From ornamental strains to monumental gains

Key, Jolyn. "From ornamental strainsto munumental gains." Cutting Tool Engineering, April 2008, pp 88-89.

For a small Michigan job shop, a technology change provided productivity gains while achieving comparable quality when making prototype and production injection molds.

Granby Mold Inc., Walled Lake Mich., has been building injection molds for automotive and nonautomotive applications since the 1970s. One niche market is molds for automotive chrome-plated emblem components, and the company has created a handful of molds for Lincoln ornaments of various sizes.

After some lean times in the auto market, Granby Mold decided it was time to upgrade its capabilities and diversify its opportunities. The moldmaker hadn't purchased any new CNC equipment since the 1990s, and John Turcotte, Granby Mold's owner and president, felt the company could not excel in this industry without upping productivity. The result was the purchase of an Okuma MB-56V VMC with THINC-OSP (the intelligent numerical control-Okuma sampling path) control. (Okuma America Corp. is based in Charlotte, N.C.)

"Before the MB-56V, we made an electrode to burn the whole cavity of the Lincoln ornament mold," said Turcotte. "We did that to achieve the fine detail required and found it was not even worth going in there with the CNC mills we had been using. However, after getting the new high-speed machining center and seeing the incredible results on other molds, we decided to cut the entire cavity and core without any EDMing."

Eliminating EDMing saved time while achieving high-quality output. The company's polisher noted that the core did not need polishing, but it was easier to polish the milled cavity to a mirror finish than the EDMed one.

Made from P-20 steel, the ornamental mold was cut with a 1mm ballnose endmill at 15,000 rpm with step-overs as small as 0.0003" on finish passes. According to Turcotte, machine tool distributor Gosiger 3D, Plymouth, Mich., gave invaluable support in recommending endmills, speeds, feeds, and programming and tool measurement techniques. "One of the difficulties in machining cavities is to blend corners that are cut using multiple-sized endmills," he said. "Gosiger 3D taught our employees how to utilize a Blum laser to achieve higher accuracy and near perfect cutter-to-cutter blends."

The laser-based measuring equipment was programmed to automatically set the correct tool length, check for any cutter wear or damage and alert the machine and operator of potential problems with a cutter before changing to the next needed one. "We start with a big cutter (4mm) and work down to the smallest size (.02") we need," Turcotte said. "The cutters have to be pretty small to achieve the amount of fine detail in an ornament cavity. We also achieve the same results in hard steels. We recently hard milled two 50- to 52-HRC stainless steel molds and three 48- to 52-HRC S-7 steel molds with great results."

In the case of the Lincoln ornament mold, the cavity and core took approximately 12 hours to machine. The .02" cutter machined about 15% of that time, and did the work that used to be done with EDM. If EDM were used instead, electrodes would have taken at least 12 hours to machine, and 3 more hours to perform the actual burn.

After cutting the cavity and core details, the operator removes the workpieces and checks the spot, which is the fit of the two sides—cavity to core. No spotting work was required on the Lincoln mold.

Paramount to Granby Mold's team was the reliability of the THINC-OSP controller. "We don't have the luxury of testing our programs," Turcotte said. "We just download them into the control, start cutting, turn off the lights and go home. Machining hundreds of details over the past 2 years, we use lights-out machining to compete. The control always does what we program it to do."

Besides satisfying the customer, Granby Mold reduced its costs by about 30 percent by switching from EDMing to high-speed machining.

In addition to automotive molds, Granby Mold designs and manufactures thermoform tooling, rubber molds, stamping die details, and thermoset injection, transfer and compression molds. Employing the VMC opened new opportunities by adding hard milling, prototype mock-up capability, special fixture work and new product design and development to the repertoire of Turcotte's five-man team. He added, "We've built some challenging tools for some pretty tough customers who were really impressed with the difference in the quality of the final work we can now produce."

Caption: Lincoln Ornament

Captions: John Turcotte, owner and president of Granby Mold shop's Okuma MB-56V