With development of the much-publicized STEP standard for product data exchange predominantly CAD-centric, it's not surprising that many engineers think STEP is just another way to transfer drawing information between customers and suppliers. However, STEP is actually much more than an advanced part geometry data exchange format. Its extension into numerical control-the STEP-NC input standard for CNC machines-provides not only a full description of the part, but the manufacturing process as well-annotating CAD design data with manufacturing information about the stock, its cutting characteristics, and tool requirements.
"STEP-NC defines data representing working steps, a library of specific machining operations performed at the CNC, so that any controller will be able to calculate the tool path based on definitions contained in formatted routines integrated within the controller itself," notes Sal Spada, research director for discrete automation at ARC Advisory Group (www.arcweb.com). "STEP-NC-compliant CNCs will result in a consistently made part independent of machine tool geometry."
Though Siemens has a prototype in development, CNC vendors don't yet offer STEP-compliant controllers. However, most controller suppliers are watching the standard's progress carefully, and waiting for users to drive the demand. Consequently, today's STEP-NC implementations rely on PC-based controllers and SoftCNC technology with STEP compilers integrated into the CAM system. In fact, there's just such an implementation program going on at STEP Tools Inc., a supplier of professional STEP software toolsets.
According to STEP Tools President Martin Hardwick, General Dynamics Land Systems organized a "Super Model" project with a STEP-NC controller comprised of an MDSI soft controller running on a PC-based NT platform that controlled a 3-axis Bridgeport mill. A STEP-NC interpreter along with VirtualGibbs CAM software was also installed to read prepared STEP-NC data. Now Boeing Aircraft Co., NASA JPL, and NIST are all involved in the project. And teams of other manufacturers, CAD/CAM software developers, and machine tool OEMs are also involved with additional Super Model projects.
For example, the OMAC (Open Modular Architecture Controls) STEP-NC working group includes personnel from the previously mentioned companies, plus others such as DaimlerChrysler, General Motors, and Pratt & Whitney. The group recently reported that the development and introduction of ISO standard 14649 advances STEP-NC significantly on the manufacturing side to overcome a weak link in the CAM process: the NC program. "By defining a two-way interface between CAM process planning systems and NC control systems," says ARC Senior Analyst Dick Slansky, "the loop from the machine tool back to the production machine planning process is finally closed."
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This means engineering can send machine-readable information to manufacturing instead of drawings, making the design process more efficient and allowing machinery building OEMs to more easily outsource machine components. For manufacturing, STEP-NC means a control can dynamically check the safety and optimize the performance of a part program for the tooling available at run time to streamline the CAD/CAM/CNC process chain. In fact, STEP Tools estimates that STEP-NC can cut machining time by 50%, reduce cutter path planning by 35%, and provide a 75% decrease in machine-process planning time based on its Super Model Project data.
Old standards don't cut it. STEP-NC, or AP-238, makes the M- and G-code link between CAM software and the NC transparent. It also alleviates one of the most annoying features of today's manufacturing environment-post processors. Today's systems use ISO 6983 global or RS 274D specification in the U.S. to tell a machine how to make a part. However, each type of CNC typically requires a dedicated post processor for correct execution of the code that instructs the machine how to move the cutting tool.
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AP-238 files can be made either using a process planning system or directly with an integrated CAD/CAM system with and AP-238 interface that supports manufacturing features. |
Because CNC vendors define certain M- and G-codes differently using customized codes, this post processed cutter path is typically unique to the motion attributes of each machine tool. Consequently, most process information today resides in the CAD/CAM system or supports on-line systems, and the CNC for the machine tool generally doesn't have access to it.
As a result, most of the production process function must be realized during the production-planning phase because they are not available on the machine tool at runtime. "Part programming based on this standard is fraught with bottlenecks and workarounds, and often forces the NC programmer to redesign the part in order to produce it," according to ARC's Slansky.
Rather than dealing with data conversions, reprocessing of incomplete format translations, and other misinterpreted information, STEP-NC lets the operator launch a Web browser to select a part from a database accessed on the Internet or an intranet. Once the workpiece's 3D image appears the operator opens a dialog box, checks a few parameters and default settings, then starts the machining cycle.
STEP-NC-compliant controllers. It will be a while before this scenario depicts how most shops will be running their tools. After all, the largest benefactors of such data exchange will be discrete manufacturing enterprises with far-flung facilities across the globe, plus manufacturers that rely on equipment from many different suppliers. That is because most CNC vendors are unwilling to commit to developing STEP-NC-compliant controllers until customers start asking for it.
Siemens is the only CNC vendor Hardwick of STEP Tools knows of that currently has a STEP-NC-compliant controller in development, but he expects others to get more involved soon. According to Rajas Sukthankar, project manager at Siemens Motion Control Systems, the company has a prototype working in a milling application. "The lofty goal behind STEP-NC is to describe more workpieces independent of geometry and technology so that you can run these programs on any milling machine or turning machine independent of the controller type," Sukthankar explains. "STEP-enabled controllers will bridge the gap between CAM vendors and different CNC controllers, going well beyond G-codes to directly address the issues of defining work piece geometry independent of the control directing its production.
Though Sukthankar admits that STEP-NC represents a revolutionary change with far-reaching implications for products and processes throughout manufacturing, he says, "it is not, at least today, universally accepted or clearly demonstrated that the technology will actually reduce manufacturing costs." Nonetheless, STEP is moving ahead. In fact, STEP Tools recently released the results of a pivotal meeting held in late May. The conference consisted of industry leading firms that will be first to implement STEP-NC technology into manufacturing operations.
The implementation programs will be executed in systematic phases within each company's production operation and across its supply chain.