Patent Publication Number: US-2004043704-A1

Title: Method and apparatus for high speed cutting

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] This invention relates to an improved method and apparatus for high-speed cutting of an object.  
       [0003] 2. Description of the Related Art  
       [0004] High-pressure fluid jets, including high-pressure abrasive water jets, are used to cut a wide variety of materials in many different industries. Systems for generating high-pressure abrasive water jets are currently available, for example the Paser 3 system manufactured by Flow International Corporation, the assignee of the present invention. An abrasive jet cutting system of this type is shown and described in Flow&#39;s U.S. Pat. No. 5,643,058, which patent is incorporated herein by reference. In such systems, high-pressure fluid, typically water, flows through an orifice in a cutting head to form a high-pressure jet, into which abrasive particles are entrained as the jet flows through a mixing tube. The high-pressure abrasive water jet is discharged from the mixing tube and directed toward an object or workpiece to cut the workpiece along a selected path. After the jet passes through the object being cut, the fluid and waste associated with the cutting are collected in a catcher tank.  
       [0005] Various systems are currently available to move a high-pressure fluid jet along a selected path. (The terms “high-pressure fluid jet” and “fluid jet” used throughout should be understood to incorporate all types of high-pressure fluid jets, including but not limited to, high-pressure water jets and high-pressure abrasive water jets.) Such systems are commonly referred to as two-axis, three-axis and five-axis machines. Conventional three-axis machines mount the cutting head assembly on a ram that imparts vertical motion along a Z-axis, namely toward and away from the workpiece to be cut. The ram, in turn, is mounted to a bridge via a carriage, the carriage being free to move parallel to a longitudinal axis of the bridge in a horizontal plane. The bridge is slideably mounted on one or more rails to move in a direction perpendicular to the longitudinal axis of the bridge. In this manner, the high-pressure fluid jet generated by the cutting head assembly is moved along a desired path in an X-Y plane, and is raised and lowered relative to the workpiece, as may be desired. Conventional five-axis machines work in a similar manner but provide for movement about two additional rotary axes, typically about one horizontal axis and one vertical axis. Similar machines are also available to manipulate other types of cutting devices.  
       [0006] Other types of machining equipment, for example metal punch press machines, move the workpiece to be cut along two axes in an X-Y plane. Regardless of whether prior art systems maintain the workpiece stationary and move the cutting device, or hold the cutting device stationary while moving the workpiece, these systems all use relatively complex mechanical drive assemblies, for example ball screw drives or rack-and-pinion assemblies.  
       [0007] Applicant believes it is desirable and possible to provide an improved system for cutting a workpiece. The present invention provides such a system.  
       BRIEF SUMMARY OF THE INVENTION  
       [0008] Briefly, the present invention provides an improved method and apparatus for cutting a workpiece, for example with a high-pressure fluid jet. (The terms “cutting” and “cut” used throughout should be understood to cover all types of cutting, including, but not limited to, engraving, scribing, etching, etc., in addition to cutting through a workpiece.)  
       [0009] More particularly, a workpiece to be cut is held and selectively moved across a top surface of a table along a first axis by a part gripper mounted to the table. In one embodiment, the part gripper and associated workpiece are selectively moved along the first axis by a linear motor mounted to a longitudinal beam of the table. A cutting device is mounted to a boom for selective movement along a second axis that is perpendicular to the first axis. In one embodiment, the cutting device is a cutting head assembly that selectively produces an ultrahigh-pressure fluid jet. Alternatively, it may be any type of cutting device, including but not limited to, a laser, plasma cutter, or scriber, etc.  
       [0010] In one embodiment, the boom is coupled to the table, such that forces generated by movement of the cutting device along the second axis are dissipated through the framework of the table. In one embodiment, the cutting device is selectively moved along the second axis by actuation of a second linear motor coupled to the boom. In this manner, the workpiece is cut using, in combination, the movement of the workpiece along one axis, and movement of the cutting device along a second axis, perpendicular to the first. An appropriate controller, including software, is used to control motion of the workpiece and of the cutting device, as will be understood by one of ordinary skill in the art.  
       [0011] Because all cutting takes places in a narrow area along the second axis, any waste generated by the cutting, including water and abrasives if used, may be collected by placing a single-axis container underneath the table, along the second axis. This is in contrast to prior art systems where cutting occurs substantially over the entire length and width of a cutting table, requiring a catcher tank to extend substantially the full length and width of the workpiece.  
       [0012] In accordance with another aspect of the present invention, a shoe is coupled to a distal end of the cutting device, the shoe being made of a material having a sufficient hardness to displace the workpiece as needed to substantially maintain a standoff distance between the cutting device and the workpiece within a desired tolerance. For example, if the workpiece is sheet metal, it will be understood by one of ordinary skill in the art that such a workpiece is not perfectly flat, and that it may distort as it is cut. By providing a shoe around a distal end of the cutting device, being formed of a material that is harder than the material being cut, the shoe will push against the workpiece and displace it as necessary to maintain a desired standoff distance. In one embodiment, the shoe surrounds the distal end of a high-pressure fluid jet cutting head, and an outlet end of the mixing tube of the cutting head assembly is inset from an end surface of the shoe.  
       [0013] By moving the workpiece along one axis while moving the cutting device along a second perpendicular axis, maximum speed of cutting and acceleration are increased. Furthermore, by using two linear motors in accordance with the present invention, in contrast to more complex mechanical drive systems of the prior art, manufacturing costs are reduced, and speed of cutting, accuracy, and reliability are increased. For example, while prior art systems allow cutting in the range of 400-3,500 inches per minute, cutting may be done in accordance with the present invention at the rate of 5,000 inches per minute. In addition, the present invention eliminates the problems associated with large, prior art catcher tanks, and handling of waste is simplified, given the relatively small container that may be used to catch the discharged jet and waste along the second axis of the machine. 
     
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS  
     [0014]FIG. 1 is a front isometric view of an assembly provided in accordance with the present invention.  
     [0015]FIG. 2 is a front isometric view of a table and boom subassembly of the cutting assembly shown in FIG. 1.  
     [0016]FIG. 3 is a rear isometric view of the subassembly show in FIG. 2.  
     [0017]FIG. 4 is an isometric view of a portion of the assembly of FIG. 1, with elements removed for clarity.  
     [0018]FIG. 5 is a schematic illustration of a side view of the assembly shown in FIG. 1.  
     [0019]FIG. 6 is a cross-sectional elevational view of a shoe provided in accordance with the present invention, shown mounted on a high-pressure fluid jet cutting head. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
     [0020] There are many systems currently available for engraving a workpiece or cutting one or more parts of any desired shape from a workpiece. For example, as discussed previously, ultrahigh-pressure abrasive water jet systems such as those manufactured by Flow International Corporation, move a cutting head along a selected path in an X-Y plane, over a fixed workpiece. Other systems, for example metal punch press machines, maintain the punch or cutting device in a stationary position, while the workpiece is moved in two directions in an X-Y plane. These conventional systems use relatively complex mechanical drive systems, that entail numerous moving parts, and the associated cost and reliability problems associated with such mechanisms. Also, the upper speed at which conventional systems may cut is restricted, given limitations of the mechanical drives. For example, currently available water jet systems allow cutting at rates of 400-1,000 inches per minute. The present invention provides an apparatus and method for cutting a workpiece that overcomes many of these limitations.  
     [0021] More particularly, as illustrated in FIG. 1, a workpiece  10  to be cut is held and selectively moved by a part gripper  11  that is coupled to a table  12 . In one embodiment, the part gripper  11  and associated workpiece  10  are selectively moved along a first axis  24  by actuating a first linear motor  17 . A cutting device, such as a cutting head assembly  13 , is coupled to a boom  14  for selective movement along a second axis  25  that is perpendicular to the first axis  24 . Boom  14  is cantilevered over the table  12  and is rigidly mounted to mast  15 , which in turn is rigidly mounted to the base frame  19  of table  12 . In one embodiment, the cutting device  13  is selectively moved along the second axis  25  by actuation of a second linear motor  18 . A container  16  is removably positioned underneath table  12 , and is aligned along the second axis  25  defined by boom  14 , to catch any waste material resulting from cutting of the workpiece.  
     [0022] A controller incorporating appropriate software, controls motion of the workpiece  10  along the first axis  24  and motion of the cutting device  13  along the second axis  25 , to move the high-pressure fluid jet along a desired path. It will be understood that cutting a workpiece in accordance with the present invention may result in scribing a selected design into a workpiece or the cutting of a part having a desired shape, or any desired combination thereof. In one embodiment of the present invention, motion of the workpiece and cutting device are controlled to cut a part in the workpiece that is connected to the workpiece by small tabs. Once one or multiple parts are cut into a workpiece in this manner, the tabs are broken and the parts are removed from the workpiece. The formation and location of the tabs may be dictated by the control system, as will be understood by one of ordinary skill in the art.  
     [0023] Although the apparatus of the present invention may take many forms, one exemplary embodiment is illustrated in FIGS. 2 and 3. As seen in these two figures, the table  12  may be formed of a base frame  19 , including a longitudinal beam  33 . The boom  14  is either rigidly coupled to or integrally formed with mast  15 , that in turn is rigidly fixed to or integrally formed with the table  12 , for example through longitudinal beam  33 . As further seen in FIGS. 2 and 3, in one embodiment, the mast  15  is further coupled to a secondary longitudinal beam  20 . Given the coupling of the boom  14  to table  12 , forces generated by motion of the cutting device  13  along the boom  14  are transmitted and dissipated through the longitudinal beams and diagonal braces  46  of the table. A slot  23  is defined in the table  12 , the high-pressure fluid jet or other cutting device being allowed to pass through slot  23  as it cuts the workpiece, if such cutting extends through the entire depth of the workpiece.  
     [0024] As best seen in FIGS. 4 and 5, a first set of rails  34  is mounted on an upper surface of the longitudinal beam  33  defining a first trough  36  therebetween. A plurality of magnets  38  are positioned within the first trough  36 . Gripper plate  28  is coupled to both bearing boxes  39  provided on rails  34 , and to motor coil  40  of first linear motor  17 . As motor coil  40  selectively moves along magnets  38 , the part gripper  11  and workpiece  10  held thereby, selectively move along the rails  34 , defining the first axis  24 . By bolting the gripper plate to the motor coil, the gripper plate not only moves with the motor coil, but also acts as a heat sink for the motor. Part gripper  11  may be of conventional design, such as those sold by Alternative Parts, Inc. Clamps  47  are selectively raised and lowered via a pneumatic cylinder to allow a workpiece to be inserted and grasped by the part gripper  11 .  
     [0025] In one embodiment, a second set of rails  35  is mounted on a side surface of the boom  14 , defining a second trough  37  therebetween to receive a second linear motor  18  having the same structure defined above for the first linear motor  17  provided on the longitudinal beam  33 . Conventional boots  51  may be placed over the motors and their respective paths of motion.  
     [0026] Although the upper region of table  12  may be formed in a variety of ways, in one embodiment, a metal plate  26  having a slot provided therein is laid on top of the frame  19 , such that the slot in the metal plate is aligned with the slot defined in the table. Alternatively, the plate may be integral to the frame, or it may be eliminated. It is desirable for an upper support surface  30  of the table to have a low coefficient of friction. In one embodiment of the present invention, this is achieved by positioning strips of polymer brushes  27  having a low coefficient of friction across the top surface of the table  12 . An insert made of high-molecular weight plastic may be placed into the slot  23  to improve the wear of this region as the cutting device passes along the length of the slot.  
     [0027] In some applications, for example when cutting sheet metal, it may be desirable to provide a shoe  21  in accordance with the present invention, as illustrated in FIG. 6. Shoe  21  is positioned around a distal end of the cutting device, and preferably is of sufficient hardness to displace the workpiece being cut as needed to substantially maintain a standoff distance  31  between the cutting device and the workpiece. For example, if the cutting device is a cutting head of a high-pressure fluid jet system, as illustrated in FIG. 6, the standoff  31  is the distance between an exit orifice  32  of the mixing tube  45  and an upper surface of the workpiece  10 . A first end  48  of shoe  21  may be coupled to the cutting device in any appropriate manner, for example via a threaded connection. A second end  49  of shoe  21  is positioned on or adjacent the workpiece  10  when in use. The body  50  of shoe  21  is formed of a material selected to be harder than the material being cut. For example, if the workpiece is sheet metal, the shoe  21  may be formed of hardened steel. In one embodiment, a distal end of the cutting head, for example the end of mixing tube  45 , is inset from the end surface of shoe  21 , such that the shoe also acts as a pierce shield.  
     [0028] As will be understood by one of ordinary skill in the art, an ultrahigh-pressure fluid jet, including an abrasive fluid jet if desired, is generated by the cutting head assembly  13  shown in FIG. 6 by allowing a quantity of pressurized fluid to flow through inlet  41 . The pressurized fluid flows through orifice  42  secured in orifice mount  43 , to generate a high-pressure fluid jet  29 . If desired, a quantity of abrasive is introduced into the assembly via abrasive inlet port  44 , where the abrasive is entrained in the fluid jet and discharged through mixing tube  45  to impinge against the workpiece  10 . Although the present invention has been described herein in the context of the cutting being performed by a high-pressure fluid jet, it will be understood that other types of cutting devices may be used, for example a laser, a plasma cutter, a scriber, etc.  
     [0029] Many advantages are achieved in accordance with the present invention. By moving the workpiece along one axis while moving the cutting device along a second perpendicular axis, maximum speed of cutting and acceleration are increased. By using direct drive linear motors, for example those manufactured by Yaskawa, high accelerations and speed of cutting are achieved, with improved accuracy. For example, while conventional water jet systems allow cutting at a rate of 400-1,000 inches per minute, the present invention allows cutting at a maximum travel velocity of at least 5,000 inches per minute, providing significant cost benefits and advantages.  
     [0030] By eliminating the more complex mechanical drives of conventional systems, the cost of the assembly is reduced, and reliability is improved. Furthermore, limitations such as “backlash” associated with conventional mechanical drive systems are eliminated by use of direct drive linear motors. Because all pieces of the assembly are integrally formed and prealigned at the time of assembly, the apparatus may be simply plugged in and used, rather than being leveled or lagged to the floor, as required in conventional systems.  
     [0031] An additional advantage is the elimination of a need for a catcher tank extending substantially the same length and width as the workpiece. More particularly, in conventional systems where the cutting occurs over almost the entire X-Y plane defined by the workpiece, it is necessary to be able to catch debris along the entire area. It has therefore been necessary to provide a large catcher tank having dimensions approximately equal to those of the workpiece. When using large catcher tanks, systems are required to maintain the abrasives in suspension, and eliminate waste from the catcher tank. In contrast, cutting in accordance with the present invention occurs over a small region along the second axis, such that a relatively small container  16  may be removably positioned under the table along this second axis, to catch debris. Given the small area of the container, the contents may be directly pumped out, thereby simplifying waste disposal. Although this may be achieved in a variety of ways, in one embodiment, the container  16  is simply provided with locking casters that allows the container to be removably positioned under the table, and provided with a drain line  22  allowing fluid communication between an interior region and exterior of the container. A slot catcher  52  extends from an underside of the table  12  along the length of slot  23  into the interior of container. In one embodiment, slot catcher  52  is coupled to container  16  such that it is removably positioned under the table  12 .  
     [0032] It will be understood that a variety of workpieces may be cut in accordance with the present invention, and that the size and weight of the workpiece that can be handled may be increased by simply increasing the size of the linear motors. By way of example only, the present invention may be used to cut a six foot by ten foot piece of 10 gauge sheet metal.  
     [0033] From the foregoing it will be appreciated that, although specific embodiments of the invention have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.