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A CNC Machine OEM Pushes the Limits of Repeatability

A CNC Machine OEM Pushes the Limits of Repeatability

The current generation of CNC machining systems has features that significantly improve metalworking operations from what they were only a few years ago. These machines are generally faster, more rigid and quieter than ever, have powerful controls and extensive automation capabilities, and fabricate a wider range of metals.

Advances in machine design also have yielded extremely high levels of metalworking precision and repeatability. Although these parameters vary by manufacturer, model and application, accuracy of around 0.00004 inches and repeatability of 0.0008 inches over a defined range of machine travel are typically claimed of jig bore machining systems.

One OEM, however, states that the precision of its machining centers, as measured by repeatability, is considerably greater than the industry average. Yasda Precision Tools, a Japanese company, reports a repeatability of 0.3 micron over 1 meter (3.281 feet) of travel on all of its 3-, 4- and 5-axis vertical and horizontal machining centers and jig borers.

For those interested in the conversion, 0.3 micron equates to 1.18110236 x 10-5 inches -- the approximate size of some bacteria. Even in the event of a little deviation during milling and boring, Yasda's stated capability here puts it among a select group of OEMs.

For this reason, I interviewed Stephen Previti, Yasda product manager at Methods Machine Tools Inc., of Sudbury, Mass., which sells the company's machines in North America. (Yasda's North American headquarters are in Elk Grove Village, Ill.) I was curious to learn how Yasda achieves such stated high repeatability and other measures of accuracy, whether these extreme specifications are really necessary on the shop floor, and what type of manufacturing customers invest in such machine capabilities.

My conversation with Previti came shortly after Methods announced, in January, an addition to the Yasda line, the YBM Vi40 5-axis vertical CNC jig-boring milling machine.

Previti said that as impressive as the repeatability is, it's only one result of a system-wide approach to machine design and construction that Yasda has practiced for decades. The company uses a bridge-style design for machining centers. Each guideway is constructed of Rockwell 62 hardened steel, and the machine design itself is symmetrical, meaning that the left and right sides of the machining area are mirror images of each other, in order to reinforce balance, accuracy and rigidity.

All of this seems conventional so far, but each machine undergoes a number of procedures to build in accuracy, Previti said.

These include an average 400 man-hours of manually scraping and hand-lapping each mating surface to eliminate differences in yaw, pitch, and roll between them. Workers use chisels to scrape surfaces perfectly square, according to Previti, and then hand-lap them using a cloth and special compound.

The manual procedure removes the effect of grinding, in the form of machine noise, on the guideway, he noted. The grinding may only have a "sub-micron influence" on accuracy, Previti said, but it's enough to affect machining operations.

The scraping is meticulous work. The personnel who do it are craftsmen whose apprentice periods last 10 years or more.

As these steps are underway, technicians use an automatic laser collimeter to measure the yaw, pitch, and roll of each surface. The laser beam is aligned down the guideway of a machine.

Previti said that these measurements determine the amount of chiseling and lapping that is necessary to "completely remove traces of yaw, pitch, and roll" from critical surfaces.

"A machine, as manufactured, will be as true 20 years from now as when it left the factory," Previti asserted.

Other design techniques that Yasda employs to fine-tune its machines include the installation of a closed-loop cooling system that reportedly maintains temperature to within 0.2C of ambient. The cooling system also regulates the temperature of the solid, highly polished ball screw in each machine, and prevents the creation of a thermal delta between the ball screw and other components.

Yasda uses an isolation coupling that prevents a direct connection between the machine spindle and motor assembly. This eliminates radial influence and mechanical noise being transferred from the motor to the spindle, which could affect accuracy and repeatability, Previti said.

Despite the effort that goes into building the machines, is this level of performance really necessary on the shop floor? According to Previti, it is, although he added that Methods seeks to qualify customers' applications before recommending Yasda machining systems.

"The machines are very application dependent," he explained. "We want to make sure that the value of the parts planned for them makes sense."

Markets for Yasda machines include any application that uses advances metals for critical components -- for example, aerospace, medical devices and transportation.

The Yasda machining centers probably aren't the best options for low-tech parts -- especially since they carry a price premium. But for part manufacturers that machine either complex, high-tech and/or expensive components, as well as very hard materials, a Yasda can be an asset.

Previti noted that in most cases a machine shop can consolidate operations with a Yasda machine. As an example, he pointed to the machining of cavities with high surface finishes for plastic injection molds. "This would typically require someone to first EDM (electrical discharge machine) the cavity, then finish-machine and hand-polish it. A Yasda can do all of these operations in one step and reduce manufacturing time by 60%," he said.

Efforts to contact competitive CNC machinery OEMs and machine shops that are known to use Yasdas were mostly unsuccessful: companies either declined to respond or did not return telephone calls.

One tooling OEM who did respond is Preben Hansen, president of Heimatec Precision Tools, a German company with US operations in Prospect Heights, Ill. Hansen said the company uses a 5-axis Yasda horizontal machining center in its German factory to fabricate critical parts for live tools, lathe components, and other products.

"The tools we build have lots of bearings in them, and their service lives depend on how well they fit," he said. The Yasda machine allows Heimatec to achieve a high degree of accuracy in boring out housings, which improves the operation and duration of the bearings.

Watch the Yasda Vi40 machining center in action:

Design engineers and professionals, the West Coast's most important design, innovation, and manufacturing event, Pacific Design & Manufacturing, is taking place in Anaheim, Feb. 10-12, 2015. A Design News event, Pacific Design & Manufacturing is your chance to meet qualified suppliers, get hands-on access to the latest technologies, be informed from a world-class conference program, and expand your network. (You might even meet a Design News editor.) Learn more about Pacific Design & Manufacturing here.

Pat Toensmeier has more than 30 years of experience writing for business-to-business publications. His main areas of coverage have been defense, design, manufacturing, technology, and chemicals, especially plastics and composites. He has reported extensively on developments in these areas from the US and Europe, and covered industry events as well in Brazil and Asia. Toensmeier has held various positions at major publishers such as the McGraw-Hill Companies and Hearst Corporation. A graduate of the University of Missouri, he is a contributing editor for several print and online publications. Toensmeier is based in suburban New Haven, Conn.

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