A granite air bearing is the quietest way to move a precision stage. Instead of rolling on balls or sliding on oil, the carriage floats on a thin film of compressed air, and the granite guide it floats over is what sets the accuracy of the motion. That combination - a granite air bearing guide driven by a linear motor - is now standard in semiconductor equipment, in femtosecond and picosecond laser systems, and in the XY tables and precision stages that sit beneath both. This article looks at why.
What a granite air bearing actually is
An air bearing works by pushing a thin film of compressed air, typically a few micrometres thick, between a moving carriage and its guide. Because there is no metal-to-metal contact, there is no wear, no stick-slip and almost no friction: a well-designed air bearing stage can move smoothly from a few micrometres per second up to metres per second, and hold position to nanometres. The guide surface is the bearing race, so its flatness and straightness set the accuracy of the whole axis. That is why the guide is usually granite.
Why the guide should be granite, not steel
Steel and cast iron can be made flat, but granite brings three properties an air bearing guide needs. It is dimensionally stable, so the air gap stays constant as the room temperature moves. It damps vibration internally, so the film does not transmit the small disturbances that would otherwise show up as position error. And it does not rust, which matters when the bearing surface must stay clean and dry for years of service. Black granite from UNPARALLELED® has a density of about 3100 kg/m³ and is non-magnetic, an advantage when the stage carries linear motors and position sensors.
Semiconductor equipment: where the demand is highest
Semiconductor tools are the most demanding users of a granite air bearing. Wafer stages, lithography and overlay tools, wafer and mask inspection, die bonding and advanced packaging all position a wafer or a tool to nanometres while moving fast. An air bearing stage gives them a guide with no wear particles, which matters inside a clean enclosure where a single particle is a defect, and it gives them motion without the vibration a rolling element would inject into the optical path.
Femtosecond and picosecond laser equipment
Ultrafast laser systems - femtosecond and picosecond tools used for precision cutting, drilling and surface texturing - depend on a stable optical path and a stage that does not overshoot. At these pulse energies and repetition rates, a small vibration or a few micrometres of straightness error turns directly into a change in the cut. A granite air bearing axis keeps the workpiece and the focusing optics in the same relative position over long travels, so the process holds its edge quality from the first feature to the last.
XY tables and precision stages
Below the machine level, the same axis appears as an XY table or a gantry stage. A granite base carries one or two air bearing axes, a linear motor drives them, and a linear encoder reads position directly off the scale. Stacking two axes on one stable granite block, or building an H-gantry across it, keeps the two directions orthogonal and level with each other. This is the standard architecture for high-end inspection, laser processing, dispensing and metrology stages, and it is built around granite for the same reason at every scale.
Machining a 20-metre air bearing guide in one piece
Air bearing guides are getting longer, because the applications keep growing: large-format inspection, panel and glass processing, and long-travel laser systems. UNPARALLELED® machines single granite components up to 20 metres long, 4,000 mm wide and 1,000 mm thick, so an air bearing guide can be produced as one continuous piece instead of being joined from several blocks. A single piece removes the joints that would otherwise interrupt straightness and the flatness of the bearing surface, and it keeps the air gap uniform over the full travel.
Large grinding capacity behind the guide
The straightness of a long guide is set on the grinding machine, so the machine has to be as capable as the part. Our large grinding and lapping capacity is sized for exactly this work, and the table below summarises what we can bring to an air bearing project.
| Capability | Range |
|---|---|
| Single-piece length | Up to 20,000 mm (20 m), machined as one piece |
| Single-piece width | Up to 4,000 mm |
| Single-piece thickness | Up to 1,000 mm |
| Single-piece weight | Up to 100 tonnes |
| Heavy grinding | Precision platforms up to 6,000 mm long |
| Environment | Grinding, lapping and inspection in a temperature-controlled workshop |
After grinding, the bearing surface is lapped and hand-scraped to bring the whole guide into a single plane, and the finished part is inspected for flatness, straightness and parallelism under stable temperature before it ships. On a long guide, that controlled environment is not a formality: a measurement taken in an uncontrolled room would say more about the room than about the guide.
Specifying a granite air bearing
If you are building an air bearing stage, the drawing we need is short: the length and width of travel, the straightness and flatness you require, the load, and the way the carriage is driven and read. From that we machine a granite air bearing guide as a single piece, prepare the mounting surfaces for the drive and the encoder, and, where the axis sits on a larger machine, combine it with custom precision granite bases to build the full frame. The result is a stage whose accuracy comes from one stable, damped, rust-free material rather than from a chain of assembled parts. Send us the specification and the application, and we will return the guide, the base and the inspection data to match.