Machine bed support key to accuracy

All images Courtesy of S. Smith

Figure 1. Supporting machine tool beds at the Airy points minimizes deflection, which is greatly exaggerated in this illustration.

A machine tool bed serves several important functions. It supports the machine’s moving elements and supports them in geometric relation to each other. The relationship between the moving elements is more important than the relationship between the moving elements and the ground the machine is based on.

It is typically desirable for the bed to be as flat as possible, which requires considerable effort to scrape or grind the surfaces on which the guide ways are mounted. The bed should be stiff, thermally stable and able to absorb or dissipate vibration. When the machine is installed, the bed must be aligned.

Bed flatness depends on the connection between the bed and the foundation. In geometry, three noncollinear points are enough to define a plane. If the machine bed is very stiff, then three connection points between the bed and the foundation are sufficient to support the bed. In such a case, machine accuracy does not depend on foundation accuracy. Machines with work spaces up to a cubic meter or more can be found with three mounting points.

Even a stiff bed is not infinitely stiff. The bed sags a little between the mounting points. The sag can be minimized by placing the supports, or feet, at the Airy points (Figure 1). (Airy points are used in precision measurement to support a length standard in such a way as to minimize bending or drooping.) The distance between the supports (d) should be about 60 percent of the bed length (L).

The distance from either end of the bed to the supports should, therefore, be about 20 percent of the bed length. In Figure 1, two of the three supports should be aligned on Airy points, and the third support should be centered on the bed (see Figure 2 on page 24). With three-point mounting, machine alignment is maintained, even if the temperature changes and the bed has a different coefficient of thermal expansion than the floor.