Robotic machining used for aerospace applications

Robotic waterjet systems use a mix of water and garnet that exits the waterjet to make precise cuts for an industry that demands perfection.

12/28/2017


Robotic machining has come a long way and has proven to be robust and accurate enough to achieve the demanding tolerances required by the aerospace industry. In one example, a robotic waterjet system cut an integrated bladed rotor (IBR) for a commercial jet engine.

Using tap water mixed with an abrasive media that shoots out of a small orifice at ultra-high velocity, a robotic waterjet system can cut through solid metal up to a foot thick. A six-axis robot can maneuver a waterjet nozzle across a part, shaping the graceful contours of jet engine airfoils with ease.

"This is 3-D cutting with waterjet," said Dylan Howes, vice president of business development for Shape Technologies Group. "The Aquarese system is the only 3-D robotic abrasive waterjet machine able to achieve 94,000 psi (6,500 bar)."

The waterjet machine integrates technology with advanced robotics to provide turnkey solutions for Shape Technologies Group's aerospace, energy, and automotive customers. The robot brings flexibility and smooth motion to the waterjet process. With six degrees of freedom, its articulated arm can approach a workpiece from virtually any angle and follow a smooth, accurate, and highly repeatable toolpath to create precision cuts and contours. In metal cutting applications, the waterjet rough-cuts the components, which subsequently undergo final milling operations.

"One of the primary benefits of waterjet is that it's extremely versatile," Howes said. "You can cut metal, composites, glass, stone, paper, food, just about anything. With waterjet, you could be cutting metal one day and cutting foam the next day on the same machine."

Robotic waterjet system produces a stream of water traveling at supersonic speed to cut a variety of materials, including superalloy components used in the aerospace industry. Courtesy: Shape Technologies Group/RIAAquarese systems are used to cut titanium alloys, Inconel, Ni-based alloys, other superalloys, stainless steel, and composites. Abrasive waterjet is required for cutting metals. Garnet is used as the abrasive media in 99% of abrasive waterjet applications. Water and garnet exit the waterjet cutting head at nearly four times the speed of sound to increase cutting power by 1,000 times.

Robotic waterjet machining is a cold-cutting process; there's no heat-affected zone (HAZ) or thermal fatigue. This is an advantage over laser and plasma cutting. Howes said there is no mechanical stress on the part. Part integrity is not compromised and only light fixturing is required.

"Waterjet is more efficient than rough milling or wire electrical discharge machining (EDM)," Howes said. "It's much faster, has a lower operating cost, and produces large offcuts which are easier to recycle than the chips that you get from a milling operation."

The waterjet process is chemical-free and environmentally friendly. The water, along with any garnet used as an abrasive, can be recycled.

"There are no hazardous fumes," Howes said. "You can use closed-loop water systems. There's none of the dross waste you would find in a laser or plasma application." 

Robotic accuracy, repeatability, rigidity

Robotic abrasive waterjet system performs 3-D cutting of a titanium blisk for a commercial aircraft. Courtesy: Shape Technologies Group/RIARobotic waterjet has been used more commonly for softer materials until the aerospace industry realized its value for cutting metals and composites.

"It has become a more common application because now we can achieve better performance," said Sebastien Schmitt, North American robotics division manager for Stäubli Corporation. "We've made so much progress with the rigidity of the arm and precision. It makes it possible today to work within that domain. Accuracy, repeatability, rigidity, all this comes from our patented gear box that we manufacture and design in-house."

The robot is a high-payload 100 kg model, which Schmitt said is needed for rigidity. It's also important for the counterforce from the ultrahigh-pressure waterjet.

"The fact that we are rigid, very precise, very repeatable gives you the ability to push the edge of performance," said Schmitt, noting they are now able to compete with traditional milling methods. "The cost of a 5-axis computer numerical control (CNC) machine is three or four times the cost of a system like you see here."

Aquarese uses a humid environment (HE) robot developed specifically for wet environments. The enclosed arm structure is IP65-rated and reinforced by arm suppression for added waterproofing. The IP67-rated wrist is corrosion-resistant and protected against low-pressure immersion. The tool flange and critical parts are made of stainless steel to hold up in corrosive environments. Longevity is also important, especially when the robot is working in harsh environments.

The proprietary robot programming language is optimized for compatibility with computer-aided design (CAD)-to-path software. According to Howes, it's a simple process to import a CAD model and generate an optimized toolpath. The waterjet systems are programmed using a software suite bundled with the system. For 3-D robotic waterjet cutting, a module with functions for specific applications is supplied. Applications include roughing blisks and fan blade trimming. 

Material savings

Robotic waterjet system is fully integrated with a six-axis robot, ultrahigh-pressure waterjet pump, cutting head, control system, and programming software. Courtesy: Shape Technologies Group/RIAMaterial savings is a major advantage of robotic waterjet. In one application, the process involved roughing out two turbine blades from one bar of lightweight alloy.

"For one slug, you get two parts that are near net shape before final machining and grinding," Howes said. "This is a huge advantage with waterjet because you're using 3-D nesting which can't be done with milling. The only other way you can do this is with wire EDM, which is very expensive."

Common cut lines also can be used when cutting sheet metal. The waterjet's thin cutting width, ranging from 0.003- to 0.015-in. for a pure waterjet stream and 0.015- to 0.070-in. for an abrasive waterjet, allows for intricate detail. Howes said this couldn't be done efficiently with conventional machining where the kerf, or width of the cut, is too wide. Common cut lines, 3-D nesting, and larger offcuts all provide significant material savings.

Robotic waterjets also can be used for stripping solutions for coating removal on aircraft engine parts, including boosters and combustors for the maintenance, repair, and overhaul (MRO) sector. They also have systems for ceramic shell and core removal for investment casting foundries typically in the aerospace or industrial gas turbine market.

"We can also integrate the core removal systems with cutting solutions for de-gating, as well as systems for removing the flashing from forged materials," Howes said. "All of these are robotic applications." 

Research is enabling technologies for robotic machining

Robotic waterjet stripping system for removing coatings from turbine components in maintenance, repair, and overhaul operations. Courtesy: Shape Technologies Group/RIARobotic machining, whether with waterjet or more conventional means, has its limitations when it comes to rigidity and accuracy. Researchers are exploring novel ways to address these limitations.

Research is underway at the Boeing Manufacturing Development Center (BMDC) on the campus of the Georgia Institute of Technology in Atlanta. The BMDC is focused on implementing industrial automation in non-traditional ways, such as shimless machining. The center is located in the 19,000-sq-ft Delta Advanced Manufacturing Pilot Facility (AMPF).

Although the ribbon cutting for the center was in June 2017, Georgia Tech's partnership with Boeing is in its tenth year, according to Shreyes Melkote, associate director of the Georgia Tech Manufacturing Institute, and Morris M. Bryan, Jr., professor of mechanical engineering for advanced manufacturing systems at Georgia Tech.

"The AMPF is a translational research facility focused on discrete parts manufacturing where we work with industry to take ideas and technology developed in the lab and translate or tailor them to applications that might be of use to the industry sponsor," said Melkote, who is also an affiliated member of Georgia Tech's Institute for Robotics and Intelligent Machines, where he serves as a bridge between manufacturing and the robotics and automation area.

Melkote's research focuses on robotic milling. His objective is to use enabling technologies to allow robots to produce more complex features and surfaces, and to do it with a high degree of accuracy.

"Lack of stiffness and accuracy are limitations that still need to be overcome," Melkote said. "Technologies such as sensing, compensation, and metrology addressing stiffness and the limitations of articulated arm robots are what we've been working on for the last four or five years."

Experimental setup for closed-loop robotic milling uses a wireless sensor system for real-time force measurement to improve machining accuracy. Courtesy: Georgia Institute of Technology/RIAFor example, Melkote said they are using laser tracking devices and other types of metrology systems, along with in-process sensing of forces, to help address accuracy issues when using robots in high-force applications. Some of these findings have been published in journals.

In the paper "A Wireless Force-Sensing and Model-Based Approach for Enhancement of Machining Accuracy in Robotic Milling," Melkote and fellow researchers at Georgia Tech test a new hybrid method that combines wireless force sensing with a mechanistic model of the milling forces to increase the accuracy of robotic milling while preserving its flexibility. Milling experiments are conducted with a high-payload articulated robot and a wireless polyvinylidene fluoride (PVDF) sensor system for real-time force measurement. The results show significant improvement, over 70%, in the dimensional accuracy of simple geometric features machined by the new method.

"Robotic machining could offer a lower-cost and more flexible, versatile technology for enabling assembly of aerospace components," Melkote said. "We're exploring how we can use other technologies, like metrology, to compensate for the limitations and achieve the tolerance requirements that the aerospace industry needs." Several technologies developed at Georgia Tech have been transitioned into production at Boeing, including new design methods for advanced commercial aircraft, flow control for 787 aircraft, material handling for F/A-18, F-15 and C-17 components, vision systems for hole countersinking, and autonomous robotics for assembly.

Other research includes teaching robots tasks through human demonstration and exploring the use of robots as flexible fixturing devices. Robotic-assisted manufacturing will continue to be a focus for researchers as advanced manufacturing grows.

Tanya M. Anandan is contributing editor for the Robotic Industries Association (RIA) and Robotics Online. RIA is a not-for-profit trade association dedicated to improving the regional, national, and global competitiveness of the North American manufacturing and service sectors through robotics and related automation. This article originally appeared on the RIA website. The RIA is a part of the Association for Advancing Automation (A3), a CFE Media content partner. Edited by Chris Vavra, production editor, Control Engineering, CFE Media, cvavra@cfemedia.com.

More answers

www.controleng.com keywords: robotics, aerospace

Key Concepts

  • Robotic waterjet machining is a cold-cutting process with no mechanical stress and part integrity is not compromised.
  • Robotic waterjets can also be used for stripping solutions for coating removal on aircraft engine parts, for the maintenance, repair, and overhaul (MRO) sector.
  • Researchers are looking at enabling technologies designed to allow robots to produce more complex features and surfaces with a high degree of accuracy. 

Consider this

What other applications would benefit from robotic waterjet machining?

ONLINE extra

See a video of a robotic waterjet system rough cutting a titanium blisk.

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