Case Studies: Portable CMM Keeps Up with Production
At Lucas Industries (Springfield, VT), a producer of tooling and fixtures for the aircraft industry, portable coordinate measuring machines (PCMMs) have become an integral part of the production process. Inspection and documentation, which the aerospace industry calls certification, is required for nearly everything the company turns out.
Jobs range from commercial airliners and helicopters to military aircraft. Customers include Boeing, Sikorsky, Northrop Grumman and Lockheed Martin. On a subcontract, Lucas has even produced tooling for European aircraft giant Airbus.
The inspection processes at Lucas must stay in sync with production from five vertical machining centers, one of them a five-axis machine. Production normally run 22 hours a day, five days a week, but can increase to 24 hours a day, seven days a week if needed. "The two portable CMMs and their two operators have to keep up," says Gary Westfall, vice president and chief designer. "We place equal stress on the CMM's accuracy and speed.
"Dimensional accuracy and repeatability are important, of course," Westfall says. "But equally important for us are speed in setup and fast and easy operation."
Nearly every tool Lucas builds is one-off, a design never seen before and unlikely to be seen again. At best, says Westfall, the company might get a "string" of tools or fixtures representing several production stages in a customer's factory. Even then, each tool is a different.
Aircraft fabrication is inherently low-volume and high variety, with a tool or fixture required for almost everything done in an aircraft production. Composite parts made from tooling built by Lucas include fuselage exterior skin panels, structural panels, airframe trusses, engine cowlings and nacelles, wings and tail fins, and landing gear doors.
Some of the Lucas's layup tools, such as those for leading edges of airliner wings and tail fins, may span 12 feet or more. They ensure precise alignment and orientation of the composites' plies, proper curves in the surfaces and exact fits with abutting components.
For its certifications, Lucas relies on two System 3000i portable CMMs from ROMER CimCore (Farmington Hills, MI). Each is equipped with PowerInspect software from Delcam Inc. (Windsor, Ontario, Canada).
"When we got the first ROMER arm, we had to sell the customers on using it, instead of the gantry CMMs they were familiar with," Westfall recalls. "We quickly made believers out of them."
Lucas often inspects five, six or seven small jobs a day, from initial setup to completed inspection report. While some inspections on complex jobs can take a full day to set up, most take just five or 10 minutes. The Lucas average is less than 15 minutes, even though Westfall has to create and download a PCMM model for each one.
"Because everything in aerospace has to be certified," Westfall points out, "the arms are used on 99% of the jobs. And because virtually no jobs repeat, there are no pre-existing setups that can be retrieved and reused. That makes PCMM setup speed important to us.
"A great feature of the 3000i is that it lets us quickly pick four start [reference] points to register the arm for each job. We do that," he explains, "with just a quick scan and a check to make sure the arm can reach the points from the initial clamping. You cannot do that with a [fixed in place conventional] gantry PCMM or a laser," Westfall adds. "They are too inflexible. Even if it were possible, it would take hours. We do it in minutes.
"We also need the ability to handle engineering data such as CATIA and IGES files, to manipulate the geometric data, to do the statistical analysis and to generate reports," he adds. "PowerInspect does that for you, automatically, right on the screen while you're watching it."
The operator then runs a best-fit routine between the tool and the CAD model to lock in the coordination. From then on, the arm and the operator "know" exactly where they are in terms of the coordinates of whatever they are inspecting. The geometry of the tool that is being inspected is overlaid on the CAD file by PowerInspect, "even if the part is several hundred inches away from the zero-zero-zero point of the arm," Westfall says.
The four start-up points, plus the 100 to 200 additional points probed in the typical Lucas measuring job, are assigned coordinates in terms of the
aircraft-waterline, station line and butt line-rather than the X, Y and Z of machine tools and the shop floor. "We always work in aircraft coordinates and not shop coordinates," Westfall notes. "The data for the jobs comes in as aircraft coordinates and goes back out to the customer the same way."
Special emphasis is given to surface contours, to the scribe lines that mark the perimeters of parts and to tool location holes, especially for drill bushings. Scribe lines mark where the composite part is to be trimmed after lay-up and curing in an autoclave. "They are usually machined 0.015-inch deep into the surface of the tool," Westfall says. "They require a special computer model to ensure that the path can be probed and certified as correct."
Aerodynamic surfaces add to the challenge. Many are doubly curved, some sharply and some very gently and have huge radii. The blended surfaces that join them are precisely specified. "When parts have complicated aerodynamic geometries, the scribe lines can become curvy and wavy," Westfall says. "PowerInspect deals well with these difficult surfaces."
PowerInspect visualization, he adds, "displays things like surface contour splines and scribe lines clearly and unambiguously, and they are easy to rotate and manipulate on screen. The 3000i was originally bought for its strengths in handling engineering data-for how much more could be done in the computer," Westfall notes.
The ROMER systems have changed the way Lucas generates inspection reports. PowerInspect also gives Lucas automated report generation-standardized, simple engineering reports in Microsoft Excel spreadsheet formats. "PowerInspect tracks, gathers and assembles all that information on every point for us automatically," he adds. "This includes statistical summaries, histograms, means, standard deviations and max/min values plus a CAD representation of the [CATIA] file with every probed point plotted on it.
"We report out every point and its deviation, if any, to whatever tolerances were specified," Westfall says. "This takes less than a minute."
"The 3000i PCMMs are at the heart of the business here," he adds. "The speed and accuracy of the arms have opened a lot of doors for us. Since we got the first arm in 1998, we have tripled our business. That's not to say that the arms brought us all that business, though they did bring in some of it. But without the arms," he notes, "we could not have gotten all those new jobs out the door.
• Inspecting against downloaded CAD files minimizes the time needed to create and program inspection routines. Westfall creates all the inspection programs in CATIA from Dassault Systemes, which all aircraft companies use.
• The system has the ability to handle complex geometry, especially scribe lines and splines.
• Inspection reports are automatically generated.