Research on geometric error modelling and decoupling of machine tool based on NURBS projection curve

As a geometric measuring instrument of machine tool, double ball bar (DBB) instrument can effectively identify the geometric error of machine tool. In the existing literature, DBB mostly uses the two-dimensional circle as the identification trajectory to identify the geometric error continuously, and the most efficient way of single recognition is still in two-dimensional plane. Aiming at the low efficiency of DBB spatial geometric error recognition, the recognition range of DBB is extended from two-dimensional plane to spatial hemisphere. A method of space recognition and decoupling based on NURBS projection curve is proposed in this paper. The error decoupling of feed axis and rotation axis is realized in an experiment. The identification method has the characteristics of high efficiency, no secondary assembly and avoiding interference. It can decouple all the five axes in hemispherical space at one time and get the effective solution.

Thermal Error Modeling of Feed Axis in Machine Tools Using Particle Swarm Optimization-Based Generalized Regression Neural Network

This paper demonstrates the development of a thermal error model that is applied on the feed axis of machine tools and based on the neural network. This model can accurately predict the value of the axial thermal error that appears on machine feed axis. In principle, there is the generalized regression neural network (GRNN), which has the good nonlinear mapping ability and serves to construct the error model. About variables, the data of temperature and axial thermal error of machine feed axis are the inputs and outputs, respectively. The particle swarm optimization (PSO) is a component of this model, which serves to optimize the smoothing factor in GRNN, and the particle swarm optimization-based generalized regression neural network (PSO-GRNN) model is built. From experiment, the datum is acquired from a machining centre in four different feed rates. Thereafter, the back propagation (BP) neural network model, the traditional GRNN model, and the PSO-GRNN model were established, and the data collected from the experimentation are input in three models for prediction. Compared with the other two models used in this paper, the PSO-GRNN model can maintain higher prediction accuracy at different feed speed, and the prediction accuracy of it changes less in different feed rates. The proposed model solved the problem of generalization ability of the neural network at different feed rate, which shows good performance and lays a good foundation for further research like thermal error compensation.

Feasibility Study of a Hybrid Spindle System with Ball and Active Magnetic Bearings for Quadrant Glitch Compensation During End Milling

Quadrant glitches are caused by friction and motion loss on the feed axis of machine tools. A previously developed method of compensating for quadrant glitches using the feed axis in which the friction model and time series data are not consistent with the actual friction behavior has some problems, making it difficult to construct a feedback system with a high response problems such as a feed axis with a large lost motion. The ultimate goal of this study is to develop an innovative method of compensating for the quadrant glitches caused by the motion of the feed axis of the machine tool using a newly proposed hybrid spindle system with an active magnetic bearing at the end near the end mill and a ball bearing at the other end in combination with a proportional-integral-derivative controller. This study aims to verify the effectiveness of the proposed quadrant glitch compensation method through experiments on the motion of the end mill using a model experimental device for the hybrid spindle system. Through experiments, a quadrant glitch with a peak of 7 μm without compensation is decreased to 1 μm by applying the proposed method using the hybrid spindle system. The undercut error is also decreased by applying the proposed method.

Werkzeuge der ultraschallunterstützten Bearbeitung*/Modelling of an Actorsystem - Development of a tool for ultrasonic assisted machining with an axial-tangential vibration vector

Die schwingungsunterstützte Bearbeitung hat sich bereits bei der Zerspanung von hartspröden Werkstoffen mit einer einachsigen Schwingung in der Kontaktzone bewährt. Untersuchungen zu schwingungsunterstützten Bohrprozessen beschränken sich bisher auf eine Schwingungserzeugung, die entlang der Vorschubachse ausgerichtet ist. Für alternative Schwingungsrichtungen fehlt in erster Linie die geeignete Aktorik. In diesem Beitrag wird eine alternative Methode zur Erzeugung einer axial-tangentialen Schwingung in der Kontaktzone untersucht.   Ultrasonic assisted machining with uniaxial vibration is a well-proven process for machining hard and brittle materials. Existing investigations of vibration assisted drilling and boring processes so far are limited to an oscillation along the feed axis, which primarily due to nonexistent actuators. This contribution will present investigations into an alternative method for creating axial-tangential vibrations in the tool contact zone.

Measurements of Temperature of CNC Machine Tool Ball Screw Utilising IR Method

In numerous papers it is proposed to use IR measurements of feed axis ball screw temperature distribution in order to compensate CNC machine tool thermal errors. The paper aims to validate reliability of the IR measurements in application to the feed axes ball screws. The identification of key factors influencing the accuracy of the IR measurements of ball screw temperature distribution has been conducted. A test-bench utilizing a ball screw assembly with built-in temperature sensors was introduced and the experimental data are presented along with conclusions.

Synchronisation of Feed Axes with Differing Bandwidths Using Set Point Delay

This paper presents an effective method for the synchronisation of multiple feed axes with differing controller bandwidths by delaying the set point trajectories of those axes with higher bandwidths. First, a simplified model of a cascade-controlled feed axis is defined, which allows the problem to be treated analytically. The problem of synchronisation of the feed axes is then analysed mathematically, leading to the hypothesis of synchronisation through a delay of the set points of the more dynamic axes. Subsequently, the dynamic error behaviour and boundaries of a feed axis are calculated. The optimal damping factor for a feed axis is shown to be 1/√2 and the dynamic error can be formulated in terms of the bandwidth and acceleration or jerk limit. The proposed method is proven through a simulation and verified based on experimental results. In addition, the stated error bounds are verified, and the limits of the applicability are determined.