Large-load-capacity oil hydrostatic bearings generate prohibitive amounts of heat in large sizes when run at speeds useful for diamond turning of optical components. The viscosity of air is more than three orders of magnitude less than the thinnest oil; therefore, the frictional heating of large-diameter air bearings is very small and very manageable. A formidable manufacturing problem with large air bearings is that the extremely low viscosity of air requires that the thickness of the bearing film is also very small. This very small bearing clearance of 5–8 micrometers means that the required accuracy of geometry and dimensions of air bearing components is extremely difficult to achieve.
In the case of meter-size air bearing dimensions, the required precision is comparable to high-quality optical components in most cases because exact dimension is required in addition to near-perfect geometry. With current technology, the cost to fabricate, and the time to produce large mirrors, is one of the limiting factors for many missions. Research tools require large-aperture, lightweight optics that will perform well in a space cryogenic environment. Currently, no mirror technology has been demonstrated that meets the mission requirements at an affordable cost.
Diamond turning has been proven to produce highly aspheric optical contours to visible wavelength tolerances with extremely smooth surfaces. Diamond turning has the additional enabling capability to not only produce extremely smooth and accurate optical surfaces, but also mechanical attachment surfaces and datums that allow extremely fast and complex optical components to be quickly and easily aligned. The productivity of diamond turning allows the production of quantities of optical components with exacting duplication of optical surfaces and datums, which allow “snap-together” optical systems.
The large diamond turning machines that have been built all have oil hydrostatic work spindles that fail in diamond-turning very large optical components because of excessive heat generation when this type bearing is made large enough to have the required load capacity for very large optics diamond turning.
The solution to this problem is to make large vertical air bearings with hard metal journals running against porous graphite bearing surfaces. Porous graphite air bearings do not friction weld in the event of the spindle bearing surface contacting the bearing journal surface, and the result is only slight polishing of the graphite surface. The porous graphite flow structure provides extremely uniform flow of the air film, and the tortuous flow path acts to damp the flow-induced dynamic instability that is a severe problem in both oil and air hydrostatic bearings.