Real-Time Prediction of Workpiece Errors for a CNC Turning Centre, Part 4. Cutting-Force-Induced Errors

A neural network method is presented for predicting cutting-force-induced errors in real-time during turning operations based on the estimated cutting forces. Workpiece errors can be considerably affected by the deflections of the machine–workpiece–tool system. A model of the elastic deflections of the machine–workpiece–tool system due to the cutting force in turning developed. A novel radial basis function (RBF) neural network is used to map the relationship between the cutting-force components (radial, axial and tangential) and the consequent dimensional deviation of the finished parts caused by the combined deflections of the machine–workpiece–tool system. Cutting tests were performed on a two-axis CNC turning centre and the experimental results showed that the prediction accuracy of the maximum diameter error of the workpiece was within 15%. The trained RBF neural network was able to predict the cutting force induced error in real-time during turning.     

Real-Time Prediction of Workpiece Errors for a CNC Turning Centre, Part 1. Measurement and Identification

This paper analyses the error sources of the workpiece in bar turning, which mainly derive from the geometric error of machine tools, i.e. the thermally induced error, the error arising from machine–workpiece–tool system deflection induced by the cutting forces. A simple and low-cost compact measuring system combining a fine touch sensor and Q-setter of machine tools (FTS FQ) is developed, and applied to measure the workpiece dimensions. An identification method for workpiece errors is also presented. The workpiece errors which are composed of the geometric error, thermal error, and cutting force error can be identified according to the measurement results of each step. The model of the geometric error of a two-axis CNC turning centre is established rapidly based on the measurement results by using an FTSFQ setter and coordinate measuring machine (CMM). Experimental results show that the geometric error can be compensated by modified NC commands in bar turning.     

Real-Time Prediction of Workpiece Errors for a CNC Turning Centre, Part 3. Cutting Force Estimation Using Current Sensors

This paper presents a new method for measurement of cutting force using reliable and inexpensive current sensors. The relationship between the various factors which affect the performance of the spindle and feed drive systems are analysed, together with models of the spindle and feed drive system. The tangential (Ft) and axial cutting forces (Fa) are measured, using a neuro-fuzzy technique, with inexpensive current sensors installed on the a.c. servomotors of a CNC turning centre. The normal cutting pressure (Kn) and effective friction coefficient (Kf ) are calculated using the model of the cutting force and the two cutting forces measured by motor current, then the radial cutting force (Fr) can be calculated based on the model of cutting force. Experimental results show that the method can measure tangential, axial and radial cutting forces within errors of 10%, 5% and 25%, respectively, so the need for controlling or monitoring the cutting processes can be met in practice.     

In-Process Modelling and Estimation of Thermally Induced Errors of a Machine Tool During Cutting

A compact measurement system was developed to measure the time variant machine tool errors during cutting. The system is composed of a gauge, a sensor holder, 5 gap-sensors and a PC. The gauge is made of invar which has a very low thermal expansion coefficient, and is often used to measure the thermally induced errors of a machine tool. A new neural network model was considered to estimate the time variant machine tool errors during cutting using a new concept of input values. The detail of the model proposed is described in the paper together with experimental methodologies using a compact measurement system to examine the validity of this approach. These schemes were implemented on a small vertical-machining centre.     

Principle and Simulation of Fixture Configuration Design for Sheet Metal Assembly with Laser Welding, Part 1: Finite-Element Modelling and a Prediction and Correction Method

The quality of the stamping process has a direct effect on laser welded sheet metal assembly. The fixture plays an important role in the satisfactory metal fit-up that laser welding requires. The traditional "3-2-1" locating scheme will no longer suffice for the deformable laser sheet metal assembly process. Because of the often poor stamping quality, a complex die fixture has to be employed to meet the metal fit-up requirements. The die fixture corresponds to an "infinite-2-1" locating scheme where the tooling cost is very high and yet lacks flexibility. This limits the application of laser welding. In this paper, a new locating scheme with both total locating and direct locating for welds is proposed. A total locating scheme is used to locate the overall assembly, and a direct locating scheme is used to locate the weld location, which is critical for ensuring correct metal fit-up. A finite-element model and a prediction and correction method for the direct locator configuration are developed in this paper. A case study is used to show that the proposed method is effective for sheet metal assembly for laser welding.     

Principle and Simulation of Fixture Configuration Design for Sheet Metal Assembly with Laser Welding, Part 2: Optimal Configuration Design with Genetic Algorithm

A fixture plays a key role in ensuring proper metal fit-up in sheet metal assembly with laser welding. In this paper, an optimal fixture configuration design model is proposed based on a new locating scheme which includes both total locating and direct locating on welds. In this model, both the number and the location of the concerned locators are taken as objectives. The proposed prediction and correction method is used for determining the number of direct locators, and a pattern sorting method is developed for determining the number of total locators. Considering the degree of metal fit-up (DMF) to be a fuzzy quantity, a feasible evaluation criterion of DMF is also developed using a fuzzy synthesis evaluation method. A powerful optimisation technique – genetic algorithm – is employed as the optimisation procedure. A case study is presented which demonstrates the effectiveness of the optimal model, and the degree of metal fit-up can be improved compared to the initial locating scheme.     

<Emphasis Type="Italic">Development of a Tissue Engineering Scaffold Structure Library for Rapid Prototyping. Part 2: Parametric Library and Assembly Program</Emphasis>

Rapid prototyping (RP) techniques have been found to be advantageous for tissue engineering (TE) scaffold fabrication due to their ability to address and overcome the problems of uncontrollable microstructure and the feasibility issues of complex three-dimensional structures found in conventional processing techniques. This research proposes a novel approach for TE scaffold manufacture using RP techniques. The approach involves the integration of medical imaging devices (CT/MRI) for the acquisition of anatomic structural data, three-dimensional CAD modelling for designing and creating the digital scaffold models and RP for fabricating the physical scaffolds. To aid the user in CAD modelling, a standard parametric library of scaffold structures is designed and developed. With the library, a user can select the geometry of the scaffold unit cell and size it to suit the end application of the TE scaffold. A developed application program will then assemble the scaffold structure from the selected unit cell, following the surface profile of the anatomic structure to be replicated. A physical scaffold will then be built using an RP system. ID="A1"Correspondance and offprint requests to: Dr C. K. Chua, School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798. E-mail: mckchua@ntu.edu.sg     

Development of a Robotic System which Assists Unmanned Production Based on Cooperation between Off-Line Robots and On-line Robots. Part 3. 1 Development of an Off-Line Robot, Autonomous Navigation, and Detection of Faulty Workpieces in a Vibrating Parts Feeder

Remarkable advances in robotics and intelligent technology have made unmanned production realisable. Based on the off-line robot concept proposed in our first paper, an off-line robot and peripheral devices have been developed for the realisation of unmanned production. The developed robot has a power-wheeled steering (PWS) type mobile platform on which a double-arm manipulator is installed. In order to detect and recognise 3D objects, double CCD cameras are mounted on the top of the robot. Two types of robot end-effectors are developed to handle workpieces and conduct trouble shooting actions. A newly developed production trouble simulator is very useful in simulating and resolving production troubles. A virtual pilot plant for production composed of the off-line robot, two on-line robots, a vibrating bowl feeder, a belt conveyor and a production trouble simulator has been constructed and tests for production simulation have been undertaken. In this paper, autonomous navigation of the developed robot in the virtual pilot plant is investigated. Then a search for faulty parts in a parts feeder using the developed real-time image extraction method is carried out.     

<Emphasis Type="Italic">Development of a Tissue Engineering Scaffold Structure Library for Rapid Prototyping. Part 1: Investigation and Classification</Emphasis>

In tissue engineering (TE), a porous scaffold structure may be required as a template to guide the proliferation, growth and development of cells appropriately in three dimensions. Although TE scaffolds can be created using one of many conventional techniques available, most will suffer from a lack of mechanical strength and/or uniformity in pore distribution and sizes. This study is focused on creating scaffolds using rapid prototyping (RP) techniques. Utilising these novel techniques, a computer-aided design (CAD) of the scaffold structure must first be modelled. The scaffold structure is then fabricated directly from CAD data using a RP system. The objective of this research is to (1) investigate and select various polyhedral shapes suitable for scaffold modelling, (2) classify the selected unit cells, (3) create a parametric library of scaffold structures and (4) verify by building the CAD models using the selective laser sintering process. The first two objectives are covered in Part 1 of this two-part paper. The remaining objectives will be described and discussed in Part 2. ID="A1"Correspondance and offprint requests to: Dr C. K. Chua, School of Mechanical and Production Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798. E-mail: mckchua@ntu.edu.sg