Through Heat or Dirt and Dark of Night ...
Ford Brazil recently integrated five of them onto its shop floor. Up north, in Michigan, GM Powertrain has eight of them that it uses to inspect cylinder blocks, heads and intake manifolds for automobile engines. DVD manufacturers have them, as does an aluminum extruder. And some companies have even designed them into systems for use in unattended, lights-out factory environments.
What is them? "Them" are coordinate measuring machines (CMMs) that have left behind the shackles of the metrology lab and found a home in the shop, near production, where they can best be used to conduct fast and accurate 3-D measurements of parts as they are made.
These companies are not alone. Manufacturers of a wide variety of products have successfully overcome the difficulties of inspecting parts near production areas. The CMMs can be found on transfer lines, inside manufacturing workcells and as part of elaborate multitechnology turnkey systems. In some cases, the CMMs are off-line but nearby, so that parts can be quickly conveyed or hauled by robot or forklift from machine tools to CMMs for inspection. These sophisticated measurement, test and inspection tools are being used for first article inspection, 100% inspection, and random inspection in pre-process, in process and post process placement-all with the idea of discovering nonconforming parts before they are shipped to customers or to the next production process.
And it is not just large automobile or aerospace companies that are using CMMs in this way. One of the strengths of today's CMM offerings is that they are affordable for smaller companies while still being factory friendly enough to allow them to be used without having to construct an expensive metrology lab.
Smaller companies such as Wire-Tech Inc. (Tempe, AZ) have taken advantage of the precise 3-D measurements possible with coordinate measuring machines, and the company did so without having to build a separate lab. Wire-Tech is a 10-man Electrical Discharge Machining (EDM) company with 12 CNC machines in its 10,000-square-foot shop. The company needed the CMM to increase its capabilities and decrease its inspection time. The CMM reduced inspection time on some products from as much as eight hours to as little as two hours. "Because we're in a competitive industry where we EDM a lot of plastic and metal injection molding, we need to check every part as fast and as accurately as possible," says president Rick Erickson. "Even though some EDM jobs require hundreds of electrodes, the CMM allows us to check all the electrodes in at least half the amount of time and more accurately than before."
CMMs were introduced in the early 1960s, but not until the 1990s did CMMs really start to come out from the clean, temperature-stable environment of the laboratory. As part measurements increasingly moved from the lab to the plant floor, CMM suppliers realized that their products must be tough enough to take their places near the machine tools and still provide the kind of accuracy, repeatability, and data-analysis capability needed.
That means that the CMMs must be shop-hardened with temperature-resistant materials, antivibration systems, protected guideways such as bearing and scale covers and scale systems that have low coefficients of thermal expansion (CTE).
Variations in temperature are generally considered to be the biggest problem to be overcome. Temperature affects the base material at the microscopic level and the cumulative effects cause structural changes in both the CMM and the parts being measured. Objects are at their correct size only at 20 C or 68 F and any differences in temperature can affect measurements. The extent of the change depends on the material that the CMM is made of, as well as the material that the part to be measured is made from.
That is why it is important for potential users to know the materials that are used to construct the CMM. For instance, the CTE for steel is approximately 11.5 parts per million per degree and aluminum is 23.1 parts per million per degree. For every temperature increase of 1 degree, steel would expand by 11.5 parts per million, while aluminum will expand at 23 parts per million. A trend among some CMM manufacturers is the integration of a nickel alloy called Invar (Quality, Aug. 2001, p. 26). The alloy contains 64% iron and 36% nickel. Invar's CTE does vary depending on the exact alloy content of the material, but it has an expansion coefficient of less than 1 part per million per degree of Fahrenheit.
PCC Airfoils (Mentor, OH), a manufacturer of turbine blade castings used in jet engines and land-based power generators, introduced CMMs to reduce its gaging costs. But, it was concerned about CMM construction materials. "That was a big consideration," said Mark Kostur, a computer applications engineer in PCC's quality inspection department in a March 2000 interview with Quality magazine. "We wanted the thermal stability that granite and ceramic construction provides, and we did not want to deal with the inaccuracies that temperature fluctuations can induce in aluminum."
Hot and cold
Built-in temperature compensation as standard equipment has become a minimum requirement for many companies appraising CMMs. Sensors on today's CMMs allow for real-time temperature data to be taken on the shop floor and the CTE to be factored. Adjustments are made to the measured results.
Experts say that sensors should be placed in at least three elevations: at the work surface; at the midsection of the machine, such as the bridge on a bridge-style CMM, for example; and at the extreme position at the high spot of the vertical axis. The actual number of sensors needed on a machine depends on the required tolerance; as many as 18 to 20 sensors could be required. The sensors can be screwed or bolted to the part, and some companies use modeling clay to affix the sensors to the part because of its ease of use.
Controlling the part temperature may come through acclimation, where the part is allowed to soak to the required temperature, or it is air or liquid cooled. Allowing a part to soak up its new surroundings can be time consuming and cause production queues to build. Sometimes as little as 15 to 30 minutes of soak time is enough for the part to acclimate itself to new temperatures. The rule of thumb is that low-mass, or thin-walled parts, will cool or heat up quickly, while high-mass, denser parts will change temperature more slowly.
Another option is to enclose the CMM in a cabinet. In addition to keeping temperatures more stable, this approach also protects the CMM from airborne contaminates, although parts still may need to acclimate to the temperature inside the cabinet.
The Powertrain Systems Div. of Federal Mogul (McConnelsville, OH) manufactures large bearings for the locomotive industry. The company uses an enclosed CMM in a manufacturing cell. But because the shop floor has airborne contaminants, Federal Mogul needs to take special precautions; even a slight piece of dirt that lands on a part could cause the machine to measure the part as out of tolerance. To alleviate this concern, a parts washer was integrated with the CMM system. An operator manually loads and unloads parts, and ensures that parts are put on the machine clean.
Another company that went with a self-enclosed CMM is SouthTech Inc. (Tappahannock, VA), an office equipment parts manufacturer. The company, which performs metal stamping, high-volume subassembly and automated plastic injection molding in a single plant, purchased an enclosed CMM. The CMM checks 75% to 85% of the measurements on each part produced, depending on the part's features. Thanks to the CMM enclosure, the company says it was able to avoid having to construct an environmentally-controlled area, which would have been expensive.
"We use the CMM to quantify all our tools," says David O'Connell, QA specialist at SouthTech. "We check a part's hole locations, overall lengths and widths, parallelism and perpendicularity."