Spindle Head Research Articles

Identification of Dynamic Parameters of a Novel 3-DOF Spindle Head

A methodology is presented to identify the dynamic parameters of a novel 3-DOF spindle head for high-speed machining of extra large components. The architecture behind the spindle head is a 3-RPS parallel mechanism having one translational and two rotational movement capabilities. The inverse kinematic and dynamic analyses are carried out in the first place, allowing the model suitable for dynamic parameter identification to be formulated by setting a characteristic reference point on each moving component.In the light of the load torques under different working conditions, a hierarchical strategy is proposed for dynamic parameter identification. The merit of this strategy lies in that it enables the influences due to the Coulomb frictions to be counteracted or eliminated via proper path planning, thereby the identification accuracy to be significantly improved. The experiments under various working conditions are carried out to verify the validity and effectiveness of the proposed approach.

C004 Characteristics of thermal displacement of cylindrical grinding machines with rotation of wheel spindle : Influence of thermal displacement of wheel spindle and wheel spindle head

Measurement devices are proposed for the relative thermal displacement between the grinding wheel and the workpiece, and for the thermal displacement of the wheel spindle itself during rotation of the wheel spindle. Then by separating the thermal deformation of the wheel spindle and the wheel spindle head from the relative thermal deformation between the wheel spindle and workpiece, the thermal deformation of the grinding machine is evaluated in detail. The following results are obtained. (1) The grinding wheel spindle and the wheel spindle head cause the thermal displacement in various directions. (2) A percentage of the influence of the wheel spindle and the wheel spindle head in the relative thermal deformation is different depending upon the direction of the thermal displacement.

Proposal of a Machining Test of Five-Axis Machining Centers Using a Truncated Square Pyramid

NAS 979 has been used for over 40 years as a performance evaluation standard for five-axis machining centers. This standard provides some finishing conditions of the cone-frustum under five-axis control, and prescribes the measuring methods and permissible tolerances of geometric deviations. However, this standard cannot be applied to the tilting rotary table type five-axis machining center but to the universal spindle head type one. When the standard is applied to the tilting rotary table type, it is not clear yet that the effects of the geometric and synchronous deviations which influence the measured results. Thus, there are no methods for evaluating the accuracy of linear interpolation movement under simultaneous five-axis control. This paper proposes a machining test method using a truncated square pyramid for checking the accuracy of the tilting rotary table type five-axis machining centers. In the simulation and experiment, the bottom of the truncated square pyramid with a half apex angle of 15° is mounted on a fixture with a slope of 10° or 20°, and feed velocity of each axis is analyzed by changing the center position.

Computation of a compact state space model for an adaptive spindle head configuration with piezo actuators using balanced truncation

Finite element models of machine tools or their building blocks are usually very large and thus do not allow for fast simulation or application in controller design. Especially when algebraic constraints come into play the models become differential algebraic equations and therefore are even more difficult to handle in the application. In this contribution we propose a method based on modern system theoretic model order reduction algorithms that allows to generate a first order standard state space reduced order model for a structural model of an adaptive spindle support that is of second order index 1 differential algebraic form due to the piezo actuation applied. The accuracy of the method is demonstrated by a numerical frequency domain error analysis.

Impact Test in Working Space of Milling Center

Experimental modal analysis is quickly developed area of science in last years. Experimental approach for solving technical problems is a means to estimate or evaluate modal properties of a mechanical structure. The paper is focused at dynamic properties of milling centers, namely at the resonance frequencies and vibration shapes of a simplified tool-work piece system. All this properties are identified by measurements. The machine tool vibration was excited by impulse force and a response of excited vibration was recorded. The measurement points for vibration were selected at the spindle head, the table and the clamping device. The paper aims to present comparative results of theoretical studies with experimental results obtained from impact test to the machining centers.

Gantry Type 대형 공작기계의 Cross Rail 설계 및 좌우 이송 편차에 관한 해석

Recently, the demands of the large scale machine tools gradually increase to machine the large parts, such as large scale crankshaft, yaw and pitch bearings for the wind power generator and the vehicle or aircraft components. But the high technology is necessary in order to develop the huge machine tools. Furthermore, the global market of it has been monopolized by a few companies. So, we need to develop the large scale machine tools and study its core technology to rush into the increasing market. In this study, we carried out the researches for the important core technology of a multi-tasking, machine tool; a large scale 5-axis machine tool of gantry type for multi-task machining. This study is focused on the design of large size gantry type multi-axis machine. In the case of large size of machine the cross rail deflection in the X-axis is significant. To reduce the deflection due to the eccentric spindle head, a special hollow type design in the cross rail with outside ram is adapted in this study. Also, the Zig-Zag motion in the Y-axis is inevitable with the gantry geometry, which is by the un-balancing, different motion at the left and the right columns moving. We tried to reduce the influence of Zig-Zag motion using FEM with different loading conditions at the left and the right side column.

Influence of Thermal Behavior of Spindle on Machining Accuracy in Micro-Endmilling

This paper describes on the influence of thermal behavior of spindle on machining accuracy in micro-endmilling. In order to make clear the reasons why the depth of slot varies during slotting repeated, temperatures around the spindle head and the thermal expansion of spindle axis were investigated. The machine tool used for experiment was a small type of vertical machining center. The spindle used for experiment was a ball bearing spindle which rotated at 10000-40000 min-1. The spindle head made of aluminum alloy had various cooling systems for spindle motor and ball bearings, and the temperature of spindle head changed only a few degrees C. Therefore it seems difficult to assume that the primary cause was thermal deformation of spindle head structure, even though the coefficient of thermal expansion of aluminum alloy is approximately twice of that of steel. Finally the thermal expansion of spindle axis was partly measured and relation between the thermal expansion and the variation of depth of slots was investigated. The main results obtained are as follows, (1) Slotting after a long period warming-up of spindle resulted in a small change of depth of slot, but slotting without any warming-up caused an initial rapid increase and then saturation of depth of slot, (2) Temperature rise of spindle head and adjacent structure seemed out of relation to the variation of depth of slot, (3) The partially measured thermal expansion of spindle as well as temperature of lower flange of spindle showed qualitatively similar change corresponding to depth of slot.

Research on Thermal Analysis of Pillow Spindle System with Multi Axes Rotary Spindle Head

Based on the theory of finite-element method, a model of pillowed multiple axes rotary spindle head is built. Taking both temperature and structure load into account of the model, a calculation is made on thermal couple deformation under the combined action of the two kinds of load. A prediction on the rationality during system designing is put forward, which has gotten a wider use.

Enhanced mixed integer programming model for a transfer line design problem

Modular machining lines with multi-spindle workstations are considered. A multi-spindle head executes a set of operations. The problem of optimal design or reconfiguration of such lines is considered here. The set of all available spindle heads, operations executed by each spindle head, spindle head times and costs are assumed to be known. There are operations which can be executed by one of several candidate spindle heads, i.e., in different configuration with other operations. The problem consists in the choice of spindle heads from the given set and their assignment to workstations. The goal is to minimize the line cost while satisfying the precedence, inclusion and exclusion constraints. This problem is an extension of well known assembly line balancing and equipment selection problem. In our previous work, we proposed a MIP model which was significantly limited as to the size of the problems treated. In this paper, quite a few original approaches are suggested to improve the previous MIP model. The numerical tests reported show that the calculation time is drastically decreased, thereby expanding the model to larger and more realistic industrial problems.

Influence of thermal displacement of the spindle axis in a lathe on the machining precision

The T-FLEX CAD 3D system is used to construct a three-dimensional model of the spindle head in an MK-3002 lathe and a model of the thermal conditions in spindle rotation. Four attachment options for the spindle head’s housing are considered. The optimal attachment option is established.

Analysis and Optimization of Thermal Characteristics for NC Lathe Spindle System

Improving thermal characteristics is a crucial approach for increasing machining accuracy of NC lathe. Modeling and analysis of thermal characteristics to a high speed NC lathe were implemented by using finite element method in this paper. Based on this, temperature rise characteristic of the whole spindle system was obtained. The thermal deformation of spindle system due to thermal-mechanical coupling was calculated. Layout and parameter were optimized for the radiating plate of spindle box. And this leads to a considerable reduction in the run-out error of spindle head. The optimization result reaches the expected goal.

Development of On-Machine Measurement System Utilizing Line Laser Displacement Sensor

Demand for precision machining of dies and molds with complex shapes has been increasing. Though high performance CNC machine tools are widely utilized for precision machining, machining error compensation is still necessary to meet accuracy requirements. For precision measurement, a workpiece must usually be unloaded from a CNC machine tool. Then, the workpiece is measured by a precision measurement device, such as 3D CMM. After the machining error is clarified according to the measurements taken, the workpiece must be re-clamped for the necessary error compensation machining. This error compensation machining is costly and time consuming, and it requires a highly skilled machinists. The re-clamping of the workpiece also causes positioning errors. Therefore, demands for on-machinemeasurement have been increasing. In this paper, an on-machine measurement device that consists of a line laser displacement sensor is developed. This measurement device, attached to the spindle head of a machine tool with magnetic clamps, has special features, such as noncontact, multi-point, high-speed measurement capabilities. Additionally, a sequential multi-point method, an extension of the two-point method, is applied for shape measurement accuracy.

高速超音波スピンドルによる硬脆材料の精密・微細研削に関する研究（第1報）

高速超音波スピンドルによる硬脆材料の精密・微細研削に関する研究（第1報）

Modeling the machining stability of a vertical milling machine under the influence of the preloaded linear guide

The prediction of machining stability is of great importance for the design of a machine tool capable of high-precision and high-speed machining. The machining performance is determined by the frequency characteristics of the machine tool structure and the dynamics of the cutting process, and can be expressed in terms of a stability lobe diagram. The aim of this study is to develop a finite element model to evaluate the dynamic characteristics and machining stability of a vertical milling system. Rolling interfaces with a contact stiffness defined by Hertz theory were used to couple the linear components and the machine structures in the finite element model. Using the model, the vibration mode that had a dominant influence on the dynamic stiffness and the machining stability was determined. The results of the finite element simulations reveal that linear guides with different preloads greatly affect the dynamic behavior and milling stability of the vertical column spindle head system. These results were validated by performing vibration and machining tests. We conclude that the proposed model can be used to accurately evaluate the dynamic performance of machine tool systems designed with various configurations and with different linear rolling components.

Dynamics Analysis of a Parallel Mill-turn Tool Spindle Head Driven by Dual-linear Motors Using Extended Transfer Matrix Method

The hybrid dynamics of multi-rigid-body and multi-flexible-body system becomes the mainstream of multi-body dynamics.Currently there lacks a compact approach to model the hybrid dynamics,especially in modern machine tool application,due to the difficulty of solving the hybrid equations or the limitation of current software when dealing with the hybrid dynamics.The extended transfer matrix method(E-TMM),which extends elements in three-dimensional space with higher matrixes,is proposed to simplify the modeling process of the hybrid dynamics.The E-TMM modeling approaches of 3 basic elements including 3D vibrant rigid body,joint and flexible body are studied in details.A parallel mill-turn tool spindle head unit driven by dual-linear motors is chosen as a plant to demonstrate the E-TMM modeling process.By using E-TMM,the spindle head unit is simplified as a topological network consisting of the three types of element,i.e.,3D vibrant rigid body,joint and flexible body,including 11 rigid bodies,14 joints and 1 3D-Timoshenko beam.Then the dynamic model of the system can be easily obtained by deducing the element-network by means of state vector transformation.The dynamic characteristics of the spindle head,such as natural frequencies,dynamic flexibility,etc.can be predicted by solving the obtained model.Experiment verification indicates that the E-TMM is valid with enough accuracy in the dynamic analysis of the parallel mill-turn tool spindle head.The E-TMM is capable of modeling the dynamics of machine tool structure with no requirements of deducing and solving the sophisticated differential equations.Moreover,the E-TMM provides a simple and elegant tool for hybrid dynamic analysis in future dynamic design of machine tools.

Development of Testing Method of Five-Axis Machining Centres Based on Finishing of Truncated Square Pyramid

This paper proposes a unique test piece for evaluating the machining performance of five-axis machining centers. A cone-frustum specified by NAS 979 standard has been used as a test piece over 40 years. In this paper, a truncated square pyramid with a half apex angle of 15 degrees is proposed as a test piece for checking the machining performance instead of the cone-frustum. The truncated square pyramid has to be installed on the slope of a fixture which is mounted on the rotary table of the machining centers in order to conduct the machining test under simultaneous five-axis control. Thus, two kinds of fixtures with different mounting angle were prepared for the experiment. The profiles of four side surfaces finished by milling were measured on machine table by an LVDT sensor which is fixed on the spindle head. The contour form of the side surfaces of the test pieces were also measured on a coordinate measuring machine. It is found from both measurement data that the straightness of each side surface, the squareness between adjacent surfaces as well as parallelism can be evaluated by on-machine measurement, and also pitch errors and backlash of tilting rotary tables can be checked. This test method gives much more information compared with the cone-frustum test method.

Dynamic Feedforward Control of a Novel 3-PSP 3-DOF Parallel Manipulator

Parallel manipulators with less than six degrees of freedom (DOF) have been increasingly used in high-speed hybrid machine tools. The structural features of parallel manipulators are dynamic, a characteristic that is particularly significant when these manipulators are used in high-speed machine tools. However, normal kinematic control method cannot satisfy the requirements of the control system. Many researchers use model-based dynamic control methods, such as the dynamic feedforward control method. However, these methods are rarely used in hybrid machine tools because of the complex dynamic model of the parallel manipulator. In order to study the dynamic control method of parallel manipulators, the dynamic feedforward control method is used in the dynamic control system of a 3-PSP (prismatic-spherical-prismatic) 3-DOF spatial parallel manipulator used as a spindle head in a high-speed hybrid machine tool. Using kinematic analysis as basis and the Newton-Euler method, we derive the dynamic model of the parallel manipulator. Furthermore, a model-based dynamic feedforward control system consisting of both kinematic control and dynamic control subsystems is established. The dynamic control subsystem consists of two modules. One is used to eliminate the influence of the dynamic characteristics of high-speed movement, and the other is used to eliminate the dynamic disturbances in the milling process. Finally, the simulation model of the dynamic feedforward control system of the 3-PSP parallel manipulator is constructed in Matlab/Simulink. The simulations of the control system eliminating the influence of the dynamic characteristics and dynamic disturbances are conducted. A comparative study between the simulations and the normal kinematic control method is also presented.The simulations prove that the dynamic feedforward control method effectively eliminates the influence of the dynamic disturbances and dynamic characteristics of the parallel manipulator on high-speed machine tools, and significantly improves the trajectory accuracy. This is the first attempt to introduce the dynamic feedfordward control method into the 3-PSP spatial parallel manipulator whose dynamic model is complex and provides a study basis for the real-time dynamic control of the high-speed hybrid machine tools.

Dynamic Simulation of the Assembly Process of Spindle Box of Machine Tool

Based on the analyzed transmission process of spindle head and combined with the analyzed assembly process of spindle box, it determined the relationships of transmission and assembly between the various components of the spindle head. Firstly, using software of computer-aided design (UG6), it established a 3D parts library of spindle box of machine tool. Secondly, we had been developed dynamic simulation and power transmission simulation software of the assembly process of the spindle box of machine tool. It could realistically reproduce the disassembly process and the power transmission process of spindle box of machine tool when running the software. The software not only could be used for the old products to analysis and improvement, but also could be used for development of new products, and to some extent, the prototype could be replaced by it. The software was one of the essential tools for both the training of related employees and teaching of mechanical engineering students.

Modeling and Estimating Thermal Error in Precision Machine Spindles

Thermal induced errors are significant factors that affect machine tool accuracy. The deformation of spindle is the main contributor to thermal error. In this paper, the thermal characteristics of the spindle system are investigated. Taking into account the coupling of elastic deformation and temperature, the heat conduction of the spindle system is modeled. The heat of bearings and heat transfer coefficient, and boundary conditions of the spindle are determined. Based on the numerical results, an iterative model of spindle's temperature and thermal deformation are acquired under the actions of thermal loads using the finite element method. Taking the spindle of precision boring machine with some reasonable assumptions and simplicities as an example, the finite element analysis model of spindle thermal characteristics is analyzed with virtual prototyping, and the static/transient temperature field and thermal-structure field are calculated using ABAQUS software. The characteristics of heat flow and thermal deformation within the spindle are analyzed according to the simulation results. The research results provide a theoretical foundation for reasonable arrangement and optimal design to reduce radial and axial deformation of the spindle head, temperature controlling, and the error compensation to the precision machining tool.

Effect of preload of linear guides on dynamic characteristics of a vertical column–spindle system

Realization on the dynamic characteristics of the column–spindle system is of importance for enhancing the structural performance of a vertical milling machine. Generally, the spindle head is fed under linear guide mechanism through a ball-screw driver. To assess the dynamic characteristics of a vertical column–spindle system under the influence of a linear guide, this study developed a finite element model integrated with the modeling of linear components with the implementation of contact stiffness at the rolling interface. Both the finite element simulations and the vibration tests reveal that the preload of a linear guide greatly affects the vibration behavior associated with a spindle head, and the dynamic stiffness of the spindle head could be enhanced by increasing the preload of the linear guide. Current results clearly indicate that the simulations agree well with the experimental measurements. This also confirms that the proposed model can be successfully applied to evaluate the dynamics characteristics of machine tool systems of various configurations.