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The Aerospace Precision-Machining Race

Jim Lorincz
By Jim Lorincz Contributing Editor, SME Media

To meet increasing demand, machine tool builders are marshaling new resources, technologies and strategies

That huge backlog of aircraft being recorded by the global giants Boeing and Airbus, along with a lengthening list of regional aircraft, is stretching the supply chain’s capabilities to machine the newest difficult-to-machine materials. Components must rotate faster, run hotter, and burn cleaner for new jet engine designs that are claimed to be 20% more fuel-efficient. Engines and designs that are structurally lighter are being developed, using advanced materials that challenge existing manufacturing processes. And demand for commercial aircraft isn’t going away any time soon. The two-billion people flying today are expected to swell to six billion by 2025, with freight hauling capacity expanding just as dramatically.

To meet the expected demand, machine tool builders are busy marshaling their resources of machine technology, customer-based experience, and process data to stake out their claims for manufacturing everything from jet engine components, airframes, structural components, to landing gear, manufactured from advanced materials.

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Parpas XS bridge mills feature X-axis travels up to 18 m for machining tools that are used for building up composites.

Technology Data Gathered Locally Shared Globally

Okuma America Corp. (Charlotte, NC) has formalized its global approach to aerospace machining to position itself to deal with the onslaught of applications and business growing out of the aircraft backlog of orders. “We’ve created a team of people from our three top markets—US, Europe, and Japan—who are sharing data, case history experience, to find the sweet spots in Okuma’s aerospace technology,” said Bob Baldizzi, principal engineer. “Our goal is to customize our machines using our single-source technologies for machining jet engine rotating components, disks, blisks, and hubs,” said Baldizzi.

“When you think of a hub or a disk on the hot side of an engine with slots that have to be milled in the periphery of these disks, it’s absolutely critical that the machines perform with utmost accuracy,” said Baldizzi. “Because we have our own control, we can get data out of the actual cutting dynamics quite effortlessly. We’ve had customers ask us for monitoring capability that coolant flow sensors, coolant temperature sensors, and vibration analysis sensors can provide. We can package these data-gathering points together and create part tracking using a bar code and a part’s serial number including inspection information for complete traceability.”

Okuma’s aerospace machine models include the Multus B750, multifunction machine with center distances up to 6 m for turning as well as machining. For blade milling, one of the Multus platforms has been modified to do five-axis machining for profiling jet engine blades. Okuma’s horizontal machines can be optioned with Turn-Cut functionality to bore tapers using standard off-the-shelf tooling. “Aircraft guys especially like that capability for doing multiple bores on hydraulic actuation systems without changing tools,” said Baldizzi.

Laser Drilling Tiny Holes in Turbine Blades

Processes vying for aircraft engine machining in addition to advanced five-axis machining center technology and multitasking machines include  recent developments in advanced laser processing, electrical discharge machining (EDM) and high-precision electro-chemical machining  (ECM), especially for  small hole drilling in turbine blades.

At its new plant in Auburn, AL, GE Aviation is using high-power lasers to drill tiny cooling holes in jet engine blades made from heat-resistant superalloys that operate inside the high-pressure turbine. “This is one of the most critical and sophisticated components in our jet engines,” said David Joyce, GE Aviation CEO. “They are perfectly shaped aerodynamically with laser-drilled cooling holes because they operate at extraordinary temperatures. We consider them a work of art.”

GE has invested $75 million in the new plant where laser tools are being developed that can not only drill, but also weld and print. The GRC lab has one of the most powerful lasers in North America at 20 kW. The researchers mounted the laser on a robot and use it to melt metal and develop new welding methods that are more efficient. The scientists tap the laser’s high-energy density to penetrate deep and fast into metal parts.

Precise ECM Processes Jet Engine Blades, Blisks

Electro-chemical machining (ECM) is most commonly associated with deburring applications, especially in injection molding technology. Specially designed ECM tools are used to remove material only at strictly localized areas to remove burrs for the creation of radii or to create annular grooves, cavities and other geometries. EMAG LLC (Farmington Hills, MI) has introduced an advanced Precise Electro-Chemical Machining (PECM) process that is capable of rough-and-finish machining of both single blades and blisks with the precision that is required for jet engine applications.

The rough-machining process is a pre-contouring operation with open tolerances and feed rates of 2-4 mm/min, while leaving enough material (approximately 0.2 mm) for the subsequent finishing process. The rough-machining operation can be carried out using a variety of tooling strategies optimized to the relevant geometry. Where the single blade may be machined with a double-sided, synchronized operation, the pre-machining of blisk geometries is best done along the blade’s axis, for example.

The ECM process has the advantage that tool geometry and suitable scaling of the power supply allow large blades and blisks to be machined at the same feed rates and at the same cycle times as smaller single blades. EMAG’s ECM/PECM technology covers a power range of up to 20,000 A DC and a pulse rate of 30,000 A. The PO 900 BF machine can accommodate workpieces of 900-mm maximum diameter and single blades up to 250-mm tall. These machines can also be equipped with hydraulic zero-point clamping systems, variable oscillators and automatic toolchangers.

Agile Process Combines Grinding, Milling

Makino’s G series grinders are well suited to grinding blades, vanes, and mostly inner components of a jet engine, particularly in the hot section. “We’ve had a lot of focus around the Viper grinding process in our G series machines for processing newer materials like titanium aluminides,” said Billy Grobe, aero engine technology manager, Makino Inc. (Mason, OH). “These particular machines can do grinding and milling, and anything that you can do on a machining center. They give us an adaptable machine platform that has the capability of being able to machine a blade or vane components complete in as little as two clampings, eliminating the stack up error that results from moving workpieces from machine to machine,” said Grobe.

At IMTS, Makino introduced its EDBV3 Fast Hole Drill EDM, a water-based five-axis machine that is designed for EDMing cooling air holes and shaped diffuser holes in blade and vane segments. All EDM drilling on the EDBV3 is performed fully submerged under water for higher part quality, improved stability and up to 10 times faster processing speeds than conventional technologies. To further improve productivity, the EDBV3 uses a single-electrode processing approach, which avoids the high cost of custom multi-electrode holders and standardizes the toolholders with a more flexible and cost-efficient system. For untended burning of varying cooling hole diameters, the EDBV3 features automatic tool change (ATC) and automatic guide change (AGC) systems. A patented electrode set that combines the electrode holder and die guide together into a common assembly provides enhanced reliability with simple and precise automated exchanges. Together, these features enable 30-second ATC and AGC exchanges.

Making the LEAP in Jet Engine Machining

“Both the GE LEAP and the Pratt Whitney Gear Fan jet engines are designed to have about a 20% better fuel efficiency burn,” said Scott Walker, president, Mitsui Seiki USA (Franklin Lakes, NJ). “Pratt & Whitney’s strategy is to use planetary gears to drive the fan. This allows running the engine hotter and faster, creating more power. But the materials required are almost impossible to machine. Some of the hot stage blades are upwards 28–30% nickel content making them extremely hard so you have to grind them,” said Walker.

Mitsui Seiki builds both machines to produce integrally bladed rotors (IBR) and single blades and is developing milling and grinding strategies to be used to produce Ti-Aluminides.  Machine models include the Vertex style machines for blade and blisk machining and 800-mm and 1-m trunnion-style machines for machining engine shrouds and casings, which are made from thin-walled Inconel with a lot of holes and angles that require five-axis machining.

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Mitsui Seiki’s aerospace models include five-axis machining centers for blade and blisk machining.

The LEAP has some challenging materials like GE’s Titanium Aluminide. Ti Aluminide is an intermetallic compound (gamma titanium alloy) that features strong interatomic ties that makes it resemble ceramics. It’s relative brittleness can be countered by the addition of elements like niobium and chromium and while it has half the density of more typical nickel alloys it is able to withstand heat up to 800° C. “The strategies for machining these materials still have to be worked out, especially with the volumes that will be required,” said Mitsui Seiki’s Walker.

“One strategy is to make jet engines like car engines, using fully automated lines. They don’t want to make them faster or quicker, they want to make them consistently good and reliable so they can plan down the production line how many engines a month they’re going to produce. As a result, you’re going to see a transition from stand-alone machines to typical automotive-type processes, which means that the machines have to be available to accept robotics and gantry cranes for loading/unloading, measuring, and doing all the processes with as few people as possible,” said Walker.

Superior Thermal Stability that Controls Tool Tip Accuracy

Parpas America Corp. (Bloomfield Hills, MI) has engineered thermal stability control into its XS overhead gantry five-axis bridge mills and OMV/Formula horizontal boring mills for controlling machining accuracy at the tool tip. In effect, all the machine structures that can affect accuracy are enclosed and provided with engineered heated and cooled conditioned air and coolant. The XS bridge mill can have X-axis travels of 22 m or more for machining lay-up molds and tools that are used to fabricate extremely large composite parts for the outer skins of aircraft like the Airbus A350 and Boeing 787.

Another machine that uses a similar method of thermal constancy is the Parpas OMV/Formula.

Two Formula machines have been in service at Lockheed Martin Fort Worth for a turnkey solution milling the F-35 fighter jet center section fuselage. That project employed both the thermal management system mentioned above for the XS, in addition to an automated cell with temp control and air filtering.

“The XS machine is an overhead gantry machine that has the ability to encapsulate the entire machine without putting it into a controlled environment, including column sides, the bridge which is the cross rail, the ram and ram saddle with the exception of the guideways of the ram [the vertical axis],” said Tom Hagey, operations manager. “The ram protrudes down to the table of the machine with the head. Boxways that are hardened and ground and exposed in ambient temperatures are gundrilled out and refrigerated coolant is passed through,” said Hagey.

“AIP Aerospace Tooling Group has ordered four of our machines, one for their Odyssey division in Michigan; the other three for their Coast Composites Irvine, CA facility. They’ll be used to produce the tools for building composite parts for the Boeing 787 and Airbus A350. One of the machines, a dual gantry model has travel of 59 × 15′ [18 × 4.5 m],” said Hagey.

Tool Selection Leads to Predictability

According to Ed Mulvey, technical support applications engineer, Horn USA (Franklin, TN), multitasking machining using tools like Horn’s spline milling and gear milling tools can complete a gear or a hub with a spline on it in one setup on the latest multitasking machines. “The real benefit for the user is that quality of the part is ensured as handling is minimized. Our tools are especially well suited to meeting the requirements of tough grooving that are required on aerospace parts. All of our tools are diameter specific for the bore size and depth of groove,” said Mulvey.

“The secret of effective machining is in getting the best combination of substrate, cutting edge and coating, especially for applications involving difficult-to-machine materials, like Inconel, titanium, high-end stainless, precipitation stainless, cobalt alloys and Stellite. Except for some of the newer hardenable titaniums, we don’t think of titanium as being as difficult to machine as in the past,” said Mulvey.

“Because all of these materials have low machinability ratings, we strive for predictability in tool life. We provide a quality tool so that the customer knows how often he has to change the insert on a consistent basis. It helps that most machines today have a tool management system that will tell the operator when it’s time to change the insert so that valuable time isn’t lost in production,” said Mulvey. “A recent example is one customer who is machining a component for the aerospace industry from premium nitriding steel. We recommended a coating that is running with phenomenal success. Previously, CBN was used and the process had to be stopped mid-cycle to allow the material to cool. Our free cutting geometry in combination with the coating allowed non-stop production. This custom solution reduced the scrap rate and improved the overall component quality.”

Toolholding Technologies Ensure Quality Results

Haimer USA (Villa Park, IL) offers three toolholding technologies that are designed to produce quality machining results, especially in high-speed machining applications. “The first is our balancing equipment that is designed to balance tooling assemblies before they go into the machine,” said Brendt Holden, president. “A tooling assembly includes the toolholder with the cutting tool and all accessories such as pull-studs, nuts, collets, face-mill cutters, inserts, etc. Balancing the complete assembly allows the machine to run fast, especially in aluminum machining, without creating vibration at the cutting edge. Balanced tooling produces excellent surface finishes, extends tool life, and prevents possible damage to the spindle,” said Holden.

The second technology is Cool Flash which is integrated into Haimer’s Power Shrink chuck with Cool Jet. “Cool Flash overcomes the tendency of coolant to flare out away from the cutting zone in high-speed applications. It is especially effective in aluminum structural machining where a lot of chips are created or in deep milling applications on vanes where it’s difficult to remove chips in jet engine machining,” said Holden. “Cool Flash allows the coolant to come around the cutting tool where it is redirected back to the shank of the cutting tool where it follows the shank of the tool with a high pressure to cutting zone.” The third technology is the Safe Lock which is designed to prevent tools, principally high helix end mills, from being pulled out of the holder in high-speed structural aluminum machining or in titanium roughing applications. ME


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