The wrong measurements could prove disastrous

January 2017 - With each rocket test launch, there’s a moment where spectators hold their breath, aware of the possibility that the rocket may either not deploy, or worse, explode much sooner than intended. The engineers and fabricators that stand behind the construction of the rocket, however, are reassured having vetted and analyzed every inch from when it arrives as raw material to the shop floor until fabricated and prepared for launch.

AMRO Fabricating Corp. in South El Monte, California, uses Verisurf software to inspect rocket subassemblies. Each panel is 12 feet wide, 24 feet long and 4 inches thick for isogrid and orthogrid components. Partially hollowed, these structures are fashioned from a single metal plate, have triangular integral stiffening ribs, or stringers. Fastened together, the panels become the rocket’s structure. Assemblers use eight panels to build one ring 27.5 feet in diameter by 24 feet tall. Five such assemblies are stacked to build a single Space Launch System (SLS) core stage. Once fully assembled, the height of the SLS will reach 371 feet in its crew configuration, according to AMRO.

Above left lies a formed grid panel while, on the right, a flat grid panel awaits forming. The precision forming check fixture sits behind the flat panel.

A gantry CNC machining process drills holes for fastening points and other features, according to Rodrigo Delgadillo, laser/tooling supervisor at AMRO. “We use Verisurf to check the part after the forming or rolling process,” he explains. “Five high-capacity brake presses finish machined parts, which are formed or rolled into specified curved profiles, which takes time. Stress has to go somewhere so if it’s not managed well, the material will warp.”

The company performs quality checks using precision fixtures “to ensure the finished curved profile and critical index locations of a part is accurate and within tolerance,” Delgadillo continues. Relying on 3D CAD model data, Verisurf software employs assembly guidance in fabricating the fixtures. After the part is formed, it is placed in a secondary trim fixture designed to precision trim the outside dimensions to tolerance. Portable CMMs guide assembly and inspect for the quality of each fixture.

Finished parts undergo a final quality inspection, now combining laser tracking and Verisurf software. “We make sure that [each panel] is always within range [during forming] by checking it [against] the software [model] so we can get a good part out of it,” Delgadillo says. “Some parts are formed before they’re machined so, if you don’t know what you’re getting after a part is machined, you could end up with a part that has too much or too little material, which takes a lot of time to remove.”

Secondary processing delays rocket assembly so, to combat nonproductive labor, machine time is recorded in real time so that fabricators can make adjustments immediately, alerting the next operation programmers to any variations or changes. “We can keep everyone informed during every step of part production,” Delgadillo says. “If we need to make adjustments, we do so in Verisurf.”

Verisurf software quality inspection images depict detailed alignment and reporting stages.

Inspection ensures that “all those hours spent drilling and machining don’t end up in the scrap bin,” he continues. “With the Verisurf software, we can preprogram parts to be inspected as soon as they’re off the machine. Anyone with basic knowledge of the software can walk you through the inspection process from start to finish.”

Modeling

Verisurf will measure features and surfaces and apply the associated tolerance directly from the authority Model-Based Definition (MBD). The software supports open standards and communicates directly with all brands and models of laser trackers, laser radars, large-volume laser scanners, projectors, portable CMM arms, as well as stationary CMMs, says Nick Merrell, technical operations manager at Verisurf. 

“Our customers face extremely complicated and critical requirements which have zero margin for error and [they] must complete the tasks effectively and efficiently,” Merrell says. “With the large and complex isogrid and orthogrid components AMRO manufactures, a lot of [its] time and effort is applied to verifying product conformance and reporting.” 

AMRO shared its workflow information with Verisurf engineers to develop custom functions to streamline the measurement processes while maintaining the data’s reliability. The automated routine they developed resulted in lowered inspection time by more than 50 percent and reduced the need to move large parts and/or inspection equipment during the process, according to Merrell.

An AMRO worker runs one of five high-capacity press brakes used to form grid panels to their finished profiles.

Over the years, Delgadillo says AMRO has requested certain features, such as modules making it easy to measure rib intersections. “We subtract measurements from the rib intersections—between the ribs,” he explains. “That feature is significantly cutting down the time it takes to get those measurements.” Operators no longer manually measure features at each intersection; instead, “the software automatically measures and calculates with minimal points.”

Thickness, midpoints of ribs, mid planes “are all complex measurements and areas where, if not monitored, are points of potential failure,” Delgadillo says. 

Verisurf’s Discrete Point Capture feature, developed in large part as a result of customer feedback, allows the first part being checked to set the inspection plan. Discrete target points inspected against the CAD model are retained by the software, so all future parts can be inspected exactly the same way. “This reduces the time needed to create an inspection plan and is highly repeatable from one part to the next, which helps to spot deviation trends,” Verifsurf’s Merrell says.

Reliable metrology and the ability to provide Verisurf with feedback that could result in added or updated features is valuable in an industry where scrap is exceptionally expensive. At AMRO Fabricating, highly targeted and exacting inspection is no small matter as each panel costs hundreds of thousands of dollars. “That’s just for one piece,” Delgadillo says. “Not being able to know the status of a part and if it’s out of tolerance is a huge risk—one that Verisurf’s software eliminates.” MM