CUTTING ASSEMBLY FOR CUTTING SHEET MATERIAL RELEASABLY RETAINED BY A PRESSURE DIFFERENTIAL

Abstract: A method and apparatus for cutting sheet material (12) upon an apertured substrate (40), the apparatus including a microporous sacrificial overlay (30) affixed to the substrate (40). The overlay (30) is selected to provide sufficient support to the sheet material (10) such that upon a cutting wheel (82) passing over an aperture (43) in the substrate, the sheet material (12) is severed by the cutting wheel (82).

Description: CUTTING ASSEMBLY FOR CUTTING SHEET MATERIAL RELEASABLY RETAINED BY A PRESSURE DIFFERENTIAL Field of the Invention The present invention relates to cutting a sheet material, and more particularly, to a cutting assembly for cutting sheet material retained upon a vacuum table having a plurality of vacuum apertures. Background of the Invention Rotary wheel cutters, in contrast to reciprocating blade cutters, have unique characteristics which are suitable for cutting plies of sheet material that are collectively relatively thin, for example, less than 1/4 inch thickness. The cutting action produced by a wheel comes about through a severance of the material when the sharp peripheral cutting edge of the wheel is brought into engagement with a support surface with the material therebetween. The edge severs the materials or fibers in what is believed to be both a crushing and cutting operation. A unique and advantageous characteristic of the rotary wheel cutting process is that there is basically no inherent limitation on the speed at which the severance of material takes place, nor upon the rate at which the cutting wheel operates producing that severing process. Consequently, a cutting wheel is a desirable tool for cutting a single ply of selected fabric material, for example, a suit. One of the principle difficulties encountered in cutting single plies of sheet material is the retention of the sheet material in a fixed position throughout the cutting process. In systems employing an automatically controlled machine that operates from a predetermined program, the material cannot shift in the course of a cutting operation, otherwise, the pattern pieces that are cut will not conform to the program lines of cut. Also, since high speed is one of the main advantages of the cutting wheel, the machine should be designed to perform at high speed with minimum inertia and extra motion. Vacuum hold down devices of the type which produce sub atmospheric pressure at a work or bearing surface on which the sheet material is spread have gained acceptance in the material cutting art. A vacuum hold down device is disclosed in U.S. Patent No. 4,444,078. The vacuum hold down device includes a cutting wheel which rolls in cutting engagement with a bearing surface which has an array of openings therethrough communicating through channels with a vacuum pump. However, problems may be encountered when such apparatus is used to cut materials such woven fabrics. As the cutting instrument passes over the apertures in the bearing surface, threads which comprise the fabric may be forced into the apertures by the cutting instrument rather than being cut by it. As a result, pattern pieces cut from the fabric are not easily separated from the waste material. Further, failure to effectively sheer all of the threads which comprise the fabric in the area of an aperture may result in cut pattern pieces with rough or ragged edges. U.S. Patent No. 4,444,078 represents an approach of the prior art to solve the present problem. In the '078 apparatus, each aperture in the support surface includes a valve movable between an open position spaced from the support surface and closed position flush with the adjacent support surface. However, a support surface having a multitude of these valves and associated mechanisms for selectively activating the valves is extremely complex and hence expensive. Further, the large number of moving parts increases maintenance requirements. United States Patent No. 5,699,707 discloses a high speed material cutter having a cutting wheel with a peripheral cutting edge. The '707 patent employs a microporous support belt on a conveyor, wherein the support belt and the convey move along a cutting table. The support belt includes a tough engagement surface against which a sharp edge of a cutting tool is engaged without causing substantial downward deformation of the support belt. However, such belt is relatively expensive and thereby significantly increases the cost of the system. A need exists for a cutting system which cooperates with an apertured vacuum table to provide continuous cutting along a cut path. Specifically, the need exists for ensuring cutting of a sheet material when the cut path cross over an aperture in the vacuum table. The need further exists for a method of cutting sheet material on an apertured vacuum table which reduces bridges of material between a desired part periphery and the remaining portion of the sheet material. The need further exists for a cutting system that can be employed in existing vacuum systems for improving the performance of the system. Summary of the Invention The present invention provides for the releasable retention of a sheet material on an apertured vacuum table, wherein cutting of the sheet material at the local regions overlying the apertures is ensured. In particular, the invention provides for the continuous severing of a sheet material by a cutting wheel along a cut path, wherein the sheet material is retained on an apertured vacuum table and the cut path crosses an aperture. The present apparatus includes a vacuum table assembly having an apertured substrate and a microporous overlay affixed to the substrate. The overlay covers the entire cutting area of the substrate, and thus overlies the apertures in the substrate. The sizing and location of the apertures in the substrate and the porosity of the overlay are selected so that at least a single ply of sheet material can be operably and releasably retained by a vacuum generator connected to the apertured substrate. The overlay is selected to provide sufficient support of the sheet material such that upon the cutting wheel passing over a portion of the overlay that overlies an aperture in the substrate, the cutting wheel severs the sheet material. Brief Description of the Drawings Figure 1 is a perspective view of a cutting system having an apertured substrate and an overlay; Figure 2 is a partial cut away elevation view of the substrate and the overlay; Figure 3 is an enlarged view showing cutting wheel cutting a sheet material and the overlay on the substrate; and Figure 4 is an enlarged view showing cutting wheel cutting a sheet material in the local area of an aperture. Figure 5 is a graph of air flow resistance to media velocity for various substrate materials. Detailed Description of the Preferred Embodiments A cutting system 10 embodying the present invention is shown in Figure 1. The cutting system 10 is particularly adapted for cutting a single sheet of material 12 or a lay up comprising relatively few sheets of material arranged in a stacked relation. The cutting system 10 generally includes a vacuum table assembly 30, a carriage assembly, 60 a cutter assembly 80 and a controller 90. Vacuum Table Assembly Referring to Figure 1, the vacuum table assembly 30 provides a substrate 40 and an overlay 50 for supporting the sheet material 12 or lay up which includes layer(s) of the sheet material arranged in a face to face, a face down or a face up vertically stacked relation. In a first configuration, the vacuum table assembly 30 has a horizontally disposed generally rectangular base. The base supports the substrate and a lower plate joined together by longitudinally extending side members and transversely extending end members defining a vacuum chamber. A vacuum generator is fluidly connected to the vacuum chamber through a duct which communicates with the chamber. Preferably, the vacuum generator has a high flow rate capacity. The substrate 40 includes an upper surface 42 and a lower surface 44 with a plurality of passageways 43 extending between the upper surface and the lower surface. The passageways 43 have corresponding apertures 45 in the upper surface. The passageways 43 thus provide fluid communication between the vacuum chamber and the upper surface 42. The apertures 45 are typically circular and have a diameter from approximately 0.005 inches to 0.060 inches, wherein the apertures are spaced by approximately 0.08 to 1.00 inches. The substrate 40 may be formed out of any of a variety of materials such as metal, plastics or composites. In addition, the substrate 40 may include an apertured replaceable wear layer. Alternatively, the substrate 40 may be a slightly penetrable plastic material having the plurality of apertures 45. Although the apertures 45 are shown as circular, it is understood that the apertures may be any of a variety of shapes such as triangular, square, rectangular or polygonal. In addition, the apertures 45 may have a cylindrical, tapered or frustoconical cross section. In each case, the aperture 45 has a maximum dimension D, which for the circular aperture is the diameter of the aperture. The apertures 45 are located in the substrate 30 in a pattern. A common alignment is in a regular repeating array of rows and columns. In a further configuration, the substrate 40 is a belt that rotates about a set path to sequentially expose sections of the belt to a reduced pressure. The belt includes an upper surface and a lower surface with the plurality of apertures extending from the lower surface to the upper surface for communicating a pressure differential. As shown in Figure 2, the overlay 30 is affixed to the upper surface 42 of the substrate 40. In a preferred configuration, the overlay 30 is a continuous piece of material. However, depending upon the number of apertures 45 in the substrate 40, the overlay 30 may be employed as a corresponding plurality of patches, wherein each patch overlies an aperture. In the discrete patch configuration, the patches are sufficiently thin, so that the sheet material is not adversely distorted in the region of the patch. In a further configuration of the overlay, the overlay may be formed by spraying fibers, such as polyester fiber in a polyester based resin carrier onto the substrate 40. The sprayed fibers may be deposited on the substrate 40 in the region of the apertures 45, or over the entire working area of the substrate. The overlay 50 covers the area of the upper surface 42 of the substrate 40 in which cutting of the sheet material 12 can occur. As the upper surface 42 of the substrate 40 includes the apertures 45 and the overlay 50 covers the upper surface, the overlay covers the apertures. The overlay 50 is connected to the substrate 40 to preclude relative motion between the substrate and the overlay. The overlay 50 is connected to the substrate by any of a variety of mechanisms such as adhesives or bonding. A suitable adhesive is Spray 77 contact cement manufactured by 3M. As an alternative to the contact cement, a solvent based adhesive that slightly dissolves the substrate or the overlay 50, then evaporates and bonds, may be employed. Alternatively, the overlay 50 may be fused to the substrate 40. Preferably, the connection of the overlay 50 to the substrate 40 does not significantly inhibit the transmission of a pressure differential through the overlay. The overlay 50 may be affixed to the substrate 40 so that the overlay is not reusably removable. That is, separation of the overlay 50 from the substrate 40 may result is destruction of the overlay. Alternatively, the overlay 50 may be affixed to the substrate 40 to permit ready release of the overlay. Thus, portions or the entire overlay 50 may be non destructively removed from the substrate 40. The affixing of the overlay 50 to the substrate 40 and the resulting adhering of the overlay is at least partially determined by the material of the substrate and any intervening adhesives. Generally, the overlay 50 is a microporous material having a sufficient porosity to readily communicate a pressure differential therethrough. In addition, the overlay is a sacrificial element. That is, upon the cutting of a sheet material 12 retained on the overlay 50, the overly is also cut. It is believed the crushing/severing interface with the cutting wheel exists substantially between the edge of the wheel and the substrate 40. However, it is believed that the material of the overlay 50 in the region of an aperture 45 may not be completely severed. That is, the overlay 50 appears to sufficiently support the sheet material 12 in the area of the aperture 45 to provide for severance of the sheet material without a cutting of the overlay in this local region. Preferably, the overlay 50 has a uniform density throughout its thickness. That is, the overlay 50 does not include surface treatment or formation steps that require secondary manufacturing techniques. Thus, the cost of the overlay 50 can be minimized. While the overlay 50 has a sufficient rigidity to preclude local sections of the sheet material 12 from passing into an underlying aperture 45 in the substrate 40, the overlay is a deformable material. Preferably, the material of the overlay 50 does not significantly wear or dull the cutting wheel. It is believed the cutting/severing force on the sheet material 12 is exerted as the sheet material is forced between the edge of a cutting wheel and the substrate. The overlay 50 is compressed as the cutting wheel is urged toward the substrate. The overlay 50 is sacrificial and cut as the cutting wheel is urged toward the substrate to cut the sheet material 12. The aperture sizing in the substrate and the overlay material 50 are sufficient to provide severing of the sheet material 12 in the local areas overlying the apertures in the substrate. A suitable material for the overlay is Technostat media manufactured by Hepworth Air Filtration, a division of Hepworth Minerals and Chemicals (www.hepair.com) of the U.K. The Technostat media found most useful has a density of 50g/m 2 . However, it is understood various densities may be employed as shown in Figure 5. A typical thickness of the material is approximately 0.020 of an inch, wherein an adhesive is sprayed onto the upper surface of the substrate and the overlay is then pressed against the substrate. Carriage Assembly The carriage assembly 60 is located with respect to the table assembly 30 to move the cutter assembly 80 in a longitudinal (x) and transverse (y) coordinate direction relative to the substrate in response to signals received from the controller, in a manner well known in the art. The cutter assembly 80 is connected to the carriage assembly 60 for angular movement about a pivot axis A-A generally perpendicular to the substrate 40 in response to control signals from the controller 90. The carriage assembly 60 may include a plunger or bias mechanism such as a hydraulic piston, cam or spring for urging the cutter assembly towards the substrate. Thus, the cutter assembly 80 is arranged for rolling engagement against the overlay and the substrate to cut the sheet material. Cutter Assembly As shown in Figure 1, the cutter assembly 80 includes a peripheral cutting wheel 82 having a corresponding cutting edge 84, disc or wheel which penetrates any cover sheet, the sheet material 12 and the overlay 50. Depending upon the configuration of the system 10, the cutting edge 84 may penetrate the substrate 40 when the sheet material 12 is cut. The cutting wheel 82 may be any of a variety of diameters. For the previously recited aperture sizes, a wheel diameter of approximately one half inch to two inches is typical. As previously stated, the cutter assembly 80 is mounted so that the assembly can be rotated about the pivot axis A-A. The cutting wheel 82 has an axis of rotation B-B about which the wheel rotates and the peripheral cutting edge 84 circumscribes the axis of rotation B-B in a plane normal to the axis of rotation. The axis of rotation B-B of the cutting wheel 82 may be rotated about the pivot axis A-A so that the cutting wheel may track against the substrate in any direction within the plane of the substrate. In addition, pivot axis A-A is non intersectingly aligned with axis of rotation B-B. That is, during cutting engagement with the substrate 40, the pivot axis A-A leads the translation of the axis of rotation B-B as the cutting wheel traverses a cutting path along the substrate. Controller The controller 90 is a standard desk top computer such as an IBM, compatible or Macintosh. The controller 90 includes a user interface for entering instructions as to the sheet material 12 and patterns to be cut. A cutting program runs in the controller for directing the orientation of the cutting assembly relative to the desired periphery. The cutting program may designate the path the cutting wheel follows or traces as the desired cut path. The cutting program includes instructions for directing the cutter 82 along the desired cut path and maintains the axis of rotation B-B perpendicular to any radius of curvature in the cut path. In order for sufficient borders or tolerances to exist, a nesting program may adjust the nest to allow for certain cuts. To ensure as complete separation as possible along the cut path, the cutting program causes a specific overlap of cut segments at non-radiused curves or apexes in the part periphery. The cutting program employs overcuts (cuts beyond the apex at the terminal end of a path segment) and heel cuts (cuts before the cut path reaches the next path segment at the apex) to insure a severance of the material. The cutting wheel 82 is lifted out of cutting engagement with the substrate and rotated about the pivot axis A-A. The axis of rotation B-B is translated about the pivot axis A-A to dispose the cutting wheel 82 relative to the apex such that upon lowering the cutting wheel into cutting engagement with the substrate to translate the cutting wheel along the second path segment, the path traced by the cutting wheel is uninterrupted. Operation The sheet material 12 is spread upon the overlay 50. Upon initiation of a vacuum in the vacuum table assembly 30, ambient air is drawn through the sheet material 12, the overlay 50 and the apertures 45 in the substrate 40. The sheet material 12 is thus retained with respect to the overlay 50 and the substrate 40. The sheet material 12 can then be cut. As shown in Figure 3, the cutting wheel passes along the cut path and the cut path extends along a non-apertured portion of the substrate 40, the cutter presses the sheet material 12 and the overlay 50 against the rigid substrate 40 and the overlay and the sheet material are cut or severed. Referring to Figure 4, the cut path may extend over an aperture 45 in the substrate 40. As the cutting wheel 82 passes over the aperture 45, the overlay 50 resists penetration of the sheet material 12 into the aperture. The overlay thus supports the sheet material 12 over the aperture 45. The overlay 50 provides sufficient support such that the cutting wheel 82 severs the fibers of the sheet material 12 in the area of the aperture 45. In the cutting process, the peripheral edge 84 penetrates the sheet material 12, the overlay 50, and depending upon the selected material of the substrate 40, penetrates the substrate. In those sections of the substrate 40 having the apertures 45, the cutter 82 passes through the sheet material 12 and contacts the overlay 50 to cut the sheet material as it is supported by the overlay in the local area of the aperture. In the patch configuration of the overlay, the peripheral edge 84 penetrates the sheet material 12 and bears against the substrate 40. Upon crossing an aperture 45, the peripheral edge bears against the patch and the patch provides sufficient support to induce cutting of the sheet material 12 as the peripheral edge passes over the aperture 45 in the substrate 40. As the overlay 50 is cut along each cut path raced by the cutter 82 the overlay is slightly degraded after each use. Upon sufficient degradation of the overlay 50, a new overlay is employed. As the overlay 50 produces very little resistance to the passage of air, the new overlay may be attached to the degraded overlay. Alternatively, the degraded overlay may be removed from the substrate and the new overlay applied to the substrate. In the patch configuration of the overlay 50, failure of the cutting wheel to locally server the sheet material in the area of an aperture 45 indicates a need to repair or replace the patch. The worn patch may be removed and replaced. Alternatively, the worn patch may be repaired with the fiber spray. It is also contemplated that a new patch may be affixed to a worn patch. The present overlay 50 provides enhanced retention of the sheet material 12 relative to the substrate 40. It is believed that the overlay 50 may include projecting fibers that extend from the overlay. These fibers may engage the threads in the sheet material and partially secure the sheet material. That is, similar to the hook and loop fasteners, the projecting fibers of the overlay may interlock with the sheet material 12. A further component of the retaining force on the sheet material may result from an enlarged distribution of the area over which the pressure differential extends. That is, for a given square foot of substrate 40 having apertures with a 0.08 inch diameter, evenly spaced on 0.05 inch centers, there are 576 apertures/square foot. These apertures provide an area of 1.6 square inches per square foot. The overlay 50 increases the effective area over which the pressure differential acts, such that areas of reduced pressure the surface of the overlay have a diameter of approximately 0.05 inches. This results in an area of 4.5 inches, an increase of nearly three times the area. While a preferred embodiment of the invention has been shown and described with particularity, it will be appreciated that various changes and modifications may suggest themselves to one having ordinary skill in the art upon being apprised of the present invention. It is intended to encompass all such changes and modifications as fall within the scope and spirit of the appended claims.

Claims: What is Claimed: 1. A vacuum hold down assembly for releasably retaining a sheet material, comprising: (a) a substrate having a plurality of spaced apertures; and (b) a sacrificial microporous overlay connected to the substrate to overlie the apertures and preclude movement of the overlay relative to the substrate, the overlay being resiliently deformable and having a uniform density. 2. The vacuum hold down assembly of Claim 1, wherein the substrate is a belt. 3. The vacuum hold down assembly of Claim 1 wherein the substrate is fixed. 4. The vacuum hold down assembly of Claim 1 , wherein the overlay covers the substrate intermediate of the spaced apertures. 5. The vacuum hold down assembly of Claim 1, wherein the overlay covers the substrate intermediate of the spaced apertures. 6. The vacuum hold down assembly of Claim 1, wherein the overlay is connected to the substrate only in a local region of the apertures. 7. The vacuum hold down assembly of Claim 1, wherein the overlay includes sufficiently tortuous flow path to distribute a pressure differential at the spaced apertures over an area greater than the spaced apertures. 8. A vacuum hold down assembly, for supporting a flexible sheet material, comprising: (a) a substrate having a plurality of spaced apart apertures; and (b) a microporous vacuum permeable overlay fixedly connected to the substrate to overlie the apertures, the overlay selected to substantially preclude penetration of the sheet material into an aperture in the substrate. 9. The vacuum hold down assembly of Claim 8, wherein the overlay is connected to the substrate to preclude non-destructive separation of the overlay from the substrate. 10. The vacuum hold down assembly of Claim 8, wherein the overlay is releasably connected to the substrate. 11. The vacuum hold down assembly of Claim 8, wherein the overlay is one of bonded, adhesively bonded, welded, and integral with the substrate. 12. A cutting assembly, comprising: (a) a substrate having a plurality of spaced apart apertures. (b) a cutter translatable with the support surface for being urged against the substrate; and (c) a sacrificial vacuum permeable overlay connected to the substrate to overlie the apertures the overlay constructed to permit contact of the cutter with the substrate. 13. A cutting assembly for cutting a flexible sheet material, comprising: (a) a substrate having a plurality of apertures; (b) a cutter translatable with respect to the substrate to pass over the apertures; and (c) a vacuum permeable overlay connected to the substrate to overlie the apertures, the overlay selected to sufficiently support the sheet material as it overlies an aperture to provide cutting of the sheet material as the cutter passes over an aperture. AMENDED CLAIMS [ received by the International Bureau on 20 October 2000 (20.10.00); original claims 1, 8, 12 and 13 amended; original claims 9 and 10 cancelled; remaining claims unchanged (2 pages)] 1. A vacuum hold down assembly for releasably retaining a sheet material, comprising: (a) a substrate having a plurality of spaced apertures; and (b) a sacrificial microporous overlay connected to the substrate to preclude nondestructive separation from the substrate and, to overlie the apertures and preclude movement of the overlay relative to the substrate, the overlay being resiliently deformable and having a uniform density. 2. The vacuum hold down assembly of Claim 1, wherein the substrate is a belt. 3. The vacuum hold down assembly of Claim 1 wherein the substrate is fixed. 4. The vacuum hold down assembly of Claim 1 , wherein the overlay covers the substrate intermediate of the spaced apertures. 5. The vacuum hold down assembly of Claim 1, wherein the overlay covers the substrate intermediate of the spaced apertures. 6. The vacuum hold down assembly of Claim 1, wherein the overlay is connected to the substrate only in a local region of the apertures. 7. The vacuum hold down assembly of Claim 1, wherein the overlay includes sufficiently tortuous flow path to distribute a pressure differential at the spaced apertures over an area greater than the spaced apertures. 8. A vacuum hold down assembly, for supporting a flexible sheet material, comprising: (a) a substrate having a plurality of spaced apart apertures; and (b) a microporous vacuum permeable overlay fixedly connected to the substrate to preclude non-destructive separation of the overlay from the substrate and to overlie the apertures, the overlay selected to substantially preclude penetration of the sheet material into an aperture in the substrate. Please cancel Claim 9. Please cancel Claim 10. 11. The vacuum hold down assembly of Claim 8, wherein the overlay is one of bonded, adhesively bonded, welded, and integral with the substrate. 12. A cutting assembly, comprising: (a) a substrate having a plurality of spaced apart apertures. (b) a cutting wheel [cutter] translatable with the support surface for being urged against the substrate; and (c) a sacrificial vacuum permeable overlay connected to the substrate to overlie the apertures, the overlay constructed to permit contact of the cutting wheel [cutter] with the substrate. 13. A cutting assembly for cutting a flexible sheet material, comprising: (a) a substrate having a plurality of apertures; (b) a cutting wheel [cutter] translatable with respect to the substrate to pass over the apertures; and (c) a vacuum permeable overlay connected to the substrate to overlie the apertures, the overlay selected to sufficiently support the sheet material as it overlies an aperture to provide cutting of the sheet material as the cuttinjg wheel [cutter] passes over an aperture.

Inventors: HAFERMAN DAN, HAFERMAN, DAN
Applicants: GERBER TECHNOLOGY INC, HAFERMAN DAN, GERBER TECHNOLOGY, INC., HAFERMAN, DAN
Application year: 2000
Main classification: B26D7
IPC codes: B25B11, B26D7, B43L5