Center Of Gravity Is Key To Reduced Vibration

The metalworking industry’s keen interest in high speed, high accuracy machining during the last decade has focused attention on one topic in particular—vibration.

As an indication of the high priority given to vibration control, consider how one machine tool builder made it the overriding design issue in its latest development in vertical and horizontal machining centers. Mori Seiki’s designers and engineers started with the premise that one of the chief sources of vibration in any machining system is the configuration of the basic machine tool structure itself. Specifically, they concentrated on relationships among the moving axes—how force is applied to move masses in several directions at once. One of the fundamental insights was that any time force is applied to a mass at any spot other than its exact center, a tendency for a twisting motion develops. This twisting inevitably causes vibration, the recurring movement error caused by inertia acting on the flexibility of machine structures such as base or column castings.

Most machine tool designs, the engineers concluded, do not adequately address this issue. It was clear that a radical approach to machine tool design had to be developed—one that put all driving forces as close to the center of an axis’s mass as possible.

Because a ballscrew can’t be inserted directly through the center of gravity when it is occupied by a machine component, designers developed an arrangement of two ballscrews aligned with the center of gravity to create virtually the same effect. For example, on a horizontal machining center, the fourth axis table lies at the center of gravity on the Z axis, which does not allow placement of a ballscrew at the optimal location. As Mori Seiki puts it, “the net driving force acts through the center of the axis.” The company is using Driven at the Center of Gravity (DCG) as its trademark to designate this design principle.

So far, the company has introduced three models of DCG machines: a vertical machining center (the NV4000 DCG) and two horizontal machining centers (the 40-taper NH4000 DCG and the larger, 50-taper NH6300 DCG). These machines follow the DCG design principle, along with two other design principles. The first allows machining forces to traverse across upright structures through the bed to form a closed loop. The second involves digital analysis to minimize mass in the castings while retaining strength and vibration damping characteristics. According to the company, by incorporating these design features, the machines exhibit 10 times less vibration in operation. The benefit is significantly higher feed acceleration/deceleration rates without degradation of surface finish or geometric accuracy. Tool life also improves.

The construction of the vertical machining center shows the most obvious departures from more conventional designs. Most notable is the arch-shaped column and how it carries the Z axis. The arch shape of the column is such that the top of the arch is recessed so that the Z-axis carriage can be centered between twin ballscrews that are aligned with the center of the column’s mass. Thus, there is zero overhang in the whole structure.

Twin ballscrews are also used in the Y axis. The drive units are positioned on opposite sides of the bed casting and aligned with the center of mass beneath them. These ballscrews are likewise in line with the center of the carriage so that drive forces are as near to the worktable’s upper surface as possible. The X axis is driven by a single ballscrew in the center of the table. This ballscrew is as close to the Y-axis ballscrews as clearance allows. Because the center of gravity for the X axis is unoccupied by any other machine components, it can be driven directly with a single ballscrew. The company says it can offer these five-ballscrew VMCs at three-ballscrew prices, thanks to manufacturing efficiencies at the factory.

The horizontal machines feature a box-in-box construction. Twin ballscrews are used on the X axis, whereas the Y axis has a single ballscrew to reduce mass. This arrangement achieves the optimal trade-off between acceleration and contouring accuracy, developers found. Ballscrews on the X axis are located at the top and bottom of the saddle and aligned with its center of mass to achieve the virtual effect of driving at the center of gravity.

Shops that need better rather than more parts will be attracted to these models, the company says. High-end manufacturers in the die/mold, medical and aerospace industries will be likely buyers.