"Driven at the Center of Gravity (DCG)" is a design principle that tackles one of the metalworking industry's top concerns--vibration. Using this radical new design concept, Mori Seiki Co. Ltd., a Japanese CNC (computer numerical control) machine-tool manufacturer with U.S. headquarters in Texas, has taken machine tools to a new level, conquering vibration in the main machine components by putting the moving parts' driving force as close to their center of gravity (or their mass) as possible. The company has unveiled three models of DCG machines: a vertical machining center (the NV4000 DCG) and two horizontal machining centers (the 40-taper NH4000 DCG and the bigger, 50-taper NH6300 DCG).

To come up with this radical new approach, the company questioned the configuration of the basic machine tool structure itself. Its engineers and designers realized that one of the main sources of vibration in any machining system originates from the way it's set up. They observed that applying a force to a mass at any place other than its exact center causes a twisting tendency, which leads to vibration.

"DCG came about when looking at how the basic components of an axis are arranged," David Austin, manager of technical communications at Mori Seiki U.S.A. Inc., tells Industry Week. "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."

By decreasing torque, the company was able to increase the machine's rigidity and reduce vibration, as components are loaded symmetrically. When compared to non-DCG machines, DCG machine tools show greatly reduced residual vibration on the X, Y and Z axes. In fact, the company claims that its new machine tools operate with only a tenth of the vibration of other machines. What's more, dramatically decreased torque and vibration have resulted in increased tool life and improved accuracy.

On the NV4000, the Z axis is driven by dual ballscrews, with the spindle positioned between them. This way it creates the effect of direct alignment with the spindle's mass or center of gravity. The Y axis also features dual ballscrews, which are also in line with the center of mass beneath them. Meanwhile, the X axis has a single ballscrew that moves through the center of gravity of the machine table.

One shop that recently bought the NV4000 has already realized its substantial benefits. "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," comments Jerry Busche, vice president of Indiana-based contract machine house Busche Enterprises, to Industry Week. "Compared to other machines, our productivity is higher, our tooling lasts longer and our cost per part has decreased."

Mori Seiki expects shops that want to improve part quality, instead of quantity, to be drawn to these new machine tools. Potential purchasers include high-end manufacturers in the die/mold, medical and aerospace industries.

Sources:

Technologies Of The Year -- Machine Tools Re-discover Gravity
Traci Purdum
Industry Week, December 1, 2004
www.industryweek.com/CurrentArticles/asp/articles.asp?ArticleID=1707

Center of Gravity is Key to Reduced Vibration
Mark Albert
Modern Machine Shop, March 2004
www.mmsonline.com/articles/0304rt3.html