Flying-machine machining

On the shop floor of aerospace job shop Protomatic, Dexter, Mich., a Haas VF-2SS vertical machining center armed with helical cutting tools was machining a billet of 1018 steel at a feed of about 250 ipm and a spindle speed around 2,000 rpm.

The company’s manufacturing manager, John Donajkowski, ran the machine; he’s been with the company for more than 30 years. Doug Wetzel, managing director, has been around even longer; the company was founded in 1971 by his father, Bill Wetzel. With Les Smith, senior machinist, another longtime employee, they demonstrated for CTE some of the ways that machining aerospace materials, particularly the challenging ones, has evolved in Protomatic’s shop.

The shop does work for military and commercial primes and startups, and not only on airframes and engines but also missile systems, satellites and rocket engine components. About 40 employees work at its 30,500-sq.-ft. facility, which houses 30 machines for turning, milling and grinding. The company is getting started on MTConnect—an open manufacturing-communications protocol for connecting, monitoring and collecting data from an entire production floor—with five multiaxis machines. Every procedure for every job, Wetzel said, is flow-charted.

A KCRA-grade ceramic tool from Kennametal. Image courtesy Kennametal.

The list of workpiece materials includes plastics, aluminum, stainless steel, nickel-base superalloys and refractory alloys—high-temperature alloys that contain elements such as tungsten or molybdenum.

Although Protomatic is a family-owned, single-location shop, it can handle the tough stuff, Wetzel said. “We have electrical and mechanical engineers on staff who understand [how to machine challenging materials] and give guidance.”

Sound Practices

That points up a difference from the old days, according to Wetzel. The newer alloys and what shops need to know to effectively cut them call for a different kind of expertise. “The technology is no longer understood from an intuitive sense,” he said.

“For example—listen.” Wetzel paused and looked at the Haas, which sang a smooth, low-volume tone as it cut.

“The machinists of yesteryear would start a machining operation, and they’d actually listen to it. They would be listening to the excitation frequency—the rpm of the cutting tool and how it responded,” he said. “And if they were getting a harmonic, they knew they were at the wrong speed. So they would adjust their speeds or feeds to remove the harmonic until it was running smoothly. They had trained ears.

Machinist Joe Wetzel sets up an operation at Protomatic. Image courtesy Protomatic.

“The problem is, that only works if the relevant sounds are in the spectrum that humans can hear with the naked ear—and that’s just no longer always the case,” Wetzel added.

Now, technology is handling what used to take human skill.

“We use a frequency-analyzer microphone that allows us to analyze where the harmonics are occurring, and, based on what it tells us, we can adjust to a smoother speed and feed rate,” he said. “And the machine tool makers are beginning to incorporate this kind of tool into their machines, with sensors and programs that listen and adjust as needed.”

For Wetzel, this is an example of tribal knowledge and experience being supplemented, if not replaced, by technology to work with these materials.

“In some ways, we’re making smarter systems while dumbing-down what the operator needs to know, so the machine [will be] making decisions on this automatically,” he said. “Regarding harmonics, for example, you won’t need a machinist with a trained ear; the machine itself will have the trained ear.”

Hard Lessons

Improvements in cutting tools make a big difference in Protomatic’s ability to compete, according to Wetzel. For example, Smith emphasized the advantages of 5-flute, helical cutting tools, especially when cutting Inconel. “The multiple flutes seem to do a lot better on the tougher materials,” he said.

Les Smith, Protomatic’s senior machinist, loads a rocket-engine component made of Inconel 718 into a Mazak Quick Turn Nexus 200-II M. Image courtesy Protomatic.

It’s not only the geometry of the tool’s cutting edge that concerns Protomatic, but also what is behind it. When machining deep pockets, “you want to keep your tool as short as possible for the sake of rigidity and to minimize vibration,” Smith said. “We’ve learned that it’s better to go part of the way down with a short tool, switch to a slightly longer one and then one that’s longer yet,” and optimize the speed and feed for each tool.

“We’ve found that reduced-neck tools work very well when you get down into the deeper pockets,” Smith said, holding up a tool, “so that the neck isn’t dragging on the wall when you get deep.”

These best-practices were developed after producing what Wetzel called “problem-child parts,” including actuator components and parts for rocket-engine fuel systems.

The Best Path Forward

Along with changes in the tools themselves, improvements in howtoolpaths are plotted have been achieved. For more than a decade, Protomatic has been using toolpath-generating and optimizing software programs such as Volumill from Celeritive Technologies and Truemill from Surfcam, according to Donajkowski. “These programs eliminate the machining of air,” he said. “They keep the tool engaged with the material at all times, so there’s no wasted motion.”

One wants to keep a carbide tool engaged continuously because “carbide is a material that doesn’t like to be shocked,” Wetzel explained. “If you’re shocking and vibrating a carbide tool with unnecessary impacts against the material and chirping it in corners, you’re going to create oscillations and opportunities to chip the tool.”

Before the advent of such programs, “we were sort of making our own paths with those dangers in mind,” Donajkowski said. “These programs certainly make it easier.”

Manufacturing manager John Donajkowski sets up a machining operation on Protomatic’s Tsugami TMA8 multitask turning center. Image courtesy Protomatic.

He recalled how such programs came into the shop. Decades ago, “the older, more experienced machinists showed us their ways of machining, what to watch out for—and listen for,” he said. “But we picked up on computers faster than they did, and we started to learn and then teach them about these new kinds of software programs.

“And now with the new generation coming in, it’s basically the same thing,” he finished. “They’re going to teach us new tricks.”