Balancing on the Fly

Ten years ago, there were no balancing requirements for tools and toolholders used on machining centers. But as spindle speeds, material-removal rates, and the demand for improved surfaces all increased, balance became an issue.

As a first step, some companies introduced systems that allow users to balance their tool-and-toolholder assemblies on balancing stands before they mount them into the machine tool spindle. Users have found that these off-line systems do provide some of the benefits of tool balancing. However, says Thomas Schulte, vice president of BalaDyne Corp. (formerly known as Balance Dynamics Corp.), Ann Arbor, MI, they also have found balancing off-line to be a time-consuming process that does not correct unbalance introduced after the tool and toolholder are on the machine.

The next step came with the introduction of portable balancing equipment and toolholders equipped with balancing rings. These mechanisms allow users to balance their tools on the machine, but they have their drawbacks as well, according to Schulte. Users typically balance their tools with these systems once, just before the tool is used for the first time. Any unbalance introduced during the process remains uncorrected. These systems also cannot interface with the machine’s controls to alert the machine to dangerous vibration levels.

Seeing these disadvantages, some in the industry have called for a balancing system that can keep tools and toolholders balanced throughout the course of an application. Such a system would make it possible to adjust tool balance on the fly, responding to all process changes before vibration levels can begin to affect the tool’s performance.

According to a survey sponsored by the Boeing Corp., Seattle, WA, and conducted by BalaDyne, machining center manufacturers and users believe that the use of such an active balancing system could result in improved machined surface finishes, extended bearing and spindle life, increased cutting speeds and feeds, extended tool life, enhanced operator safety, and a reduction in the degree tool-and-toolholder assemblies need to be balanced before they are put to use cutting chips. BalaDyne has developed such a system. To further develop the system, BalaDyne has been awarded an Advanced Technology Program grant by the National Institute of Standards and Technology, Gaithersburg, MD. Tests conducted to date on the system indicate that it is capable of fulfilling users’ expectations.

Becoming Unbalanced

By monitoring and correcting the balance of the tool-and-toolholder assembly from the beginning to the end of an application, an integrated balancing system can keep tool vibrations at an acceptable level. Tool vibration has become a serious concern of late because of the increased speeds shops are using to machine metal. When a tool’s center of gravity is not in line with its axis of rotation, the unbalance can generate a large degree of force. The magnitude of this force is related to the square of the tool’s speed of rotation. In other words, when the operator doubles the speed of the tool, the force generated by any unbalance will be quadrupled.

There are many factors that can cause a new tool or a tool that was once balanced to be unbalanced. Some unbalance may be introduced by variances in the manufacturing of the toolholder. Sources of unbalance might be the drive slots in a CAT-type toolholder or the unground base of the V-flange. Variations in the position of the retention knob also can cause a tool to be unbalanced.

A tool-and-toolholder assembly that was balanced before it began making chips can become unbalanced after a component is replaced, the tool is reground, or the tool or toolholder is adjusted or modified in some other way. More unbalance may be introduced when the tool-and-toolholder assembly is mounted in the spindle, even if a prebalanced assembly is used. It is almost certain that a toolholder will not be in the same position in the spindle as it was in the off-line balancer’s mount, and even small changes in its position will throw off the balance that was so carefully achieved on the balance stand. In addition, there will be variations in the spindle and drawbar assemblies that will affect the tool’s center of gravity, especially if an HSK holder is used. Variations in the Belleville washer assembly stack can be another source of unbalance.

Off-line balancing can correct some of the balance problems mentioned, but it clearly cannot compensate for unbalance introduced between the time the tool-and-toolholder assembly leaves the off-line balancer and the time it is actually making chips. The logical step is to integrate the balancing system into the spindle. This allows the operator to balance the tool-and-toolholder assembly as the last step before machining begins and at regular intervals throughout the application.

Integrated System Components

Figure 1: The components of an integrated balancing system provide a feedback loop by which tool balance can be monitored and maintained. Click here for a larger version of this image.

The first integrated system to be marketed consists of a balance actuator mounted permanently to the machine tool’s spindle and a controller mounted at the operator’s pedestal or control board (Figure 1). To monitor tool vibrations, the system uses a sensor, such as an accelerometer, mounted on the spindle housing near the plane of the balancing actuator.

The two parts of the actuator are the balancer-ring assembly mounted on the spindle shaft and the stationary coil assembly. Power is passed from the stationary coil to the rotating ring by inducing magnetic fields across an air gap. This magnetic force changes the position of two counterweighted rotors inside the ring assembly. The rotors can be positioned relative to each other as the balancer rotates with the toolholder. Once moved into position, the rotors are held there with permanent magnets. BalaDyne’s literature says that the magnetic force is strong enough to keep the rotors in place even during rapid acceleration and deceleration and periods of high vibration.

Figure 2: In an integrated balancing system, the degree and direction of the balance correction is determined by the relative position of the unit’s two counterweighted rotors. 

The correction for any unbalance comes from the position of the rotors relative to each other (Figure 2). If the counterweighted portions of the rotors are positioned opposite each other, they will counteract each other and no correction is made. If they are positioned adjacent to each other, a maximum correction is made. The rotors can be positioned anywhere between these two extremes. BalaDyne designs each model to provide the maximum amount of correction that is appropriate for the machine on which the model is mounted. For instance, the unit designed for a 15,000-rpm Fischer spindle is capable of countering 230g-mm of unbalance. This matches the capacity of a Kennametal HSK 100 balanceable toolholder. Other units are designed with different capacities. Some units used in nonmachining applications are capable of correcting up to 50,000g-mm of unbalance.

The balancer’s controller determines the amount of correction needed using data it receives from the vibration sensor. The sensor is normally oriented to measure radial spindle-housing vibration. The controller also receives information from other sensors about the spindle speed, phase reference, and balance-weight positions. This information is processed to compute the vibration amplitude and phase angle. The controller then displays the results of its calculations.

When the sensor readings tell the controller that the tool-and-toolholder assembly is unbalanced, the controller tries to determine the position of the rotors that will best reduce the amount of vibration at the sensor. The controller then sends power pulses to the coil assembly to move the rotors to these positions. A continuous-feedback system tells the controller that the rotors are in position. If the first trial of the automatic cycle does not reduce vibration to acceptable levels, additional cycles are performed until acceptable results are achieved.

The balancing system’s controller can also be set up to communicate with the machining center’s controller. This allows the balancing process to be integrated automatically into the machining process. Schulte says the balancing routine is typically performed after tool changes. On lengthy operations, the tool may be checked periodically to detect any degradation in its balance.

When the balancer’s controller is linked with the machine’s controller, the balancer’s controller will wait for the machine’s controller to send a "read" signal, indicating that the spindle has reached operating speed and is ready for a vibration reading. This reading is taken while the cutting tool is out of the cut.

On receiving the read signal, the balancer’s controller reads the vibration and compares it to a user-set high limit. If the vibration exceeds this limit, the balancer’s controller sends an alarm to the machine’s controller. This tells the controller to stop the machining cycle until the unbalance can be corrected. Upon receiving the alarm, the machine’s controller tells the balancer to run its automatic balancing routine.

Through successive trials, the balancing system reduces the vibration level. When this level drops below a low limit set by the user, the balancer’s controller stops sending an alarm signal to the machine’s controller. At this point, the controller can resume the machining cycle.

Putting It To the Test

BalaDyne ran a series of tests on its integrated balancing systems. These tests indicate thaactive balancing using a system integrated with the machine’s spindle can reduce vibrations significantly. In one test, a long tool with unbalance at its tip was run in a spindle at various speeds. When the integrated balancer was activated, tool vibrations remained at a low level throughout the range of speeds. Without the balancer, vibration levels rose dramatically as the speed increased (Figure 3).

Figure 3: As this chart shows, vibrations increased as the spindle speed rose when an integrated balancer was not used, but remained at acceptably low levels when the balancer was activated. 

Figure 4: The rise and fall of vibration levels on this chart are the result of the integrated balancing system seeking the proper degree of correction through trial and error. 

The results of another test show how the system uses quick series of trials to arrive at the optimal balance. In Figure 4, the vibration recorded actually gets worse immediately after the balancer is enabled. This is because the balancer’s controller initially moved the balance rotors the wrong way. It took the system less than a second to correct itself, however. By analyzing feedback from the vibration sensor at the spindle, the controller was able to reverse itself and bring the vibration level down to an acceptable level. From the initial trial to the final correction took less than three seconds.

The balancer uses parameters to calculate the amount of unbalance and the location of the unbalance. According to Schulte, the controller can improve upon and refine these parameters by comparing the balance result to the balance calculated. These refinements then allow the balancer to bring vibrations down to an acceptable level faster in subsequent balancing cycles.

In a third test, an HSK toolholder with a 310mm-long test rod mounted in it was used. The toolholder was mounted in a 15,000-rpm Fischer spindle. An unbalance of 70g-mm was imposed on the end of the tool. This degree of unbalance represented about a third of the total balancing capacity of the balancing system installed on the machine. Figure 5 shows the results of this test. As the chart indicates, once the active control was enabled, the system took less than a second to bring the vibration level to a point below the required limit.

Figure 5: In this trial, the integrated balancing system took less than a second to correct for unbalance and reduce vibrations to an acceptable level. 

Through further analysis of the test data, BalaDyne researchers found that an integrated, active balancing system could also reduce the level of harmonic vibrations that occur at frequencies above the principal vibration frequency. These harmonics are high-frequency forces on the insert or cutting edge that can cause tool degradation, according to Schulte. By eliminating these vibrations, he says, tool life should be enhanced.

At present, machining center end users cannot purchase an integrated balancing system to upgrade their existing machines. At some point in the future, however, end users may be able to retrofit their machines with replacement or rebuilt spindles that are equipped with integrated balancers, Schulte says. The system is available to spindle manufacturers, who can integrate it into units that are to be installed on new machines.

Integrating an active balancing system into a spindle does increase the cost of the spindle significantly. Schulte says this cost is offset by the benefits, which include longer tool and spindle life and improved surface finishes. In addition, the balancer can provide a level of safety should vibrations reach a dangerously high level. In this capacity, the unit can shut down the machine before a vibrating spindle destroys itself catastrophically.

Some industry experts have said balancing tools so carefully that they generate less force than the operation itself is a waste of time and resources. Schulte agrees this may be the case with slower spindle speeds and heavy cuts. But, he says, a high degree of balance is critical to the success of a high-speed machining operation, because a small degree of unbalance can result in large amounts of force. According to Schulte, this becomes even more necessary in those high-speed operations, such as tool and die machining, that generate little tool force. Schulte recommends using the balancer for any processes being machined at speeds above 10,000 rpm. For processes run at 24,000 rpm or higher, active balancing is a necessity, he says.

Even with the use of an integrated balancing system, users still may need to prebalance their tools or purchase balanced toolholders. But Schulte believes the degree of balance a tool has before it is mounted on the machine is not as critical when an integrated balancing system is used. As long as the integrated system is available to make corrections on-line, some unbalance will not jeopardize the operation, according to Schulte.

Active tool balancing using a system integrated into the machine’s spindle represents the next step in balancing technology. In the future, as more and more users turn to high-speed machining to remain competitive, integrated balancing may become a feature they look for when they shop for new machining centers.