Annual Journal of IIE Hong Kong

This paper describes a PC-based CAD/CAM package for the design and manufacture of mould cavities for producing products with sculptured surfaces. The surface of a product is described by specifying control points on its cross sections. Spline curves are fitted through these control points along both traverse and longitudinal axes. The number of curves fitted depends on the required accuracy. The designed surface can be converted to obtain the shape of a mould cavity by manipulating it with geometric operations such as scaling, left-right inversion and male-female inversion. An utility has also been developed to link the package with a 3-D digitizer to capture the surface geometry of a given pattern effectively. It may be used to filter digitized surface data of the pattern and subsequently convert the data into control points suitable for the package to re-construct or modify the pattern geometry. The package also includes an efficient and easy-to-use cutter path simulation. Triangular facets representing a wire-frame model of the surface can be generated for further analysis, such as shading and calculation of geometrical data of the surface. Cutter paths and hence the control codes for both rough and finish machining the mould cavity on an NC machine can be generated. The cutter paths are automatically planned in accordance with the specified cutter dimensions and allowable tolerance for efficient utilization of the NC machine. Introduction Plastic containers are examples of products with sculptured surfaces which are commonly used for packaging liquid products, such as shampoo and detergent for general consumption. They are generally manufactured by blow moulding or injection moulding. The shapes of these containers have become more irregular as market competition demands a higher degree of aesthetics in packaging. The demand for aesthetical shapes, coupled with a general tendency to shorten the production lead time, have presented some problems the mould and die makers who are generally small or medium sized. Traditionally, the mould cavities machined by copying from a pattern of the bottle, which is normally hand-made by a skilled craftsman. General CAD/CAM systems based on surface modelling techniques have been available for the design of products or dies with free-formed surfaces[1]. However, specific functionality required to facilitate the design process of a product surface may not be available in these CAD/CAM systems. This includes the conversion of digitized surface data of a given pattern to a CAD file in a format convenient for re-modelling of the pattern geometry. Generally, the amount of data involved in digitizing the surface of a pattern with reasonable accuracy is huge. Hence it is a tedious and indeed difficult process to manipulate these data for subsequent re-creation of modification the pattern geometry in a CAD system. Furthermore, the machining functionalities of most of these systems may not as strong and easy-to-use as desired. In general, these systems may not be able to generate cutter paths for direct rough machining. They also may not plan the cutter paths automatically to achieve a specified machining accuracy. The user could only guess the possible machining accuracy by specifying the number of cutter paths to be generated. Hence they may not be particularly suitable or economical for the design and manufacture of mould cavities for products with simple sculptured surfaces. This paper presents a CAD/CAM package with specific utilities developed to help the design of a product or reconstruction of a given pattern, and subsequent manufacture of mould cavities. The CAD/CAM package is based on the widely proliferated IBM PC compatible micro-computers. It is aimed to provide a simple and useful tool for the local mould and die manufacturers. The CAD/CAM Package The CAD/CAM package consists of the following major modules: (1) Design of the surface of a bottle or mould cavity. (2) Generation of data for geometrical analysis of the surface. (3) Filtering and conversion of digitized surface data of a pattern to a CAD file for reconstruction of the pattern geometry. (4) Simulation of cutter paths for rough machining the surface or mould cavity. (5) Simulation of cutter paths for finish machining of the surface or mould cavity. Design of a Surface Surface Contours The surface of a product, such as a bottle, can be constructed from the contours of its cross sections. The cross sections are defined by the coordinates of some control points on traverse (x-y) planes perpendicular to the longitudinal (z) axis of the bottle, as shown in Fig. 1. If the cross section is symmetrical about the longitudinal axis, only a half of the control points is needed and a mirror image of these control points is taken to complete the definition of the cross section. Fig. 1: Control points for cross sections 1995-96 Annual Issue of IIE (HK) 13 The package creates contours \v hick describe the product surface along both traverse and longitudinal axes by fitting spline curves[2] to the control points. Fig. 2 shows an example of the contours of a bottle surface. The coordinates of the control points are normally specified with a mouse or the keyboard. The package automatically inserts intermediate contours between each pair of the cross sections to give a finer representation of the product surface. The number of intermediate contours is adjusted according to the degree of accuracy required. A mesh patch consisting of triangular facets is then generated to form a wire-frame model of the bottle surface. The wire-frame model can be further processed to generate a shaded surface to give a more realistic visualization of the designed bottle. Geometrical data of the bottle, such as volume and surface area, can also be calculated accordingly for analytical purposes, such as heat flow etc.. Fig. 2: An example of a bottle surface Shape Control along Longitudinal Axis The traverse cross sections are generally taken at critical locations along the longitudinal axis so that the surface can be well described with a minimum number of cross sections. Better control of the shape can, of course, be achiev ed by increasing the number of cross sections. To fine tune the local smoothness of the surface, the control points of a particular cross section can be slightly shifted about. It is also possible to change the end-point conditions of the spline curves to obtain the required local smoothness. Furthermore, the contour curves can be forced to pass through the control points or made tangent to the line segments joining the control points. The plane of all traverse cross sections are assumed to be perpendicular to the longitudinal axis of the bottle surface. This assumption facilitates the design of the surface because the longitudinal axis of a bottle is generally straight. However, it is possible to describe a 3-D spline curve for the longitudinal axis. This allows bottle surfaces with relatively complicated shape along the longitudinal axis to be designed easily. Fig. 3 shows a surface with a straight longitudinal axis and one with a curved longitudinal axis. 14 1995-95 Annual Issue of IIE (HK) Fig. 3: Surfaces with a straight z-axis and a curved z-axis Geometrical Accuracy The geometrical accuracy of a modelled surface depends on the number of intermediate sections, which is affected by the specified maximum allowable chordal error. As shown in Fi g. 4, the chordal error, e, is controlled by the number of intermediate contours along both traverse and longitudinal axes. The number of intermediate contours required to achieve a specified degree of geometrical accuracy is calculated according to the following method: Fig. 4: Chordal error (1) The chordal error, e, of a curve segment and the line representing the plane of a facet is calculated. If a is greater than the specified precision, one intermediate control point is inserted. (2) Repeat step (1) until all chordal errors are within the specified precision. (3) Fit spline curves to the intermediate control points inserted above. (4) Generate facets to form a wire-frame model of the surface, if necessary. Filtering and Conversion of Digitized Surface Data If a pattern of the product is available, its geometry can be captured by measuring some strategic points on various cross-sections of the pattern surface on a 3-D digitizer or a coordinates measuring machine. The package treats these strategic surface points as the control points, and reconstructs the pattern geometry by fitting both traverse and longitudinal spline curves through them. The reconstructed geometry may then be modified by manipulating the positions of control points. In general the amount of digitized surface data may be in term of hundreds of kilobytes and tens of megabytes, with no obvious strategic points to be regarded as control points. It is therefore very difficult to manipulate these data for reconstruction or re-design of the model geometry. To alleviate this problem, an utility has been developed to (1) filter and reduce the digitized surface data while maintaining a specified degree of accuracy and, (2) convert the processed data into control points suitable for the package to reconstruct the model geometry. The filtering algorithm is based on determining the changes of slopes of two line segments formed by three consecutive points, as shown in Fig. 5, in both traverse and longitudinal directions. If the changes of slopes are all smaller than a pre defined tolerance, the middle point is ignored. The tolerance may be varied to adjust the speed of processing and the overall reduction of the surface data. Fig. 5: Slopes of line segements formed by consecutive points The processed surface data are then converted to a CAD file by electing suitable numbers of control points and cross sections. The package reads the control points from the CAD file to reconstruct and hence modify the model geometry. Shape Manipulation Three types of operations are available for the manipulation of the surface or its cavities, namely scaling, male female inversion and left-right inversion. The scaling operation is used to enlarge or reduce the size of the bottle surface either independently or uniformly along the x, y and z axes. This operation allows a family of bottles of different sizes to designed easily. The leftright inversion allows a surface of the mirror image of an existing surface to be obtained. This is useful for modelling cavities for blow moulding. The male female inversion is used to obtain a convex surface from an existing concave one, or vice versa. The cross sections of the existing surface are firstly turned up-side down, and then left right inverted. This operation is very useful for modelling cavities for injection moulding. Generation of Surface Mesh Patch if the surface designed is satisfactory, a mesh patch is generated by joining up each of the grids formed by the intersections of the traverse and longitudinal contours with two triangular facets. A wire-frame mode of the surface is formed by arranging these facets in a suitable format which is easily accessible by other modules or packages. The wire frame model can be further analyzed for shading of the surface, calculation of heat transfer, simulation of plastic flow pattern, and calculation of geometrical properties, such as surface area and volume. Machining of the Surface A rough cut module and a finish cut module have been implemented for generating cutter paths for machining the surface or its cavities. The user specifies the cutter dimensions, maximum allowable depth of cut and the required degree of accuracy. The cutter paths will then be generated according to these specifications. Rough Cut The rough cut module simulates the cutting process for bulk removal of the excess material from a rectangular block within which the surface or cavity is contained. This module scans a traverse section of the surface and feeds the cutter to remove the material until the cutter touches the surface. The cutter is then lifted and moved horizontally to avoid the crest of the section. This rough cutting process is repeated for all traverse sections by feeding the cutter layer by layer as specified by the allowable depth of cut until the parting plane is reached. This module allows a rough profile of the bottle surface to be machined easily. Fig. 6 shows part of the simulated cutter paths for rough machining a bottle surface. Fig. 6: Part of the simulated cutter paths for rough machining Finish Cut To obtain a good machined surface texture, the machining asperities, or cusps, must be controlled to an acceptable level. If the maximum allowable cusp height h as shown in Fig. 7 is specified, the centre distance s between two paths of a spherical cutter with a radius r can be calculated using the 1995-96 Annual Issue of IIE (HK)15 (1) Calculate the maximum possible centre distance, s, between two neighbouring cutter paths; (2) Calculate the length of each longitudinal curve segment between each pair of adjacent layers, as shown in Fig. 8; (3) Selected the maximum length, L, of the curve segments calculated in step (2); (4) Assuming that s is small so that the curve segment in question approximates a straight line, insert [INTEGER(L/s) + 1] intermediate sections between the adjacent layers; (5) If the distance between any pair of sections is less than s, insert one more section; (6) Repeat steps (2) to (5) for another pair of adjacent layers until enough sections have been added to the whole model; (7) Repeat steps (2) to (6) for traverse layers; (8) Generate triangular facets; (9) Generate cutter paths that run across the facets smoothly while maintaining the cutter tangent to the normal vector through the centroid of the facets[3,4] until the parting plane is reached; (10) Generate the cutter paths for machining the parting plane with a flat-ended cutter, if necessary. Fig. 8: Insertion of intermediate sections for finish machining 16 1995-95 Annual Issue of IIE (HK) This algorithm has been found to be relatively efficient in that the simulation of machining a surface of general size and accuracy can be completed within a period of about 2 minutes. Fig. 9 shows a bottle surface being machined. Fig. 9: A Photograph of a bottle surface being machined NC Post processor This module processes the data files containing the cutter paths for rough cut and finish cut. It generates NC part programs suitable for machining the bottle surface or its mould cavity on a CNC milling machine with a FUNAC 6M controller. The NC part programs can be downloaded to the controller for DNC machinin g of the surface. Conclusions It can be concluded that a CAD/CAM package, based on an IBM compatible micro-computer, has been developed for the design and manufacture of mould cavities for production of products with sculptured surfaces. Graphical menus and functional utilities have been developed to facilitate the description of a bottle surface and its subsequent manipulation to obtain the mould cavities. The utility for filtering digitized surface data may effectively reduce the file size by a large proportion while maintaining a reasonable degree of accuracy. The processed data may be further converted to a CAD file format for the package to reconstruct the model geometry. Modules for cutter path simulation have been developed for both rough machining and finish machining. These modules are capable of generating cutter paths for machining the surface with the required accuracy efficiently. References [1] J.S. Gunasekera, "CAD/CAM of Dies", Ellis Horwood, 1989. [2] Carl de Boor, "A Practical Guide to Splines", Springer Verlag, 1978. [31 J.P. Ducan, "Sculptured Surfaces in Polyhedral Machining"", McGraw-Hill, 1981. [4] J.P. Ducan, "Introduction to Polyhedral NC Concept", Lectures in Hong Kong 1989. A Computer Integrated System for the Automation of Electrical Tester Fixture Fabrication H.W. LAW Department of Manufacturing Engineering City University of Hong Kong JACKSON H.K. LAM Chemical & Electronic Equipment Division Dainippon Screen (Hong Kong) Ltd. Introduction Keen competition, stringent quality requirements, tight design -to-market time and low profit margin are the main challenges of consumer electronic industry. Use of problemfree printed circuit boards (PCBs), which are a crucial component in electronic products, is continue to be an important issue in electronic industry. In face of the electronic products' shorter life cycle and higher density circuit requirements, it is, therefore, important that PCB manufacturers continuously strive for better board quality and automate their processes in order to maintain their competitiveness. Due to high repair costs for defective boards found after assembly, PCB manufacturers usually carry out electrical test (E-test) on all their boards produced before delivery in order to save guard their board quality. Any improvement on the rapidity and the reliability of the fabrication of E-test fixture could, thus, benefit the board turn around time, quality and profit. This paper describes the development of a computer integrated system for the automation of E-test fixture fabrication. Circuit board design data in Gerber format from electronic CAD systems is used and the fixture fabrication data is then automatically generated by the system. The generated data can be downloaded to CNC drilling machines and Etesters directly. As a result, the accuracy of fixture data preparation is improved and the fixture fabrication turn around time is reduced. Fig. 1: Manual PCB E-Test Fixture Preparation Steps Bare Board Electrical Testing and Manual Fixture Fabrication Open/short circuit is an un -acceptable quality problem of PCBs. Electrical test is an effect[l][2], reliable and nondestructive test method to ensure a PCB meets the required electrical connective. Different testing fixtures are required for the testing of different circuit boards. For fabricating fixtures of large board area with high circuit density, the erroneous human judgment is likely increasing the fixture fabrication time and costs. Fig. 2: Different Type of Circuit Test Point A Computer Integrated Automatic Fixture Fabrication System In order to reduce the human effect in the fixture fabrication process, a computer integrated fixture fabrication system is proposed and developed. The configuration of the system is shown in Figure 4. 1995-96 Annual Issue of IIE (HK )17 Current manual fixture fabrication practice [3] is shown in Figure 1. It requires experienced technicians to identify test points, select probe size, prepare NC drill program for the drilling of the fixture base plate, draw wire map and create netlist. The practice starts from the identification of end points and isolated points of the circuit manually based on the circuit artwork provided (Figure 2). A wire map with suitable numbering of the identified test points is, then, drawn on a transparency. Test pointsi probe sizes are also selected and the tip style of the probes (Figure 3) are identified according to the circuit pattern. Drill paths for the holes of different probesi receptacles are then identified manually and marked with different colours for the drilling of the fixture base plates. The hole locations are then digitized manually before the fixture base plates are drilled. The supporting pins and probe sockets are then assembled. The sockets of the probes are wire wrapped to the edge connector and the tooling pins are placed into the fixture base plates. The fixture is then mounted onto an E-tester. The correct open-short relationship of the circuit ("OK" data) is then "learnt" by the tester using a "golden board" which is a circuit board checked manually beforehand for opens and shorts. The tester with the testing fixture is now ready for testing of PCBs.