May 2008 Edition

CAD/CAM

‘Cutting’ Air is No Way to Make a Profit

A user-friendly interface helped a mold-maker machine electrodes in about half the time

Producing electrodes for electrical discharge machining – EDM – is challenging because the toughness of the material and the need for a fine finish requires many machining passes. The machine often wastes time cutting air over areas that were already cut to size on previous passes.

Modern Moulds and Tools Ltd., Lancing, West Sussex, UK, is familiar with this problem. The company is a leading producer of tooling for injection and blow molding, die-casting, and thermo-forming. It uses EDM to produce the cavities for most of these tools. Up to 50 electrodes are sometimes required to produce
a single mold cavity.

Previously, the company programmed the vertical machining centers used to produce these electrodes at the machine control using machining macros. The controller was not user-friendly, so it took a considerable amount of time to train operators to program the machines.

But, the biggest problem with this approach was that there was no way to determine where metal was left after each pass. Programmers had to move the tool over the entire part contour for each roughing, semi-finishing, and finishing cut.

Operators were also limited to two dimensions in setting the boundaries for the surfaces requiring machining. As a result, the boundaries often exceeded the area that needed to be cut by a wide margin. The machine spent so much time "cutting" air that it took 15 hours to machine a typical electrode.

Finding a More Efficient Process

"A lot of time was wasted during the machining operations," John Horspool, works director for Modern Moulds, said. "Machining electrodes is critical to our success so we looked for a way to do it more efficiently."

They discovered that Pro/Toolmaker, from Parametric Technology Corp., Needham, MA, provided far more productive methods to rough out electrodes and other parts. Its graphical user interface makes it much easier for operators to produce CNC programs.

"It only takes one day to train new operators to create their own programs," Horspool said. "The graphical user interface is intuitive to anyone that is familiar with metal cutting operations. Our training focuses on the specialized toolpaths and machining strategies that are provided by the software package."

Accurate Surface Boundaries Equal Reduced Air Cutting

The first step in producing a CNC program is importing the solid model geometry of the electrode. Importing the solid model eliminates the need to manually define the electrode surfaces and also ensures that the surfaces are trimmed to their exact size. This saves machining time by avoiding cutting air on the outer boundaries of the surfaces.

Modern Moulds operators next identify the surfaces to be machined and begin matching them up with machining operations. Pro/Toolmaker makes this process more productive by providing a wide range of high-speed machining strategies that can be used to optimize nearly any type of cut – exit and connections for roughing, rest-roughing, finishing, and rest milling, including pencil trace.

By using Pro/Toolmaker, Modern Moulds has developed a library of machining operations, each of which includes the cutting tool, machining speeds, feed rates, ramping, coolant, and air. These operations have been developed by the company’s most experienced operators to optimize the machining operations. The machining library helps spread best practices throughout the company and ensures that every machining function is performed at the ideal operating conditions. The company typically uses machine speeds of 1,600 rpm and feed rates of 8 meters per minute.

Modern Moulds produces electrodes, core, and cavity detail for mold tools by machining them with a series of progressively smaller end mill and indexable cutters. Normally, depending on the size of the part being machined, the first roughing pass would be programmed using a 25mm diameter by 5mm corner radius, and the second with a 15mm by 3.5mm. The last two passes are made with 12mm by 1mm and 6mm by 1mm coated solid carbide torodial cutters.

Machining of an electrode has gone from 15 hours to six hours with the use of Pro/Toolmaker

The rest-machining function determines what metal remains to be cut after each machining operation. Then, when it generates the next pass, it traverses to the areas that need to be machined and skips over all the others. This feature substantially reduces air cutting.

Realistic Simulation Avoids Crashes

After defining all of the operations required to produce the electrode, the program generates a realistic simulation of the complete machining operation. The simulation makes use of pre-existing models of the machine, tooling, and fixturing as part of the program so that possible clashes can be eliminated before the program is run on the machine. The simulation also helps operators identify areas for further productivity improvements.

Finally, when the operator is happy with the program, the operator uses it to generate ready-to-run G-code. Rest-machining, surface trimming, the machining library, and other features make the code so much more efficient that the typical electrode can now be machined in only six hours, 60 percent less time than was required in the past. Parametric Technology Corp.

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What do you think?
Will the information in this article increase efficiency or save time, money, or effort? Let us know by e-mail from our website at www.ModernApplicationsNews.com or e-mail the editor at pnofel@nelsonpub.com.