Patent Application
20080002923
The embodiments of the invention address the problems associated with positioning a workpiece relative to one or more of its surfaces, which can be difficult when trying to achieve high accuracy. These problems were solved by developing a workholder comprising a body comprising a fluid bearing and a fluid, wherein the fluid bearing is adapted to releasably retain a workpiece by maintaining the fluid in a gap between the workholder and the workpiece, wherein the fluid bearing is adapted to produce a radial and/or an axial force on the workpiece, and wherein the workholder does not include a mechanical interface to releasably retain the workpiece.
The workholder of the embodiments of this invention may use hydrostatic or hydrodynamic bearings. One embodiment may further include a source of fluid. The fluid may be a liquid or gas delivered to the gap between the workholder and the workpiece by an external source. In another embodiment, the fluid may be pressurized internally by viscous shear.
In order to eliminate one of the problems associated with conventional workholders, jaws for gripping a surface of a workpiece have been eliminated. Instead of using a mechanical type interface, for example, a jaw, collet, chuck, magnetic chuck or clamping system, a fluid is maintained in a gap between the inner surface of the workholder such that the workpiece itself becomes one element of a fluid bearing. The workpiece may be in almost any form, such as cylindrical, conical, spherical, flat or irregular.
In one embodiment, the workpiece may be constrained to only allow rotation along an axis of the workpiece. In another embodiment, the rotational error motion of the workpiece is preferably less than 1 μm, and more preferably less than 0.1 μm.
In one embodiment, the fluid bearing can use virtually any fluid, including air, oil, water, alcohol or aqueous solutions of NaCl, NaNo3, HF, HCl, HNO3, NaOH or the like. Lubricant fluids may include an electrically non-conductive lubricant and an electrically conductive, non-metallic, non-magnetic additive that improves electrical conductivity of the lubricant without sacrificing desirable lubricating properties such as viscosity, anti-oxidation, anti-corrosion and anti-wear performance. Base lubricants may include a mineral-based hydrocarbon, a synthetic hydrocarbon, an ester or a combination of base lubricants. Mineral-based hydrocarbons are preferably highly refined (highly purified). Preferred additives may include organic polymers, such as a commercially available solution of a quartemized polymeric aminoamide ester, a nitrilo polymer, chlorobenzene and ethylene dichloride in aromatic and aliphatic hydrocarbons. The aromatic and aliphatic hydrocarbons have a 40-70% concentration as compared to the remaining elements of the additive solution. One example of such a commercially available solution is Tolad 511 from Petrolite Corporation, U.S.A. Another example of a suitable commercially available organic polymer includes a solution of a solvent (tolune, isopropyl alcohol, and other aromatic solvents C9-C16), dodecyl, benzene and sulfonic acid. Other commercially available solutions can also be used. Since the additives are non-metallic and non-magnetic, the additives do not adversely affect wear and viscosity performance. Other non-metallic additive solutions can also be used.
The concentration of the additive in the lubricant can be varied to achieve a desired conductivity. However, the concentration is preferably kept low such that the overall viscosity of the lubricant is not changed. Formulation of fluids for appropriate fluid bearing properties therefore may require different considerations than for fluids intended as general-purpose lubricants.
Referring to
The workpiece 12 may have a spin imparted to it by an external driver 18, which may be designed so that it minimizes any influence on the spin axis, which preferably is determined by the workpiece 12 surfaces. The bearings in the external driver assembly 18 may be designed so that they have minimal tilt resistance. The external driver/rotor assembly 18 may include its own magnet and stator and its own hydrostatic bearing set that is separate from the workpiece 12 holder bearing. The workpiece 12 may also remain static, with no imparted spin, if only precise centering is the desired effect.
In another embodiment, the workpiece 12 may be rotated by non-mechanical means such as an eddy current clutch magnetic coupling or by using the fluid escaping across the bearing pads to induce rotational motion by shearing the fluid.
In one embodiment, hydraulic pistons 20 may be used to create an axial loading to force the thrust hydrostatic bearing 16 onto the workpiece 12, which in turn forces the workpiece 12 onto the external driver 18 so that the part can be spun.
The gap between the workpiece 12 and the inner bearing surface may be matched based on the viscosity of the fluid used in the bearing. In one embodiment, the gap between the workpiece 12 and the inner bearing surface is preferably 12-15 μm.
Again referring to
In one embodiment, the components of the workholder 10 bearing assembly may be fabricated from series 430 stainless steel. Other suitable materials include titanium, metals with high resistance to anodic corrosion, plastics with low absorbtivity of water and ceramics. The components may be made by casting, molding or powder metallurgy.
The above description is presented to enable a person skilled in the art to make and use the invention, and is provided in the context of a particular application and its requirements. Various modifications to the preferred embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Thus, this invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.
This application discloses several numerical range limitations that support any range within the disclosed numerical ranges even though a precise range limitation is not stated verbatim in the specification because this invention can be practiced throughout the disclosed numerical ranges. Finally, the entire disclosure of the patents and publications referred in this application are hereby incorporated herein in entirety by reference.
