When it comes to high-speed machining, there is a huge concentration on (and, perhaps, fascination with) spindle speeds. Which makes sense. For one thing, die makers like the ability to go fast because it provides the means by which they can achieve smoother surfaces. Which minimizes the amount of required hard work. And prismatic part producers like it because it facilitates high stock removal rates.
But there are a couple of things to consider:
1.Die makers don't want chatter.
2.The prismatic part producer doesn't want the bearings in the spindle to go bad, which results in major downtime.
The problem is a simple one. Although great care is typically taken by spindle manufacturers to produce units that run fast and true, the spindles are used to carry tools. Should the tools be off balance, then, as they go spinning around, the centrifugal forces generated create problematic conditions—such as chatter or broken bearings in the spindle.
A solution, of course, is simply to balance the tools before they are placed in the spindle. This is a good practice regardless of the spindle speeds, be it high or low.
But there can be some balancing problems remaining when the tools are to be run at high speeds because chances are the balancing has been performed at low speed. Dynamic behavior is different at high speeds as compared with low. So although a tool may work just fine at below, say, 10,000 rpm, it may shake like mad (comparatively speaking, after all, we are talking microns here) above the 10K mark.
According to Stephen W. Dyer, senior development engineer, Balance Dynamics Corp. (Ann Arbor, MI), "Unbalance forces increase as a square of the spindle speed."
Balance Dynamics has been working on balancing spindles-in-action for several years. Engineers there concentrated on balancing grinding wheels. Apparently, the systems they supplied worked well, based on studies they collected from installations on grinders in sites including the Ford Cleveland Engine Plant 1, the Chrysler Trenton Engine Plant, Cummins Jamestown Engine Plant, and others.
But there isn't a direct translation from grinding machines to machining centers, even higher speed grinders. At issue is the fact that grinding wheels tend to be setup and run for a long time, at least compared to the tools in a machining center. Machining center tool changes are typically frequent. So whereas it would be okay to spend a minute getting the balance right for a grinding wheel (taking into account how long it will be fitted to the spindle), anyone who is frequently changing tools in a machining center, where tool-to-tool time must be within a handful of seconds, can't contend with minute after minute after minute.
Given that causes of unbalance include the drawbar grip and taper fit, it is almost a vicious circle: the more often tools are changed, the greater the possibility for unbalance, yet there's little time to contend with the problem.
In 1994 Balance Dynamics received a patent for a dynamic balancing solution. The dynamic balancing system they've developed is called "BalaDyne."
The mechanism is integral to the spindle. There is an accelerometer that senses vibration. It causes a signal to be sent to a coil in one of two rotating rings that are part of the balancer. In total, there are three rings, with the third fixed to the spindle nose. The rotating rings—or rotors—have a known unbalance.
The rings are rotated by the spindle shaft. Permanent magnets that are in the rotors hold the rotors in the required orientation, so that the points of unbalance offset each other. Which brings us back to the signal sent to the coil. The coil sends specifically tuned electrical pulses to one of the rotors; these pulses create a magnetic field. This field interacts with the fields of the permanent magnets in the rings. The generated field is such that it causes one of the rotors to move in the direction and to the position necessary to balance the tool, toolholder and spindle setup. That is, it takes into account where the unbalance is of the rotating elements, then adjusts the positions of the unbalance on the two rotors so that one of the rotors is repositioned to offset the heavy point on the rotating mass.
(There is an RS-232 port that permits the creation of a display of what's happening. Essentially, picture a clock face with dots at the end of the hands. That represents the two rotors. When the spindle is revved up and the accelerometer detects the vibration caused by the heavy spot, a third dot is placed on the face, representing it. When the BalaDyne balancing kicks in, the positions of the hands move, indicating that balance is being achieved.)
The whole thing happens fairly quickly. The rotors can be repositioned within 0.8 seconds. The BalaDyne controller interfaces with the machine tool control (which, of course, controls the spindle speed). So, by the time there is back and forth between the controllers, the whole balancing action takes about two seconds or less. Which is certainly more reasonable than a minute.
Wayne L. Winzenz, executive vice president of Balance Dynamics, emphasizes that this is not an add-on or retrofit device. Presently, they are working with a spindle from Fischer Precision Spindles Inc. (Middlefield, CT), a model (the 2612/15) that's capable of rotating at up to 15,000 rpm, and are working on a system for a machine tool manufacturer for a spindle that rotates at 12,000 rpm. Ideally, the system is designed right into the spindle for improved effectiveness. (In some cases, it may be that there is something else in the spindle—such as coolant piping—that would prohibit the installation of the system.)
The Fischer spindle was selected because Boeing expressed an interest in dynamic balancing. But Thomas E. Schulte, vice president of Strategic Business Development, says that there is a tremendous opportunity in the auto industry for dynamic balancing.
He isn't the only one who thinks so. In October the company received a $2-million grant from the National Institute of Standard and Technology, part of the 1997 Motor Vehicle Manufacturing competition that's within the U.S. Commerce Dept.'s Advanced Technology Program. In this work they'll be initially developing a balancing system capable of working at 24,000 rpm, then will do further work to bring it up to 40,000 rpm.
For Stiff Spindles
"We've designed the HX series to provide higher metal removal rates without sacrificing load-carrying capability," says Jack Wheeler, business manager, Machine Tool Products, Fafnir Bearing Div., The Torrington Co. (Torrington, CT).
He's talking about a new series of Fafnir high-speed bearings that has been developed for applications in automotive metal cutting. The bearings can be used in grinding, milling and boring machines.
The bearings are manufactured with a 15° contact angle. This has the result of increasing spindle stiffness during cutting (a characteristic that can be enhanced through the use of optional ceramic balls), which means reduced deflection and increased machining precision.
The HX series bearings can be fitted into existing spindle designs.