سیستم برنامه ریزی عملیات سازگار STEP برای CNC تبدیل عملیات

Over the last 50 years, there have been many significant enhancements in computer aided systems which have influenced the CNC technology. One area that can be considered as a bottleneck to these CNC enhancements, and in particular to interoperability in CNC manufacturing is G&M part programming (ISO 6983). To overcome this bottleneck, the new standard ISO 14649, known as STEP-NC, is being developed to provide detailed information on component design, process planning and machining strategies to manufacture parts for the next generation of intelligent CNCs. This standard forms the basis of a new paradigm shift in the CNC domain to support digital modelling of CNC manufacturing resources. The research in this paper aims to identify major issues and develop new software tools to demonstrate the feasibility of interoperable CNC manufacturing based on STEP-NC. Besides the literature review on recent research and development on STEP-NC, this paper proposes a Process Planning System (PPS) with surface roughness chosen as the process planning objective. PPS consists of five modules: program reader, process planner, STEP-NC CAD viewer, STEP-NC CAM viewer and program writer. The reader is responsible for interpreting the geometry and the manufacturing data from a STEP-NC text file into a stored data list. The process planner uses this data list and enables users to evaluate surface roughness based on a mathematical model. Through the STEP-NC CAD viewer, the part geometry can be shown and via the STEP-NC CAM viewer the toolpath can be verified. Finally, the writer converts the stored STEP-NC data of the system into an updated STEP-NC file. An example case study component is used to demonstrate the PPS and show the interfacing of the STEP-NC data.

From its emergence in 1952, the Numerical Control (NC) machine tool has undergone significant improvements and has provided an ever-increasing important part in manufacturing. Many other relating technologies, including Computer Aided Design (CAD), Computer Aided manufacturing (CAM), Computer Aided Process Planning (CAPP), have advanced greatly coupling with the enhancement of computer technology. The term used to refer to an NC machine has evolved into Computer Numerically Controlled (CNC) machine, while the capability has been upgraded to support multi process, large volume range and high precision and geometrical complexity component production. It is possible to machine a complicated part involving different processes such as milling, turning and laser hardening on one CNC machining centre in a single setup, which is also contributing to the efficiency and machining quality like precision and surface roughness. Compared with these machine tool and process developments however, the early NC machines and today’s modern CNCs utilise the same standard for programming, namely G&M codes formalised as ISO 6983. This old fashioned programming language is famous for its low band-width information transferring ability, as it just describes the machine switch functions and the cutting tool movements. The manufacturing knowledge from the previous CAx (CAD/CAM/CAPP) systems has to be transformed into a set of low level machine tool actions by a post-processor, which isolates the CNC from the manufacturing chain. As a result of this single direction flow means, any information on the shopfloor level cannot be relayed back to the planning department. On the other hand, the increasing computing capability enables the CNC controller to manipulate more manufacturing information. However, only with the availability of comprehensive manufacturing knowledge the next-generation CNCs can realise the dream of intelligent control. Generally, it is commonly recognised that the ISO 6983 standard has become a bottleneck for the advancement of CNC manufacturing because of the data noncompliance through the CAD/CAPP/CAM/CNC chain [1]. To eliminate this problem, a new standard known as STEP-NC has been developed since the late 1990s, which is formalized as an ISO 14649 [2]. As the replacing data interface for CNC, one of the expected benefits from STEP-NC is bringing the component geometry information into the controller. However, STEP-NC goes much further with a comprehensive data model that overcomes the lack of process planning information in ISO 6983 files [3]. Unlike the G&M codes, STEP-NC associates the machining objectives (CAD design data) with solutions (CAM process data required) in an object-oriented way. It does not need to define precisely the detailed axis movement of the machine tool, although it has the mechanism to incorporate it in the STEP-NC file. The aim of an STEP-NC is to provide the CNC with a comprehensive manufacturing data model and an interface to establish an intelligent controller. Furthermore, the new data interface is compliant with an STEP (ISO 10303), which is the major ISO standard for product information exchange throughout the product lifecycle. Through incorporating STEP data, STEP-NC builds up a bi-directional information highway between the CAD/CAM and CNC systems, without using the post-processor [4] and makes interoperable process planning and manufacturing feasible and a future reality [5]. STEP-NC provides a real opportunity and challenge to promote the improvement of manufacturing capability utilising high level and detailed information. As the potential interface for CNC, STEP-NC is still an evolving and improving international standard. It is expected that the implementation of STEP-NC will bring great changes to current CAD, CAM and CNC. These changes will provide industrial users and software vendors with new challenges to identify the new boundaries of current CAx systems [6]. For example, the STEP-NC compliant CNC will incorporate a CAM system which will take over some of the tasks from the offline CAM system, namely to adjust and optimise the STEP-NC programs, according to the available manufacturing resources and implement greater intelligence across the CAx process chain. Thus, both offline and shopfloor (CNC) CAM systems will play a critical role for STEP-NC to be a success and gain any significant advantage of an STEP-NC compliant CNC manufacture. To date, the majority of STEP-NC research has focused on milling operations. In this research, a Process Planning System (PPS) for the next generation controller for turning is proposed. This paper introduces the PPS and outlines the mathematical model on which process planning is carried out. Finally, an example component is used to demonstrate the application of the system.

This paper has proposed and demonstrated the PPS process planning system based on an STEP-NC, which can serve as the interpreter and process planner for the whole architecture of the STEP compliant NC controller. The major conclusions of this work are: i) STEP-NC data can be used for generating a standardized data for generic process planning and toolpath generation for turning operations. ii) With the incorporation of the surface roughness module, PPS can be integrated with STEP-NC data to generate process plans to manufacture a part with a prediction of surface quality. iii) This research has investigated the opportunity of process optimisation utilising the manufacturing knowledge contained in STEP-NC. With the more sufficient information in STEP-NC, the shop-floor is entitled more privileges to optimise the manufacturing variables towards intelligent manufacturing. This research illustrates the potential that a STEP-NC data model provides the basis for standardized process planning and CNC machining.