Tool balancing basics

Unbalanced tools are a problem. They generate vibration, which may lead to broken tools, shorter spindle life and higher workpiece roughness.

Implementing balancing protocols for a machining process helps protect a machine tool. Balanced tools reduce vibrations and are needed whether cutting at 44,000 or 3,000 rpm.

Like many things, tool balancing merits a cost-benefit analysis. While new technology might affect the way balancing is performed and justified, the need for analysis is unchanged.

Cause of Imbalance

Tool imbalance is caused when the center of gravity of a rotating mass is not aligned with its rotating axis.

“So the unbalance is a distance that creates an eccentricity,” said Manager Francesco D’Alessandro of Balance Systems Corp. in Wixom, Michigan. “This eccentricity creates vibration.”

The Tool Dynamic TD Comfort balancing machine (above) is equipped with a PC, keyboard, mouse and monitor. Image courtesy of Haimer USA

Defined as the distance from the center of rotation of a tool to its true center of mass, eccentricity measures the extent to which tool weight is off-center. If eccentricity is measured in microns and tool mass is measured in kilograms, these units yield imbalance in gram millimeters, a common unit.

President Thomas Hoenig of GTI Spindle Technology Inc. in Manchester, New Hampshire, agrees that tool balancing is extremely important. Imbalance damages spindle bearings and other spindle components. Imbalance also causes chatter on machined and ground parts. And unbalanced tools cause premature tooling wear through taper damage to the tooling interface.

“Tooling balance should be done almost always,” he said. “The only time it does not apply is on very slow rpm machining operations, such as tapping holes, or some slow lathe cuts. Any machining operation over 3,000 rpm, balance is critical.”

However, President Brendt Holden of Haimer USA LLC in Villa Park, Illinois, said in many cases, some of the most dramatic advantages of balancing tooling assemblies can be found at low rpm for things like boring heads or facemill arbor assemblies.

“Our advice is to always look at applications that present challenges that no one can figure out how to solve,” he said. “Maybe they looked at the fixturing or the machine condition or the cutting tools, but still they are not getting the performance they wanted. Did they look at balancing? If not, this would be the time to investigate balancing.”

Holden said people today want to reduce cycle time, which may mean decreasing the number of tool changes. If a shop formerly used roughing, semifinishing and finishing tools to complete a part but now prefers to use only one tool for the job, that can cause issues with balance. Also, a special tool might be designed to drill, counterbore and chamfer. With pockets in different positions, more attention must be paid to balancing to achieve quality specifications.

Necessary Balancing

“Theoretically speaking, balancing is always necessary,” D’Alessandro said. “Although in case of very low spindle rpm (less than 3,000 rpm) and poor workpiece finishing goals, tool balancing might be skipped.”

That said, modern machine tools tend to have high spindle speeds to achieve greater cutting volumes or reach high cutting speed requirements — for example, when machining Inconel.

“The increases of these values have also raised the demands for better tools and holders, which then require to balance the tool more frequently than before and at a lower residual imbalance,” D’Alessandro said. “In order to monitor this phenomenon, it may be helpful installing on the machine a permanent spindle monitoring system, such as the B-Safe sensor, which is able to give visual aids and alerts to the operator when the vibration is too high.”

The Best Balance 1000 is a portable device for measuring and correcting vibrations of rotating components due to imbalance. Image courtesy of Balance Systems

Sources said companies gain 15% to 25% in productivity and tool life on average when they begin balancing tooling assemblies, with even better gains if they started with poor balance situations. Although balancing will not solve every problem with a cutting process, estimates are that balancing can solve at least a third of machining issues, especially ones that never seem to go away.

D’Alessandro said the most common problems that end users experience before calling Balance Systems are tool breakage and workpiece chatter. Analysis often reveals that these difficulties are due to vibration, which is generated mainly by tool imbalance.

“The imbalance is sneaky because it cannot be seen and can be felt only when it is too late,” he said. “For this reason, I like to call it the silent killer of machine tools.”

Holden said his most frequent questions from customers are not about balancing — at least at first.

“However, during our discussions the topic of balancing comes up as a clear need as it relates to solving their problems,” he said. “For example, our customer might call about surface finish or tool life issues. Then upon examination, we determine that they have a runout problem. While that runout might not be visible via static checking on a tool presetter or during a slow manual rotation in the machine spindle, once the tool is spinning dynamically at the machine-set rpm, the runout increases if the toolholder assembly is unbalanced. Then we run a test to balance the assembly and find that once balanced, the runout goes away and the customer is happy.”

Holden said customers also complain that they are not able to run at desired speeds and feeds when machining a part.

“Diving into things, we realize that some of the toolholder assemblies used in the production process had unbalance,” he said. “Then they balance the assemblies, and the customers are then able to machine their parts at the appropriate feeds and speeds as they had originally planned.”