The role of balance in high speed finish boring: Precision boring is particularly vulnerable to unbalance, but not every high speed boring application

A single-point boring tool with adjustable cutting diameter may be used for the tightest-tolerance cutting that a given machining center is likely to perform. At the same time, this tool is probably the most inherently unbalanced object the machine will ever use to take a cut.

This irony has real consequence as spindle speed increases, because the centrifugal force resulting from unbalance increases as the square of rotational speed. Raising the speed from 4,000 to 8,000 rpm, without any change in tooling, causes centrifugal force to quadruple. Raising the speed of the same process to 12,000 rpm raises the force from unbalance to 9 times what it was at the original speed. Because of the exponential increase, an effect that was once negligible may be amplified to the point that tight tolerances can no longer be held.

One company providing variable-diameter precision boring tools is KPT Kaiser of Elk Grove Village, Illinois. During the past decade, as maximum spindle speeds for all classes of machining centers were increasing significantly, this company introduced updated versions of its tools better suited to boring at high rpm. Among the newer versions are adjustable-balance models that use moving counterweights to compensate for the change in unbalance that comes from changing the radial position of the cutting insert But not every boring application--not even every high speed application--is a candidate for one of these balanceable tools. Vice president of engineering Jack Burley says that tools with adjustable balance account for only 10 percent of the company's sales of the variable-diameter tooling. Tools that are not balanceable by means of a moving counterweight are nevertheless engineered to be balanced at the middle of the tool's adjustable diameter range. And for most finish boring applications, just this level of balance is enough.

Understanding unbalance as it applies to single-point boring is useful for shops that want to machine precision holes at faster cutting speeds. In addition, the same understanding is also useful in a larger context. Shops moving to higher spindle speeds for various applications are often concerned about the potential effects of unbalance. The example of high speed finish boring, which combines a highly asymmetric tool with a very light depth of cut, can provide a useful benchmark for these shops. The extreme set of conditions shows the extent of the impact that unbalance can have on the performance of the process.

Mr. Burley says a common mistake in high speed machining is to give too little consideration to balance. Another common mistake, he says, is to give too much consideration to balance. Higher speeds do make it worthwhile to use high-quality tools and toolholders manufactured to tight balance requirements, but where end mills and other inherently symmetrical tools are concerned, just choosing quality tooling is probably enough to ensure that balance is sufficient. Trying to improve balance further through some off-line adjustment to the tool is likely to be overkill, resulting in a change in centrifugal force that's tiny compared to the force from the cut. The reason is that single-point boring tools are more prone to require balance adjustment precisely is that the light depth of cut makes a small unbalance force more significant by comparison.

And even at that, the effect of unbalance would not be considered significant to everyone. Comparison testing at 10,000 rpm between balanceable and non-balanceable single-point tools showed a difference in hole roundness error of about 5 microns (see Figure 1). Many machining operations could tolerate error of a comparable magnitude. But in precision boring, error of this magnitude has to be addressed.