Toolholder and tool balance

Machine users often hear that they need to balance their toolholders and wonder if it is true. Is the machine or workpiece being damaged because a toolholder is not balanced? The right question is not “should the tools and holders be balanced?” but “when?”

Let’s start with a general discussion of unbalance. The rotating components in a machine tool spindle are many. There is the spindle shaft itself. There may be the rotor of an electric motor or gears or pulleys if the motor is not integral with the spindle. In milling, there is the toolholder, tool, drawbar, grippers and a stack of Belleville washers. In turning, there is the workpiece and possibly clamping devices like chuck jaws.

All of these components are made and assembled by human beings and are not perfect. The rotating assembly’s center of mass and the center of rotation, defined by the spindle bearings that support the rotating assembly, are not the same center. The net effect is the same as if there is a mass attached off center on the spindle. The size of the unbalanced mass and its distance from the center of rotation play the same role, and those terms usually are grouped together. The unbalance is expressed in g-mm or oz.-in.

In most cases, the user has no ability to adjust the balance of the spindle, and the balancing actions, if any, are applied to the toolholder and spindle.

The permissible unbalance (U) in g-mm is given by

U = (G × 9,549 × W) / rpm

Where G is the balance grade, 9,549 is a constant that makes the units consistent, W is the mass of the rotating object in kg and rpm is the spindle speed.

The balance grade depends on the application. For example, G6.3 is typically specified for general-purpose machines, G2.5 for high-speed machines and G1.0 for precision machines. The permitted unbalance depends strongly on the mass of the object to be balanced, and you will get different results for the tool, the tool and holder, and the entire spindle assembly.