Method For Holding Sheet Material By A Vacuum Holddown

Abstract

A method is disclosed for holding relatively stiff sheet material such as denim while it is being worked upon in a multi-ply layup on a supporting surface. The top ply of the layup is covered along its peripheral region by an air-impervious material extending from the supporting surface adjacent the side of the layup up to the top ply at its periphery. The central region of the top ply within the covered peripheral region is left uncovered and exposed to the ambient air. A vacuum is generated between the air-impervious material and the sheet material of the layup. This vacuum produces forces across the air-impervious material to compress the layup against the supporting surface thereby holding the layup in place.

Description

BACKGROUND OF THE INVENTION

The present invention relates to the automated cutting of fabrics especially the stiff or coarse fabrics such as denim.

The conventional automated cutting process involves passing a reciprocating cutting tool through a multi-ply layup of the sheet material as the tool is automatically guided along a prescribed cutting path in response to signals from a computer or numerical controller. One method of holding the layup in position on a support surface during the cutting operation is disclosed in U.S. Pat. No. 3,598,006 entitled METHOD FOR WORKING ON SHEET MATERIAL AND OTHER OBJECTS, having the same assignee as the present invention. In the patent, the layup is covered with a sheet of air-impervious material such as polyethylene and then a vacuum is generated between the supporting surface and the air-impervious material. The vacuum creates forces across the air-impervious material which compress the whole layup against the supporting surface. When working with a stiff fabric such as denim, however, this holding method causes the layup, particularly in the central region to become too tightly compressed and compacted. The stiff, compressed material becomes so hard that it is very difficult to advance a cutting tool through the layup especially along a curved cutting path where the tool is required to rotate and maintain an orientation generally tangent to the path.

The present invention seeks to avoid this result by providing means whereby the layup is held in place by compressing it only at its periphery and leaving the central region within the periphery more loosely packed. The material thereby offers much less resistance to the cutting tool and allows the material to "flow" around the tool as it advances and turns within the layup.

SUMMARY OF THE INVENTION

This invention resides in a method of holding down a multi-ply layup of sheet material by compressing only the periphery of the layup against a supporting surface. The layup is first positioned on the supporting surface and then is covered by air-impervious material at the periphery of the top ply and along the sides of the layup between the top ply and the supporting surface. The air-impervious material may be a flexible film, a rigid material or combinations of flexible and rigid material. The central region of the top ply is left exposed to ambient air.

With the layup covered in this manner, a vacuum is generated between the air-impervious material and the sheet material of the layup. This produces forces across the air-impervious material which compress the lateral edges of the layup against the supporting surface. The layup is thus held firmly in position along the supporting surface where it may be worked upon. By compressing only the periphery of the layup against the supporting surface, the central region of the layup within the periphery is permitted to remain non-compressed in a loosely stacked condition for easier working with a penetrating tool.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an automatically controlled cutting device having a vacuum table with a layup of sheet material positioned thereon and covered with a sheet of air-impervious material in accordance with the present invention.

FIG. 2 is a cross-sectional view of the vacuum table and cutting tool of FIG. 1 with a vacuum imposed on the layup.

FIG. 3 is a top plan view of a layup showing a cutting tool in cross section moving through flexible or pliable material.

FIG. 4 is a cross-sectional view of a layup formed of stiff sheet material and shows the cutting tool during a cutting operation conducted in accordance with the present invention.

FIG. 5 is a fragmentary, cross-sectional view showing one manner of covering the periphery of the layup on the vacuum table with an air-impervious material.

FIG. 6 is a fragmentary, cross-sectional view showing another manner of covering the periphery of the layup with an air-impervious sheet material.

FIG. 7 is a fragmentary, cross-sectional view showing a manner of covering the periphery of the layup on a table without a built-in vacuum system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An automatically controlled cutting device such as described in U.S. Pat. No. 3,598,006 referenced above is shown generally at 10 in FIG. 1 to illustrate the present invention. In the device, a vacuum support table 12 includes an air-permeable bed 14 comprised of closely packed bristles or the like which define a flat supporting surface 16. The exact nature of the supporting bed is not critical to the present invention; however, it is advantageous in the cutting device if the bed is air-permeable and of a type which may be repeatedly penetrated by a cutting tool. The sheets of material or fabric to be cut may then be positioned on the supporting surface in overlying relation to each other to form a multi-ply layup 30 which is completely penetrated by the tool.

A cutting tool, typically in the form of a reciprocating knife blade 18 as shown in FIG. 2, is movably mounted on a cutting head 20 for rotation about an axis perpendicular to the supporting surface 16. The blade may then move tangentially along curved cutting paths in the layup. A presser foot 21 having a central cutout accommodating the blade 18 is also supported from the cutting head 20 for adjustment above the supporting surface 16. The presser foot slides over the upper surface of the layup under its own weight, or under the influence of other adjusting devices, to prevent the sheet material from lifting excessively with the blade on its upstroke. The cutting head 20 is translatably mounted on a carriage 22 for movement laterally of the table and the carriage 22 is translatable longitudinally of the table 12 on a system of racks or ways 24. Suitable drive means move the carriage 22, the cutting head 20 and the knife blade 18 relative to each other and the table 12 in response to commands from a control computer 26. In this way, the blade 18 may be moved automatically along a predetermined cutting path through the layup of material to be cut.

To hold the layup 30 in fixed position on the supporting surface 16 in accordance with the method described in U.S. Pat. No. 3,598,006, the layup is first covered with a sheet of flexible, air-impervious material 32 such as polyethylene. A vacuum is then generated on the supporting surface 16 and along the lateral edges of the layup between the polyethylene sheet and the sheet material of the layup. This is done by drawing air downwardly through the air permeable bed 14. The decreased pressure associated with the vacuum results in a distribution of forces across the polyethylene sheet which causes the sheet to bear down on the layup compressing it against the supporting surface 16.

Where a continuous sheet of air-impervious material is used to cover the layup completely, the entire layup is compressed against the supporting surface. This result is undesirable where the layup material is initially relatively inflexible and stiff as, for example, in the case of denim fabric used in overalls and the like. When a layup of such fabric is compressed, it becomes extremely firm and hard and thereby severely retards the movement of the knife blade through the layup. The fabric in this hardened condition does not "flow" around or make an opening for the knife blade to permit the blade to move freely in the layup. Instead, the fabric closes in upon the blade and binds against the blade, the obvious results of which are increased friction between the fabric and the blade, higher loading upon the carriages and blade with possible blade failures and increased machine wear.

An illustration of what is meant by the "flow" of material around the cutting blade 18 is shown in FIG. 3. Because some fabrics are normally flexible or pliable, they tend to separate easily when cut and are pushed outwardly away from the advancing cutting edge of the tapered blade insert 37 to leave an opening for the rest of the insert and the thicker insert retainer 38. When the layup 30 is compacted, lighter more flexible fabrics such as silk or linen continue to flow around the blade in the desired manner with little difficulty, but the stiffer and heavier fabrics such as denim do not.

The present invention eliminates the problem associated with stiffer fabrics by utilizing a modified form of the foregoing holddown technique in conjunction with the inherent stiffness of the layup material. Instead of covering the layup completely with a sheet or film of air-impervious material 32, only the lateral sides 33 of the layup and the perimeter or peripheral region 34 of the top ply are covered. The central region 36 of the top ply within the peripheral region is left exposed to the ambient air. When a vacuum is generated on the supporting surface 16 and between the air-impervious material 34 and the layup, the forces produced across the air-impervious material compress only the peripheral region of the layup and hold the layup in position on the supporting surface. The central region 36 remains non-compressed as seen most clearly in FIG. 2, but due to the stiffness of the layup material, the material in the center is held in place on the supporting surface 16 by the compressed peripheral region.

FIG. 4 is a cross-sectional view of the layup 30 at the rear of the blade 18 and illustrates the cutting operation when the stiffer and heavier fabrics are cut in accordance with the present invention. Without the strong holddown forces in the central region 36 of the layup 30, the upper plies of sheet material may be lifted slightly upward to form a peak at the cutting blade 18. The presser foot 21 limits the upward movement and serves as a cutting anvil for the blade. The peaking action in the absence of the overlay occurs because the stiff material fails to yield laterally in the plane of the material and the only direction free for movement is upward away from the supporting surface 16. The peaking action opens a small slot for the blade as in FIG. 3 or at least reduces the binding forces and resulting friction which would otherwise be experienced by the blade throughout the entire depth of the layup.

In a preferred form of the invention, air-impervious sheet material 32 is placed in direct contact with the layup at the peripheral region 34 of the top ply and along the lateral sides 33 of the layup as shown in FIGS. 2 and 5. A vacuum is then generated between the air-impervious material and the layup to produce holddown forces only along the peripheral region of the layup and thereby prevent the plies of the layup from shearing relative to each other or shifting on the supporting surface.

In FIG. 6, an air permeable block 40 of plastic foam or similar material is placed around the layup between the lateral sides 33 of the layup and the air-impervious sheet material 32. Air is then withdrawn from the layup through the bed 14 and through the air permeable block 40. This method of holding the layup is particularly advantageous where the sheet material of the layup itself is air-impervious. The permeable block ensures that air is withdrawn from the peripheral region of the layup at each ply. In this way, air is withdrawn more uniformly over the entire side of the layup.

The form of the invention shown in FIG. 7 is similar in operation to that shown in FIG. 6, but is particularly adapted for use with tables which have no vacuum system built into the surface supporting the layup. Also, the peripheral regions of the layup are covered by a combination of flexible and rigid materials which are air impervious. A slotted or apertured channel member 42 of an air-impervious material such as aluminum, is utilized in place of the air permeable block 40. A strip of flexible, air-impervious material 32 is placed on the peripheral region of the top ply and overlaps the top of the relatively rigid channel member 42. A vacuum pump (not shown) is connected to the interior of the channel member and air is drawn out of the layup through slots 44 in the channel member to create the desired holddown forces.

In this case, it is assumed that the channel member 42 and the support bed 46 establish an air-tight seal. The support bed 46 may be of the type suitable for penetration by a cutting tool, but need not be air permeable.

While the present invention has been described in several preferred forms, it will be readily apparent that numerous modifications and substitutions can be had without departing from the spirit of the invention. For example, if the channel member 42 and the support bed 46 in FIG. 7 are incapable of establishing an adequate seal to generate a vacuum within the peripheral region of the layup 30, the flexible sheet of air impervious material 32 may extend from the top ply of the layup over the entire channel member 42 onto the supporting surface of the bed 46. The flexible material in this case would be better adapted to establish the necessary seal. Accordingly, the present invention has been described in several preferred forms by way of illustration rather than limitation.

Claims

I claim:

1. A method of holding sheet material on a supporting surface comprising the steps of:

a. positioning sheets of the material one upon the other on a supporting surface to form a multi-ply layup of the material;

b. covering the layup at the periphery of the top ply and along the sides of the layup between the top ply and the supporting surface with air-impervious material;

c. exposing the central region of the top ply within the periphery to the ambient air; and

d. generating a vacuum between the air impervious material and the sheet material of the layup to produce forces across the air impervious material and to hold the layup against the supporting surface.

2. A method of holding sheet material as recited in claim 1 wherein the step of covering the layup comprises spreading an air-impervious sheet material from the supporting surface adjacent the side of the layup to the periphery of the top ply.

3. A method of holding sheet material as recited in claim 1 wherein the step of positioning comprises positioning the sheet material on an air-permeable supporting surface; and the step of generating a vacuum comprises drawing air from between the layup and the air-impervious material through said air-permeable supporting surface.

4. A method of holding sheet material as recited in claim 1 wherein the steps of covering and exposing together comprise laying air-impervious material along the peripheral region of the top ply of the layup.

5. A method of holding as defined in claim 1 wherein the step of covering comprises spreading a sheet of flexible air-impervious material along the peripheral region of the top ply of the layup.