Derek G. Ford

Dynamic calibration of CNC machine tools

Machine calibration has become an important tool for assessing and maintaining machine accuracy, and for providing a measure of production quality. The calibration techniques supported by the calibration standards and the available metrology technology use static measurement cycles [BS3800, Part 2 (1991); ISO 230-2, Part 2 (1988)]. Static calibration cycles can be time-consuming to perform, and with coarse step sizes cannot give a complete picture of the machine performance. Recent advances in the laser interferometer technology used for machine calibration allow data to be captured dynamically. Dynamic data-capture technology provides the potential for dynamic machine calibration. Dynamic calibration overcomes the inherent problems of static calibration, being quick to perform and providing detailed information on machine performance. This paper describes the concept of dynamic machine calibration. In particular a novel dynamic calibration technique, conceived at the University of Huddersfield, is described. This technique utilizes existing calibration technology. Example dynamic calibrations, that highlight the potential of the technique, are presented and discussed.

Hybrid modelling and simulation of a computer numerical control machine tool feed drive

Abstract The paper presents a new approach to the modelling and simulation of a computer numerical control (CNC) machine tool feed drive. The hybrid model of the drive incorporates a distributed load, explicit damping factors and measured non-linear effects in order to achieve a realistic dynamic performance. In this way, the shortcomings of traditional modelling methods applied to CNC machine tool axis drives are overcome. A MATLAB/SIMULINK package was used to simulate this new model for a CNC machine tool feed drive. A novel hybrid model with a distributed load, explicit damping factors, backlash and friction was developed and shown to have a similar dynamic response to the machine tool feed drive system under the same conditions. The simulated results display resonance frequencies, while the lumped-parameter models generate only the response of a second-order element.

Active vibration control for a CNC milling machine

There is a requirement for improved three-dimensional surface characterisation and reduced tool wear when modern computer numerical control (CNC) machine tools are operating at high cutting velocities, spindle speeds and feed rates. For large depths of cut and large material removal rates, there is a tendency for machines to chatter caused by self-excited vibration in the machine tools leading to precision errors, poor surface finish quality, tool wear and possible machine damage. This study illustrates a method for improving machine tool performance by understanding and adaptively controlling the machine structural vibration. The first step taken is to measure and interpret machine tool vibration and produce a structural model. As a consequence, appropriate sensors need to be selected and/or designed and then integrated to measure all self-excited vibrations. The vibrations of the machine under investigation need to be clearly understood by analysis of sensor signals and surface finish measurement. The active vibration control system has been implemented on a CNC machine tool and validated under controlled conditions by compensating for machine tool vibrations on time-varying multi-point cutting operations for a vertical milling machine. The design of the adaptive control system using modelling, filtering, active vibration platform and sensor feedback techniques has been demonstrated to be successful.

Modal parameter identification for CNC machine tools using Wavelet Transform

Abstract The paper presents a new use of the Continuous Wavelet Transform for modal parameter identification applied to CNC machine tools. Firstly, the resonant frequencies and damping ratios of the CNC machine tool axis drive are estimated in the frequency domain using the transmissibility relation at resonance. The experimental Bode diagrams are determined using a novel measurement practice for the decoding of signals generated by a position encoder. This paper focuses on a novel application of the Continuous Wavelet Transform to identify the resonance frequencies and corresponding damping ratios of the CNC machine tool axis drive. The proposed method has the ability to detect variations in the amplitude levels of weak components embedded in strong noise and non-stationary processes. The superior ability of the Wavelet Transform to identify accurately modal parameters is demonstrated by comparing the results of the two different methods.

Practical experience of thermal testing with reference to ISO 230 Part 3

Thermal errors in machine tools can represent up to 70% of the total positional accuracy of machine tools [l]. The significance of temperature-induced errors led to the introduction of an international standard method of testing for thermal effects [2]. A wide range of machme tools has been measured and assessed by the Ultra Precision Engineering Centre (UPEC) to advise on thermal error avoidance and to implement error correction where appropriate. This paper presents some results taken fiom these machines and discusses the implications for manufactured parts. Attention is also drawn to the problem of distinguishing from the various sources of thermal effects during tests that may take several hours in a variety of workshop conditions. The results suggest that some deviation from the standard can provide more process-specific information.

Modelling and simulation of a feed drive using the transmission line modelling technique

The paper focuses on the modelling and simulation of a test rig and a digital drive is based on the transmission line modelling technique. The rig is a positioning system equivalent to a CNC machine tool feed drive. Accurate representations of dynamic systems included in electromagnetic, power electronic and some mechanical applications have been obtained until now by applying the transmission line modelling technique. The paper proposes a novel application of th~s method for simulation of closed loop control of the position of a test rig. The model includes the distributed parameter dynamics of the feed drive and the effect of mass distribution. This is based on the flexibility of transmission line modelling to develop lumped and distributed parameter systems. Also a transmission line model for the controller integrated into the digital drive is developed. Encouraging simulation results are obtained by using MATLAB. In this way, the CNC machine tools feed drives could be modelled with increasing accuracy as hybrid, distributed-lumped representations using numerical methods.

A New Approach To The Modelling AndSimulation Of A CNC Machine Tool Axis Drive

The paper presents a new approach to the modelling and simulation of a CNC machine tool axis drive. Modules have been created for different parts of the CNC machine tool. This allows greater flexibility in the construction of the model and an investigation of the interaction between model components. In this way all the shortcomings of the traditional methods are overcome. MATLAB / SIMULINK package has been used to simulate this new model for a CNC machine tool. Simulation results are very good, in accordance with CNC machine tool theory. The described approach allows the easy construction of detailed machine tool drive models. It represents the basis for future incorporation of geometric, non-rigid and thermal models of machine tool behaviour.

A general purpose thermal error compensation system for CNC machine tools

Thermal effects cause the majority of machining errors on many types of machine tool, with linear expansion and distortion of the structural elements causing unwanted movement between the tool and workpiece. Heat inputs that cause temperature elevation and gradients come from many sources mternal and external to the machine tool and make thermal errors difficult to control without some form of compensation. Many thermal error modelling and compensation systems have been proposed which use neural networks, multi-regression analysis, heat modelling or probing techniques. However, each method suffers from one or more major drawbacks that limits its effectiveness when used in a practical machining environment. One feature of all the thermal error compensation techniques is their lack of flexibility that makes them difficult to apply to more than one machme type in a timely and cost-effective way. This paper describes a combined thermal and geometric error compensation system with a flexible structure that is general purpose in its application to any machine tool. The system can accept input from any number of temperature sensors. Information from the temperature sensors is acted upon by a novel programming language based model that estimates thermal movement and directs error components to a number of outputs for compensation by axis position modification. The entire compensation system can be applied either in a stand-alone computer that accepts a wide range of feedback signal types, or integrated into an open archtecture machine controller. The system allows the management of temporary or permanent input failures and displays every thermal error component as an aid to fault diagnosis. Both position independent and position dependent thermal errors can be reduced through compensation. The system has been applied to several machine tools.

Determination of stiffness and damping sensitivity for computer numerically controlled machine tool drives

Abstract A generalized eigenvalue method is used to determine the undamped and damped natural frequencies, coefficients of damping and mode shapes of a computer numerically controlled (CNC) machine tool drive. The calculated results compare well with the measured results obtained by using vibration analysis equipment. Then the sensitivity of the various modes to changes in the stiffness and damping characteristics of the machine drives is estimated. These values prove to be useful in adjusting the parameters of a hybrid model for a CNC machine tool axis drive in order to alter the simulated results in accordance with the measured results.

Measuring And Modelling Thermal Distortion On CNC Machine Tools

Heat sources internal and external to a machme tool cause temperature gradients to arise due to resistance to heat flow in and around the machine tool and workpiece. This causes linear expansion and distortion of the structural elements from which the machine tool is constructed. The connectivity of the structural elements sets the effect of the expansion and distortion on the relative positions of the tool and workpiece, leading to thermal errors on the workpiece. Measurement on a wide range of machine tools confirms that temperature gradients are significant in their effect on machining accuracy and that they move and change shape during the machining process. An analysis of machine tool structures and connectivity shows that both the position and magnitude of temperature gradients is important in its effect upon the relative movement of the tool and workpiece. A bending model that estimates the effects of thermal distortion using knowledge of the position of the temperature gradient is derived. The performance of the bending model is compared with a finite element analysis model and a model that has no knowledge of the position of temperature gradients. Results obtained from a vertical machining centre show that knowledge of the position and magnitude of temperature gradients is an essential part of predicting thermal distortion accurately.