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
Up to 50 EDM electrodes are needed to produce a single mold cavity at Modern Moulds
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.
By using Pro/Toolmaker, Modern Moulds has reduced the time needed to machine cavity electrodes by 60 percent
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.