Developing an integrated STEP-compliant CNC prototype

STEP-compliant CNC is the next-generation CNC controller, taking the STEP-NC data model as the interface scheme between CAM and CNC and carrying out various intelligent functions. At the moment, efforts are being made worldwide to establish an international standard for the new interface scheme (so-called STEP-NC), formal-- ized as ISO 14649. In the near future, the new interface scheme will be completed and announced as the international standard.

Upon completion, the standard will replace the conventional scheme based on ISO 6983, so-called M&G codes. Further, the new interface scheme will impact the CAD-CAM-CNC process chain and the advancement of the CNC controller. This paper develops an integrated STEP-compliant CNC system (or STEP-CNC) based on the new interface scheme. The system is composed of five modules: (1) Shop Floor Programming System (PosSFP), (2) Tool Path Generator (PosTPG), (3) Tool Path Viewer (PosTPV), (4) Man Machine Interface (PosMMI), and (5) CNC Kernel (PosCNC). The developed system is a prototype but very comprehensive, including all the modules required for realizing "art-to-part" through the new CADCAM-CNC chain. Architecture and functional details are presented together with a realistic demonstration.

As the brain for industrial machinery, computer numerical control (CNC) is the core element in modern manufacturing systems. In spite of a great deal of technological achievement, contemporary CNC still needs further enhancement to overcome the existing drawbacks; that is, (1) it is basically an executing mechanism without intelligence, (2) it is based on low-level language [ISO 6983 (ISO 1982), so-called M&G codes], and (3) its architecture is vendor specific and black-box-styled proprietary without allowing user access.

Therefore, the next-generation CNC is required: (1) to use high-level language for seamless integration in the CAD-CAM-CNC chain, (2) to be multifunctional, intelligent, and autonomous, and (3) to have an open architecture based on modular/software implementation technology. These requirements should be accounted for in developing the next-generation CNC, for which researches are being undertaken in various aspects. In particular, a new interface scheme between CAM and CNC, often called STEP-NC, is under active progress by ISO TC 184 SCI and SC4 (ISO 2000).

The new scheme is based on the internationally standardized product model (STEP: STandard for the Exchange of Product model data), formalized as ISO 10303, as well as the process plan information. As described in the next section, ISO 14649 specifies information contents and semantics (ICS) for various CNC manufacturing processes. It is currently available in FDIS (Final Draft International Standard) version, with its final version to be completed in the near future. Upon completion, it will become a new CNC language replacing ISO 6983. The impact of the new interface scheme can be visualized in many ways. As the new data model will be an information highway for e-manufacturing, encompassing CAD, CAM, and CNC, the `art-to-part' dream (Albert 2000) can be realized, thereby producing 3-D models in physical parts by CNC, like producing a hard copy from the printer. Significant gains are expected in the process chain of CAD, CAM, and CNC, as will be discussed in the next section. Furthermore, complete elimination of postprocessing is possible (Albert 2000). Soon, the new interface scheme will be used as a means for implementing the Internet's B2B activities, e-design, and e-manufacturing (Hardwick 2001a, Leyrich 2001, Albert 2001, Maniscalo 2001, Teresko 2001, Hardwick and Loffredo 2001, American Machinist 2000, Hardwick 2000).

From the perspective of CNC, the new data model is very significant, providing CNC with all the information about `what-to-make' (geometry) and `how-tomake' (process plan) with its machine tools. In other words, depending on how the new data model is implemented, CNC would be able to incorporate various intelligent functions, which is not feasible in the conventional control based on ISO 6983. Thus, as the new language is under establishment, increasing attention has been paid to the development of new CNC based on the new interface between CAM and CNC. Major developments include Super Model in the United States and NC Prototype in Europe.

The U.S. Super Model, whose prototype was presented at the 2001 SC4 meeting held in San Francisco (Hardwick 2001b), places emphasis on the development of an `intelligent interface' between the ISO 14649 database and CNC via XML and is implemented on the commercial systems of FB Mach and Virtual Gibbs. The demonstration was actually done with a Bridgeport CNC via G-code conversion. The European prototype, NC Prototype, presented at the 2000 SC4 meeting held in Charleston, South Carolina (Glantschnig 2000), focuses on CNC execution based on the part program written in the ISO 14649 physical file. The STEP-NC was implemented on a Siemens 840D NC Kernel interfaced with the commercial systems of Open Mind and CATIA. Interface between ISO 14649 and CNC was made via Interpreter built in CNC.


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