Today, CAM programs typically produce CNC instructions by reading CAD data. Then through a graphical user interface, human interaction, and some automation, a machining process is established. This resulting instruction set for the CNC machine is termed generically G-Code, or machine control data. First developed in the 1950s, more recent revisions to the standard date to the 1980s.
“G-Code is a legacy of the paper tape days. It is a primitive description of a tool path” that does not address critical questions such as material removal rate or tolerances among others, said Fred Proctor, Group Leader of the Control Systems Group of the National Institutes of Standards (NIST). Because it is “dumb,” the machine cannot adapt to real-time changes in machine dynamics or tool wear. Portability is another issue. Proctor points out that today there is not just one G-Code standard but more than 5000 dialects adapted for each machine, requiring postprocessors to convert tool-path data for each machine.
Enter STEP NC. STEP is the Standard for the Exchange of Product model data, also termed the ISO 10303 for computer exchange of product data. The STEP standard has a number of application protocols (AP) of which AP238 (ISO 14649) is designated STEP NC. It includes the CAD design along with cutter information. Tool paths are now full three-dimensional bounded curves rather than a simple set of moves or machine axes motion. “The analogy I like to make is that G-Code is like plotter language for the old dot-matrix printers, where as STEP NC is like PostScript.”
The implications of successful implementation of STEP NC are twofold for a large aerospace OEM such as Boeing. First is the elimination of the complicated and redundant CAD to CAM to CNC software infrastructure. Portability is critical in this—not only from machine to machine but also from supplier to supplier. The second reason is that with more information available to the controller, including material properties, full GD&T, cutter information, and the CAD geometry itself, the controller could use that information to optimize the process.
Controllers that can read the data, and CAM programs that can output the data in AP238 format, are key to more widespread use of STEP NC. Even if today’s infrastructure is complex and not overly portable, parts are machined. It does work.
Fanuc CNC America is a company that supplies CNC controllers. Rick Shultz, the company’s Program Manager for Aerospace and Aerostructures, noted that “with today’s improvements in processing power, CAD/CAM, and CNC algorithms, the urgency to replace G-Code is reduced. If traditional G-Code is replaced in the near future, STEP NC would be a likely candidate.”
He also acknowledged that today’s built-up infrastructure means expensive-to-develop and -maintain postprocessing. There are benefits in upgrading CNC programming to a higher-order language. “Eliminating or minimizing the need for a traditional post can result in significant savings, especially when a different post is required for each machine and/or CNC in a facility,” he said.
He pointed to improved CAD/CAM and G-Code through advanced capabilities offered by the company. “For example, changing five-axis applications to support tool vector orientation definition makes the generated geometry—with minimal reformatting of the program—machine-independent.” Why? Because, he said, traditional posts directly program the rotary axes for the particular machine. “Directly addressing the rotary axes means the geometry is not portable to another machine without reposting. By changing the programming method to tool vector representation—sometimes referred to as {i,j,k} programming—the geometry portion of the part program is transferable with minimal modifications.”
Schultz noted that a prototype Fanuc CNC that accepts STEP NC is operating in a laboratory at Boeing. Martin Hardwick, CEO of STEP Tools, said two popular CAM packages, MasterCAM and GibbsCAM, already produce AP238 standard output. “More are needed for the standard to grow in acceptance, and more CNC vendors must join Fanuc in processing the data in real time on their controls.”
