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Within the past five years, machine tools and the operations they perform have continually become more sophisticated along with the parts they machine. Instead of machining many simple parts to be riveted or fastened later, five-axis machines now cut "unitized" parts that combine many parts into one.

"Some of these aerospace structures that used to be done in a half dozen or a dozen pieces are all being cut in a single piece," says Bill Hasenjaeger, product marketing manager for CGTech, Irvine, CA. That means more complex metal cutting, and machining simulation has followed suit. The technology has expanded beyond just simulating cutting toolpaths to avoid collisions and other errors, to simulating entire machines (Figures 1, 2).

It also has pushed accuracy. The goal: What happens on the computer screen should be exactly what happens on the shop floor. This is particularly valuable for the complex machining applications in the aerospace industry, where individual parts can reach into the tens to even hundreds of thousands of dollars. In these applications, "you can't just throw a test part away," says Rajas Sukthankar, marketing and business development manager for the Machine Tool Business of Siemens Energy & Automation, Elk Grove Village, IL.

Machine shops want the first part be a good part—always—all the while reducing time it takes to get that part from the designer's desk to the shop floor. They're doing this by streamlining inspection, increasing metal removal rates and attempting to eliminate all inaccuracies from a simulation.

USING EMPIRICAL EVIDENCE

Applications can vary tremendously from shop to shop and machine to machine. For this reason, simulation software relies on test results taken from actual cuts on the shop floor. "We go out to the end-user and ask, 'What do you use to cut this?' Then we take that empirical knowledge and plug that into [CGTech's simulation package] VERICUT, then spend some time on-site performing testing, until we build up a library of cutting tools they use," Hasenjaeger says.

Though the process can increase efficiencies on the shop floor many times over, it isn't error-proof. Occasionally, the simulation can be slightly different than what happens in the real world, particularly with five-axis work. The culprit has usually been the control emulation software used to detect idiosyncrasies behind machine kinematics. "Generally, when we create a control emulation for a new control, we don't support 100 percent of the control," Hasenjaeger says. "For most, this isn't a problem since we've been doing control emulation for many years, and we continually enhance the control models." But at times, a simulation may encounter a feature on a control that has not been emulated yet, "so we have to go back, learn it and build the simulation support."

He uses the Siemens Sinumerik 840D as an example. The control has fairly unique five-axis interpolation methods—one built on a cone, another on a sphere, and so on. "From a control emulation point of view, we have to do a lot of research and software development to support that simulation accurately," he says.

NC programs and the G codes they entail are in essence a set of destination points—and lack information on what happens between those points. "Imagine a linear cut where the orientation of the tool doesn't change; you just move 5 inches to the right," Hasenjaeger explains. "There's not a lot of question about what happens."

But add more axes of motion and the story changes. The tool may tilt backward 5 degrees, then forward by the same amount, or the table may tilt. On most machine tools, the specific behavior of motion between points depends on the kinematics of the machine. On a very detailed level, sources say, such differences in tool interpolation can show on the finished product.

GETTING THE SOURCE CODE

Enter the virtual NC kernel. Siemens has been working with several software vendors, including CGTech, to integrate its NC kernel into machining simulation itself.

The kernel "emulates nothing more than the actual moves of the machine in real-time," says Sukthankar. "It gives you a very good depiction of what the tool tip is really going to do on the surface."

The virtual NC kernel is open code, which means algorithms that account for the kinematic transformation of five axes, "or any esoteric kinematic transfer of the machine," can be built directly into the system, he says. "Before, you were really working with an estimation." Now, what you see on the PC workstation is what you get on the shop floor.

"Any time you're talking about a five-axis contouring profile, you'll see a difference [using the virtual NC kernel], especially when you're using a kinematic transformation to do the parts cutting," Sukthankar says. Hasenjaeger adds that the technology makes his company's job a tad easier. "Siemens is handing us the process-digested NC program in the form of axis positions," he says. "You get 100-percent coverage of all control features, whereas with control emulation, [the simulation] may support everything you're doing, or it may not."

IN-PROCESS INSPECTION

Machining simulation has made great strides to streamline in-process inspection, particularly of aircraft parts, where traceability requirements can take up a lot of man-hours. Most only have a CAD model of the finished piece, and some have one of the beginning material, yet not many have anything in between. Lacking is a precise picture of what the part should look like in-process. This means that, often, such inspections are prone to error.

That can change by using an inspection module within the simulation program. "The inspection module allows you to use a simulated cut piece to serve as the 'master part,' so to speak, a part that's dimensionally correct per the NC program," explains Hasenjaeger. That part allows users to choose specific critical features to inspect, be it a 10-mm hole or a 5-mm-deep pocket. It streamlines in-process inspection procedures and, ideally, makes them more reliable and accurate. The technology is currently being used by at least one company machining complex parts for Airbus, including bulkheads and wing ribs.