Below a micron of depth of cut, the machine's bearings stop being support hardware and become the accuracy itself. Two fluid-film technologies divide the ultra-precision field: hydrostatic oil bearings and aerostatic air bearings. Machines like the LT Ultra MMC 600H class — a 100,000 rpm diamond-milling spindle on a granite base — exist in both flavors, and the choice between them is a choice about stiffness, cleanliness and the geometry of the work.
What hydrostatic oil buys
Stiffness and damping. The oil film resists deflection hard, which multi-axis micro-milling needs: a cutting edge taking side loads on a curved freeform surface leans on bearing rigidity that air, compressible by nature, delivers reluctantly. Oil also damps — chatter at micron depths is death by a thousand marks, and the oil film eats it. The trade is plumbing: supply, filtration, temperature control, and oil management in a space that may prefer not to have it.
What air buys
Cleanliness and straightness. Air bearings run without lubricant, so there is no oil to migrate onto optics — the decisive property for fly-cutting flat optical surfaces where a film of oil is a defect. Air spindles also rotate with near-zero runout, which planar fly-cutting rewards. The trade is load capacity and damping, both lower than oil — planar work, not aggressive 3D paths.
The decision rule
Freeform, multi-axis, interrupted micro-cuts: hydrostatic oil for its stiffness and damping. Flat optical fly-cutting where cleanliness rules: air. Both live on granite bases for thermal and damping reasons, and both answer to motion systems a class beyond ordinary motion controllers — the drive ripple that a normal axis would hide is, at this scale, surface texture.
Can one spindle do both optical and freeform work?
Hybrid machines exist, but every bearing choice compromises the other duty. Shops that genuinely span both worlds own both spindles — the workpieces are too valuable to feed to a compromise.
Why 100,000 rpm for diamond milling?
Depth per revolution scales with feed per tooth: at micron depths, only high rpm buys productive feed rates without loading the diamond. The bearing technology that survives that speed is precisely the fluid-film pair described here.
