Patent ID: RE43736

Claim:
A vacuum hold down table which includes: a) a perforated table surface sheet for receiving a workpiece , said workpiece being a sheet of material; b) a frame and support system to locate and support said table surface sheet while providing fluid access to the bottom of said table surface sheet; c) at least one plenum assembly located beneath said table surface sheet, said at least one plenum assembly having side walls which abut said table surface sheet, at least about the periphery of said table surface sheet; d) a vacuum system adapted to reduce the pressure in the plenum assembly to below ambient pressure; thereby creating a pressure differential across the thickness of the said table surface sheet; e) said table surface sheet having perforations located in at least two zones, with different aperture densities in different zones, whereby different effective hold down forces are produced on a workpiece in the different zones, wherein there is a central zone, whose size and shape corresponds approximately to the size and shape of the workpiece and an outer zone which is outside of the central zone; f) wherein the number and size of the perforations in the inner central zone combine to produce a first hold down force on the sheet workpiece located in the inner central zone, and the number and size of the perforations in the outer zone combine to produce a second, lesser, hold down force on sheet material located in the outer zone.