There used to be a big gulf between the processes and materials in auto and aero. And it used to be, more or less, that aero would borrow processes or practices from auto for the simple reason that auto had managed to come up with approaches that were, in order to be done in automotive volumes, more or less optimized, or at least dialed in to the extent that would make them cost-effective enough to perform mass production. Aero, where things are generally done at a more leisurely pace (no 60-per-hour airframes) and with a significantly more advanced materials. After all, the budgets are not only bigger, but the conditions, rigors and requirements are greater.
But now things are changing, says Greg Hyatt, vice president of the Mori Seiki Machining Technology Laboratory. "As automotive is pursuing higher and higher fuel economy, they're driving up the combustion temperatures, and we're starting to see the use of materials normally associated with turbo machinery and jet aircraft," he says. Among the materials are stainless alloys, nickel-based alloys, and titanium. "The cutting conditions used for those materials in aerospace are completely inappropriate for automotive production. They're extremely slow."
But Hyatt points out that the fundamental processes are being modified so that there is applicability in automotive: comparatively speaking, they become "high-speed machining." As he observes, "High speed is relative. Machining titanium valves and stainless manifolds is not high speed relative to aluminum machining, but it is high speed relative to the aerospace industry. But to get costs under control and to get parts produced for a price consumers are going to accept, we need more aggressive cutting conditions than have been the case in aerospace."
Here are four developments from aerospace that are being brought to automotive. And while the cutting speeds aren't necessarily "high" as regards the RPMs and the traverse rates typically found in what's become "high-speed machining," Hyatt points out that what is becoming more relevant is high productivity rather than high speed: Getting more parts out more quickly than just operating machinery at higher speeds and feeds.
Grinding Rather Than Machining.
It's called "VIPER grinding," which is an acronym for either Very Impressive Performance Extreme Removal or Vitreous Improved Performance Extreme Removal. This technology was developed at Rolls-Royce aircraft engine operations (www.rolls-royce.com) along with Tyrolit, the Austrian grinding wheel manufacturer. Hyatt says that in the process, extremely porous vitrified grinding wheels are used-so porous "they're more air than wheel." The reason: the wheel is flooded with coolant, and it acts like a reservoir and consequently puts massive quantities of coolant right in the cut. "For difficult-to-machine alloys," Hyatt says, "we can actually rough grind the material and have higher material rates than we could with milling or turning." He says that parts made with stainless or those with a high nickel content are well suited to the process. It should be noted that they are not performing the grinding on a traditional grinding machine, but they are integrating the grinding into a conventional machine tool so that any additional turning, milling or drilling can be performed in the same setup.
Spinning and Turning.
Here it is a case where parts like titanium valves can be produced. Hyatt admits, however, that even the gains made by this process-he cites a 300% increase in cutting speed "as a starting point"-aren't what they might seem to be: "Ten times normal productivity for producing titanium parts is getting close to the normal productivity that Detroit is used to." Essentially, this is a development that comes from both Kennametal, a cutting tool manufacturer, and Mori Seiki. In the process, both the round insert and the workpiece is rotated. Not only is the cutting speed increased, but so too is the feed per revolution, also about three times the norm. But because of the radius of the tool, good surface finish can be attained. Hyatt says that it is possible to machine with the minimum quantity lubrication (MQL) approach that's being pursued by more companies because the spinning tool brings the coolant into the cutting zone such that it is "much more effective than static lathe tools." Again, this process is being performed on standard machines, like the Mori Seiki NZ series of turning machines that include multiple turrets and integral motors to drive the tools.
You may be familiar with creep-feed grinding, wherein the workpiece is moved in the direction of grinding wheel rotation very slowly. But because of that creeping, the depth-of-cut is significant. Speed-feed grinding is just the opposite. The wheel is reciprocated across the workpiece-it works best on cylindrical surfaces (e.g., valve stems) rather than prismatic parts-at an extremely high velocity. Hyatt explains, "There is a shallow depth of cut and the high feed, so the heat generated is distributed across a wide area." Because of this, there is no thermal damage to the workpiece. "With the high acceleration and traverse rates of modern machining centers," he observes, "we can do this on a modified standard machine." Mori Seiki is working with Tyrolit on developing this process.
This is a process that is being used in lower-volume applications-like producing high-performance parts for race cars or components for agricultural equipment-and the metal removal speed isn't particularly fast, but the total processing time for the parts are greatly minimized compared to conventional processing methods. Again, Mori Seiki is working with Tyrolit on what is certainly an unconventional approach. "We've done everything wrong intentionally," Hyatt says. Whereas the normal process of machining some medium carbon steel components is to machine, take the parts out of the machine and induction-flame-harden the surface, then put it back into a machine for finishing. What they're doing "wrong" in the grind hardening process is machining the part in its annealed state, then using the grinding process to put so much heat into the workpiece ("The material is glowing," Hyatt notes) that it is hardened. Then it can be finished with hard turning or grinding. This helps reduce the throughput time as it isn't necessary to move the part into a secondary heat-treating area. Because this process eats up the tools rather quickly, they're using their Hydrogage system, which uses the coolant in a way analogous to air in air gaging, but this is being performed in process. They are able to achieve a submicron resolution-better than the repeatability of the machine-and the measured results allow them to provide closed-loop control and tool compensation.