INTELLIGENT PROCESS PLANNING FOR RAPID PROTOTYPING

Abstract

Rapid prototyping (RP) is a new manufacturing technique compared with the conventional methods. This is because RP processes build parts by adding instead of removing material, although the parts can be made using the traditional ways such as NC milling or hand carving. By accelerating the design process and speeding up tooling development, RP technology helps designers cut down product development time and cost. While RP technology has advanced significantly in the past few years, it has also set a number of new requirements for process planning. Process planning is a highly complex and intuitive process. The quality of process planning is highly dependent on heuristic knowledge and craftsmanship. Similar to process planning being the link between design and manufacturing, computer-aided process planning (CAPP) can serve as a bridge between computer-aided design (CAD) and computer-aided manufacturing (CAM). The contents of CAPP for stereo lithography (SL) include the determination of part building orientation, creation of support structure, selection of process parameters, etc. The work presented in this thesis is an attempt to introduce a complete CAPP system for SL systematically. Emphasis is placed on determining the optimal part-building orientation, one of the modules of the CAPP system. In rapid prototyping, such as SLA and fused deposition modelling (FDM), the orientation of the part during fabrication is critical as it can affect part accuracy, reduce the production time, and minimise the requirement for supports and thus the cost of building the model. This thesis presents a multi-objective approach for determining the optimal part-building orientation. Different objectives such as part accuracy, building time, etc. are considered. Objective functions have been developed based on known sources of errors affecting part accuracy and the requirements of good orientations during the building of a model. The objective functions employ weights assigned to the various surface types affecting part accuracy. The primary objective is to attain the specified accuracy achievable with the process. The secondary objective is to minimise the building time. New concept of product design by rapid prototyping processes is also studied and some part design considerations for rapid prototyping are summarized. The algorithm of variable layer thickness has been developed and implemented. The number of layers of a part is used to estimate the building time. The possibility of directly slicing CAD data of the model instead of using the STL file of the model is also explored. The interface between the CAD system and RP processes was built on Pro/Engineer. The cross-sectional data of the model is saved in a data file which can be sent to the solid creation system (SCS) for fabrication. The system was implemented on a Hewlett-Packard series 712 workstation and Silicon Graphics (HL7963KW-SG) workstation. The system uses Pro/Engineer as a platform, and Pro/Develop as an interface to access the CAD database directly. Finally, examples are given to illustrate the algorithm for deriving the optimal orientation which can assure better part quality and higher building efficiency.