You may not realize it, but a balanced toolholder assembly can help you get the most from your machine tools.
Do balanced tools increase productivity?
The effect of unbalance is one of the simplest physics theories for anyone to visualize. Consider what happens when you have too many wet towels on one side of your washing machine as it rotates. Or how your ride would be if you didn’t have balanced wheels on your car.
These scenarios are easy to imagine, but the effect of unbalance is not so evident when it comes to toolholders for milling applications because of several misunderstandings that have persisted for years within the moldmaking industry. On top of that, the benefits of balancing toolholder assemblies at all speeds are often simply overlooked.
A Balanced Look Back
It was the moldmaking industry that first identified the importance of running truly balanced toolholder assemblies. A toolholder assembly is made up of the toolholder, cutting tool, pull-stud if needed, nut, collet, etc. In the early to mid-‘90s, with machining centers able to run as fast as 15,000 rpm, there was a rash of spindle failures. The less-massive spindles on these high-speed machines and the extreme unbalance of most of the toolholders that were being used made unbalance the issue to be resolved.
In the early ‘90s, there was no known machine on the market designed specifically to accurately balance the toolholder assembly. Consequently, state-of-the-art companies attempting to stay ahead of the technology curve purchased balancing machines designed to balance parts such as rotors, wheels, crank-shafts, turbines, etc.—not toolholders. These balancing machines actually were unbalancing the assemblies, however, so customers did not see a noticeable difference when machining with their “balanced” toolholders.
In the late ‘90s, a balancing machine that truly balanced toolholder assemblies finally came to market. In addition, other balancing advancements introduced around this time included finer, “pre-balanced” toolholders, as well as toolholder systems that provide more consistent balance repeatability when changing cutting tools (i.e., shrink-fit toolholders). This combination of developments enabled moldmakers to accelerate and optimize their use of the newest high-speed technologies and produce molds with finer finishes, resulting in greatly reduced polishing and/or EDM work.
Even though the mold industry identified the need to use balanced toolholders for its high-speed applications two decades ago, the industry is still somewhat confused about the importance of running truly balanced assemblies.
A misconception about toolholder balancing is the perception that only using pre-balanced toolholders is sufficient. While using these pre-balanced toolholders is highly recommended, it does not guarantee full machine tool utilization. After changing cutting tools, measuring the unbalance of a toolholder assembly is essential to fully realize the maximum potential of all the machine tools in a shop. Even slower-rpm machines (ones that run at 8,000 rpm) can be used to their fullest potential if they are using balanced toolholder assemblies.
Finely balanced toolholder assemblies offer many advantages, including increased productivity, safety, and long cutting tool and spindle life. Only when the toolholder assembly demonstrates a precise concentricity and balance can the optimal cutting conditions be utilized. In this case, the rule of thumb is: Too much balance is better than not enough.
What happens to the unbalanced holders already in the plant? Although in practice it is impossible to avoid mixing balanced and unbalanced holders, keep in mind that a single machining process with an unbalanced holder at a high rotational speed can undo required machining accuracy and damage the spindle. For this reason, toolholders should be balanced, preferably in the plant itself. If not, it is advised to make toolholder balancing a part of goods reception and quality control. Let’s face it, how do you know what condition your toolholder assemblies are in unless you have a machine to check them?
Causes and Consequences of Unbalance
Unbalance is caused by uneven weight distribution during rotation, which creates centrifugal forces that increase to the square with the rotational speed. This means that if the unbalance is the same, the spindle creates a 25-times-higher centrifugal force at a rotational speed of 10,000 rpm than at a rotational speed of 2,000 rpm. As a result, an unbalance in toolholder assemblies has a particularly noticeable negative effect on high-speed machining, and on heavy tools or cutting tools with complex geometries.
A main consequence of unbalance is this centrifugal force that puts a strain on the spindle bearings, which can cut spindle lifetime in half. As a result, the use of balanced tools is always recommended; otherwise, spindle warranties are nonexistent or limited.
Another consequence is the vibration that is created when the effective direction of the centrifugal force changes as the spindle rotates. These vibrations are transferred throughout the whole machine and cutting tool, reducing cutting tool life. It has been reported that the tool life of an unbalanced toolholder assembly is decreased by 10 percent, on average, which results in increases in cutting tool costs.
These vibrations are not only harmful to the spindle and the cutting tool, they also impair the reliability of the entire process and are transferred to the outcome of the machining. Vibrations create chatter marks that have to be removed by additional fine machining or hand work.
To reach the correct process reliability level and achieve the final results demanded, vibrations must be reduced. One method for doing so requires reducing spindle speed, feed
rate and cutting depth. This can lead to smoother cutting, but also decreases metal removal rates and productivity.
The problem with decreased metal removal rates and productivity becomes clear after a simple analysis: A machining center costs $100 per hour to operate (one-shift operation, 1,600 operating hours per year). With a 10-percent increase in metal removal time, you save $10 per hour, which equals $16,000 per year.
Another benefit of balanced toolholder assembly is increased spindle life, which is key for cost savings and machine reliability. It enables spindle replacement to be planned, eliminating unplanned machine downtime. A truly balanced system can realize savings of more than $20,000 per year, per machine. And this does not even include additional savings that can be realized from improved part surface quality, increased size accuracy, and fewer machine breakdowns or downtimes.
These figures put the objection over the cost of a balancing machine into perspective. One could argue that an investment is only expensive when it does not pay for itself quickly enough or at all. The aforementioned calculations show that a balancing machine can, in fact, pay for itself very quickly. And keep in mind that a single machining process with an unbalanced holder at a high rotational speed can undo required machining accuracy and damage the spindle. Consider that a spindle replacement alone can cost more than a balancing machine. Figure 1 illustrates the annual savings potential per processing center for one-, two- and three-shift operation.
Figure 1 - Potential cost savings per machining center that can be achieved through proper balancing.