GB2109716A

GB2109716A - Vacuum workpiece holder

Info

Publication number: GB2109716A
Application number: GB08226389A
Authority: GB
Inventors: Kenneth O Wood; Thomas A Gordon
Original assignee: Gerber Scientific Products Inc
Current assignee: Gerber Scientific Products Inc
Priority date: 1981-09-21
Filing date: 1982-09-16
Publication date: 1983-06-08
Also published as: JPS5859740A; GB2109716B; DE3234969A1; DE3234969C2; FR2513160A1; FR2513160B1

Abstract

A vacuum chuck for holding a workpiece 31 has a chuck or work holding area over which a large number of suction cells 22 are distributed. When a workpiece is placed on the work area each suction cell covered by the workpiece applies vacuum to the workpiece bottom over a relatively large area to produce a force holding the workpiece to the chuck. Each suction cell is connected to a vacuum space 30 by a flow path 34 exclusively serving that cell and including a restriction permitting air to flow from the suction cell to the vacuum space at only a very low rate when the cell is uncovered. The total air flow to the vacuum space when all of the suction cells are uncovered is sufficiently small that a relatively small capacity suction pump or similar vacuum source can maintain a significant vacuum in the vacuum space sufficient to operate the holder. Therefore, workpieces of different sizes may be used with the holder without having to modify or adjust the holder in any way and without having to seal the exposed portions of the work holding area. A manually operable vacuum venting valve, associated with the vacuum space, is provided for releasing the workpiece from the holding force to facilitate transferral of workpieces to and from the holder. The restrictive flow paths may be provided in an alternative embodiment in an air-impermeable flexible material 53 spread over the work-holding area. <IMAGE>

Description

SPECIFICATION Vacuum workpiece holder Background of the Invention This invention relates to a vacuum chuck or similar device for holding workpieces; and deals more particularly with improvements in such a device whereby the chuck is capable of handling various different sizes of workpieces without adjustment requires only a relatively small capacity vacuum source, and needs no masking or other sealing of areas not covered by the workpiece.

Vacuum workpiece holders of the general type with which this invention is concerned are well known and are shown for example by prior U.S.

Patents No. 2,694,337; No. 2,993,824; and No.

3,294,392. In such prior art devices a workpiece support surface is provided by a porous member overlying a vacuum chamber or other vacuum space. When a workpiece is placed on the support surface vacuum pressure passing through the porous member from the vacuum space is applied to the bottom surface of the workpiece causing, in conjunction with atmospheric pressure, a force to be applied to the workpiece pushing is tightly against the support surface. However, if the workpiece fails to cover the entire support surface air leaks through its exposed portion often causing an inadequate holding force to be applied to the workpiece, due to lowering of the vacuum pressure in the vacuum space, or requiring a very large capacity vacuum pump capable of maintaining a substantial vacuum in the vacuum space despite the leakage.To overcome this leaking problem, Patent No. 2,694,337 shows a large number of valves associated with the support member which may be individually opened or closed to make porous only that portion of the support member covered by the workpiece.

Patent No. 3,294,392 shows the use of an airimpermeable plastic sheet or blanket over the exposed portion of the support surface to seal against air leakage into the vacuum space.

Both of the solutions to the leakage problem suggested by Patent No. 2,694,337 and Patent No. 3,294,392 have some disadvantages, and the object of the present invention is to provide an improved vacuum chuck or workpiece holder which is an improvement on the type of device shown by these Patents.

A more particular object of the invention is to provide a vacuum chuck or workpiece holder capable of handling a wide variety of workpiece sizes without requiring any masking, valving or other conditioning of the support member when switching from one workpiece to another regardless of whether the successive workpieces are of the same or different size. That is, the workpiece holder of the invention will immmediately accept and satisfactorily hold any workpiece presented to it and falling within a wide size range, provided only that the workpiece is relatively nonporous and has a bottom surface generally conforming, or capable of conforming, to the shape of the support surface.

Another object of the invention is to provide a vacuum chuck or workpiece holder of the foregoing character which requires only a relatively low capacity pump, or other vacuum source, which is relatively inexpensive to obtain and operate.

Another object of the invention is to provide a workpiece holding device of the foregoing character which may be designed for and used with a variety of different types of workpieces, with either curved or flat bottom surfaces, such as rigid or semi-rigid blocks, plates or sheets of metal, wood or plastic or such as flexible sheets of paper, cardboard, plastic or composite material.

Other objects and advantages of the invention will be apparent from the following detailed description and from the accompanying drawings and claims.

Summary of the Invention The invention resides in a workpiece holder having means defining a work holding area consisting in part of a workpiece engageable surface and in other part by a plurality of suction cells isolated from one another and distributed over the area. The outwardly directed face of each suction cell is either entirely void or is of a screenlike nature so as to consist in part of voids and in other part of material surfaces engageable with the workpiece to provide additional workpiece support.Each suction cell is connected to a vacuum space by its own exclusively associated flow path including a restriction having an effective cross sectional area many times smaller than the area of the void portion of the cell face, thereby permitting only a very low flow of air from the atmosphere to the vacuum space when the cell is uncovered by a workpiece, but nevertheless permitting the void area of the cell face to become vacuumised when covered by a workpiece.

The invention also resides in various different constructions of the suction cells, in the provision of a vent valve for venting vacuum from the vacuum space to facilitate placement of a workpiece on, and its removal from, the holder, and in the suction cells being of either uniform size and/or distribution over the work holding area or of nonuniform size and/or distribution over the work holding area.

Brief Description of the Drawings Fig. 1 is a perspective view showing a vacuum work holder or chuck embodying the present invention.

Fig. 2 is a fragmentary vertical cross sectional view taken on the line 2-2 of Fig. 1.

Fig. 3 is a perspective view showing the work holder of Fig. 1 combined with a vacuum pump.

Fig. 4 is a perspective view showing a work holder comprising another embodiment of this invention.

Fig. 5 is a fragmentary vertical sectional view taken on the line 5-5 of Fig. 4 and also showing a portion of a workpiece received on the holder.

Fig. 6 is a fragmentary plan view showing the face of one of the vacuum cells of the Fig. 4 device.

Fig. 7 is a view similar to Fig. 6 but shows an alternative form of restriction defining opening in the top sheet of the holder.

Fig. 8 is a view similar to Fig. 5 but shows an alternative embodiment of the invention.

Fig. 9 is a fragmentary perspective view, partly in section, showing a portion of a work holder comprising still another embodiment of this invention.

Fig. 10 is a view similar to Fig. 9 showing a work holder comprising another embodiment of the invention.

Fig. 11 is a view similar to Fig. 9 showing a work holder comprising still another embodiment of the invention.

Fig. 1 2 is a perspective view showing a work holder comprising still another embodiment of the invention.

Detailed Description of the Preferred Embodiments A workpiece holder embodying this invention may be designed for use with various different sizes and types of workpieces which are substantially nonporous and have a relatively smooth bottom surface. The workpieces may be rigid with either flat or curved bottom surfaces, in which case the vacuum holder is designed with a conforming flat or curved work support surface; or the workpieces may be pieces of flexible material capable of conforming to various different shapes of support surface, in which case the work holder is designed with whatever shape support surface is necessary to hold the workpiece in the desired condition while being worked upon.The work holder may also either be stationary relative to earth or some support and used with a tool or instrument moved by hand or machine, or the holder may be movable by hand or machine and used with a stationary tool or instrument. In the embodiments of the invention shown and described herein the work holder is shown to have a flat work support surface and to be of a generally stationary character, but it should be understood that such flatness of the support surface and stationary character of the holder are not essential to the invention and have been chosen for illustration purposes only.

Turning to the drawings and first considering Figs.1,2 and 3, a work holder embodying the invention is there shown at 14 and comprises a main plate 1 6 providing an upwardly facing work area 18 including a workpiece engageable surface 20 and a plurality of suction cells 22, 22 distributed over the area. The plate 1 6 consists of a rigid body 24 of air-impermeable material such as metal or plastic and has a layer 26 of an elastomeric material such as latex, neoprene, silicone rubber or urethane on its top surface. The layer 26 serves as a friction surface to inhibit sliding of a workpiece, such as that indicated at 31 in Fig. 1, over the work holding area and to promote sealing between the workpiece and the cells 22, 22 which it covers. The layer 26 may not, however, be needed in all cases and can be omitted if desired.It can be applied by either spraying or painting it onto the top surface of the body 24 or by applying it as a separate sheet glued or otherwise secured to the top surface of the body.

The vacuum cells 22, 22 of the holder 18 are formed by cylindrical recesses 28, 28 having open upwardly directed faces 29, 29 flush with the workpiece engageable surface 20. The workpiece engageable surface 20 extends between and completely surrounds the recesses 28, 28 and therefore pneumatically isolates the recesses from one another along lines located in the surface 20.

The recesses are of substantially uniform size and shape and are distributed uniformly over the work holding area 1 8. Although the recesses are shown to be of circular cross section this is not critical and they may be of any other convenient cross section if desired.

Below the recesses 28, 28 is a vacuum space or chamber 30 defined by a cavity in the bottom of the main plate 16 covered by a bottom plate 32 sealingly secured to the plate 16 as shown. A portion of the vacuum space 30 underlies each of the recesses 28, 28 and each recess is individually connected with the vacuum space by its own exclusive flow path consisting of a small opening 34 extending from the bottom of the recess to the vacuum space. The effective diameter of the opening 34 is many times smaller than the area of the outwardly directed face of its associated recess 28 so that when such face is uncovered by a workpiece only a very small amount of air will flow from atmosphere through the opening to the vacuum space in a given amount of time.That is, the opening 34 serves as a restriction providing, both by its small cross sectional area and its length, a high resistance to the flow of air therethrough. Thus, when a workpiece 31 is first placed on the holder 14 the holes 34, 34 associated with the covered recesses 28, 28 allow air to be evacuated from such recesses to vacuumise the recesses and create a workpiece holding force. This vacuumising of the covered recesses occurs fairly rapidly due to the relatively small volume of the recesses; however, as to the recesses 28, 28 left exposed by the workpiece the total amount of air flowing through their openings 34, 34 is still sufficiently small that an acceptable vacuum can be maintained in the vacuum space 30 by a relatively low capacity vacuum pump such as shown at 35 in Fig. 3. The number of suction cells, their size, shape and spacing, the size of the openings 34, 34 and the size of the vacuum pump 35 may vary widely depending on the use to which the holder is to be put. As an example, however, the holder may have, as shown, one hundred forty-four suction cells 22, 22 defined by cylindrical recesses 28, 28 of 3/4 inch diameter which are spaced one inch from one another along twelve rows and twelve columns, and the openings 34, 34 may have diameters of approximately 0.010 inch and lengths of about 1/8 inch. With such a construction of the holder a vacuum pump 35 driven by a 1/8 horsepower motor should be sufficient to properly service the holder.In any event, the capacity of the pump or other vacuum source used with the holder should be such as to be capable of maintaining an appreciable vacuum, of about 5 inches of mercury of greater, in the vacuum space 30 when all of the suction cells 22, 22 are exposed as a result of no workpiece being received on the work area 1 8.

As shown, the vacuum pump 35 is connected to the holder 14 by a vacuum line 36 communicating with the vacuum space 30. The vacuum pump preferably is one which operates continuously while the holder is used with different successive workpieces. To facilitate the transfer of workpieces to and from the holder the holder may include, as shown, a vent 38 and a manually operable valve 40 in the vent.

The illustrated valve 40 is biased upwardly to a closed position by a spring 42 and may be manually depressed to an open position, at which its port 44 aligns with the vent 38 to vent the vacuum space 30 to the atmosphere. Therefore, when a workpiece is to be placed on the holder 18 the valve 40 may be depressed to vent the vacuum space 30. While this venting is in effect the workpiece may be placed on the holding area 1 8 and can be easily slid over the holding area to bring it to any desired position.The valve 40 is then released to close the vent 38, vacuumising the space 30 and applying vacuum to the workpiece 31 through the covered suction cells 22, 22. To remove the workpiece from the holder the valve 40 is again depressed, releasing vacuum from the vacuum space and from the covered suction cells thereby removing the holding force from the workpiece and allowing it to be easily lifted from the hoider.

Figs. 4, 5 and 6 show a workpiece holder 46 comprising another embodiment. The holder 46 comprises a main plate 48 having a flat top face 50 containing a plurality of cylindrical recesses 52, 52 of uniform size distributed uniformly over the face 50. The top face 50 is in turn covered by a flexible sheet 53 the top surface of which provides the work area 56 for the holder; and the sheet is preferably made of a material providing its surface with a frictional quality inhibiting sliding of workpieces thereover, the material also preferably being one which is elastomeric so as to promote sealing between the bottom surface of a workpiece and the suction cells of the holder.

Various different materials may be used for the sheet 53 and for example it may be a sheet of urethane or neoprene. The areas of the sheet 53 which overly and register with the recesses 52, 52 comprise the suction cells 58, 58 of the holder 46, and the area of the top sheet surrounding the suction cells 58, 58 comprise a workpiece engageable surface 60.

A vacuum space 30, similar to that of the holder 14, is included in the holder 46 and underlies each of the recesses 52, 52.

Communication between each recess 52 and the vacuum space 30 is provided by a relatively large opening 62 in the bottom of each recess which provides no substantial resistance to the flow of air between the recess and the vacuum space. A means providing a restriction in the flow path between each suction cell 58 and the vacuum space 30 is provided by a small opening 64 in the sheet 53, there being one such opening 64 for each suction cell 58.

The vacuum produced in each recess 52 draws onto it the overlying portion of the sheet 53 as illustrated in Fig. 5, thereby giving the suction cells 58, 58 the shape and appearance of suction cups, and the upper or outwardly directed face 66 of each suction cell, which is flush with the work engageable surface 60, is therefore completely void. When a workpiece, such as shown at 31 in Fig. 5, is placed on the holder 46 the cells 58, 58 covered by the workpiece become vacuumised by the withdrawl of air through their openings 64, 64.

This vacuum pressure is applied to the bottom surface of the workpiece 28 through the open top faces 66, 66 of the covered cells creating a holding force urging the workpiece downwardly toward the holder. As this occurs, the sheet 53 is compressed between the workpiece 28 and the plate 48 forming a seal around the edge of each covered suction cell 58 and the workpiece bottom inhibiting leakage of outside air into the cell.

Again, as with the openings 34, 34 of the holder 14, the size of the restriction forming openings 64, 64 is so small that the total amount of air flowing through the uncovered cells 58, 58 is small enough that a suitable degree of vacuum can be maintained in the vacuum space 30 by a relatively small capacity vacuum source regardless of the number of cells uncovered.

As shown in Fig. 6 each hole 64 of the holder 46 is a discrete opening as may be formed by means of a drill or by a heated piercing tool pushed through the sheet 53. Other type openings may, however, be provided, and as an example of one such other type Fig. 7 shows an opening 64' consisting of a small slit formed by pushing a sharp blade through the sheet 53 without removing any substantial material. When the associated recess is vacuumised, the slit 64' opens sufficiently to allow air to flow through it into the recess.

In the holder 46 of Figs. 4, 5 and 6, the recesses 52, 52 are empty so that when the recesses are vacuumised the overlying portions of the sheet 53 are pulled into the recesses by a variable amount depending on the degree of vacuum in the recesses and the elasticity of the sheet material. If the sheet 53 is very thin, as may be desirable in some cases, this pulling of the sheet into the recesses 52, 52 may tend to rupture the sheet or may pull the sheet into engagement with the bottoms of the recesses closing off some of the openings 64, 64. To prevent this each recess may, as shown in the holder 46' of Fig. 8, be provided with an air-porous insert 68, such as a block of felt or sintered metal, having a top surface located only a small distance below the top surface 50 of the plate 48.Therefore, as shown in Fig. 8, the inserts 68, 68 engage the portions of the sheet 53 drawn into the recesses 52, 52 allowing the creation of suction cells 58, 58 without undue stretching or other undesired deformation of the sheet 53.

In each of the holders 14 and 46 described above each suction cell has an entirely void outer face which provides no support for the workpiece, workpiece support and being provided entirely by the workpiece engageable surface which surrounds the various cells. These holders are therefore suitable for use with relatively rigid workpieces which are not significantly deformed by the absence of support in the areas occupied by the suction cells. They may not however perform satisfactorily for holding paper or similar thin flexible workpieces.To overcome this problem and provide a holder for such flexible workpieces the suction cells may be designed so as to have screen-like outer faces consisting partially of voids and partially of material engageable with the bottom of the workpiece to provide workpiece support in the area of the suction cells. Figs. 9,10 and 11 show three holders constructed in this manner. These holders are in large part similar to the holder 14 of Figs. 1 to 3 and parts which are or may be identical with those of the holder 14 have been given the same reference numbers and need not be redescribed.

Fig. 9 shows a holder 76 in which each recess 28 is filled with an air-porous insert 78 having a top surface 80 flush with the work engageable surface 20. Each insert 80 may, for example, be made of sintered metal or rigid felt.

Fig. 10 shows a holder 82 wherein each recess has associated with it an aperture plate made of rigid metal or plastic received in a shouldered upper portion of the recess. Each plate 84 has a top surface 86 flush with the workpiece engageable surface 20 of the plate 1 6 and includes a large number of relatively large diameter apertures 94, 94 passing through it and communicating with each of the underlying recesses 28, 28. Therefore, each cluster of apertures 94, 94 associated with one recess 28 and encircled by a broken line 96 in Fig. 11 comprises one suction cell and the top surface of the sheet 92, between the suction cell areas 96, 96 provides a workpiece engageable area 20' similar to the workpiece engageable area 20 of the holder 14.

In all of the previously described holders the suction cells have been shown or taken to be of substantially uniform size and to be distributed uniformly over the work holding area. This, however, is not necessary and if it is known that small workpieces are always to be placed at one location of the work holding area it may be desirable to provide a large number of relatively small sized suction cells in such location and to in general gradually increase the size and spacing of the suction cells as the distance from such location is increased.Such a holder is shown, for example, at 98 in Fig. 12 and includes a plurality of suction cells 100, 100. These suction cells except for their size and distribution may be similar to any one of the cells previously described and the remaining structure of the holder 98 may also be similar to any one of the previously described holders. The holder 98 has a work holding area 102 and along two margins of the area 102 has two straight edges 104 and 106 to assist in placing workpieces on the holding area.

When dealing with rectangular workpieces a workpiece is placed on the holder 98 by moving two of its edges into engagement with the straight edges 104 and 106 respectively. Therefore, smaller workpieces will always be located in the upper left-hand corner of the holder, as seen in Fig. 12. In keeping with this, the vacuum spaces 1 00, 1 00 in the upper left-hand corner of the work holding area 102 are of relatively small size and are relatively closely spaced to one another, and other suction cells 100, 100 are increased in size and more widely spaced as their distance from the upper left-hand corner is increased.This arrangement of the suction cells assures that a fair number of suction cells will be covered by a small workpiece to provide an adequate holding force for it while at the same time reducing air leakage into the vacuum space by leaving exposed a smaller number of suction cells than would be the case with a uniform size and a uniform distribution of cells. When a large workpiece is placed on the holder 98 the covered suction cells provide ample force to secure it to the holder with the widely spaced large suction cells in the areas remote from the upper left-hand corner being sufficient to hold down the extremities of the workpiece.

It should also be noted that in connection with the holder 46 of Figs. 4, 5 and 6 the top sheet 53 may be supplied as an imperforate one to the user with the user punching the openings 64, 64 therein as needed for particular workpieces. That is, for example, the holder may have a large work holding area and a vast number of recesses 52, 52. Then when using the holder to hold a workpiece substantially smaller than the available work holding area the operator may punch openings 64, 64 in the sheet to provide such openings only for such recesses as will be covered by the workpieces. Of course, the top sheet is replaceable and new imperforate sheets may be used and punched as needed. Also, the punching process is one which is easy and capable of being performed at little expense.

Claims

1. A workpiece holder comprising: means defining a work holding area, a portion of which area consists of a workpiece engageable surface defined by air-impermeable material and another portion of which consists of a plurality of suction cells distributed over said area, each of said cells being completely surrounded by portions of said workpiece engageable surface so that said cells are isolated from one another along lines located in said surface, each of said cells having an outwardly directed face flush with said workpiece Jngaging surface which face consists at least in part of d void portion, means defining a vacuum space adapted for connection to a source of vacuum, and means defining a plurality of flow paths each extending between said outwardly directed face of a respective one of said cells and said vacuum space, said flow path defining means including for each of said flow paths a means defining a restriction having an effective cross sectional area many times smaller than the area of said void portion of the face of the associated one of said suction cells.

2. A workpiece holder as defined in Claim 1 further characterised by said means defining a work holding area including a plate having a top face with a plurality of recesses distributed thereover, said recesses comprising said suction cells and the portion of said top face located between said recesses comprising said workpiece engageable surface.

3. A workpiece holder as defined in Claim 2 further characterised by a portion of said vacuum space being located below each of said recesses, and for each of said flow paths said means defining a restriction comprising a small opening passing from the bottom of the associated recess to said vacuum space.

4. A workpiece holder as defined in Claim 1 further characterised by said means defining a work holding area comprising a plate having a top face with a plurality of recesses distributed thereover, a sheet of air-impermeable flexible material spread over said top face of said plate and covering said recesses, said sheet including a plurality of small openings each passing through said sheet and communicating with a respective one of said recesses, the spaces located on the outside of said sheet and registering with said recesses comprising said suction cells, and said small openings in said sheet comprising said restrictions.

5. A workpiece holder as defined in Claim 4 further characterised by said sheet of airimpermeable flexible material being a sheet of elastomeric material.

6. A workpiece holder as defined in Claim 4 further characterised by said sheet of airimpermeable flexible material being a sheet of material comprised substantially of urethane.

7. A workpiece holder as defined in Claim 4 further characterised by each of said small openings being in the form of a discrete hole in said sheet material.

8. A workpiece holder as defined in Claim 4 further characterised by each of said small openings being in the form of a slit in said sheet material.

9. A workpiece holder as defined in Claim 4, 5, 6, 7 or 8 further characterised by a portion of said vacuum space being located below each of said recesses and said flow path defining means including a relatively large opening extending from the bottom of each of said recesses to said vacuum space.

10. A workpiece holder as defined in any one of Claims 4, 5, 6, 7 or 8 further characterised by a plurality of relatively rigid air-permeable inserts tdch received in a respective one of said recesses and arranged so as to engage and support the bottom surface of the associated portion of said sheet of flexible material when said sheet of flexible material is drawn into said recess by a pressure difference existing between the bottom and top surfaces of said sheet.

11. A workpiece holder as defined in Claim 10 further characterised by each of said inserts comprising a piece of felt.

12. A workpiece holder as defined in Claim 10 further characterised by each of said inserts comprising a piece of sintered metal.

13. A workpiece holder as defined in Claim 1 further characterised by said means defining a work holding area including a plate having a top face with a plurality of recesses distributed thereover, and a plurality of relatively rigid airpermeable inserts each received in a respective one of said recesses, the portion of said plate top face located between said recesses comprising said workpiece engageable surface, and said inserts having outwardly directed faces flush with said workpiece engageable surface, each of said inserts comprising a respective one of said suction cells.

14. A workpiece holder as defined in Claim 13 further characterised by a portion of said vacuum space being located below each of said recesses, and said means defining a restriction for each of said flow paths comprising a small opening passing from the bottom of the associated recess to said vacuum space.

1 5. A workpiece holder as defined in Claim 1 further characterised by means defining a vent extending between said vacuum space and atmosphere, and a valve for selectively opening and closing said vent.

1 6. A workpiece holder as defined in Claim 1 5 further characterised by said valve having a means biasing it to the position at which it closes said vent and a manually operable means for moving it against the force of said biasing means to the position at which it opens said vent.

1 7. A workpiece holder as defined in Claim 1 further characterised by said work holding area having one section containing suction cells with outwardly directed faces of small area and closely spaced with respect to another and by said work holding area having another section containing suction cells with outwardly directed faces of relatively larger area and relatively more widely spaced with respect to one another.

1 8. A workpiece holder comprising: a plate having a top face with a plurality of recesses distributed thereover and a workpiece engageable surface between said recesses, a plurality of relatively rigid air-permeable inserts each received in a respective one of said recesses and having an outwardly directed face flush with said workpiece engageable surface and including void portions, means defining a vacuum space, and means defining a plurality of flow paths each extending between a respective one of said recesses and said vacuum space, each of said flow paths including a means defining a restriction having an effective cross sectional area many times smaller than the area of the void portions of the face of the associated one of said inserts.

1 9. A workpiece holder substantially as herein described and shown in the accompanying drawings.

14. A workpiece holder as defined in Claim 13 further characterised by each of said inserts being a piece of sintered metal.

1 5. A workpiece holder as defined in Claim 13 further characterised by each of said inserts being a rigid member having a plurality of discrete apertures passing therethrough generally perpendicular to its top face.

1 6. A workpiece holder as defined in Claim 13 further characterised by a portion of said vacuum space being located below each of said recesses, and said means defining a restriction for each of said flow paths comprising a small opening passing from the bottom of the associated recess to said vacuum space.

1 7. A workpiece holder as defined in Claim 1 further characterised by said means defining a work holding area including a plate having a top face with a plurality of recesses distributed thereover, and a rigid sheet overlying said top face of said plate, said sheet having for each of said recesses a plurality of discrete apertures passing through said sheet and communicating with said recesses to define one of said suction cells.

1 8. A workpiece holder as defined in Claim 1 7 further characterised by a portion of said vacuum space being located below each of said recesses, and said means defining a restriction for each of said flow paths comprising a small opening passing from the bottom of the associated recess to said vacuum space.

19. A workpiece holder as defined in Claim 1, 2, 4, 13 or 17 further characterised by means defining a vent extending between said vacuum space and atmosphere, and a valve for selectively opening and closing said vent.

20. A workpiece holder as defined in Claim 19 further characterised by said valve having a means biasing it to the position at which it closes said vent and a manually operable means for moving it against the force of said biasing means to the position at which it opens said vent.

21. A workpiece holder as defined in Claim 1 further characterised by said all of said suction cells having outwardly directed faces of substantially uniform size and shape distributed substantially uniformly over said work holding area.

22. A workpiece holder as defined in Claim 1 further characterised by said work holding area having one section containing suction cells with outwardly directed faces of small area and closely spaced with respect to another and by said work holding area having another section containing suction cells with outwardly directed faces of relatively larger area and relatively more widely spaced with respect to one another.

23. A workpiece holder as defined in Claim 1 further characterised by a vacuum source connected with said vacuum space, said restrictions being of such size and said vacuum source being of such capacity that when all of said suction cells are uncovered by a workpiece a substantial vacuum can be maintained in said vacuum space by said vacuum source.

24. A workpiece holder as defined in Claim 1 further characterised by a source of vacuum connected with said vacuum space, said restrictions being of such size and said vacuum source being of such capacity that when all of said suction cells are uncovered by a workpiece a vacuum of approximately five inches of mercury or greater is maintained in said vacuum space by operation of said vacuum source.

25. A workpiece holder comprising: a plate having a top face with a plurality of recesses distributed thereover and a workpiece engageable surface between said recesses, means providing a vacuum space, and means defining a plurality of flow paths each extending between a respective one of said recesses and said vacuum space, said flow path defining means including for each of said flow paths a means defining a restriction having a cross sectional area many times smaller than the area of its associated recess as measured flush with said workpiece engageable surface.

26. A workpiece holder as defined in Claim 25 further characterised by said workpiece engageable surface being provided by an elastomeric material.

27. A workpiece holder comprising: a plate having a top face with a plurality of recesses distributed thereover, a sheet of air-impermeable flexible material spread over said top face of said plate and covering said recesses. said sheet including a plurality of small openings each passing through said sheet and communicating with a respective one of said recesses, each of said openings having an effective cross sectional area many times smaller than the cross sectional area of its associated recess. means defining a vacuum space, and means connecting all of said recesses to said vacuum space.

28. A workpiece holder comprising: a plate having a top face with a plurality of recesses distributed thereover and a workpiece engageable surface between said recesses, a plurality of relatively rigid air-permeable inserts each received in a respective one of said recesses and having an outwardly directed face flush with said workpiece engageable surface and including void portions, means defining a vacuum space, and means defining a plurality of flow paths each extending between a respective one of said recesses and said vacuum space, each of said flow paths including a means defining a restriction having an effective cross sectional area many times smaller than the area of the void portions of the face of the associated one of said inserts.

29. A workpiece holder substantially as herein described and shown in the accompanying drawings.

New claims or amendments to claims filed on 23 February 1 983.

Superseded claims 1-29.

New or amended claims:

1. A workpiece holder comprising: means defining a work holding area, a portion of which area consists of a workpiece engageable surface defined by air-impermeable material and another portion of which consists of a plurality of suction cells distributed over said area, each of said cells being completely surrounded by portions of said workpiece engageable surface so that said cells are pneumatically isolated from one another along lines located in said surface, each of said cells having an outwardly directed face flush with said workpiece engaging surface which face consists at least in part of a void portion, means defining a vacuum space adapted for connection to a source of vacuum, and means defining a plurality of flow paths each extending between said outwardly directed face of a respective one of said cells and said vacuum space, said flow path defining means including for each of said flow paths a means defining a restriction having an effective cross sectional area many times smaller than the area of said void portion of the face of the associated one of said suction cells, said means defining a work holding area comprising a plate having a top face with a plurality of recesses distributed thereover, and a sheet of an elastomeric air-impermeable flexible material spread over said top face of said plate and covering said recesses, said sheet including a plurality of small openings each passing through said sheet and communicating with a respective one of said recesses, the spaces located on the outside of said sheet and registering with said recesses comprising said suction cells, and said small openings in said sheet comprising said restrictions.

2. A workpiece holder as defined in Claim 1 further characterised by said sheet of an elastomeric air-impermeable flexible material being a sheet of material comprised substantially of urethane.

3. A workpiece holder as defined in Claim 1 or Claim 2 further characterised by each of said small openings being in the form of a discrete hole in said sheet material.

4. A workpiece holder as defined in Claim 1 or Claim 2 further characterised by each of said small openings being in the form of a slit in said sheet material.

5. A workpiece holder as defined in any preceding Claim further characterised by a portion of said vacuum space being located below each of said recesses and said flow path defining means including a relatively large opening extending from the bottom of each of said recesses to said vacuum space.

6. A workpiece holder as defined in any one of Claims 1 to 4 further characterised by a plurality of relatively rigid air-permeable inserts each received in a respective one of said recesses and arranged so as to engage and support the bottom surface of the associated portion of said sheet of flexible material when said sheet of flexible material is drawn into said recess by a pressure difference existing between the bottom and top surfaces of said sheet.

7. A workpiece holder as defined in Claim 6, further characterised by each of said inserts comprising a piece of felt.

8. A workpiece holder as defined in Claim 6 further characterised by each of said inserts comprising a piece of sintered metal.

9. A workpiece holder comprising: means defining a work holding area, a portion of which area consists of a workpiece engageable surface defined by air-impermeable material and another portion of which consists of a plurality of suction cells distributed over said area, each of said cells being completely surrounded by portions of said workpiece engageable surface so that said cells are pneumatically isolated from one another along lines located in said surface, each of said cells having an outwardly directed face flush with said workpiece engaging surface which face consists at least in part of a void portion, means defining a vacuum space adapted for connection to a source of vacuum, and means defining a plurality of flow paths each extending between said outwardly directed face of a respective one of said cells and said vacuum space, said flow path defining means including for each of said flow paths a means defining a restriction having an effective cross sectional area many times smaller than the area of said void portion of the face of the associated one of said suction cells said means defining a work holding area including d plate having a top face with a plurality of recesses distributed thereover, and a plurality of relatively rigid air-permeable inserts each received in a respective one of said recesses, the portion of said plate top face located between said recesses comprising said workpiece engageable surface, and said inserts having outwardly directed face flush with said workpiece engageable surface, each of said inserts comprising a respective one of said suction cells.

10. A workpiece holder as defined in Claim 9 further characterised by each of said inserts being a piece of sintered metal.

11. A workpiece holder as defined in Claim 9 further characterised by each of said inserts being a rigid member having a plurality of discrete apertures passing therethrough generally perpendicular to its top face.

12. A workpiece holder as defined in Claim 9 further characterised by a portion of said vacuum space being located below each of said recesses, and said means defining a restriction for each of said flow paths comprising a small opening passing from the bottom of the associated recess to said vacuum space.

13. A workpiece holder comprising: means defining a work holding area, a portion of which area consists of a workpiece engageable surface defined by air-impermeable material and another portion of which consists of a plurality of suction cells distributed over said area, each of said cells being completely surrounded by portions of said workpiece engageable surface so that said cells are isolated from one another along lines located in said surface, each of said cells having an outwardly directed face flush with said workpiece engaging surface which face consists at least in part of a void portion, means defining a vacuum space adapted for connection to a source of vacuum, and means defining a plurality of flow paths each extending between said outwardly directed face of a respective one of said cells and said vacuum space said flow path defining means including for each of said flow paths a means defining a restriction having an effective cross sectional area many times smaller than the area of said void portion of the face of the associated one of said suction cells, said means defining a work holding area including a plate having a top face with a plurality of recesses distributed thereover, and a rigid sheet overlying said top face of said plate, said sheet having for each of said recesses a plurality of discrete apertures passing through said sheet and communicating with said recesses to define one of said suction cells.