Workpiece positioning table with air bearing pads

A workpiece positioning table, comprising:

• a granite slab having a first horizontal ground and lapped top surface;
• a straightedge affixed to said top surface and defining a first horizontal axis;
• first and second opposed edges on said straightedge;
• a first table;
• a first plurality of air bearing pads on said first table effective to support said first table above said top surface on films of air;
• a first plurality of air guide bearings facing said first and second opposed edges and effective to guide said first table for motion along said first axis;
• a U-shaped granite bridge having first and second legs resting on said top surface;
• a U-shaped cavity between said first and second legs providing clearance for passage thereinto of said first table;
• a ground and lapped horizontal second top surface on said granite bridge;
• said second top surface being parallel to the first-mentioned top surface;
• a second table;
• a plurality of air bearing pads on said second table effective to support said second table above said second top surface on films of air;
• a second straightedge affixed to said second top surface and defining a second horizontal axis orthogonal to said first horizontal axis;
• third and fourth opposed edges on said second straightedge;
• a second plurality of air guide bearings facing said third and fourth opposed edges and effective to guide said second table for motion along said second axis;
• each of said air bearing pads being formed of a single block of metal and including means for permitting deflection of bottom surfaces thereof to accommodate irregularities in the surface above which it is supported;
• a source of pressurized air;
• a first air pressure regulator operative to receive said pressurized air and to regulate its output air pressure to a first substantially constant air pressure;
• a first plurality of flow control valves each of which receives said first substantially constant air pressure and feeds air pressure to one of said first plurality of air bearing pads;
• a second air pressure regulator operative to receive said pressurized air and to regulate its output air pressure to a second substantially constant air pressure;
• a second plurality of flow control valves each of which receives said second substantially constant air pressure and feeds air pressure to one of said second plurality of air bearing pads;
• and
• means for independently adjusting a constant flow of air through each of said flow control valves in said first and second pluralities of flow control valves to independently control a height of a lift above said top surface and said second top surface developed by each of said air bearing pads.