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.

For machine tools, a more important consideration is whether the force produced by the rotating unbalance is an appreciable fraction of the force produced by the cutting operation. All of the forces, whether derived from cutting or from unbalance, must be carried by the bearings and by the connections between the tool and the spindle. When roughing, where the cutting force can be hundreds of newtons, an unbalance force of several newtons is irrelevant. However, in a precision machining operation, where the cutting force is fractions of a newton, that same unbalance would be a big problem. So how can the unbalance force be computed? 

The force is given by 

F = meω2

Where F is the force, m is the unbalanced mass, e is the eccentricity (the distance of the mass from the center of rotation), and ω is the spindle speed. 

The force increases with the square of the spindle speed. If the spindle speed doubles, the force produced by the rotating unbalance increases by a factor of four. For example, a tool and holder with an unbalance of 5 g-mm on a spindle rotating at 5,000 rpm produces a force of 1.37 N.

That 1.37 N force is about 0.3 lbs. The same tool and holder in a spindle rotating at 40,000 rpm produces a force of 87.7 N, or about 20 lbs. 

Reducing those forces through balancing is beneficial, but balancing also comes with costs. The shop either has to buy a balancing machine or pay for a balancing service. Balanceable toolholders usually have one or more weighted rings whose position can be adjusted to change the balance condition. Such holders are more expensive than conventional holders and require additional length on the toolholder to accommodate the rings. The added length reduces stiffness and typically reduces the achievable metal-removal rate. 

A good strategy for deciding about whether or not to balance is to compare the unbalance force to the cutting force. If the unbalance force is as large as 5 percent of the cutting force, then improved balance is desirable. In general, balancing is much less important for machines with low spindle speeds, and more important for higher-speed machines. Even with high-speed machines, many shops can get by using prebalanced holders and prebalanced tools. Very precise and high-speed applications, however, require balancing the toolholder and tool assemblies. CTE

About the Author: Dr. Scott Smith is a professor and chairman of the Department of Mechanical Engineering at the William States Lee College of Engineering, University of North Carolina at Charlotte. He specializes in machine tool structural dynamics. Contact him via e-mail at kssmith@uncc.edu.