The Case For Hydrostatic Ways
Moving on oil can let a grinding machine deliver more power along a smoother pass.
Few machine tool components are more fundamental than the way system, which provides the interface between moving and stationary elements of the machine. In many machines, the moving elements ride on way systems using rolling elements—that is, way systems with metal-to-metal contact. Hydrostatic ways, by comparison, do not have this contact. In a hydrostatic system, the guideways use a pressurized film of oil, with pockets in the way system distributing the oil to center the moving element. Without the surface-to-surface contact, the slides can operate without wear. Furthermore, because the moving element is suspended by fluid, the impact of vibration is reduced.
However, the way system does not work in isolation. Each machine design is an integrated system. Speaking of the “advantages” of a way system is a bit misleading, says Chris Stine, because the machine design has to leverage any potential benefits the way system affords. Mr. Stine is vice president of United Grinding, a company that offers grinding machines using various way-system options. When the machine design does leverage the benefits of hydrostatic ways, he says, those benefits can include any of the following:
Acceleration And Weight-Carrying Capacity
With the moving mass sliding along oil, additional energy does not have to be spent to overcome the friction in systems with metal-to-metal contact. The resulting productivity improvement can be seen in applications involving wide wheels, high horsepower and/or heavy stock removal.
For example, in an application for grinding a power-generation turbine blade using an 8-inch-wide wheel, a machine with hydrostatic ways can get much more mass in motion compared to a similar machine using conventional guideways. The hydrostatic machine can also accelerate a given mass at a higher rate. The difference on a turbine blade, which typically requires repeated movements, can be enough to grind the work in 30 percent less time.
United Grinding’s Mägerle machines with hydrostatic ways are used in grinding applications along the lines of the one above. The company relayed comments from some users. Eric Reutimann, owner of Spectra-Tech in Hanover Park, Illinois, says, “The table almost ‘floats,’ as if it were on water. There is no stick-slip, as in the case with some machines.”
Another advantage is long life. It is incorrect to say hydrostatic-way machines are lower maintenance machines, because a different kind of maintenance is required. Fluid and filters have to be tended to, and so on. However, because these ways eliminate metal-to-metal contact, there is (in theory) no wear.
Gary Treichler, vice president and general manager of Form Grind Corporation in Rancho Santa Margarita, California, says that he has watched his own hydrostatic-way grinding machines “keep producing great parts,” even though they have been in use for more than 30 years.
Hydrostatic ways can “wrap around” the machine’s moving elements so that the moving element is buffered from surface-to-surface contact in all directions (see drawing on the right). This buffer damps vibration.
The actual importance of this vibration-damping can vary significantly from application to application. Linear guideways with rolling elements are used in many precision-machining applications without the vibration necessarily making a difference. However, in some cases, vibration damping can prove crucial to achieving a needed appearance, smoothness or flatness of the part.
In one recent example, a manufacturer needed to grind 33-inch-long plates of optical glass to a flatness better than 100 microinches. The machine with hydrostatic ways not only achieved a flatness well within this requirement without a temperature-controlled room, but it also kept the glass from chipping or breaking during grinding because of the damping effect.
Making The Choice
While a hydrostatic-way machine may cost more than one using mechanical ways, a shop facing the wide-grinding-wheel application mentioned above might gladly pay more for a machine that is able to grind the part 30 percent faster.
On the other hand, a different shop machining exactly the same part might prefer the redundancy of having multiple machines available for the work. This shop might prefer to have a process that uses a larger number of less expensive machines, even if those machines are individually less productive. In other words, performance and price, by themselves, do not indicate which machine tool is better.
Mr. Stine says whether or not the shop is a job shop might also affect the choice. To stretch their capital equipment dollars, job shops often embrace a philosophy that has the shop buying no more capability than necessary in each of its machines.
Then again, perhaps the very same job shop might embrace the opposite mindset in some cases. A job shop that needs to buy a bigger machine tool than any it currently has available, just for the sake of one job, might choose to buy the most capable big machine tool it can. That is, it might buy a hydrostatic-way machine offering greater performance so that if the shop needs to enter large-envelope machining anyway, it can provide the most aggressive big-envelope machining services possible.
In short: It all depends. The way system doesn’t work in isolation. In an interconnected system such as a machine tool, the true significance of a feature such as hydrostatic ways depends not only on the design of the machine, nor only on the part that is being machined, but also on the very outlook of the shop that is doing the machining.
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