Multi-axis Machine Research Articles

Bending and Torsion Fatigue-Testing Machine Developed for Multiaxial Non-Proportional Loading

A new fatigue-testing machine was developed to perform high-cycle multiaxial fatigue tests at 50 Hz, in order to reduce testing time. The developed machine can combine bending and torsion loading and perform fatigue tests at a high frequency, under proportional and non-proportional loading conditions, where the principal stress direction changes during a cycle. The proportional loading is cyclic bending loading, and the non-proportional loading is cyclic, combining bending and reversed torsion loading. In this study, the effectiveness of the testing machine was verified by conducting tests under these loading conditions, using specimens of type 490A hot-rolled steel and type 304 stainless steel. The fatigue life linked to bending loading obtained using the new testing machine was slightly extended compared with that obtained using the conventional fatigue-testing machine. The fatigue life derived as a result of a combination of bending and torsion was comparable to that obtained using the conventional fatigue-testing machine, although a fatigue limit reduction of 100 MPa was observed compared to the former study. The feasibility of tests using the developed multiaxial fatigue-testing machine was confirmed.

Badania efektywności ekonomicznej oprzyrządowania technologicznego wydłużającego łańcuchy kinematyczne obrabiarek sterowanych numerycznie

The article describes the selection of machining holders included in the so called special machining fixture-tools. The equipment presented in the work is readily used by technologists as a solution extending the kinematic chain of the machine tool. It enables multi-axis machining (from many sides) using one item clamping. This paper presents the calculation of productivity and its growth through the use of special equipment. The research was carried out for medium-volume production, which is quite a difficult area in the design of technologies and the study of production costs. In the technical development of tooling and modeling of its usage costs, it was assumed to maintain the required level of dimensional and shape accuracy with a simultaneous significant reduction of operating costs. As part of the work, a model was also developed for determining the break-even point of investment in special machining tools that are used on triaxial milling machining centers.

Special Issue on the Latest Machine Tool Technologies and Manufacturing Processes

With the 2011 launch of Industrie 4.0, a German project aiming to promote the computerization of manufacturing, the integration of physical or actual manufacturing systems with cyber-physical systems (CPS) using various technologies, such as the Internet of things (IoT), industrial Internet of things (IIOT), and artificial intelligence, is considered to be more important than ever before. One of the goals of the Industrie 4.0 is to realize smart factories or smart manufacturing using advanced digital technologies. However, the core component in the manufacturing systems is still machine tools. This special issue, composed of eleven excellent research papers, focuses on the latest research advances in machine tools and manufacturing processes. It covers various topics, including machine tool control, tool path generation for multi-axis machining, and machine tool components. Furthermore, this special issue includes innovative machining technologies, including not only cutting and grinding processes but also the EDM process and burnishing process connected effectively with force control techniques. All the research contributions were presented at IMEC2018, a joint event with JIMTOF2018, held in Tokyo, Japan in 2018. The editors would like to sincerely thank the authors for their dedication and for their well written and illustrated manuscripts. We are also profoundly grateful for the efforts of all the reviewers who ensured their quality. Finally, we sincerely hope that studies on machine tools and related manufacturing technologies will further contribute to the development of our global society.

An Automated Setting Operation of Tool and Workpiece Based on Setting Errors Compensation for High Efficient Multi-axis Control Ultraprecision Machining

An Automated Setting Operation of Tool and Workpiece Based on Setting Errors Compensation for High Efficient Multi-axis Control Ultraprecision Machining

An improvement in the production flexibility and accuracy of rotor milling process employing Finger Cutters

Finger Cutters (FCs) are the recently SRM-developed rotor cutting technology which can be easily integrated into multi-axis flexible milling machines. Opposite to the traditional disc cutters, FC operates inside the rotor body and as a result of that requires much less torque. This opens a new possibility to utilize multi-axis machines in rotor milling process, bearing in mind that they generally have less torque than specialized milling machines. An introduction to FC technology including mathematical modeling, geometrical definition and cutter body grinding has been presented in ICSM2018 conference in Dortmund [2].In this work, we will present the recent development in FC technology. In particular, we will focus on an innovative method to increase the production accuracy of the FC-cut rotor bodies. This method incorporates an iterative procedure of milling, measuring and adjusting. First, a theoretical target surface, so-called adjustment cut, is defined with a safety margin from the final finished surface. FC removes the material aiming to match the adjustment cut. Measuring the surface and comparing with the adjustment cut, a set of correction factors are calculated for the next iteration. Employing this iterative procedure, a notable improvement in the rotor manufacturing accuracy is achieved while risk for scrap rotors is reduced significantly.

Improving the process efficiency of helical gears’ toothed rims at the stage of pre-production

The article reveals modes of increasing the efficiency of preproduction process for manufacturing nodes with helical gears. The increased efficiency is ensured by processing toothed rims of helical gears with end tools on multi-axis CNC machines. The advantage of the proposed processing method is the minimum deviation of actual and theoretical profiles of gear tooth side surfaces.

Kinematic modeling and machining precision analysis of multi-axis CNC machine tools based on screw theory

This paper presents the solution to evaluate and predict machining precision of a multi-axis computer numerical control (CNC) machine tool. The “function-motion-action” method is used to describe the motion relationships of a multi-axis CNC machine tool, and the multi-body system structure model of the machining system is further constructed. By analyzing the motion process of the machine tool based on the meta-action layer and considering the motion transfer relations in meta-action chains, the kinematic models of each motion shaft are established in twist exponential form. According to the structural and kinematic relationships of these motion shafts, the kinematic and kinematic error models of the machine tool can be easily obtained. Then, the distance error formula is used to deduce the general model of machining allowance error. A simulation test and practical experiment are performed on a type of CNC controlled 5-axis machining center. The experiment results show that the proposed method is universal, and it can greatly simplify the kinematic analysis of multi-axis CNC machine tools.

Investigation of fretting fatigue behavior and micro-structure evolution in LZ50 steel subjected to torsional load

In this paper, torsional fretting fatigue in LZ50 steel was investigated experimentally using a multiaxial fatigue testing machine. After subjecting the samples to fatigue, they were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The details of the fretting damage and the evolution of the samples’ microstructures in the contact zone were studied under different torsional loads. The results showed how the combined action of fretting and fatigue led to the failure of the specimen. Fretting wear in the contact zone produced a fretting scar and cracks in the specimens initiated in the fretting contact zone. These were caused by the local stress concentrations, and the cracks then propagated due to cyclical torsional fatigue stresses. The modes of microstructural evolution occurring during torsional fretting fatigue were dislocation angling and climbing, and these formed dislocation cells in the structure of the material. Within the LZ50 steel, this pattern of behavior began in the ferrite areas of the structure. For the harder phases, the evolution started later, and in the pearlite structures. The microstructural evolution in the pearlite was accompanied by bending, torsion, and even cracking of the cementite lamellae. The evolution of the microstructures showed that the micro-cracking nucleation mechanism was the deformation of dislocation cells.

Optimization of cutter orientation for multi-axis NC machining based on minimum energy consumption of motion axes

A cutter axis optimization method for multi-axis machining based on minimum energy consumption of motion axes is presented in this paper to avoid the unstable running status of the machine tool induced by unreasonable cutter directions in multi-axis machining of sculptured surfaces. Firstly, a mathematic model based on minimum energy consumption of motion axes of the machine tool is established to achieve minimum energy consumption of the machine tool feed system. Secondly, the feasible space meeting the requirements of collision avoidance of the cutter orientation and the travel limit of each movement axis are constructed in the machine tool coordinate system, and the change of rotation axis angle of the machine tool is restrained by introducing the rule curve of sinusoidal acceleration. Then, the smooth feasible cutter axis sequences are obtained with no rigid impact or flexible impact on the machine tool. Finally, a machining experiment is carried out, and the results show that the presented method can save the energy consumption of the feed system by an average of 3.36% as well as smoothing the cutting process.

Optimal design of kinematic performance for a novel 2R1T parallel mechanism with pantograph units

A new type of 2R1T 3-PzPxS parallel mechanism is presented in this paper to satisfy the needs of configuration innovation for multi-axis machining platform. The limb configuration with a planar pantograph unit and the 3-PzPxS mechanism configuration are proposed and optimized. The analytical solutions of the forward and inverse position for the mechanism are derived, which are convenient to subsequent motion planning and control. After obtaining the workspace of the mechanism by the boundary search algorithm, the influence of key scale parameters on the rotation capacity of the mechanism in the whole workspace is investigated, and the reasonable ranges of the scale parameters required for large rotation capacity of the mechanism are confirmed. Furthermore, based on the Jacobian matrix condition number, the global performance indices of the angular velocity and the angular acceleration of 3-PzPxS parallel mechanism are defined, and ultimately the optimal ranges of the scale parameters are determined according to these two performance indices. The above results lay a theoretical foundation for the further development of high-efficiency hybrid NC machining center.

An optimized single-point offset method for reducing the theoretical error of S-shaped test piece

The S-shaped test piece is utilized to comprehensively detect the machining accuracy of multi-axis machine tools. However, due to the existence of the influence of theoretical error for S-shaped test piece on the acceptance of multi-axis machine tools, therefore, a new methodology to reduce the theoretical error is proposed in this study. Firstly, the uniform double cubic B-spline surface model is applied to a surface representation. By utilizing this model, a model of the S-shaped test piece is established. Furthermore, the distribution of the twist angle at the selected three section lines is obtained. Meanwhile, the influence of twist angle on the theoretical error of S-shaped test pieces is analyzed. Then, the single-point offset (SPO) method is analyzed, which has some drawbacks. In order to significantly reduce the theoretical error of S-shaped test piece, the optimized single-point offset (OSPO) method is proposed. Moreover, the tool path of the S-shaped test piece is generated by CAD/CAM software based on the OSPO method. Finally, in order to verify the feasibility of the presented method, a machining and measurement experiment is carried out on the gantry-type five-axis milling machine tool (XKAS2525) and coordinate measuring machine (CMM) based on the S-shaped test piece, respectively. Experiment results show that the average theoretical error of S-shaped test pieces based on the OSPO method is reduced by about 50.1% than that on the SPO method. In addition, the vast majority of theoretical errors based on the OSPO method are approximately less than 5 μm, which can be negligible. It is therefore reasonable to conclude that, compared with the SPO method, the proposed method in this paper can avoid the influence of theoretical error on the acceptance of multi-axis machine tools intuitively and efficiently.

Jerk-limited feedrate scheduling and optimization for five-axis machining using new piecewise linear programming approach

In this paper, a new computation method and an optimization algorithm are presented for feedrate scheduling of five-axis machining in compliance with both machine drive limits and process limits. Five-axis machine tool with its ability of controlling tool orientation to follow the sculptured surface contour has been widely used in modern manufacturing industry. Feedrate scheduling serving as a kernel of CNC control system plays a critical role to ensure the required machining accuracy and reliability for five-axis machining. Due to the nonlinear coupling effects of all involved drive axes and the saturation limit of servo motors, the feedrate scheduling for multi-axis machining has long been recognized and remains as a critical challenge for achieving five-axis machine tools’ full capacity and advantage. To solve the nonlinearity nature of the five-axis feedrate scheduling problems, a relaxation mathematical process is presented for relaxing both the drive motors’ physical limitations and the kinematic constraints of five-axis tool motions. Based on the primary optimization variable of feedrate, the presented method analytically linearizes the machining-related constraints, in terms of the machines’ axis velocities, axis accelerations and axis jerks. The nonlinear multi-constrained feedrate scheduling problem is transformed into a manageable linear programming problem. An optimization algorithm is presented to find the optimal feedrate scheduling solution for the five-axis machining problems. Both computer implementation and laboratorial experiment testing by actual machine cutting were conducted and presented in this paper. The experiment results demonstrate that the proposed method can effectively generate efficient feedrate scheduling for five-axis machining with constraints of the machine tool physical constraints and limits. Compared with other existing numerical methods, the proposed method is able to find an accurate analytical solution for the nonlinear constrained five-axis feedrate scheduling problems without compromising the efficiency of the machining processes.

Investigation of Carbon Fiber Reinforced Polymer Composite Welding with a New Tool in Friction Stir Welding Method

Application of thermoplastic materials has increased dramatically in recent decades due to its recyclability, low density, resistance to chemical changes. The friction stir welding process is one of the new methods of solid state welding, which has recently undergone significant improvement. In this research, using a new tool Made of plain carbon steel st37 in friction stir welding and low cost turning machine, composite sheets of thermoplastic polymer base have 12% continuous carbon fiber in the form of buttocks with two rotational speeds of 250 and 355 rpm and two advance speeds of 5/6 and 9 mm/min Optical microscope images (OM) showed the complete connection of materials. Increasing the inlet temperature resulted in the formation and growth of cavities and converting them into tunnel cavities. In general, the parameters affecting the connection quality in this study included the main shoulder diameter and rotational speed, so that, based on the results of scanning electron microscopy (SEM), the increase in rotational speed resulted in the grinding of continuous carbon fibers and thus increased tensile strength. The results of the tensile test showed that the failure of the samples is due to microstructural changes in the HAZ, in the joint zone of the welding zone and the base materials. According to the results, it can be said that using this new tool in the friction stir welding method, because of the reduction of rotational speed compared to previous studies and the lack of use of a multi-axial milling machine, can save energy.

Study on tooth profile error of cylindrical gears manufactured by flexible free-form milling

The free-form milling of gears is a modern and flexible machining process that allows the manufacturing of external gears by using standard milling tools on universal machine center. In this paper, tooth profile error of cylindrical gears manufactured by flexible free-form milling is studied. Firstly, a mathematical model is established to investigate the tool path with respect to geometric characteristic of gear surface according to the theory of spatial form milling. Then, the influence of tooth profile error related to the processing parameters such as the type of tool, the feeding strategy of tool, and the basic parameters of gear is studied. Finally, for a validation, an experiment is performed using a universal multiaxis machine center with standard end mill. The experimental results show that free-form milling of gears is an effective and economical solution for manufacturing high-quality gears while a universal machine center is available. Furthermore, the study results support the engineers in the selection of optimal processing parameters by using free-form milling method.

A space cutter compensation method for multi-axis machining using triple NURBS trajectory

Aiming at the space cutter compensation problem for multi-axis machining of complex surfaces, firstly, a space cutter compensation method for multi-axis machining based on triple NURBS (non-uniform rational B-splines) trajectory is proposed, which can describe the machining trajectory information more completely and accurately, and is also expected to achieve dynamic radius and length compensation of multi-axis machine’s space cutter. Secondly, for the generation of NURBS trajectory of cutter axis vector and cutting contact vector, three error sources in NURBS curve offset process are analyzed in detail. The error scaling problem is especially pointed out, and a NURBS curve fast offset method with user-definable tolerance is proposed. Finally, the specific calculation method and system implementation scheme of space cutter radius and length compensation using triple NURBS curve trajectory are given. The effectiveness of the proposed space cutter compensation method was verified by simulation and machining experiments.

Automatic Support Removal for Additive Manufacturing Post Processing

An additive manufacturing (AM) process often produces a near-net shape that closely conforms to the intended design to be manufactured. It sometimes contains additional support structure (also called scaffolding), which has to be removed in post-processing. We describe an approach to automatically generate process plans for support removal using a multi-axis machining instrument. The goal is to fracture the contact regions between each support component and the part, and to do it in the most cost-effective order while avoiding collisions with evolving near-net shape, including the remaining support components. A recursive algorithm identifies a maximal collection of support components whose connection regions to the part are accessible as well as the orientations at which they can be removed at a given round. For every such region, the accessible orientations appear as a ‘fiber’ in the collision-free space of the evolving near-net shape and the tool assembly. To order the removal of accessible supports, the algorithm constructs a search graph whose edges are weighted by the Riemannian distance between the fibers. The least expensive process plan is obtained by solving a traveling salesman problem (TSP) over the search graph. The sequence of configurations obtained by solving TSP is used as the input to a motion planner that finds collision free paths to visit all accessible features. The resulting part without the support structure can then be finished using traditional machining to produce the intended design. The effectiveness of the method is demonstrated through benchmark examples in 3D.

High-Speed Multiaxial Machining in an Updated Longitudinal Milling Machine

An alternative to reequipping metalworking enterprises is to modernize outdated CNC machines. In modernization, the aggregation principle permits the introduction of up-to-date high-speed cutting and multiaxial machining technology.

Experimental and FEM numerical assessment of multiaxial fatigue failure criteria for a rolling Stock's seats structure

This paper deals with the experimental and numerical investigations of the structural behaviour of a rolling stock's seats system under static and fatigue loading conditions, aimed to assess the reliability of some multiaxial fatigue failure criteria implemented in the FE model. According to the NF F31–119 standard, a specific full-scale multiaxial testing machine has been used for the fatigue test. During the test, a visual inspection approach with a detection interval of 3·105 cycles has been adopted. After 1.2·106 cycles the test has been stopped and a Liquid Penetrant Inspection (LPI) has been carried out in order to detect accurately cracks nucleation and propagation. Moreover, numerical analyses, based on the Finite Elements (FE) Method, have been performed on the EN-AW 6060 T5 aluminium alloy structure of a rolling stock's seats to predict the most affected zones where the cracks nucleation may occur under multiaxial fatigue loads. Since the in-service applied loads belong to Multiaxial High-Cycle Fatigue (MHCF) load class, Sines, Crossland and Dang Van criteria have been used to post-process the predicted results achieved by the FE analyses. For validation purpose, numerical and experimental results have been compared. According to the numerical-experimental results comparison, Dang Van criterion provided the best level of accuracy.

Error qualification for multi-axis BC-type machine tools

Multi-axis machining processes are often used to fabricate complex components with tight geometric tolerances. Thus, the need for highly accurate 5-axis machine tools is imperative in high precision industries such as aerospace and mold and die. Often, construction errors result in geometric errors within the machine tool. These errors must be identified and compensated in order to guarantee accuracy of the machine. In this work, kinematic equations of motion for a BC-style machine tool were derived while incorporating the 8 distinct kinematic error constants associated with a 5-axis machine tool. A method is presented to derive these kinematic error constants from eccentricity values obtained using 3-axis simultaneous tests for table-table style 5-axis machine tools. To validate this method, error constants were input into the kinematic simulation. Eccentricity values were then output from the simulation and error constants were derived and compared to the input values. It was shown that if the procedure is followed, the error constants can be correctly derived and compensated. This method was then implemented on a BC-style machine tool and error constants were derived.

5-axis grinding path generation for free-form surface based on plane instantaneous grinding engagements

Grinding is an important process to remove the material efficiently. Before the finish machining, it is necessary to remove the cusps by the wider grinding band for obtaining more uniform allowance. In this paper, the grinding wheel hold by the multi-axis machine tool is introduced into the free-form surface grinding. The engagement between the wheel and the to-be-ground surface is established for generating the paths. An instantaneous engagement model is proposed in terms of machine tool kinematic chain, and the spatial engagement is represented on a group of planes. Based on the engagement model, the grinding path generation for enlarging bands is transformed into an optimization. Different with traditional path generation, a slight local overcut is allowed on the wheel locations. However, the envelope of grinding engagements is limited in the required boundaries to satisfy the design tolerance. They are realized by adjusting the contact locations and orientations of the wheel. At last, a compressor blade is used to verify the advantages about the proposed path generation method.