Rethinking machine tool spindles - Rapid Traverse - protect spindles from contamination

Over the long term, spindles bear the brunt of punishment meted out by day-to-day machining forces. For metalworking professionals, spindle failures are among the most costly and disruptive events that occur in the course of daily operations. The proliferation of enclosed and guarded machines that incorporate high pressure coolant delivery systems exacerbates the difficult conditions to which spindles have traditionally been exposed.

The conventional means of protecting precision spindle bearings from contamination is to continuously purge pressurized air through labyrinth-type bearing seals during operation. In the past, it has been assumed that, simply by maintaining sufficient air-pressure through the spindle, this contamination would be prevented. The laws of physics, however, dictate some shortcomings to this approach that can lead to nagging failures, particularly under extremely demanding machining conditions.

North America's largest independent spindle builder, SETCO (Cincinnati, Ohio), made a decision in 1996 to address the problems of contamination and early failure. Through extensive research involving more than 4,000 individual tests, SETCO determined that unequal air pressure-caused by the location of ports around the circumference of a spindle's seals--creates adjacent zones of high and low pressure inside the seal. As a result, coolant--the primary cause of bearing failure--is driven from high-pressure to low-pressure zones. In effect, this phenomenon causes coolant to be sucked into the bearings. This can occur during machining or when the bearings cool down between machining operations. Under these conditions, increasing the air pressure cannot solve the problem. In fact, higher air pressure only makes the problem worse.

As a solution to this dilemma, SETCO developed the AirShield line of spindles. These products incorporate a proprietary, tangential air-purge system that equalizes pressure around the seal but does not inhibit bearing lubrication. Performance maps generated during the manufacturer's testing illustrate this pressure equalization. Tests revealed that internal pressures of the AirShield products varied within a maximum range of only 10 percent of the total pressure, compared to a typical 40-percent variation when standard labyrinth seals were tested.

Future tool: created at the University of Michigan with funding from the National Science Foundation, the Reconfigurable Machine Tool was designed for

"In the 1990s," says Dr. Zbigniew Pasek, operations manager and assistant research scientist at the Engineering Research Center for Reconfigurable Manufacturing Systems, College of Engineering, University of Michigan, "the important issue was making machine tool investments effective in a changing landscape. In short, accommodating fast product changes given a set initial investment in machinery." Pasek and his colleagues recognized that there was a wild divergence between machine tool lifecycles and product lifecycles: the machines are in use much longer than the products they were developed to make are in production. This led to a National Science Foundation (NSF) grant to help fund the establishment of the Engineering Research Center ... and eventually brought about the development of what is called the "Reconfigurable Machine Tool" (RMT).

The RMT would offer functionality that would place it somewhere between general-purpose and dedicated machines. It would have flexibility within applications. "The main premise is that if you can define the part family--generally, an application area that requires a second set of functions--then we offer something that is matched in value to the need," says Pasek.

Cylinder heads, to name one example, may have major differences based on the included valve angle, number of cylinders, etc., while the basic configuration of each is quite similar. Designing an RMT to perform the machining operations on all members of that part family--even those cylinder heads that have yet to be designed--with equal quality requires modifying the structure in such a way that repeatability is not lost. "The idea is to, within a certain work footprint, change the angular values without having to use a different set of machine tools," says Pasek. So U of M's prototype RMT has the spindle on an arcing mount that covers an area from -15[degrees] to +45[degrees], with hard stops every 15[degrees] of travel. "The hard stops assure accuracy," he says, "though we can place the spindle anywhere within that angular range. If you know where you are with the spindle--and that is defined very accurately at the hard stops--this value goes into the control system to provide the quality and repeatability required." (An associated fast calibration project is underway to increase accuracy between the hard stops.)

Motor type vs. machine design: to control vibration, this machine tool builder says the choice of linear motors versus ballscrews is not as important

Do linear motors make a machining center better? Or, more specifically, do they improve dynamic stiffness? A study conducted by machine tool builder Mori Seiki suggests that the answer is yes, but with an important caveat. Applying linear motors alone is not necessarily the most effective way to improve stability. When it comes to dynamic stiffness, changing the machine design may offer more potential for improvement.

Mori Seiki managing director Kazuyuki Hiramoto supplied the data for the graph on the following page. The numbers represent the test results for trials run on a test machine with and without linear motors, and with and without a machine-tool design concept called "Driven at the Center of Gravity," or DCG. What the numbers specifically measure is the amount of vibration that resulted after the machine was moved and stopped. The point of the comparison is that some vibration can be attributed to how the axis motors work, but potentially more of the vibration results from where the force of those motors is directed.

Axis motors on machining centers tend not to apply their force through the center of gravity of the moving elements, but instead they tend to apply it to one side or the other. Dr. Hiramoto says this runs contrary to a principle we all intuitively know. That is, when moving a heavy weight, we try to push at the center. The problem with not driving through the center of gravity on a machining center is that machine tools are--in a sense--squishy. Even a push by hand might deform a machine by 10 microns. The slight twisting of machine elements that comes from linear axis forces pushing off-center can cause vibration that may ultimately affect the surface quality of the part and the life of the tool. Machining center users sometimes deal with this vibration by making gradual starts and stops.

The company's DCG machines do try to push through the center of gravity. The machines use ballscrews, and the very challenge in realizing the DCG concept is that the lines through the center of a machining center may not leave room for a ballscrew. The spindle occupies a central location on a machine, and so might a rotary table. In these cases where some other basic component of the machine interferes at the center, the DCG machine uses two ballscrews instead, one ballscrew on either side, so that the resultant force still does drive through the center.