Perforated vacuum hold down surface

Abstract

A vacuum hold down table includes a surface sheet having perforations arranged so as to reduce surface cracking when the surface sheet is subject to forces during use of the table; the perforations may also be arranged to that a greater hold down force is produced in that portion of the table where the workpiece will be located, this may be accompanied by varying the hole diameter and/or hole spacing.

1. 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. 
 
     
     
       2. A vacuum hold down table which includes:
 a) a perforated surface sheet for supporting a workpiece; 
 b) a frame and support system to locate and support said surface sheet, while providing fluid access to the bottom of said surface sheet; 
 c) at least one plenum assembly located beneath said surface sheet, said at least one plenum assembly abutting said 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 surface sheet; 
 e) said surface sheet having perforations, wherein said perforations are located on curved lines, wherein the average radius of curvature is from about 0.1 and 100 inches. 
 
     
     
       3. A vacuum hold down table as in  claim 2  in which the perforations in the said surface sheet are distributed so that there is a, said surface sheet having a workpiece zone and an outer zone, the workpiece zone have having an increased hole density relative to an the outer zone. 
     
     
       4. A vacuum hold down table as in  claim 3  in which the density and size of the perforations in the workpiece zone combine to produce a hold down force per unit which is greater than the hold down force per unit area produced in the outer zone. 
     
     
       5. A vacuum hold down table comprising a perforated work table surface, and means to cause a pressure differential across said perforated work table surface so that ambient pressure causes a down force on workpieces placed on said perforated work table surface, said perforated work table surface having at least 1,000 perforations per square foot, wherein said workpiece perforated work table surface has at least one zone where the aperture density, in combination with the pressure differential produces a hold down force on a workpiece which is at least 20% greater than the hold down force produced on a workpiece on the balance of the perforated work table surface, wherein the apertures in at least the a center zone are arranged on curved lines. 
     
     
       6. A vacuum hold down table as in claim 5 wherein the perforations are about 2,100 perforations per square foot.  
     
     
       7. A vacuum hold down table for holding a workpiece, the vacuum hold down table including:
 a table surface sheet having a first zone and a second zone, each zone having perforations;   a enclosed space under said table surface sheet; and   a vacuum system adapted to reduce the pressure in the enclosed space below the ambient pressure and create a pressure differential across the table surface sheet;   wherein the perforations in the first zone produce a first hold down force, and the perforations in the second zone produce a second hold down force, the second hold down force different than the first hold down force, wherein the perforations in the first zone have a first density and the perforations in the second zone have a second density, wherein the first density differs from the second density.   
     
     
       8. The vacuum hold down table of claim 7 wherein spacing of the perforations, the number of perforations, or the effective diameter of the perforations is varied to produce a first perforations density in the first zone which differs from a second perforations density in the second zone. 
     
     
       9. The vacuum hold down table of claim 7 wherein the first zone has a size or a shape that corresponds approximately to the workpiece. 
     
     
       10. The vacuum hold down table of claim 7 wherein at least a number of the perforations are arranged along at least one curved line. 
     
     
       11. The vacuum hold down table of claim 7 wherein at least a number of the perforations are arranged along a generally sinusoidal line. 
     
     
       12. A vacuum hold down table for holding a workpiece, the vacuum hold down table including:
 a surface sheet having perforations located on at least one curved line;   an enclosed space located under the surface sheet; and   a vacuum system adapted to reduce the pressure in the enclosed space below the ambient pressure and create a pressure differential across the surface sheet.   
     
     
       13. The vacuum hold down table according to claim 12 wherein the at least one curved line having an average radius of curvature from about 0.1 inches to about 100 inches. 
     
     
       14. The vacuum hold down table according to claim 12 wherein the perforations in the surface sheet are distributed, the surface sheet having a first zone and a second zone, the perforations in the first zone having a first density different than a second density of the perforations in the second zone. 
     
     
       15. The vacuum hold down table according to claim 12 wherein the perforations in the surface sheet are distributed, the surface sheet having a workpiece zone and a second zone, and wherein the number of perforations and the size of the perforations in the workpiece zone produce a workpiece hold down force per unit area and wherein the number of perforations and the size of the perforations in the second zone produce an outer hold down force per unit area when the surface sheet is subject to a pressure differential, the workpiece hold down force per unit area different than the second hold down force per unit area. 
     
     
       16. A vacuum hold down table comprising:
 a table surface having a perforation density of at least 1,000 perforations per square foot, the perforations operative to produce a down force across the table surface when subject to pressure differential, the table surface having a first zone in which the perforation density is adapted to subject a workpiece positioned therein to a down force which is greater than the down force produced on a workpiece positioned on the table surface but not positioned in the first zone; and   a plenum system for subjecting the table surface to a pressure differential and generating a down force across the table surface.   
     
     
       17. The vacuum hold down table of claim 16 wherein at least a number of the perforations in the first zone arranged on curved lines. 
     
     
       18. The vacuum hold down table of claim 16 wherein at least a number of the perforations are arranged along a generally sinusoidal line. 
     
     
       19. A vacuum hold down table of claim 16 wherein the perforation density is about 2,100 perforations per square foot. 
     
     
       20. A hold down table for holding a workpiece, the hold down table including:
 a table surface sheet having a first zone and a second zone, wherein the first zone and the second zone have perforations operative to generate a hold down force when subject to a pressure differential across the table surface sheet, the hold down force produced in the first zone different from the hold down force produced in the second zone, wherein the first zone has a greater perforation density than the second zone.   
     
     
       21. The hold down table according to claim 20 further including a plenum system adapted to create a pressure differential across the table surface sheet. 
     
     
       22. The hold down table according to claim 20 wherein at least a number of the perforations in at least the first zone are located on curved lines. 
     
     
       23. The hold down table according to claim 20 wherein at least a number of the perforations are arranged along a generally sinusoidal line. 
     
     
       24. The hold down table according to claim 20 wherein the number, the effective diameter, or spacing of the perforations in the first zone is varied such that the hold down force produced in the first zone is greater than the hold down force produced in the second zone. 
     
     
       25. The hold down table according to claim 20 wherein the first zone has a size or a shape that corresponds approximately to the workpiece. 
     
     
       26. A hold down table comprising:
 a table surface the table surface having a first zone and having a perforation density of at least 200 perforations per square foot or 10% of the first zone, the perforations operative to produce a down force across the table surface when subject to pressure differential, the perforation density is adapted to produce a down force which is greater than the down force produced outside the first zone, a predetermined number of the perforations in the first zone arranged on curved lines.   
     
     
       27. A vacuum hold down surface according to claim 26 wherein the perforations have a perforation density of about 2,100 perforations per square foot. 
     
     
       28. A hold down table comprising:
 a table surface having a perforation density of at least 1,000 perforations per square foot, the perforations operative to produce a down force across the table surface when subject to pressure differential, the table surface having a first zone in which the perforation density is adapted to produce a down force which is greater than the down force produced outside the first zone, a predetermined number of the perforations in the first zone arranged on curved lines.   
     
     
       29. A vacuum hold down surface according to claim 28 wherein the perforations have a perforation density of about 2,100 perforations per square foot. 
     
     
       30. The hold down table according to claim 28 further including:
 means to subject the table surface to a pressure differential and generate a down force across the table surface.   
     
     
       31. The hold down table according to claim 28 wherein a predetermined number of the perforations are arranged along a generally sinusoidal line. 
     
     
       32. The hold down table according to claim 28 wherein the size or a shape of the first zone corresponds approximately to the workpiece. 
     
     
       33. A vacuum hold-down surface for holding a workpiece, comprising:
 a surface sheet having perforations located along at least one curved line;   the perforations of the surface sheet being arranged to generally conform to a surface of the workpiece;   the perforations of the surface sheet operative to provide a retention force on the workpiece when a pressure differential is applied across the surface sheet.   
     
     
       34. The vacuum hold down surface according to claim 33 wherein the at least one curved line has an average radius of curvature from about 0.1 inches to about 100 inches. 
     
     
       35. The vacuum hold down surface according to claim 33 wherein the perforations in the surface sheet are distributed, the surface sheet having a first zone and a second zone, the perforations in the first zone having a first density that differs from a second density of the perforations in the second zone. 
     
     
       36. The vacuum hold down surface according to claim 35 wherein the first zone corresponds approximately in size or shape to the workpiece. 
     
     
       37. The vacuum hold down surface according to claim 33 wherein the curved line is a generally sinusoidal line. 
     
     
       38. A vacuum hold-down surface for holding a workpiece, comprising:
 a table surface sheet for receiving the workpiece;   a central zone having a size and a shape that corresponds approximately to the workpiece and an outer zone deposed about the central zone;   said table surface sheet having perforations located in at least the central zone and the outer zone, said zones having different aperture densities for providing different effective hold down forces on the workpiece.   
     
     
       39. The vacuum hold-down surface according to claim 38, wherein the perforations in the central zone conveys a first hold down force on the workpiece in the central zone, and the perforations in the outer zone conveys a second hold down force on the workpiece in the outer zone. 
     
     
       40. The vacuum hold-down surface according to claim 38, wherein the second hold down force is lesser than the first hold down force. 
     
     
       41. The vacuum hold-down surface according to claim 38, wherein at least a number of the perforations in the central zone are arranged on curved lines. 
     
     
       42. The vacuum hold-down surface according to claim 38, wherein at least a number of the perforations are arranged along a generally sinusoidal line. 
     
     
       43. The vacuum hold-down surface according to claim 38, wherein the perforations have a perforation density of at least 1,000 perforations per square foot. 
     
     
       44. A vacuum hold down surface according to claim 38 wherein the perforations have a perforation density of at least 2,100 perforations per square foot. 
     
     
       45. A vacuum hold down surface according to claim 38 wherein the perforations have a perforation density of at least 200 perforations per square foot or 10% of the central zone. 
     
     
       46. A vacuum hold down table comprising a perforated work table surface having at least one inner zone and at least one outer zone, and means to cause a pressure differential across said perforated work table surface so that ambient pressure causes a down force on workpieces placed on said perforated work table surface, said perforated work table surface having an aperture density of at least 200 perforations per square foot or 10% of the at least one inner zone, wherein the aperture density, in combination with the pressure differential produces a hold down force on a workpiece which is at least 20% greater than the hold down force produced on a workpiece on the balance of the perforated work table surface, wherein the apertures in at least a center zone are arranged on curved lines. 
     
     
       47. A vacuum hold down surface according to claim 46 wherein the perforations have a aperture density of about 2,100 perforations per square foot. 
     
     
       48. A vacuum hold-down surface for holding a workpiece, comprising:
 a table surface sheet for receiving the workpiece;   a central zone having a size and a shape that corresponds approximately to the workpiece and an outer zone deposed about the central zone;   said table surface sheet having perforations located in at least the central zone and the outer zone, said zones having different aperture densities for providing different effective hold down forces on the workpiece;   wherein said table surface sheet has an aperture density of at least 200 perforations per square foot or 10% of the central zone.   
     
     
       49. A vacuum hold down surface according to claim 48 wherein the perforations have an aperture density of about 2,100 perforations per square foot.