Faster Milling of EDM Electrodes With Retrofit CNC Package

Dana Heckendorf of D. Heck Tool LLC (Thompson, Connecticut), a precision moldmaker working out of his garage, wanted to compete with bigger shops that had Computer Numerical Control (CNC) equipment. This led him to consider retrofitting his manual Kent milling machine with a CNC retrofit package. With a Mitutoyo MillStar CNC from Mitutoyo Corp. (Aurora, Illinois), the shop owner can program the shape of a complex EDM electrode in 45 minutes and mill the part in 30 minutes. According to Mr. Heckendorf, the single electrode does the work of a dozen simpler ones that would take most of a day to mill or grind manually.

"Basically, we've boosted electrode throughput more than seven to one and streamlined our EDM die sinking as well," Mr. Heckendorf says. With two-axis CNC, the moldmaker quickly mills the more intricate electrodes he needs to cut mold cavities faster and with a better finish than before. Faster electrode milling and subsequent cavity cutting trimmed delivery time on what was a six-week job by seven days. "With the MillStar, I'm doing very high quality EDM electrodes in far less time," says Mr. Heckendorf.

D. Heck Tool specializes in prototype and small production molds for plastic parts. Much of the tooling produces tiny medical and switch components. The stainless and tool steel mold frames are generally about 8 inches square by 10 inches high. While mold cavities are usually cut by a Hansvedt 201 CNC EDM, the graphite electrodes used to cut the pockets must be milled.

The smallest electrodes used by D. Heck Tool are just 0.010 inch wide. "I could mill them manually," says Mr. Heckendorf, "but electrodes are very time consuming parts to make manually."

When Mr. Heckendorf started his business three years ago, he thought CNC conversion was the smart way to go. "It's hard to sell mold-making services without talking CNC," he says. "The first question they ask you is, `what do you have for CNC equipment?'"

Sophisticated computer control was a new endeavor for the small shop owner. As a lead moldmaker, Mr. Heckendorf had long made molds with a manual finish grinder and knew nothing about CNC programming. However, the milling machine he'd bought had a bolt-on power feed and Mitutoyo digital readout, so he spoke to his MTI representative about a new CNC retrofit package.

About a year ago, he upgraded his milling machine with the Mitutoyo MillStar control package. The expandable two-axis control promised the right capability, given the type of work to be done and the background of the user. "I knew I wanted a two-axis CNC because I had no hands-on experience programming," says Mr. Heckendorf.

Mr. Heckendorf visited surrounding job shops and tried several CNC packages. "The Mitutoyo package was one of the very few to run through Windows 95, which is an easy operating system to learn," he says. "It's also got good graphics and can be programmed in G-code or plain shop language."

In addition, canned cycles automate routine operations such as hole patterns, lines and arcs. More complex jobs are set up through drop-down menus on the 10-inch color screen. Function keys let the user override the automatic control and change feed rates and other parameters on the fly. Mr. Heckendorf was able to use the control after only four hours of instruction.

Precision EDM electrodes for cutting molds are milled to the shape of the finished plastic part. Customers typically supply Mr. Heckendorf with hard copy drawings of their molded parts. Mr. Heckendorf extrapolates electrode dimensions from the drawings allowing for plastic shrinkage, overburn, and other factors.

Most electrode jobs at D. Heck Tools are programmed in conversational mode. Programs stored in the MillStar memory or on disk also give D. Heck Tool repeatability difficult to achieve with manual operation.

The EDM uses the precision electrodes to cut mold contours within 0.0002 inch. Using more complicated electrodes closer to the shape of the finished part burns away more of the mold cavity faster and leaves better quality finishes.

One complex mold for a speedometer/tachometer case previously required 28 different electrodes that were set up and machined manually. By comparison, a similar gage case mold required just 12 electrodes milled under CNC. As a result, the typical six-week job was shortened by seven working days. For D. Heck Tool, the switch to CNC is paying off in shorter mold delivery times and lower costs.

Time saved in setup also means more effective use of the milling machine. Mr. Heckendorf estimates his milling machine actually ran just four days a month in the days of manual setups. Setup occupied the rest of the time. With CNC operation and stored programs, the same machine now makes chips three weeks a month, a great increase.

For a one-man shop in a competitive market, a CNC retrofit package has proven itself a powerful business advantage. "The control allows me to quote five weeks on a job someone else is quoting nine weeks on. That's what my business is built on," he says.