Robots Tackle 3D Waterjet Cutting for Aerospace

March 20, 2018
Waterjets can quickly cut a variety of materials, including aerospace metals, and turn them into complex and curved parts.

Waterjets use a stream of high-pressure water mixed with abrasives to slice through titanium up to a foot thick, under control of a six-axis robot that can maneuver the waterjet nozzle across the part, shaping the graceful contours of jet engine turbine blades and blisks (bladed rotors).

An abrasive waterjet controlled by a robot cuts a 3D part, a titanium blisk for a commercial jet engine. (Courtesy of Shape Technologies Group)

Traditionally, robotic waterjet has been more common for softer materials and other industries. More recently, though, it’s been adopted by the aerospace industry for cutting metals and composites, as robots have proven to be durable and accurate enough to meet the demanding tolerances required by the aerospace industry.

One company in the forefront of this technology is Shape Technologies Group in Kent, Wash. It includes Aquarese, a firm that assembles waterjet cutting robots, including the only abrasive waterjet that shoots water out at 94,000 psi (6,500 bar). Flow International Corp. manufacturers the ultra-high-pressure pumps and waterjets used by Aquarese in its turnkey machines for the aerospace, energy, and automotive industries. The robots themselves are provided by Stäbuli.

Robotic waterjet cutters use an articulated arm with six degrees of freedom, letting the cutting nozzle approach the workpiece from virtually any angle. Once cutting, the nozzle follows a smooth, accurate, and repeatable toolpath to create precision cuts and contours. For metal cutting, waterjets typically rough-cut components which are then sent to final milling operations.

“One of the primary benefits of waterjet is that it’s extremely versatile,” says Dylan Howes, Shape’s VP for business development. “You can cut metal, composites, glass, stone, paper, food…just about anything. With a waterjet, you can cut metal one day and cut foam the next day on the same machine.”

Waterjets from Aquarese can cut titanium alloys, Inconel, Ni-based alloys, other superalloys, stainless steels, and composites. Abrasives must be added to the water, and garnet is the abrasive of choice for 99% of abrasive waterjets. Water and garnet are shot out of the nozzle (cutting head) at nearly four times the speed of sound.

Despite its power, waterjet machining is a cold-cutting process, so there’s no heat-affected zone (HAZ) or thermal fatigue as there are in laser and plasma cutters. There are also no mechanical stresses on the part, so part integrity is not compromised and the fixturing needed is light compared to that needed for milling or conventional machining.

A robotic waterjet cutter form Shape Technologies Group uses supersonic water and garnet (an abrasive) to slice through and shape a variety of metals, including superalloys, used in the aerospace industry. (Courtesy of Shape Technologies Group)

“Waterjet is more efficient than rough milling or wire EDM (electrical discharge machining),” says Howes. “It’s much faster, has a lower operating cost, and produces large offcuts which are easier to recycle than the chips created by milling machines.”

The waterjet process is chemical-free and environmentally friendly. The water, as well as any garnet used as an abrasive, can be recycled. “There are no hazardous fumes,” he explains. “You can use closed-loop water systems. There’s none of the dross waste you would find in a laser or plasma application.

“We use Stäubli robots because of their durability and path accuracy,” Howes adds. “We worked closely with Stäubli to refine this process for our needs and it’s been a great partnership.”

“Waterjet machining has become more common because we can now achieve the desired level of performance,” says Sebastien Schmitt, North American Robotics division manager for Stäubli Corp., Duncan, S.C. “We’ve made so much progress with the rigidity of the arm and precision, it’s possible today to work in the aerospace industry.

“Accuracy, repeatability, and rigidity all come from our patented gear box we manufacture and design in-house,” continues Schmitt. “We’re the only robot manufacturer that designs its own gear box. That gives us better trajectory performance.”

The robot commonly used is a high-payload, 100 kg TX200 HE robot, which Schmitt says you need for rigidity. But it’s also important for handling the counterforces from the ultrahigh-pressure waterjet. Aquarese found minimal to no pushback with the Stäubli robots.

The HE in the models name stands for humid environment. This robot was developed specifically for wet environments. The enclosed arm structure carries an IP65 rating, and it’s reinforced by arm suppression for added waterproofing. The IP67-rated wrist resists corrosion and is protected against low-pressure immersion. The tool flange and critical parts are made of stainless steel to hold up in corrosive environments.

The TX200 HE was 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.

“The rigidity, precision, and repeatability lets companies push the edge of performance,” says Schmitt, noting they can now compete with traditional milling methods. “For example, the cost of a 5-axis CNC machine is three or four times the cost of the waterjet cutting machine.”

Material savings is another major advantage of robotic waterjet. It commonly roughs 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,” says Howes. “This is a huge advantage with waterjet and comes courtesy of 3D nesting, which can’t be done with milling. The only other way you can do this is with wire EDM, which is expensive.”

Companies can also use common cut lines 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 abrasive waterjet—can handle intricate detail. Howes says you can’t do this efficiently with conventional machining where the kerf, or width of the cut, is too wide. Common cut lines, 3D nesting, and larger offcuts all provide significant material savings. 

Aquarese’s machines can also handle robotic waterjet stripping for removing coatings on aircraft engine parts, including boosters and combustors, for the maintenance, repair and overhaul (MRO) companies. They also have versions that remove ceramic shells and cores for investment casting foundries typically in the aerospace or industrial gas turbine market.

Waterjet cutters can be modified to handle stripping to remove coatings from turbine engine components used in jet engines. (Courtesy of Shape Technologies Group)

“We can also combine core removal with cutting for de-gating, as well as removing the flashing from forged materials,” says Howes. “All of these are robotic applications.”

Still, robotic 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.

For example, research at the recently inaugurated Boeing Manufacturing Development Center on the campus of Georgia Institute of Technology, focuses on implementing industrial automation in non-traditional ways, such as shimless machining. To compensate for the lack of stiffness and accuracy, they are developing sensing, compensation, and metrology approaches. For example, the researchers are developing laser trackers and new types of measuring systems, along with in-process sensing of forces, to address accuracy issues when using robots in high-force applications.

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