George Vosniakos*. Department of ... 1. Introduction. The topic of process planning with its associated links ..... used to calculate the minimum corner radius of a.
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PII:SO951-5240(98)00009-3
An intelligent software system for the automatic generation of NC programs from wireframe models of 242D mechanical parts George Vosniakos* Department of Mechanical Engineering, Manufacturing Division, National Technical lJniver.@ of Athens, Pat&ion 42, GR 1.57 80Athens, Greece
The aim of this work is to generate automatically NC part programs for machined components of 2-l/2 dimensions starting from a wireframe CAD model. An IGES post-processor was developed to interface the system to any CAD system, then a conversion program sought to obtain solid modelling geometry from the wireframe model. Subsequently, drawing annotation information was allocated to solid model entities through pattern matching techniques in Prolog. The product model was completed by deriving feature instances according to pre-defined feature templates constructed in Prolog according to a simple model. Operation selection uses the feature model as well as process planning knowledge structured into a hierarchy of Prolog rules processed by a context sensitive inference engine. The resulting operation sequence is communicated to a module that calculates cutting conditions and produces NC code. The system was proved to work satisfactorily with components of an industrial standard. 0 1998 Elsevier Science Ltd. All rights reserved Keywords: process programming
planning,
feature
recognition,
CAD-CAM
1. Introduction
knowledge
bases, NC
at least as far as generative process planning is concerned. Variant process planning is still the dominant type in industry and its automation via software represents a lower challenge than that of the generative type. However, construction of design and NC programming interfaces of process planning systems of either type is of comparable difficulty. Research into the integration of design and manufacture proposed new product modellers handling not only geometry, but also manufacturerelated information. Product modelling, interactive process planning, NC part programming and even machining simulation have been integrated in the same environment. Although such approaches were demonstrated to work satisfactorily, they were still heavily dependent on the human user, like, for example, the system described in Ref. [2]. In most cases the process planning system requires an accurate representation of shape, i.e. a solid
The topic of process planning with its associated links upstream to design and downstream to NC programming has been most popular in the Manufacturing Engineering research community. Many prototype software systems were presented over the last 10 years’, the majority of them addressing system philosophy and architecture issues, whereas other published work dealt with the individual technical problems in this domain, notably representation of feature recognition, tool path process plans, algorithms, cutting tool selection etc. The topic is not at the peak of its popularity any more, in spite of or, perhaps, because of the obvious failure of commercially available software to adopt to any significant extent academically devised solutions, *Tel: +30-l-77-21-457;
integration,
Fax: +30-l-77-21-197.
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Intelligent software systems: G Vosniakos
model of some type. Some systems use features directly built into the part designed3. Among those systems a few constrain the designer to think in terms of combinations of manufacturing features, each of which has direct correspondence to one or a concrete set of operations on a machine4. CAD data, in the form of shape features, is believed to be directly usable by process planning systems despite some still unresolved issues such as how to handle unforeseen features, how to map design to manufacturing features and how to deal with overlapping features or more generically with change of designer’s intent’. Feature recognition can be used in parallel to feature modelling to cover some of those problems, a hybrid solution being acknowledged as the best one, but rarely having been really implemented in practice. Software-based automation of process planning refers to three aspects: Derivation and communication to the process planning module of the necessary shape regions to be processed, including related designer’s intent. Streamlining of the process planning tasks and elimination of the need for interaction in decision making (operation sequence, tools, processing conditions). Derivation of NC CL data and code in direct correspondence to each operation of the process plan. An account of most trends in the above directions in Design for Manufacture through Computer Aided Process Planning and the use of Features is given in Refs [l] and [6]. These trends and partial achievements can now be seen in the light of Concurrent Engineering. Concurrent Engineering encompasses two trends in modern manufacturing: adding value to the product, not only by material processing, but also by information processing, and taking into account the whole life cycle of the product as early as possible during design. The organisational approach to Concurrent Engineering is based on cross-disciplinary teams and on communication enablers such as data exchange, shared databases, change propagation methods etc. The technological approach is based on product modellers, analysis and simulation tools, enablers being their interfaces or integrated data structures. As far as the technological side is concerned, the research trend is in Concurrent Engineering Toolkits and the integration/sharing of data and knowledge, because most of the individual components/tools have been known for some time7. However, it is still not clear what kind of interactions/transactions should take place, between which disciplines, and how often in product development stage this has to happen; these also depend very much on the type of manufacturing system under consideration. This paper represents an attempt at such data sharing/integration in the Design for Manufacturing side of Concurrent Engineering. The development of
the system presented was triggered by a wish to automate an existing interactive process planning system of the variant type, known as ICAPP, equipped with an IGES 3.0 interface to design wireframe geometry and feeding a semi-automatic NC programming systems,9. The two parts of that system which had reached an acceptable level of automation were the calculation of cutting conditions module and the derivation of NC code module for typical machining centre operations. These were the parts that were retained and integrated into the software developed. Next, the overall system architecture will be described, followed by selective individual element description.
2. System architecture The structure of the system is shown in Figure 1. The system, rather uncommonly, interfaces to wireframe CAD. The reason is the popularity of wireframe design in industry despite its obvious progressive withdrawal in favour of solid modelling. Nevertheless, the system can also interface to solid modelling systems, because all geometry is converted and internally represented in a simplified solid modelling data structure. The wireframe interface is materialised via IGES, the neutral format data exchange standard, which enables access to virtually any wireframe CAD system. In view of the significance of engineering drawing annotation entities for process planning such information is processed, too. Since solid modelling entities are too low level for reasoning with at the process planning stage, their further abstraction was necessary so that shape features were defined. Therefore, the planning software takes as input geometry and annotation data at a relatively high level of abstraction. Planning of operations (type and sequence) has to be empirical, because of the very nature of this process. As such, the best way to implement this part was an expert system approach. The resulting data, i.e. a process plan without processing condition details can then be passed to the purely algorithmic condition and tool selection part and then to the NC code generator. Although there may be revisions of the original choice of operations and of their sequence, e.g. should certain tools be unavailable, the premise is that the choices made are not exotic but common enough to withstand modification. As a result there are no automatic feedback loops between modules of the system, see Figure 1. Modules run in tandem mode and the user is informed on the state of execution and is given warning messages about errors encountered. It is at the user’s discretion to stop the system and relieve a problem manually or to let it terminate normally should that be possible. It was also the philosophy pursued to give the user flexibility in defining possible shapes to process and possible ways to process these shapes in the intended environment. This was enabled by the rule based solution where the actual body of the knowledge for
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Intelligentsoftiare systems: G‘ Vosniakos
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