Machine Tool Workpiece Research Articles

Research on vibration behavior and chatter stability of big-sized gear hobbing machine

Self-excited chatter vibrations are the most detrimental for the quality and safety of the machining operations. To reduce the undesired vibrations, the dynamic behaviors of the machine tool-workpiece system and the chatter stability models need to be studied. In this paper, gear hobbing is studied using analytical and experimental approaches. Firstly, based on the chatter stability theory in relation to milling, analytical model of chatter stability in gear hobbing process is derived. Secondly, a new approach is presented to identify dynamic modal parameters using operational modal analysis (OMA). Using natural excitation technique (NExT) and auto regressive moving average (ARMA) model time series analysis method. At last, the prediction capability is verified using machining experiments, and the results show that the stability diagrams is of great potential to supply guidance for the processing parameters development.

Calibration Method for In-Process Identification of Tangential Specific Cutting Force in Milling

The cutting forces that occur during machining cause static and dynamic deformations of the machine–tool–workpiece system. In general, the cutting force is spatial. However, knowledge of the tangential component of the cutting force is crucial for the optimum use of the power installed on the spindle. An important parameter for cutting force modeling is the tangential cutting force coefficient. This paper focuses on the approach of the in-process identification of the cutting and edge components of the tangential cutting force coefficient, using the spindle power signal read directly from the machine tool control system. Such procedures have already been described in the available literature. The key point for the successful implementation of these methods is the identification and avoidance of the passive torque signal. This paper describes the operational calibration of the spindle drive system. The calibration procedure is based on the tap test using modal hammer excitation and the spindle power signal response. The proposed procedure was successfully validated using a machine–machine comparison approach and a machine–dynamometer validation approach. The results are consistent with those in the available literature. With this method, we were able to determine the cutting component of the tangential specific cutting force with a deviation of 1% from the reference system and an edge component of 10%. The modal hammer method only enables the avoidance of dynamometers in the machine calibration procedure.

A comprehensive error compensation strategy for machining process with general fixture layouts

The modeling of equivalent fixture error (EFE) in machining process has been proved to be an effective way for variation reduction. However, previously developed approaches did not describe the EFE phenomenon of workpiece deformation. This paper proposes a comprehensive EFE model that can simultaneously transform the datum errors, machine tool path error and workpiece deformation to EFE. The developed model can also describe the EFE phenomenon for general fixture layouts rather than being limited to an orthogonal 3–2-1 layout case. Based on this comprehensive error compensation strategy, the datum errors, machine tool path error and workpiece deformation can be compensated for machining process with general fixture layouts. The case studies demonstrate the model validity through the real machining experiments and also show a simple simulation process for error compensation.

Implementation of an Algorithm to Prevent Chatter Vibration in a CNC System.

Machining of shafts characterized by a high compliance is difficult due to the occurrence of self-excited chatter vibrations. It is possible to limit their occurrence through the appropriate selection of technological parameters. For a proper selection of these parameters it is necessary to know the dynamic properties of the machine–tool–workpiece. This study proposes an approach through which these properties can be determined as a result of the synthesis of the dynamic properties of the system, using the receptance coupling method. Knowledge of these properties allows us to select the technological parameters of the lathe using the assistance system integrated into the CNC (Computerized Numerical Control). The final section of this work presents the experimental validation of the assistant and proposed procedures.

Milling stability analysis with considering process damping and mode shapes of in-process thin-walled workpiece

As an unfavorable factor of machining process, chatter threatens the machined quality of workpiece, which determines the assembly and fatigue performance of the workpiece. During the interaction between machine tool and thin-walled workpiece, the process damping effect, multiple modes response and dynamic changes caused by the material removal of the in-process workpiece (IPW) will make the machining process more complicated and introduce great difficulties to dynamic modeling and performance prediction. In this paper, we considered the process damping determined by the indentation volume between flank face of milling tool and machined surface, and used multi-mode model to describe this behavior. In order to establish the assembled material removal model of the IPW dynamics with multiple modes, the structure dynamic modification (SDM) and finite element method (FEM) were combined together. The updated third-order full discretization method was applied to solve the dynamic equation in modal space. Then, the three-dimensional stability lobe diagrams (SLDs) with and without material removal along the tool path were obtained respectively by enveloping multiple modes of the IPW and milling tool together. Finally, the cutting tests were carried out. The experiments showed that the assembled model could predict the dynamics of IPW accurately, and the proposed stability analysis model was relatively close to experimental results. Besides, the modes of thin-walled workpiece with weak rigidity do not always play a dominant role in the process of machining. Although the material removal rate is limited by considering the multiple modes of the system, the processing quality can be ensured.

Identification of a Lathe Spindle Dynamics Using Extended Inverse Receptance Coupling

In order to ensure machining stability, it is essential to properly determine the dynamic properties of machine tool–workpiece system. Experimental modal analysis provides good results; however, due to high time consumption, in some cases, its use is not practically justified. Then, a receptance coupling method can be used, that allows for the synthesis of the experimental models of the machine tool components and analytical models of the workpiece. However, a significant disadvantage of this method is the need for the experimental identification of the rotational degrees-of-freedom, fully defining dynamic properties of the spindle. This paper presents an improved method based on inverse receptance coupling, which enables effective identification of the spindle dynamics with the properties of the joint. Then, a measurement procedure and results of the experimental validation are presented.

Monitoring of the condition of turning machine bearing housing through vibrations

Mechanical vibration is a phenomenon that occurs very often in the process of metal cutting. It is one of many factors that have a significant impact on the quality of machining and thus prevent a rise in productivity. These vibrations are undesirable not only because they cause noise and dynamic stresses, which result in fatigue and failure of the machine, but also energy loss or performance reduction. An experimental analysis of mechanical vibrations in the machining process is important because vibrations may cause spindle damage, machine tool wear or workpiece wear and may be left with poor surface finish. Therefore, this material is devoted to the study of the impact of input cutting parameters—spindle speed and depth of cut on the formation and size of mechanical vibrations on lathe bearing housings. The measured experimental values of the acceleration amplitude of mechanical vibrations on bearing housings were compared with theoretical values.

On process damping induced by vibration-dependency of cutting direction in milling

The process damping effect is analyzed for regenerative machine tool chatter in milling via a velocity-dependent cutting force model. This model takes into account that the effective cutting direction depends on the vibrations of the machine tool-workpiece system, which modifies the effective rake angle, the chip thickness, and the cutting force. The model was originally introduced for turning operations where it results in a process damping term that improves the stability of metal cutting at low cutting speeds. Now this model is extended to milling. It is shown that the vibration-dependency of the cutting direction induces a time-periodic process damping term that is negative when the radial immersion is low. This decreases the stability at low cutting speeds, thus the low-speed stability improvement phenomenon in low radial immersion milling can be explained by extended process damping models only.

Design for Manufacturing of Variable Microgeometry Cutting Tools

Cutting edge microgeometry has gained special attention of late in the machining research community. Machine tool vibration, tool life, and workpiece surface integrity are all influenced by cutting edge size/shape. To optimize the machining process, variable microgeometry (VMG) cutting tools, in which the edge microgeometry varies along the edgeline with respect to specific variables (such as machining parameters or expected tool wear), are manufactured. Despite the advantages of VMG tools, a major hindrance in their development is the manufacturing complexity that demands high precision multi-axis edge preparation processes following extensive machine setup, fixturing, and programming. This paper details the proof of concept of a design criterion, which leads to the manufacturing of VMG cutting tools by only traditional edge preparation processes. The present method relies on the existing relationship between the edge radius subsequent to the edge preparation process and the tool wedge angle. The validity of the proposed method is first examined by a numerical simulation of the edge preparation. Carbide cutting tool inserts are then designed based on the proposed idea. Robust VMG generation subsequent to edge preparation by microblasting is demonstrated through microgeometric measurements. VMG chemical vapor deposition-coated carbide tools manufactured by the proposed approach are evaluated for turning hardened steel, and optimal designs are identified with respect to tool life and workpiece surface roughness. To address the design consideration, finite element (FE) modeling provides valuable insight into the machining process. FE modeled stress and temperature distribution clarify the experimental observations and reveal the design constraints.

Root Cause Identification of Machining Error Based on Statistical Process Control and Fault Diagnosis of Machine Tools

The essence of the machining process is the interaction that occurs between machine tools and a workpiece under certain conditions of cutting parameters. Root cause identification (RCI) is critical to the quality control and productivity improvement of machining processes. The geometric error caused by fixture faults can be identified in most RCI methods; however, the influence of machine tool degradation on workpiece quality is usually neglected. In this paper, a novel root cause identification scheme of machining error based on statistical process control and fault diagnosis of machine tools is proposed. With the pattern recognition of control charts, quality fluctuations can be detected in a timely manner. Once the machining error occurs, the fault diagnosis of machine tools are carried out. The relationship between machine tool condition and workpiece quality is established and the root cause identification of the machining error can be achieved. A case study of the machining of a complex welded box-type workpiece is presented to illustrate the feasibility of the proposed scheme. It is found that the coaxiality error of the two holes in two sides of the box’s wall is caused by the wear of the worm gear in the rotary work table of the machine tool.

Effects of cutting conditions on excitation and dynamic stiffness in milling

The effect of cutting conditions on vibration has a great significance on machining parameter selection and active vibration control. Cutting parameters affecting vibration, such as spindle speed, feed rate, and axial depth of cut, have been studied by many researchers, but the study on effect mechanism of radial depth of cut on vibration remains deficient. Radial depth of cut may change the cutting teeth, which has a great influence on vibration. This paper proposes a method to study the effect mechanism of radial depth of cut on cutting vibration based on excitation and dynamic stiffness. The dynamic model from the perspective of the contact between teeth on the cutter (TC) and the workpiece is built. The number of cutting teeth on the cutter (NCTC) is expressed as radial depth of cut; then, the bandwidth and energy of excitation at different radial depths of cut are deduced by the theory of cutting dynamics. The variation of the machine–tool–workpiece system (MTWs) dynamic stiffness at different radial depths of cut is estimated with the measured output responses and cutting force excitation. The accuracy of effects is checked by a time and frequency domain analysis of cutting force and vibration measured in the cutting tests. The results show that cutting force excitation and MTWs dynamic stiffness are significantly affected by the radial depth of cut, which are consistent with the mechanism analysis. Moreover, the effectiveness of the effects is verified by the machined surface profile.

Thermal error compensation of dry hobbing machine tool considering workpiece thermal deformation

Dry hobbing is a new gear tooth forming process, which would replace the traditional wet hobbing process due to its high efficiency and environmental friendliness. However, due to the absence of metal cutting fluid in dry hobbing, temperature of hobbed workpiece is relatively high, which will lead to the increase of dimension errors, especially tooth thickness errors. In this paper, the features of workpiece thermal deformation errors are identified and an error compensation model is developed to compensate both machine tool thermal errors and tooth thickness errors which is induced by workpiece thermal deformation. In this model, the representative temperature variables of a dry hobbing machine tool are obtained based on experimental data with fuzzy clustering method. A compensation model is proposed to map the relationship between representative temperature variables and compensation value. In a series of experiments, the tooth thickness errors ranged from −22 to −59 μm. After implementing compensation of machine tool thermal errors, the tooth thickness errors ranged from −18 to −28 μm, which demonstrates that workpiece thermal deformation has a great influence on tooth thickness errors in dry hobbing. Furtherly, the tooth thickness errors ranged from −4 to 8 μm, after implementing compensation of both machine tool thermal errors and workpiece thermal deformation errors, which demonstrate that the compensation model is effective.

Vibration Analysis of Reinforced Beam Structure Using Partially Inserted Longitudinal Core

The beam structures are considered as one of the most fundamental building elements of technical equipment. Many of the machining tools (for example lathe tool) may be also regarded as the beam structures. The various excitation effects are affecting the machining tools during the machining process. Cutting speeds, cutting forces, chip-making manner, the stiffness of MTW (machine-tool-workpiece) system are predominant effects affecting the dynamics of MTW system and also influencing the machining process (roughness of the machined surface, tool wear, tool or workpiece damage, noise generated by the machining process, etc.). Depending on the excitation effects, the machining tool body (beam structure) must have the required dynamic properties. The modification of beam modal properties based on embedding of inner reinforcing core is presented in this paper. The main aim of the modification is to change the modal properties (mode shapes, natural frequencies) of the beam body.

Modeling/analysis of four-half axis machine tool via modified denavit-hartenberg notation

Four-half axis and five-axis machine tools have five moving members (X, Y, Z, A, and B). Four-half axis systems cannot simultaneously operate the B axis with translational or rotational axes, thus limiting tool machining ability and complicating NC codes. This study employs modified Denavit-Hartenberg notation to model four-half axis machine tool kinematics and obtain corresponding ability matrixes. NC data equations for the A, B, X, Y, and Z axes with respect to the workpiece frame are derived by inverse kinematics. Machine tool active parameters and workpiece home position are simultaneously measured by using a ball-end milling cutter as a measuring probe.

The Flexible Control System Development of the Sawing and Milling Machining Center for PVC Door and Window Profile

In the paper, with the secondary development of FANUC-0MD system, the flexible control system which meets the requirements of the sawing and milling machining center for PVC door and window profile is realized. Based on researching the structure, the function, the process of the technologies and the control requirements of the machining center, the CNC system has implemented, including the machine tool origin and work piece coordinate origin establishment and auxiliary control system M code development. In this flexible control system, through studying the geometric character, the technical character and the mutual relation between them, various windows shape on the structure and the machining figuration is possessed of. We gain the feature subclasses aggregate of the machining technology of windows shape. Consequently, CAM system is achieved.

Modification of Modal Characteristics of Machining Tool Body by Reinforcement with Non-Uniform Cross-Section

The various excitation effects are affecting the machining tools during the machining process. Cutting speeds, cutting forces, chip-making manner, the stiffness of MTW (machine-tool-workpiece) system are predominant effects affecting the dynamics of MTW system and also influencing the machining process (roughness of the machined surface, tool wear, tool or workpiece damage, noise generated by the machining process, etc.). In many cases, to achieve of adequate dynamic properties of the tool, the change of modal properties (mode shapes, natural frequencies) of the body tool must be performed using structural modification. The main objective of this paper will be to formulate, analyse and evaluate the proposed methods and procedures for structural modifications of the tool, represented as a beam. A modification of modal properties by insertion of non-uniform cross-section reinforcing core into body tool is studied and the effect of material properties and geometrical parameters of core on modal properties is analysed. The applications of structural modifications of tool body are leading to better conditions in the cutting process.

The Construction of the Workpiece Coordinate in CNC System Based on Motion Control Card

Based on motion control card, it analyzed the methods and principles of the workpiece coordinate construction used the G54; created the G54 function modules, embedded CNC system of independent research and development and improved the system functions, processing efficiency and quality. The method described in the article can be directly applied to the machine tools controlled by numerical control system based on the card, and had a high practical value to the establish of other CNC machine tool workpiece coordinate system.

Analysis of the tool tip radial stiffness of turn-milling centers

Modern industry requires high-precision machines and processes. This requirement implies the determination and reduction of all possible sources of error. For turning, a main error is related to machine–tool–workpiece deformation caused by cutting force. On the other hand, current turning centers are structurally more complicated than conventional lathes. However, stiffness values for this kind of machine are not described yet in literature. Thus, the aim of this work is to provide stiffness values for this type of machines clamping a common workpiece. To obtain this information an experimental methodology to measure static stiffness along kinematic chain of the turning center is proposed. The method may be useful for both users and machine manufacturers in order to consider this factor in the general error budget of the turning process. At the same time, stiffness values for lathes with a Y-axis incorporated in a three-axis turret and equipped with living tools are registered.

An approach based on tool mode control for surface roughness reduction in high-frequency vibration cutting

The presented research work, aimed at deeper understanding of vibrational process during high-frequency vibration cutting, is accomplished by treating cutting tool as an elastic structure which is characterized by several modes of natural vibrations. An approach for surface quality improvement is proposed in this paper by taking into account that quality of machined surface is related to the intensity of tool–tip (cutting edge) vibrations. It is based on the excitation of a particular higher vibration mode of a turning tool, which leads to the reduction of deleterious vibrations in the machine–tool–workpiece system through intensification of internal energy dissipation in the tool material. The combined application of numerical analysis with accurate finite element model as well as different experimental methods during investigation of the vibration turning process allowed to determine that the most favorable is the second flexural vibration mode of the tool in the direction of vertical cutting force component. This mode is excited by means of piezoelectric transducer vibrating in axial tool direction at the corresponding natural frequency, thereby enabling minimization of surface roughness and tool wear.

Study on the Stability of High-Speed Milling of Thin-Walled Workpiece

Thin-walled structure is easy to vibrate in machining. The dynamic milling model of thin-walled workpiece is analyzed based on the analysis of degrees in two perpendicular directions of machine tool-workpiece system. In high speed milling of 2A12 aluminum alloy, the compensation method based on the modification of inertia effect is proposed and accurate cutting force coefficients are obtained. The machining system is divided into “spindle-cutter” and “workpiece-fixture” two sub-systems and the modal parameters of two sub-systems are acquired via modal analysis experiments. Finally, the stability lobes for high speed milling of 2A12 thin-walled workpiece are obtained by the use of these parameters. The results are verified against cutting tests.