3D Digitising for Reverse Engineering to Manufacture Rapid Tooling Prof. J. C. Ferreira 1 *, Prof. Nuno F. Alves 2 , Prof. Artur S. Mateus 3 1
Instituto Superior Técnico - IDMEC, Lisbon, Portugal Instituto Politécnico de Leiria - ESTG , Leiria, Portugal Email :
[email protected] 1 ; web site* : http://www.ist.utl.pt Email:
[email protected] 2 &
[email protected] 3 ;web site2 - 3 : http://www.estg.iplei.pt 2-3
ABSTRACT The aim of this R&D work is to present some know-how of Institute Superior Technique of Lisbon (IST) (1and of Institute Polytechnic of Leiria (IPLEI) (5-11) in the application of 3D digitising and propose some approaches to transform the surface from the point cloud obtained during digitising processes in parametric 3DCAD to manufacture Rapid Tooling (RT). This approach is also fundamental to verify prototype's geometry for tooling and for simulation modelling by Finite Element Analysis (FEA), to assure the quality of tooling geometry and process parameters. These tasks are a multi-disciplinary optimisation methodology for quality assurance. Based on case studies presented, the paper will reach some conclusions for the appropriate use of 3D digitising and Reverse Engineering (RE) to assist product development and Rapid Tooling before manufacturing, illustrating the achievement integrating these new methodologies and technologies. Keywords: 3D digitising, reverse engineering, surface modelling, rapid tooling
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1. INTRODUCTION 1.1 Survey of the R&D studies
The 3D digitising(12,13) and reconstruction of 3D shapes(14) has numerous applications in areas that include manufacturing, virtual simulation, science, medicine(15), and consumer marketing. This is an actual research and development field that is related to the problem of processing images acquired from accurate optical triangulation(16), and is presented as a methodology for surface reconstructing from sets of data known as range images(17). The standard methods for extracting range data from optical triangulation scanners are accurate only for planar objects of uniform reflectance. Using these methods, curved surfaces, discontinuous surfaces, and surfaces of varying reflectance cause systematic distortions of the range data. When using coherent illumination such as lasers, however, the laser speckle places a fundamental limit on accuracy for both traditional and spacetime triangulation(18). The range data acquired by 3D digitisers such as optical triangulation scanners commonly consists of depths sampled on a regular grid; a sample set known as a range image. A number of techniques have been developed for reconstructing surfaces by integrating groups of aligned range images(19,20). A desirable set of properties for such algorithms includes: incremental updating, representation of directional uncertainty, the ability to fill gaps in the re-construction, and robustness in the presence of outliers and distortions(21). Using this methodologies one is able to merge a large number of range images yielding seamless(22), to assemble high-detail models to develop Rapid Tooling.
2. SCOPE FOR 3D-SCANNING 2.1 3D digitising for Reverse Engineering Certain recent fields of technology innovation related to computer based manufacturing, such as the use of 3D digitising, are being explored to be integrated in the chain process of some Portuguese industries, for applications as: Reverse Engineering; Quality Control; Differential Inspection; Direct Replication; Detection of Inaccuracies; Redesign of Parts; Manufacturing Tools. For about six years research with 3D-Scanning technology has been carried out at the Laboratory for Modelling Prototypes (LMP) at IST with 3D-Scanners equipment, mainly for Reverse Engineering and Quality Control purposes. The developed Reverse Engineering methodology integrated with CAD / CAE to manufacture Rapid Tooling is schematised in figure 1.
Fig. 1 : Developed Reverse Engineering methodology integrated with CAD / CAE for tooling. Laser 3D-scanning digitisers bring more automation to data gathering. These devices scan without contact by a striped laser beam the profile of a physical model and CCD cameras capture profile images that by triangulation algorithms generate digital data, figure 2. By exploiting the laser stripe by triangulation, Reversa sensor measures thousands of surface points per second, taking only a few minutes to digitise typical objects, no matter how complex their surface geometry.
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Fig. 2 : Replica scan machine configuration and 3D-scanning laser digitiser principle. A Reversa 25H scanning head mounted on a Replica 250 machine-stepper driven composes the equipment at Laboratory to Model Prototypes in IST. Several scanning tests with Rapid Prototyping SLA/SLS/LOM prototypes have been carried out at the LMP-IST with this 3D Scanners equipment. The 3D scanners system has the software RI-scan that outputs ASCII X-Y-Z point clouds in range data configuration after triangulation. With the Ri-Tools software digital points are transformed into scanning outlines of the part and after discrete triangulated mesh calculation, with CopyCAD software (DELCAM - UK), the contours are processed to represent the 3D surface model of the part.
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3. EXPERIMENTAL WORK 3.1 Case study 1 – Non-contact metrology of RP patterns before casting Some scanning tests for evaluate the accuracy of RP-SL patterns (figure 3 a-c) have been carried out at LMP with a 3D Scanners equipment, figure 3d. This 3D-scanning system laser based could stripe co-ordinate point data at a standard scanning speed of 60,000 points/min., with an accuracy on ±(Z)=25µm. Some actual limitations concerns scan surface preparation for RP-SL prototypes that must be sprayed with matt white paint and for this reason the scanning accuracy is reduced for about ±20µm in Z-axes, figure 3d. A 3D digitised plot representing the scan lines is represented in figure 3e and the final casting is shown in figure 3f.
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Fig. 3 : a) 3D-CAD b) STL file c) RP-SL patterns d) 3D digitising e) Point cloud with splines f) Casting.
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To estimate the capabilities of different Rapid Prototyping processes, in the manufacture of sand casting patterns, three pairs of RP-models (SLA / SLS / LOM) have been measured at LMP. After measurements the dimensional analysis have allowed to plot the Error Distribution Functions (EDF) for each RP-model, figure 4. For each pair of RP-models have been registered 118 measurements per bin. Upon analysis of each EDF histogram it’s recognised that for the RP-models the X-Y-Z dimensions peaks within the tolerance ±0.1mm with frequencies of: 75% for RP-SLA; 70% for RP-LOM ; 25% for RP-SLS. The Cumulative Error Distribution (CED) for each RP model measured, i.e., the normalised integral of the EDF from zero error to infinity has been also calculated for a graphical representation. A CED plot of the percentage of measurements falling within a given error versus the magnitude of the error (steps=0.1mm) is shown at figure 5.
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Deviation of models from nominal dimensions (mm)
Fig. 5 : CED histograms for the RP-models SLA / LOM / SLS. From the CED it is possible to choice practical tolerance levels as well as realistic accuracy expectations to stay within the 3D-CAD specifications. Confidence limits for a given specification could be obtained by the value ∈(90) (23), i.e., the value within which 90% of all dimensions measured fall. The ∈(90) tolerance level for each RP-model is within: RP-SLA recorded ∈(90) values of ±0.25mm; RP-LOM recorded ∈(90) values of ±0.20mm ; and the RP-SLS recorded ∈(90) values of ±0.75mm .
3.2 Case study 2 – 3D digitising a casting part to make RP-SL/SLS When a casting part (figure 6a) must be reengineered one could utilize the 3D digitising technique (figure 6b) to capture the 3D-shape of the part. Then for Rapid Prototyping technology based upon Reverse Engineering the reworked 3D-CAD model (figure 6d) can be converted in triangulated mesh (ex:*.stl, figure 6e) and sent to different RP machines to create a re-engineered physical Rapid Prototyping model, as the examples RP-SL and RP-SLS shown in figure 6f.
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Fig. 6 : a) Casting b) 3D digitising c) Splines from scanning d) CAD 3D from RE e) STL format f) RP-SL/SLS prototypes.
3.3 Case study 3 – Non-contact metrology of core-box made by RP-LOM The non-contact metrology could be made with 3D digitising apparatus that have dedicated software for data processing, figure 7e. The data translation with replication software Ri-tools allows manipulating the digitised point data from the 3D-scanning. Manipulation features in 2D by sections provide a range of point that makes it
easier to measure usable point data. Teaming up replication Ri-tools software lets one construct 3D-splines from point data, wireframe triangular mesh models from splines, and finally surface models, as shown in figure 7f. The image processing utilises accurate numerical methods, based on geometric triangulation to calculate coordinates and the advanced software CopyCAD transform this data on the surface of models, figure 7g. Capabilities in the replication software let one define cutting planes that pass through the object’s surface and capture 3D co-ordinates that fall on the intersection of two planes and detect maximum and minimum points, as reference surface points, allowing Quality Control. To look for inaccuracies the shape of prototypes was checked by comparing the physical model digitised by 3D-scanning against the corresponding 3D-CAD reference points, figure 7h. The benefits of this technique is the immediate visualisation and identification of shape errors in a shorter time when comparing with conventional CMM metrology techniques. The background at LMP and made labours have permit to correct shape inaccuracies of the physical LOM core-box by transforming it in a rebuilt 3D-CAD via reverse engineering, and the time spanned was only four days. The 3D scanning of the two core-box parts (drag and cope) for digitising collected data took two days, leaving the rest of the time for the core-box re-modelling via reverse engineering with 3D-CAD.
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Fig. 7 : a-b) Study of a casting (STL) c) ½ core box (STL) d) RT-LOM core box e) 3D digitising f) Points/lines/meshs/ surfaces g) Reverse engineering h) Deviation 3D digitising versus 3D-CAD i) Sand core made in the RP-LOM core-box. For Rapid Tooling technology based upon reverse engineering the reworked numeric model can be converted (ex: RP-LOM or SLS plastic or sand) and sent to different RP machines to create a re-engineering physical pattern or RT model. 3.3 Case study 4 – 3D digitizing of RP-SLS for Reverse Engineering The digitising of 3D shapes, as the foundry patterns made by RP-SLS (figure 8a-b), allows getting surface data and representing it as a range image, as visualised at figure 8d. Accurate numerical methods, based on geometric triangulation, calculate co-ordinates and advanced software transform this on the surface of models.
The captured data was inspected using planar 2D viewing profiles, from Ri-tools software, and in almost all zones was been found a high metrological quality. The link that connects digitised 3D models to physical models for manufacturing comes from Reverse Engineering process in which a 3D scanner digitises points from the surface of a physical model and specialised software transform that into CAD surfaces or solids. At LMP the CopyCAD Reverse Engineering software is used for a range of Reverse Engineering operations. Such operations allow creating 3D-CAD models from physical models for analysis and engineering operations The CopyCAD have a complete control over the selection of boundaries to optimise surfaces for subsequent modelling and have also the option to maintain tangent continuity between adjacent Bézier curves, or to preserve sharp discontinuities between surface edges. An error map displays variations between the original data and the surfaces produced in terms of the specified tolerances. For Rapid Prototyping technology based upon Reverse Engineering (figure 8e-h) the reworked 3D-CAD model can be converted in triangulated mesh (ex:*.stl) and sent to different RP machines to create a reengineered RP-SLS casting patterns to make castings, as the one shown in figure 8i.
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Fig. 8 : a) STL file b) RP-SLS pattern c) 3D digitising d) Range image e-h) Reverse Engineering surfaces i) Casting.
4. CONCLUSIONS • • • •
The Reverse Engineering methodology aided by 3D digitising make available a faster shape metrology control of prototype tools before manufacturing processes. The Reverse Engineering methodology starting from 3D digitising of physical parts allows to rebuild promptly physical models and to manufacture faster Rapid Prototypes and/or Tools. The Reverse Engineering via 3D digitising it is a potential methodology to make Virtual Prototyping (VP) models for computer simulation. The computer simulation analysis grant optimised shapes to manufacture improved Rapid Tooling. The integration of Reverse Engineering with Rapid Tooling technology reduces lead-time and associated costs. 5. AKNOWLEDGMENTS
The authors gratefully acknowledge the funding of “Programa Operacional Ciência, Tecnologia e Inovação” (POCTI) from the “Quadro Comunitário de Apoio III”.
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