Cutting Tool Engineering
May 2010 / Volume 62 / Issue 5

Racking up ribs

By CTE Staff

Prospect Mold Inc., Cuyahoga Falls, Ohio, makes compression and injection molds, dies, fixtures and jigs.

Moldmaking is one of the industries most acutely affected by offshore competition. “We are definitely competing in a global market,” said Duane Shroyer, the company’s vice president of manufacturing. “We have to do what we can to stay competitive.”

In machining a mold for a truck’s interior panel, Prospect Mold applied advanced tools to maximize its competitiveness. The P-20 steel mold was about 80 " long × 45 " wide, and the mold cavity required metal removal to a depth of about 4 ". In addition to other contours and details, the cavity was crisscrossed with more than 750 linear inches of 0.090 "- to 0.100 "-wide × 0.400 "- to 0.900 "-deep grooves to create reinforcing ribs on the back of the molded plastic part. The groove sidewalls featured 2° of draft, or inclination, to allow the panel to release from the mold.

Courtesy of OSG

Prospect Mold replaced a three-step EDM process with one machining operation using rib-cutting endmills from OSG Tap & Die and reduced the time needed to machine narrow grooves in a large mold.

Prospect Mold roughed the cavity with indexable round inserts on an FPT Raid vertical machining center. The majority of the mold details were then cut with solid-carbide cutting tools as small as 1⁄8 " in diameter. Those initial machining operations consumed about 180 hours.

Then, to burn the narrow ribs, Prospect Mold moved the block to a sinker EDM. The EDM process included three steps: modeling the electrodes in CAD/CAM software, milling the graphite electrodes and then EDMing the ribs. For the truck panel mold, the EDM process consumed more than 300 hours. Moving the mold from the machining center and setting it up on the sinker EDM required additional time.

Prospect Mold looked for ways to save time and reduce the cost of the rib-cutting process. The shop investigated applying endmills from OSG Tap & Die Inc., Glendale Heights, Ill., engineered to cut narrow grooves. Gregg Townsend, OSG regional manager, said the toolmaker presents training seminars across the country discussing new machines, workpiece materials and tools. Among the topics is rib cutting because “it is very important and occurs quite frequently in moldmaking,” Townsend said.

The submicron-grain-carbide endmills for cutting ribs have a 5μm radius accuracy, according to OSG.

When Prospect Mold applied 2.0mm- and 2.5mm-dia. OSG Exocarb-SHP rib cutters with flute lengths of 16mm and 25mm to the panel mold, the time required to produce the rib grooves fell to just under 100 hours.

In addition to the time savings of more than 60 percent, using the cutters simplified the mold production process. “If you can eliminate three steps by doing one, instantly you are ahead of the game,” Shroyer said.

Another factor in the success of the job was that the endmills could be run at higher than a 300-ipm feed rate with acceptable tool life. Finally, because the machined surface didn’t have a recast layer from EDMing, polishing time was cut in half.

The we’ve-always-done-it-this-way reluctance many shops have towards change is “the opposite of the way we are,” Shroyer said. “We are always interested in trying something new.”

Shroyer added: “In this case, it paid off very well. Now our theory here is, if we can cut it, we are going to try. Burning is going to be a last resort.”

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