A little more than a decade ago, aerospace companies shifted how they made many of their aluminum components. Instead of stamping or casting several small components and riveting them together, they began using large blocks and machining away the vast majority of material, leaving behind single-piece components. The process change lowered weights and improved quality by giving shops repeatability – a CNC machine could produce hundreds of identical parts by following the same program.
But as those companies milled away material, the walls between part features became thinner and thinner, creating a new problem – tool chatter.
“When you get into thin-walls, it tends to vibrate and chatter, and the vibration is self-excited,” says Mike MacArthur, vice president of technology at RobbJack Corp. in Lincoln, California. “The tool vibrates at one frequency and the part vibrates at a different frequency, and they excite each other and build on each other. That’s where you get the really bad sounds when it’s cutting, and you get a very poor finish on the part.”
With thick-walled features, the workpiece could absorb some of that vibration, but as walls got thinner, chatter marred surfaces and led to parts being out of spec. To counter that, RobbJack engineers developed geometries for milling and drilling that harmonized vibration between the tool and the workpiece.
MacArthur adds, “Everything’s still vibrating, but they’re vibrating in unison.”
When RobbJack received a patent on the process in 2002, the company was thinking almost entirely of aircraft manufacturing. But with major automakers increasing their use of aluminum in several new vehicles, the company has found an entirely new customer base, winning work as a tooling supplier to the motor vehicles industry.
Increased part complexity
Though Ford’s F-150 pickup is the heaviest aluminum user in the automotive world, it’s far from the first. Most truck hoods and many car trunk lids have been made from aluminum for several years, but MacArthur said those were simpler parts – primarily single-piece constructions made from a stamping or forging. As automakers make doors and structural components from aluminum, they have to machine parts to create holes and pockets. It’s a tougher challenge both for the assemblers and toolmakers.
“There are ways of getting rid of chatter and vibration, but they generally involve slowing down and really cutting the feed rates,” MacArthur says. “A one-second deviation in cycle time isn’t that big of a deal to an aircraft manufacture that may be making 10 parts a day. It’s a much bigger deal to automotive customers that are making thousands and thousands of parts.”
He adds that by matching vibration between tool and workpiece, RobbJack’s aerospace-derived A1-303 end mill allows users to run machines up to full capacity without special calibration.
“We have a lot of customers that use all kinds of technology – putting an accelerometer on the spindle, getting the natural frequency of it – determining everything mathematically to get the frequency response. They’d get a number that at exactly 14,152rpm, it won’t chatter,” MacArthur says. “But in a manufacturing environment, you could have 20 machines of the same brand and even with the same model, and every one of those numbers is going to be different.”
While RobbJack’s cutting tools were derived from those aerospace parts, company engineers did have to make one significant change to work with automotive components. Similar to 99% of cutting tools, the flutes on the aerospace tools move up the body in a right-hand, upward-helix direction. That basic design grabs chips as they form and pulls them away from the workpiece surface toward the tool.
MacArthur says that basic tool layout creates a small outward-facing burr on the surface of the component. Those burrs were a problem for automakers because the milled and drilled surfaces on door panels and other components tend to be pathways for electrical cables and fluid hoses. Even small burrs could cut components. For exterior automotive components, burrs could mar the perfect surface finish that companies want before sending vehicles to the paint shop.
“This doesn’t happen too often in aerospace, because those guys don’t have as many hollow-bodied parts,” MacArthur explains. “When you’re working with a monolithic part, there’s only one direction that the chips can flow, so we designed the tools for that.”
For the automotive tools, company engineers reversed the helix, making the tools spiral to the left instead of the right. So instead of drawing chips away from the workpiece, it pushes them into the hollow space between inner and outer panels. It’s a subtle change, but it’s enough to improve the surface finish on those hollow parts.
Inreased use of aluminum has sped up technology transfer that already has been taking place between the automotive and aerospace industries. Plane producers looking to produce multi-billion dollar backlogs of aircraft on order have been turning to automotive suppliers to speed production rates. And automotive companies looking to cut weight and boost fuel economy have created new opportunities for companies supplying lightweight materials and components to the aerospace industry.
MacArthur says he expects that trend to continue, especially with aluminum and carbon fiber reinforced plastic materials.
“There’s a lot of cross-discipline development going on,” MacArthur says. “We’re doing a lot of carbon fiber work on the aerospace side; not such much on the automotive side. But it’s only a matter of time. As soon as they get those costs down, you’re going to see adoption.”
About the author: Robert Schoenberger is the editor of TMV and can be reached at 216.393.0271 or firstname.lastname@example.org.