Masaomi Tsutsumi

Identification and compensation of systematic deviations particular to 5-axis machining centers

Abstract This paper presents an algorithm for identifying particular deviations such as angular deviations around linear axes relating to rotary axes in 5-axis machining centers. In this study, three kinds of simultaneous three-axis control motions are designed for each rotary axis to identify the deviations. In the measurement, two translational axes and one rotary axis are simultaneously controlled keeping the distance between a tool and a worktable constant. Telescoping ball bar is an effective instrument for measuring the relative displacement to the reference length in the work volume because its attitude is freely changed. In these three-axis control motions, the sensitive direction of the ball bar is kept constant. In order to determine the deviations, we derive eight equations from the relationship between the eccentricities obtained from the measured circular trajectories and the approximations derived from the mathematical model based on the simulation. In the simulation, a mathematical model considering the particular deviations is developed and then the characteristic diagrams are prepared for every deviation and every three-axis control motion. Based on the results, we propose a procedure for identifying the particular deviations in 5-axis machining centers and its procedure has been applied to identify the deviations actually. From both the simulation and the experiment, it has been confirmed that the proposed method gives precision results and is able to apply to the measurement of 5-axis machining center which is a tilting rotary table type.

Modeling and Controller Tuning Techniques for Feed Drive Systems

In this paper, a new modeling, and controller tuning method for feed drive systems is described. Typical feed drive systems consist of an AC servo motor, a ball screw, linear guides, and a servo controller. In order to design high performance systems, it is effective to make a model and analyze its behavior. In this study, a feed drive system is modeled by a vibration model with two degrees of freedom. Various kinds of motions are measured and simulated. The results of the experiment and simulation show that these motions are well simulated by the model. This means that the proposed model can accurately estimate the transfer function of the actual system. As a result, it is easy to design a controller based on the transfer function. The gains in the velocity control loop are calculated based on the partial model-matching method. Two PI and I-P velocity controllers are applied to the feed drive system. The step responses are then compared to each other. The position loop gain is calculated from the frequency response of the velocity control system. The proposed method is applied to an actual feed drive system, and it is confirmed that the proposed method yields comparable performance to the system designed by the conventional tuning way.Copyright

Study on the load distribution of ball screws with errors

A model is developed to analyze the load distribution of ball screws with geometry errors. The load distribution of the contact of the balls and grooves is theoretically investigated under various load conditions. The results demonstrate that the negative geometry errors of the ball screw result in the decrease of the load on balls or guiding grooves, and when the negative errors reach certain value there is no contact between the balls and the groove. On the other hand, if geometry errors of the ball screw are positive, the load increases. If the error heights are assumed to be a normally random distribution, the load distribution without geometry error is the average value of that in random error. It is valuable for equilibrating the load distribution to correct nut grooves. The simulation results show that the model proposed in this paper is significant to the study of the characteristics of ball screws with regard of geometry errors.

Dynamic synchronous accuracy of translational and rotary axes

This paper describes the dynamic synchronous accuracy of the translational and rotational axes in five-axis machining centres. In this study, non-uniform three-axis synchronous motion is investigated in order to estimate the dynamic synchronous accuracy. A dynamic model of each axis including a rotary axis is developed, and the synchronous motion is simulated. The dynamic model consists of a moment of inertia, Coulomb’s friction, viscous friction, and controllers. As a result of the experiment and simulation, it is clarified that the developed model can express the experimental results accurately. In addition, a controller tuning method that can improve the synchronous accuracy is proposed, and its effectiveness is clarified by the developed dynamic model.

Control of Surface Pattern of Mold Generated by Ball-end Milling

In this research work, the generation mechanism of surface pattern in ball-end milling is analyzed theoretically and experimentally. In the experiment, the eccentricity of the tool axis and the angular position of the cutting edge of a ball-end mill are controlled according to the proposed method. From the experimental and theoretical results, it is found that the surface pattern generated on planes or cylindrical surfaces can be controlled by the proposed method.

Accuracy evaluation method for multi-tasking turning centre

Multi-tasking turning centre has ten inherent deviations which were identified by considering the axis configuration and mutual motions of axes. To estimate all these deviations, in this paper, a new methodology which is based on simultaneous three-axis motion technique is proposed. The effects of deviations on predefined trajectories were investigated by simulations. Five measurements which were extracted by conducting four simultaneous motions were used in the identification algorithm. All the ten inherent deviations were estimated accurately by means of the centre offset values of the trajectory measurement. Furthermore, the effects of setting errors of balls of ball bar and clamp-unclamp process of B-axis on measurement were discussed and eliminating techniques were proposed. All the results were verified by means of simulations and experiments. Based on the results, it can be said that the proposed method can be used as an accuracy evaluation and identification tool for multi-tasking turning centres.

Motion Characteristics of High Performance Rotary Tables for CNC Machines

In this paper, the characteristics of two rotary tables driven by worm gear and roller gear cam are measured and compared. The positioning accuracy and repeatability as specified in ISO 230-2 are measured together with the rotational fluctuation, backlash, friction torque, frequency response of the systems and also the influence of unbalance mass on rotational motion. Two rotary encoders which were attached to motor and output axis were used for measurements. The motor, controller, and the rotary encoders were kept the same for both tables to ignore the effects of these units on results. Furthermore, the simulations were carried out by mathematical models which were proposed by two of the authors and the results were compared with measured results. From the simulation results, the torsional stiffness and friction torque were identified and also compared. The results show that the measured and simulated data have a good agreement and therefore it can be said that the identified parameters from simulations are accurate. The result shows that the performances of the rotary table driven by roller gear cam is better than that of rotary table driven by worm gear.© 2008 ASME

Development of Calibration Methods of 5-axis Controlled Machining Centers. (2nd Report). Estimation of Positional and Angular Deviation by Means of Simultaneous 3-axis Motion.

Various types of 5-axis controlled machining centers have been developed to produce molds, dies and mechanical parts with complicated shape as well as aero parts. The most fundamental cause of the machining errors is the deviation of the configuration of the machining center. However, any calibration method for the 5-axis machining centers including two rotary axes has not been investigated. In this paper, a calibration method using simultaneous 3-axis control technique is proposed for 5-axis machining centers with a tilting rotary table. The ball-bar system is used as a measurement instrument. In this method, one rotary axis and two linear axes are simultaneously controlled and the sensitive direction of the ball bar system is kept constant in radial, tangential or axial accordingly. The influence of the positional and angular deviations on 3-axis motion is evaluated through the characteristic diagrams. In the experiments, the positional and angular deviations existing in the 5-axis machining center are estimated by the proposed method. The results show that the deviations can be estimated with high accuracy .

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.

Quantitative Evaluation Method for Damping Capacity of Linear Rolling Bearings for Feed Drive Mechanisms

The vibration damping of guide ways is an important factor in the design of feed drive mechanisms for machine tools. Linear rolling bearings are often used as guide ways because of their low friction. However, there are no quantitative evaluation methods for its damping. Thus, this paper describes a quantitative estimation method for the stiffness and damping capacity of a linear roller guide through an experiment and a mathematical model. The stiffness and damping coefficient of the linear roller guide are identified through the frequency resonance function and mathematical model of the feed drive mechanism. It is shown that a lubricant film can effectively increase the stiffness and damping capacity of the linear roller guide.