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Metrology Takes Flight


Look for more integration between metrology data, sensors and automation as aircraft manufacturing enters era of unprecedented growth

By Bruce Morey
Contributing Editor

“About 20 years ago, airframe manufacturers started building their manufacturing and assembly processes around laser trackers,” said Joel Martin of Hexagon Metrology (North Kingstown, RI). The motivation was simple—eliminate large, expensive tooling systems for precisely locating wings or fuselages when mating them together. Today laser trackers guided by retro-reflector targets are commonplace, establishing a baseline of metrology in three degrees of freedom in position (X, Y, and Z).

The next step is using metrology to assist assembly in six degrees of freedom by adding to position the direction (roll, pitch, and yaw). “To get six degrees of freedom, a laser tracker or scanner system is coupled with LED measurement systems,” said Martin. “Leica provides such a system off the shelf that has all six degrees of freedom built into it.” These expanded devices are now being used to guide tools mounted on robots that cut waterjet openings in barrel sections of fuselages made of carbon fiber reinforced plastic (CFRP.) They also drill holes. Both are operations where knowing position and direction of the tool is vital.

Verisurf coordinate metrology software and portable laser trackers are used to build and inspect large composite wing tools for commercial aircraft.

“The big advantage here is that those six-axis robots do not need to be calibrated,” said Martin. Robot calibration is a time-consuming process, performed in special calibration cells. Robots also need to be recalibrated periodically to ensure absolute accuracy. Giving them “senses” via metrology devices like laser trackers and LED mountings means the robot system can figure out accurately for itself its position relative to the part it is cutting, drilling, or fastening, to a typical accuracy of 50 µm, according to Martin.

Fusing metrology data from different sensors and turning it into guidance for a robot is provided by software from New River Kinematics, the company that became a Hexagon brand in January 2013. This is done through Spatial Analyzer along with a newer package called SA Machine. “SA Machine provides that needed interface for the robot,” said Martin, who reports that it currently operates KUKA robots. More brand-name robot modules are on the way.

Not Just for Assembly

Metrology has migrated into not only airframe assembly, but also original fabrication of those parts. Metrology devices are helping machine both airframe and engines, according to Leo Somerville, president and CEO of Renishaw (Hoffman Estates, IL). Airframe parts include large monolithic stringers, wings, and ribs while precise jet engine parts include blades, disks, blisks, and stators.
Laser trackers guided by additional LEDs are providing six-degrees-of-freedom guidance to aerospace manufacturers.
The challenge in larger airframe parts is the need to machine them in temperature uncontrolled environments. On machine, in-process metrology is the only practical approach, since attempting to machine and then move a 10 or 20' (3.1–6.1-m) piece represents quite a challenge, according to Somerville. “On these large CNC machines, aircraft manufacturers have developed some clever techniques to re-establish datums and perform metrology checks on the parts,” he said. Since they are using CNC machines as metrology devices, they periodically calibrate them against NIST-certified artifacts the same as for CMMs. For on-machine probing, Renishaw offers its RMP 60 and RMP 600 for large, gantry-style machining centers. These offer repeatabilities of 0.25 and 1.0 µm respectively. Its AXISET software establishes and re-establishes datums and centers of rotation for rotary axes as temperatures change during fabrication.

A newer product especially targeting critical jet engine blades is Renishaw’s OSP60 SPRINT high-speed scanning analog probe for use on machine tools. The sensor resolution is 0.1 µm and the probe is capable of delivering 1000 points per second over its infrared transmission link. The head itself can travel at 15 m/min while taking data.

Renishaw also recognized that a complete solution is more than just the probe. The Blade Toolkit solution is software integrated with the SPRINT probe to help manufacturers specifically machine blades, blade tips, and blend roots of bladed disks, according to the company. “Jet engine blades can have long machining cycles, 20–30 hours,” explained Somerville, “making a mistake can be very expensive.” The Blade Toolkit not only helps with original blades, but also helps repair blades that may no longer have a CAD model associated with them. The software extrapolates where the machining surface needs to be, providing data to the CNC controller to provide an optimum blend,  said Somerville.

While accuracy is important, speed of measurement is also an issue when using probes on a precision CNC machine tool. “When you have to probe, you are losing production time, so if you probe, you want to do it as quickly as possible,” said Jon Kulikowski, of Blum LMT (Erlanger, KY). To satisfy the need for both speed and accuracy, he pointed to their TC model probes, and to their TC64-Digilog analog touch probe for in-process workpiece inspection for special applications. “The Digilog probe is a combination of a digital touch trigger probe and analog probe,” he said. It collects data at a speed of 2 m/min and with a repeatability of 0.4 µm (two sigma), according to data from the company. Measurement data is communicated via Blum’s BRC wireless radio transmission technology. The advantage of the combined probe is in scanning bores or shafts for true roundness, scanning surface contours for deviations from a master part or generally scanning for flatness.

Kulikowski describes an ideal scenario is to use the probe in comparative measurements. “A master part is scanned ahead of time, those measurements are stored, and then during production, a comparative measurement can be made to know whether the part is within its tolerances before it is removed from its fixturing,” he said. An application where the TC64-Digilog has been found to be particularly useful is in precision gear grinding, turbine blades, and blisks. “It is ideal for the low-volume, very-high-value parts one often finds in aerospace,” he explained.

  The SPRINT analog scanning sensor comes with its own software package designed for turbine blades.

Inspection as a Bottleneck

“The bottleneck in production flow today is inspection,” said Danny Shacham, chief technology officer for Nextec Laser Metrology LLC (Eastlake, OH). After reviewing existing touch probes, Nextec felt something better was needed. Its solution was to develop a unique laser probe to replace traditional probes used on CMMs. Nextec also understood that a fundamental problem with lasers as metrology instruments is their sensitivity to optical properties of the part. Color, reflectivity, material, surface finish, and viewing angle all can adversely affect the accuracy of simple laser measurements.

Enter the WIZprobe, which combines a laser with image processing, performing what Shacham calls an infinite number of triangulations simultaneously through a toroidal lens. “The lens looks like a bagel,” he explained. The probe is adaptive, so it modulates both the laser power and the sensitivity of the CCD camera, performing self-calibration on-the-fly in response to changing conditions during the scanning process. Advertised key benefits include measuring highly shiny surfaces, soft material, edges, and sharp angles among others. He describes the output as similar to that of a touch probe, a single, highly accurate point, in contrast to scanning technologies that produce a cloud of thousands to millions of points within seconds, often with embedded noise. It collects data at 50 points per second to an accuracy of 2 µm at two sigma (described as a best-fit accuracy over 100 points).

Combining their probe on a CMM chassis with special purpose software tailored for jet-engine turbine blades resulted in the WIZblade inspection system. The software automatically compares actual measurements to part CAD data for easy reporting, with special emphasis on important turbine blade features. Why? “The leading edge of the blade is critical to the fuel-efficiency of the engine,” he said. Engine efficiencies are becoming ever more critical, since companies he serves, like Rolls-Royce or Pratt & Whitney actually have to commit to a stated engine efficiency—or pay penalties if their engines do not meet agreed-to fuel consumption targets. “Using that data to create cross sections or make scan processes gives all the data the inspector needs.”

Automation Plus Scanning Equals Speed

Johan Gout, director of operations for Capture3D (Costa Mesa, CA) reports that automating inspections of jet engine components ranging from turbine blades to fan casings is also growing. “The great news is that noncontact scanning and automation are viewed more favorably now,” he said. “Engine manufacturers realize that the speed of noncontact scanning systems lets them determine issues faster. That is a great advantage to them.”

Automation combined with the speed, accuracy and fidelity of blue/white structured light scanning technology is especially useful in measuring the organic shapes of turbine blades, blisks, and stators, according to Gout. Instead of measuring various cross sections at discrete locations, a blue/ white-light scanner measures the entire airfoil and captures the small leading and trailing radii as well as the twists and turns of the airfoil's organic shapes, he said.

Gout also noted casting manufacturers are using an automated system at different points in the process, first to ensure wax cores are accurate, verification of the resulting castings and then confirming a Maximum Material Condition (MMC) “nesting of the part” within the casting before proceeding to final machining. “The last thing you want to do is start machining a part and then find out you did not have enough material to begin with,” he said.

To automate the ATOS Triple Scan 3D scanner, Capture3D developed a multiaxis motion control system called MC-XL. The six-axis system uses a closed-loop servo and belt driven actuators to move an ATOS sensor through 1710 × 750 × 2150 mm of travel. “This is ideal for meeting FAA requirements, since each turbine blade needs to be inspected individually, thus a high-volume, high-capacity inspection process is mandatory,” said Gout. Scan volumes range from 38 mm to 2 m. “Capture 3D also offers commercial off-the-shelf [COTS] robotic scanning and inspection cells, some of which can be installed and operational in two days,” said Gout.

He also explained the automated system can be equipped with a ‘kiosk mode’ in which operators identify themselves by scanning their badge  barcode, choose an inspection routine for a particular part, load the part, and push touchscreen buttons to initiate the 3D scanning and  inspection. Their Virtual Measuring Room (VMR) software accomplishes programming of the inspection routines, both online and offline. VMR aids in collision avoidance and provides feedback to ensure part-feature inspection coverage. “This software also has a module specifically designed for blades, blisks, and stators for inspection analysis,” he said.

Stresses of Aircraft Production Ramp-ups

As commercial aircraft manufacturers step up production, there are some lesser-known requirements they need to keep in mind. “One thing that is important is CAD model verification. If you are inspecting a part by relating your measurements to the CAD model, that model better be correct,” advised David Olson marketing and sales director for Verisurf (Anaheim, CA). This is especially important from his perspective, since Verisurf’s comprehensive quality planning and reporting package is built on Model Based Definition. Such data is intimately tied to a correct CAD model. In the speed and rush of high-manufacturing volumes, with multiple variants of parts on the same line, it is a caution worth observing. Verisurf provides a handy tool in its software to validate any CAD model back to the original source, producing a report that can be included with, for example, First Article Inspections.

Just as importantly, as aircraft production ramps up so, too, does the volume of metrology tasks that are required. These will need to be performed by trained metrologists. The growth in new technologies, such as parts made of CFRP, is requiring new skills,too. “It is important now to train the next generation of metrologists,” said Olson. In response, the company developed on-line training in its virtual Verisurf University. “This is important to us. We consider it our responsibility to help train the next generation of metrologists,” he said.

Interestingly, the plethora of new metrology devices such as scanners that can collect vast volumes of 3D point cloud data poses a ramp-up challenge of its own, according to Olson. “Collecting, editing, and making useful information out of massive point clouds is something Verisurf does very well,” he said. The growing use of ever more accurate, faster metrology devices also requires metrologists to simply know more. “Professionals who can do this work correctly and fast are doing well in terms of pay,” he relates.

Data, Languages, and Communication

There was a time when communicating quality data to make parts was all done on paper. While paper-based transfer of some important data has not completely disappeared—especially for tolerance and GD&T in some environments—it is fair to say the age of digital factory information is here.

While this is good in many ways, as John Horst of the NIST (Gaithersburg, MD) maintained, the digital age has created its own problems. “There is a confusion of language throughout the factory,” Horst said. “As we rely on digital representations of quality measurement information, you find different software vendors providing excellent tools performing roughly the same tasks, but they commonly use different formats for consuming and producing information exchanged between those tasks.” This is especially relevant in aerospace, where complex, dynamic supply chains span multiple continents.

He said that common tasks in the measurement process include: planning for measurement by interfacing with CAD designs, creating part programs for use on metrology devices, executing the measurements on parts, generating the measurement results, and analyzing results via statistics.

The new Quality Information Framework (QIF) standard is designed to harmonize these tasks. QIF provides freely accessible schemas defining the common information transferred between the tasks, said Horst. He noted that the first version of QIF defines measurement plans and measurement results. Other standards ‘incorporated’ in the framework include DMIS 5.2 for programming CMMs, and I++ DME as the interface between the execution software and the CMM controller.

Horst said that a key element of this standard is that it is based on XML Schema. “We used XML Schema because it provides low cost of development, agility, and ease of implementation for solution providers, end users, and suppliers,” he said. Since the success of any standard requires broad implementation by solution providers, convenience and low cost is vital to adoption. He said that solution providers have created demos quickly and cheaply because of the decision to use XML Schema.

Metrologists working within the Dimensional Metrology Standards Consortium (DMSC) defined QIF, and QIF version 1.0 is now an official ANSI standard.

Improvements planned for version 2.0 in calendar year 2014 include CAD-to-metrology interface. It is a top priority, since cost-effective XML exchange of product definition with various conformance levels of semantic GD&T will satisfy many CAD-to-model-based metrology-use cases. ME


This article was first published in the February 2014 edition of Manufacturing Engineering magazine.  Click here for PDF. 

Published Date : 2/1/2014

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