Big Parts Demand Large CMMs
Aerospace, heavy equipment, and wind-energy components require large-scale, high-precision metrology gear
By Patrick Waurzyniak
Huge workpieces require equally large inspection systems to properly measure high-precision part tolerances. With giant parts such as airframes and wing structures, heavy-equipment chassis components, and large gear assemblies used in windmill turbines, only the biggest CMMs are up to the task of measuring and inspecting these components to very tight tolerances.
The largest CMMs typically come in either bridge or gantry-style configurations, with many systems used to inspect gears in turbines for the burgeoning wind-energy industry. Among the metrology builders surveyed, the wind-energy industry is said to offer great potential for future growth, as demand wanes for more traditional applications such as the automotive industry.
Sensor technologies used with large CMMs from Zeiss enable users to maintain high accuracy on very big parts. "The sensors on our machines set us apart," says Gerrit DeGlee, MMZ-G product manager, Carl Zeiss Industrial Measurement Technology IMT Corp. (Maple Grove, MN). "Zeiss still has its own in-house-developed sensor technology, and that gives us better control over the mechanism used to gather data for the measurements. We can accurately monitor the probe pressure that the stylus is making when it touches the part, which is important, because on large parts you've got a long stylus, or long extension, and if you put pressure on it, it bends. If you don't know exactly how much pressure you're applying, you don't know how much that sensor is bending, which can be many times what your part tolerance is, if you're not careful."
Aerospace, off-road equipment, and wind-energy components are good examples of applications currently employing large-scale metrology machines. The Zeiss MMZG gantry-style CMM is being used in high-profile applications including the Joint Strike Fighter (JSF) defense program, where primary JSF builder Lockheed Martin (Bethesda, MD) uses Zeiss CMMs to measure large wing skins for the new F-35 fighter (see "Aerospace Demands Precision Inspection," in the March 2009 issue of Manufacturing Engineering).
The range of big CMMs from Zeiss starts with its MMZ-T, a bridge-style machine with a 2-m width and varying Z heights and lengths. Zeiss' lower-cost model, the MMZ-B, is available in a variety of sizes, typically from 2 to 4-m wide with variable Z ranges," says DeGlee. "It's essentially made to order, and it's similar to the MMZ-G, our gantry machine that is designed to be high-precision in a very large machine volume, whereas the MMZ-B is designed to be more cost-effective in the large-machine arena. If you want a machine with a measurement range of 5-m wide by 11-m long with a 3.5-m Z, that's one of the bigger ones."
Applications for large CMMs run the gamut from aircraft to mining equipment, he adds. "It's really any higher-precision machined part. I'm even talking to one customer right now that works in the optics industry," DeGlee says. "We're looking at measuring large lens components, and again, we can do that because of the sensor mechanism on this machine.
"The sensor mechanism that we have as one of the options on this machine is the same as what we have on one of our most accurate laboratory machines," he adds. "That gives us the ability to carry very long sensors, because we have to reach into deep bores, maybe 800-mm long, and we want to accurately measure the diameter of that area. This can be up to 800 mm, almost 3' [0.9-m] long, reaching into the part and trying to measure something within 4µm."
Specializing in large-scale metrology, Wenzel Xspect Solutions Inc. (Wixom, MI), a subsidiary of Wenzel GmbH (Germany), builds several types of big CMMs used in precision measurement of huge parts, as well as machines for inspecting gigantic gears that can weigh many tons. "Large CMMs have been around for decades, and obviously the tool-and-die guys in this town [Detroit] use them extensively," notes Keith Mills, Xspect Solutions president. "Anybody who is machining large fabrications and manufacturing large parts can have some pretty exacting tolerances to meet. With printing presses, some of the true-position tolerances on some of these large frames are very close."
The best large CMMs today are capable of precision to the 15–20µm range, he says. "Everybody used to use the old 10-to-1 rule of thumb—you give up 10% of the tolerance to the measuring instrument. Obviously, with the repeatability of a CMM being pretty good, they've relaxed that, and we get to see 3:1, 4:1 ratios on some of the tighter tolerances," Mills states. "There always was a market for the large-volume CMM, especially with aerospace companies like Boeing. But some new technologies have come on the market, things like laser trackers, that obviously have allowed portability. So you can almost characterize the market for these large CMMs was pretty much the industrializing countries. As the US became saturated with these machines, other industrializing nations, like Korea and China, have picked up demand."
Customers installing Wenzel's large CMMs include Liebherr (Newport News, VA), which recently ordered a second inspection cell from Wenzel. The Liebherr inspection system, to be installed at a new wind-turbine facility in Mexico, combines dual-arm CMM measurement technology with the precision air-bearing mechanics of the Wenzel WGT series gear-checkers and Renishaw scanning probes. "It's measuring the slew gears that actually do the rotation of the wind turbine," Mills notes that the machine, a Wenzel LAF 2510 system, is capable of inspecting bearings and ring gears up to 19.68' (6 m) in diam. The machine is scheduled for use at Liebherr's Nuevo Leon, Mexico, factory and will be operational in late 2010.
"What has fueled these large-volume gantry machines is the wind-energy business," Mills notes. "It's not that the market's evaporated in this country. We have supplied large-volume gantry machines to some of the first-tier Caterpillar suppliers. We recently put a machine into General Dynamics' Lima, OH, facility for measuring a new amphibious vehicle. That was a 4-m-span bridge machine, and it was measuring parts 12-m long and 2.5 m-tall, so the complete cast-aluminum body of this amphibious tank is being measured."
Mixing CMM and gear technology, Wenzel's gear inspection systems feature a large-volume, dual-arm CMM with a granite base equipped with high-accuracy linear guideways for the X axis and a large hydrostatic rotary table capable of handling loads up to 100,000 lb (45,359 kg).
Processing its own granite offers Wenzel an edge over the competition, Mills contends. "All these machines are granite, and Wenzel is unique in that they're the only CMM manufacturer that actually is processing its own granite," Mills states. "Everybody else is getting their granite from China, where they have very rough milling machines rough the blocks of granite. They get them somewhere within, let's say, ±5 mm, but the geometry is all over the place, then it goes to a someone using lapping tools to lap these blocks of granite. Apart from the fact that it takes a long time to do it, it's impossible to keep all the sides orthogonal to each other.
"What Wenzel does is take big blocks of granite and rough-machine them, then put them on very expensive four, five, or six-axis grinding machines. They'll grind the precision into the ways, and so the only hand-lapping that is going on is pretty much to break up the surface and to take out the last micron or half-micron. We're actually machining the geometry of these machines."
Wind-energy applications present great opportunities for large CMM builders for inspecting high-precision gears used in wind turbines, Mills says. "Nobody wants to put a liability on top of a tall pole, so the precision of the gears is critical to the long-term performance of these wind-energy turbines. The CMM measuring these large gears is a requirement, and, of course, what's happened is that the traditional gear guys have tried to stretch their technology. They're struggling with it. Just because a machine is structured to measure a 0.5-m gear doesn't mean to say it can measure a 3-m gear. So Wenzel is able to make these large CMMs that are accurate to half a dozen microns, and we're using very large hydrostatic rotary tables with these machines."
At a bearing supplier in Germany, Wenzel sold a very large machine for measuring the bearing journals on wind-energy turbines, Mills recalls, but the company wasn't using a rotary table. "We showed them this 1.5-m-diam hydrostatic table. The central pinion of the table is suspended in 15 µm of oil film and the table-top itself is suspended hydrostatically. We did a performance run on this table, and the runout of the rotary table was 0.4µm and the wobble, the vertical runout as you rotated the table there, was approximately 0.3µm."
A wide range of large-scale CMMs are available from Hexagon Metrology (North Kingstown, RI) through its DEA and Leitz brands that offer large-volume measurement with horizontal, bridge, and gantry-style machines. "What differentiates the two product lines basically is accuracy," notes Gary Card, DEA product manager. "The Leitz product line is considerably more accurate than the DEA product line, and that's something that shows up in the applications."
The DEA large-volume CMMs offer medium-level accuracies for applications that don't require the utmost precision, Card notes. "We quite often see automotive applications—sheetmetal, castings, forgings, any kind of machined prismatic part—that don't require the high accuracy of the Leitz product," he says regarding DEA CMMs, "and that could be machined parts for cars, airplanes, locomotives, and so forth. We do a lot of applications with airframes such as those from Boeing. We make a full line of standard models, anywhere from our smallest carriage size—meaning the width of the machine, on a gantry-style CMM—from say 2 to 5 m wide, even 6-m wide on specials, and as far as length, anywhere from 3 m to a very long gantry-style CMM."
DEA's largest system, the Lambda, has been deployed for measurement of massive heavy-equipment components, including bulldozers from Caterpillar Inc. (Peoria, IL). "That particular application was for heavy equipment, a D9 bulldozer chassis that weighed several tons, on our largest gantry-style machine, the Lambda," Card notes. "One of the things we have to consider is whether there's a proper foundation underneath the machine. These machines require large reinforced concrete blocks underneath them. Even though we're not the highest accuracy CMM in the Hexagon product line, to maintain the accuracy that the machine is designed for, quite often, we would need to isolate the foundation, so we put pneumatic isolation underneath the concrete block.
"Pneumatic isolation involves a number of large cylindrical components that have a pneumatic reservoir and bladders to absorb any ambient vibration that would be coming up through the floor or the ground underneath. By the time it goes through these isolators, vibration is filtered to a level that the machine can withstand," Card adds. "The other things that we employ in all of our gantry products is something that DEA patented several years back, called the Slant Bridge. It's a triangular-shaped beam that extends the width of the measuring volume. And in our case, it's an aluminum extrusion. Stiffness is very important in a CMM, and by laying the air bearings down on a 45° or lower angle, you can get a much wider spread on the air bearings than if you had them placed in a vertical, rectangular carriage or bridge."
At the high end, the Leitz PMM-G gantry CMM is Hexagon's most accurate large-volume system. The PMMG, which comes in 55 standard sizes, can be used to inspect gears ranging in sizes up to 5-m diam and unlimited weight. The company recently showed an updated model at the Indianapolis Gear Expo, notes Peter Edge, Leitz product manager. "If you look at the design of the frame itself, it has highly machined granite guideways that sit on top of two concrete walls on each side," Edge says. "We had a smaller design and enlarged it. You get more thermal stability, and you can control the environment of the measuring cube much better with the design."
Like some of the DEA machines, the PMM-G includes a ceramic Z ram, and dual drives and dual scales in the X axis. Another feature adding to its accuracy is the addition of the Leitz LSP-S2 probe system, Edge adds. "That's one of the more important factors on the machine. It is a probe head developed by Leitz internally. The significance of that probe is its capacity. A probe head's going to have some styli hanging out of it. It's going to have some probe extension to access the part. It could be at right angles, it could be carbon fiber, titanium, or other materials, but every probe head has limits as to how much weight it can carry. The LSP probe head for the PMM-G can have a maximum extension of 800 mm, and a maximum weight of 1 kg. As you can imagine, when measuring these large components there's going to be some very long probing reaches. If you're trying to measure a cylinder, for example, you've got to get into the cylinder some way, especially if it's horizontal. Sometimes you can have 800-mm extensions."
CNC-driven CMMs from Mitutoyo America Inc. (Aurora, IL) are said to offer advantages over the industry's many DCC-driven large CMMs in the industry today, according to Mitutoyo's John Knutson. Two higher-end bridge units from Mitutoyo, the Crysta and the Bright-Strato lines, offer an integrated Y-axis bearing way that's cut into the granite, Knutson says. "It's then lapped with the air bearings and the pre-loads are against this lapped surface," he says. "What that does, before any kind of compensation is done to the machine, is to make it as mechanically accurate as possible. We make the CMM as mechanically parallel and perpendicular to all three axes as we can.
"The gage mentality is a huge factor with our machines, as well as the fact that these machines are CNC-driven, as opposed to DCC," Knutson adds. "A lot of people use the terms interchangeably, but they're really not. These are CNC, actual computer numeric controlled through a controller on the machine, so it uses the same technology as a high-end machine tool.
"That's huge. Obviously there's pricing differences, because a controller, a CNC controller, is more expensive than an off-the-shelf DCC [direct computer control] control," he states. "Where it comes into play is the capability to drive to a known location within a very small target. If you're measuring small piece parts, or small holes in a large part, and you've got parts nested, you can drive to those locations much more accurately than you can otherwise.
"A good example would be if you issued a command for that CNC to go to a certain location, and say the Z ram carrying your $25,000 probe is an inch above the granite surface plate, and you accidentally tell it to go negative, if you have a DCC machine it's going to try to do that, with horrible results for your $25,000 probe system. But if you have a CNC machine, it knows its volume, and it's going to come back to you and say 'That's out of my measurement range—I can't go there.' You'll get error warnings from it, so it's got some built-in intelligence that keeps you from doing harm."
Aluminum and ceramic-construction large-scale CMMs are offered by Nikon Metrology NV, which recently acquired Metris and now operates as Nikon Metrology (Brighton, MI) in the US. "We have our aluminum machines, the C3 machines made in Italy, and we have our ceramic performance line built in the UK, the LK line, which are bridge machines made in England," notes Joe Szymanski of Nikon Metrology.
Higher-accuracy large machines are being employed in large locomotive transmission cars, large blades for the emerging wind-energy industry, and aerospace applications, Szymanski notes. "We have seen some again in the windtower industry measuring big blades for windmills. We also have a product called laser radar, which is a CNC non-contact automated laser tracker where you don't need to follow and track a SMR [spherically mounted retro-reflector]. You can just set it up like a CMM, write a program and automated script, hit the button, and off it goes to the inspectors. We're finding that in a lot of applications on large structures we almost compete against ourselves."
The large-scale Nikon CMMs often are customized machines, he adds. "We do fairly well on the bridge machines, which have a granite table," Szymanski says. "We'll typically do them up to 5-m long by 2-m wide, then 40 and 60" (1 and 1.5 m) in Z. From our standpoint, when you start getting over that size then we would probably consider the benefits, or at least the cost benefits, of whether you want to go to a gantry-style machine."
Most large parts still require CMMs, although some big workpieces can use laser radar as an alternative means of measurement, he notes. "If you really need a CMM where you need to access parts from both sides and you need to reorient the probe, conventional CMMs are still the standard. But on bigger structures, where possibly you might need two guys to do the measurement or you might have to put somebody on a scissor lift, scaffolding, or a crane, then certainly radar makes the most sense. When you've got to accurately measure hole locations and distances, then a CMM would still be the proper way to do it."
This article was first published in the January 2010 edition of Manufacturing Engineering magazine.