Technologies Of The Year -- Machine Tools Re-discover Gravity

Jan. 14, 2005
Mori Seiki's driven-at-the-center-of-gravity technology improves accuracy and lengthens machine-tool life.

Driven at the Center of Gravity (DCG). The phrase conjures up various images -- one being an apple falling dead center on Sir Isaac Newton's head. The other image is that of Mori Seiki Co. Ltd., a Japanese CNC machine-tool manufacturer with U.S. headquarters in Dallas. To Mori Seiki, DCG technology means a better machine tool that reduces vibration in the primary machine components by minimizing the distance between the center of gravity of the moving components and the driving force of that component. Its newest vertical machining center, NV4000 DCG, introduces the DCG technology. "DCG came about when looking at how the basic components of an axis are arranged," says David Austin, manager of technical communications at Mori Seiki U.S.A. Inc. "Simply driving an axis by placing the ballscrew in between the linear guides is not always the best way to do it. The best way to do it is to align where you are driving the axis with where the thrust of the axis should be. By aligning that, we eliminate the torque in the axis." Reducing torque not only makes the machine more rigid because the deflection has been reduced, DCG reduces the amount of vibration because components are loaded symmetrically. In fact, when comparing residual vibration between a non-DCG machine tool and a DCG machine tool, there are significant improvements on the X, Y and Z axes (18.07 _m to 0.13; 7.20 to 1.38; 11.22 to 0.59 respectively.) "We always knew that decreasing vibration and decreasing [torque] would increase tool life and increase accuracy, but really what has driven this innovation is the state of the art of the machines," says Austin. "Machine tools today are very accurate, very good at delivering tool life. We needed to take the technology to the next level of tool life vibration and accuracy." On the NV4000 DCG, dual ballscrews drive the Z-axis with the spindle mounted between the ballscrews. The Y-axis also has dual ballscrews, which are mounted outside the center of gravity. The X-axis has a single ballscrew positioned to act through the center of gravity of the machine table. While dual-ballscrew technology has been around for a long time, the focus was never the reduction of vibration -- it was increasing acceleration and deceleration, according to Austin. "We're able to offer most of the benefit in terms of speed, but the biggest benefit we offer is to reduce vibration, which means better tool life and better accuracy," explains Austin. "The simulated center of gravity that results from placing the spindle between twin ballscrews has led to a lot more rigidity than we had in the past," says Jerry Busche, vice president of Busche Enterprises, an Albion, Ind.-based contract CNC machine house that recently purchased a Mori Seiki NV4000. "Compared to other machines, our productivity is higher, our tooling lasts longer and our cost per part has decreased." To aid in production of its DCG machine tools, Mori Seiki was able to pool from 450 engineers from around the globe. Additionally, its West Sacramento, Calif.-based Digital Technology Laboratory (DTL), a wholly owned subsidiary of Mori Seiki Co. Ltd., provided the software and computer resources required to perform extensive computer analysis on the DCG machines. "[DTL] looks at modeling every performance aspect of the machine before prototypes are created," says Austin. "They not only look at the basic strengths of the machines, but more importantly how the machine responds to heat from the spindle . . . [and] the linear guide? The extent that the engineers at this division analyze this machine is just incredible -- I'm amazed by it every time I go there and visit."

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