Air bearing modules rely on a micron-level thin air film to realize non-contact levitation. The air film is extremely thin, and tiny undulations on the base surface of the granite worktable will directly change the air gap distribution, resulting in air film pressure fluctuation and unstable levitation stiffness. Many equipment integrators easily fall into a misunderstanding when assembling air bearing modules: they only focus on the flatness reading of the worktable at the moment of delivery, ignoring the uniformity and long-term stability of flatness. Even if the single-point accuracy meets the standard, uneven surface undulations will cause posture jitter and positioning jump when the air-bearing slide moves at high speed, which will directly damage the performance of the whole nano-level motion system. Therefore, the flatness of the base surface of the air-bearing worktable cannot be judged only by a single numerical index. Instead, it is a comprehensive standard combining materials, processing technology and full-area inspection. UNPARALLELED Group has established a special flatness control system for working conditions matched with air bearing modules.
The core of air bearing operation is to maintain a stable and uniform micron-level air film. Usually the thickness of the air film is only a few microns. Any local protrusion or depression on the base surface will change the gap size. The flatness indicators of ordinary granite platforms mostly only meet the needs of static measurement and are not suitable for the dynamic motion working conditions of air bearings. UNPARALLELED® black granite worktable substrate has a density of 3100kg/m³, low expansion and high damping. The raw material undergoes bulk raw stone aging in advance to release native internal stress, preventing slow deformation of the reference surface of the worktable under long-term load and temperature fluctuation. It ensures that the flatness index can be maintained for a long time fundamentally, rather than meeting the standard temporarily in the short term.
For granite worktables equipped with air bearing modules, the core flatness standard is that the flatness of the full-area reference surface reaches the nanometer level, while local waviness is controlled, and the plane deviation within each meter is strictly controlled within the micron range. In addition to the overall flatness, local short-wave surface waviness is a key assessment item for air bearing applications. Long and gentle undulations can be moderately averaged by the air film, but tiny short-distance ripples will directly disturb the pressure field of the air film and cause height fluctuation while the slide travels. We complete ultra-precision grinding in a constant-temperature, constant-humidity and anti-vibration workshop. The workshop floor is thicker than 1000mm and equipped with shock-proof trenches to isolate external vibration interference and prevent surface ripple errors introduced by ground vibration during grinding. Staged aging is adopted in the processing. The workpiece is kept static after each grinding procedure to release processing stress and avoid surface deformation caused by stress accumulation.
The flatness inspection method is also different from conventional spot checks. Instead of measuring only several diagonal points on the tabletop, grid multi-point full-area collection is adopted. Laser interferometers, Mahr dial gauges and Swiss WYLER electronic levels are used for continuous scanning measurement to obtain complete topography data of the whole working surface and evaluate the plane error of the whole area. The whole metrology chain can be traced to the national metrology institute, and an inspection report with complete traceability certificates will be issued. According to the application of customers' equipment, different precision grades can be distinguished: the basic ultra-high flatness standard is adopted for ordinary optical inspection air-bearing worktables; for high-end air-bearing platforms matched with semiconductors and lithography, local waviness and plane tolerances will be further tightened to meet nano-positioning scenarios.
Structure and surface technology also help guarantee the operation effect of air bearings. If customers need to apply it in clean workshops, the worktable surface can be processed by sealing, passivation and mirror polishing to close the micropores of stone, reduce particle precipitation, and lower surface roughness, preventing the air bearing pad from being scratched by rough surfaces and avoiding interference in air film formation. We can also optimize the supporting structure of the tabletop according to the layout of air bearing modules to distribute loads and prevent the deterioration of base flatness caused by uneven stress after assembling air bearing modules. If weight reduction is required, topological lightweight hole machining can be carried out to reduce self-weight on the premise of ensuring overall rigidity without damaging the flatness of the reference surface.
The stability of the air bearing system is largely determined by the flatness uniformity of the granite base surface. It is easy to achieve high single-point precision, but to realize long-time stable levitation without posture jitter for air bearing modules, the joint support of stable material stress, precision grinding technology and full-area topography inspection is required. UNPARALLELED runs flatness control through the whole process including raw stone aging, grinding processing and pre-delivery inspection, so that the surface topography of granite worktables can be stably controlled. It ensures uniform and stable air film of air bearing modules under high-speed reciprocating motion working conditions and provides reliable benchmarks for all kinds of ultra-precision frictionless motion platforms.