performance were examined.
In the case of a complex shaped helix bevel gear, the forging of complete gear tips is very difficult to achieve. In almost all cases, tooth profile is finished by cutting machine from simple shaped forged parts, therefore requiring considerable machining time and cost. However, there are many approaches to forging. Forging is mainly classified as hot and cold forging, and uses a single motion press. In the case of hot forging takeoff of products from die is difficult by the cooling shrinkage from die and accuracy of products is lower level than cold forging. In addition, in the case of cold forging, a complicated shape is difficult to achieve based on the lack of ductility of the materials. To realize a helix bevel gear using a single forging operation, we applied a tool heating system and three-axis forging press. The tool heating system is applied to prevent a temperature decrease in the material by contact between the tool and forging material during the forging process. Further, to optimize the forging direction and timing, we used a three-axis forging press. We confirmed good forging capability of this special forging process, as well as the high precision of the forged parts. Moreover, through the thermo-mechanical control of steel and the tool temperature, the forged parts showed good mechanical properties, such as high hardness.
Additive manufacturing (AM) using metal materials can be used to manufacture metal parts with complex shapes that are difficult to manufacture with subtractive processing. Recently, numerous commercial AM machines for metallic materials have been developed. The primary types of AM using metallic materials are powder bed fusion or direct energy deposition. Other types using metallic materials have not been adequately studied. In this study, the use of the material extrusion (ME) type of AM is investigated. The aim is to use metallic materials not only for fabricating metal parts but also for adding various properties to base materials, e.g., electric conductivity, thermal conductivity, weight, strength, and color of plastics. ME is appropriate for use with various materials by mixing different types of filler. However, there is a problem in that the high density of metal fillers generates unstable extrusion. Therefore, ultrasonic vibration was used for assisting extrusion. A prototype system was developed using an extrusion nozzle vibrated by an ultrasonic homogenizer. The experimental results showed that the ultrasonic vibration allows materials to be extruded smoothly. Three dimensional (3D) shapes could be built by multi-layer deposition with a thixotropic polymer containing a highly concentrated steel powder. As one application, a 3D-shaped object was fabricated as a sintered object. After the vibration effect in the extrusion process of steel powder and clay was confirmed, a 3D object built by the proposed method was sintered through a baking process.
The orientation compensation of a three-degrees-of-freedom inchworm stage with optical navigation is described. As the stage does not use any guide or preload, a closed loop feedback control system is employed to retain the accurate orientation of the stage. The stage consists of piezoelectric actuators (piezos) for thrusting and electromagnets for positioning. A non-excited electromagnet is moved by the deformation of piezos, and excited electromagnets retain their positions. However, a weak electromagnetic force prevents the stage from retaining its accurate position. In addition, a friction force reduces the displacement of the non-excited electromagnet. Therefore, the orientation of the stage is measured using a light source and an optical detector, and the deformation of the piezos is controlled. The orientation error is reduced by using optical navigation.