Abstract:
A system and method for operating a machine whereby a workpiece is held on a worktable of the machine with a suction force. When the work piece has a surface area insufficient to receive a full cut from a tool mounted in a tool head of the machine, the machine automatically adjusts the depth and the cutting speed of the tool to lesson the cut so the workpiece is not moved or ejected from the worktable. The depth and speed of the tool may be adjusted in a single cut pass of the tool or they may be adjusted in successive cut passes.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     Computer numerical control (CNC) machine tools such as routers used in the woodworking, plastics and nonferrous metal industries, typically consist of a base unit, a stationary or movable workpiece support table mounted on a base unit, a stationery or movable gantry mounted on or adjacent the base unit, and a toolhead assembly mounted on a transversely disposed bridge member of the gantry. Either the table or the gantry is displaceable relative to the base unit longitudinally or along an x-axis, the toolhead assembly is displaceable transversely or along a y-axis and the toolhead is displaceable vertically or along a z-axis. Each is displaced along their respective axes by feedscrews driven by servomotors. The motions of the various components of the machine are controlled by a controller that operates the various servomotors of the machine according to instructions of a program inputted into the controller.  
         [0002]     Workpieces to be machined are positioned on the table in predetermined locations, and are held down by various means including clamps and vacuum systems. Vacuum systems may consist of conventional systems that provide high vacuum, suitable for large production runs, and universal systems that are more suitable for short production runs. A conventional vacuum system generally includes a vacuum port provided in the worktable, connected to a vacuum pump and a vacuum fixture positioned on the workpiece table about the vacuum port on which the workpiece is positioned. The fixture is provided with a peripheral rubber seal engaged by the workpiece seated thereon, which permits the evacuation of air between the fixture and the workpiece to hold the workpiece in place. A universal vacuum system also known as a high-flow system, generally includes a table having a lower rigid plate, an arrangement of spacers attached thereto in a grid pattern, a perimeter wall, an upper spoilboard formed of a porous material such as particleboard supported on the spacers and perimeter wall, closing the spacer grid area to form a plenum, and vacuum pump operatively connected to the plenum. As a vacuum is applied to the plenum, air is drawn through the porous upper board material, producing a low-pressure zone at the surface, which functions to hold a workpiece positioned thereon.  
         [0003]     Each of the above-described systems has certain disadvantages. In the case of the conventional system, a discrete NC program must be developed to facilitate the routing of the perimeter gasket groove for each individual workpiece to be mounted. Each fixture then becomes dedicated to the workpieces for which it was adapted to hold.  
         [0004]     Multiple fixtures must be prepared to facilitate the fastening of a variety of different workpieces. Consequently, the maintenance and storage of a sizable quantity of these fixtures is often required, which in turn, consumes a considerable amount of time and resources.  
         [0005]     The alternate, high-flow method was developed in an effort to reduce the cost of preparing, storing, and changing spoil boards. However, because this method requires air to flow at a very high volume, it necessitates the use of an expensive, high volume pump that consumes a considerable amount of electrical energy.  
         [0006]     Because air is drawn through the entire surface of the worktable, even while the work function is being performed, most of the energy required to operate the high-flow system is wasted. In addition to expensive vacuum pumps, both systems described in the foregoing paragraphs require an extensive array of peripheral items such as pipes, hoses, and valves.  
         [0007]     In certain operations, such as nested based panel processing, the high-flow system is the only workable alternative. The expense involved in such a system adds considerably to the cost of a CNC machine installation. In some cases, the cost of a high-flow system can amount to as much as 30 percent of the total cost of a machine installation. CNC machine ownership thus becomes cost-prohibitive for many small business owners, who would otherwise stand to derive considerable benefit from CNC machining technology.  
       SUMMARY OF THE INVENTION  
       [0008]     An object of the present invention is to provide a cost-effective vacuum operated hold-down system. A second object of the present invention is to provide a vacuum hold-down system that is an integral component of a CNC machining system. A third object of the present invention is to provide a modified cutting technique that allows the processing of smaller panels utilizing the high-flow system. Further objects of the present invention will become apparent, based on the following detailed description.  
         [0009]     These and other objects are overcome in a system that utilizes a relatively low-cost centrifugal type impeller, coupled to an a-c induction motor. It is relatively light in weight, which facilitates the mounting of the entire system directly to the underside of the moving table of a CNC machine. By building the system into the machine, it is possible to place the inlet of the blower directly under the worktable vacuum port, thus eliminating all external piping. Since there is no need for piping within system, static pressure losses between the vacuum source and the worktable plenum are virtually non-existent. The system is simple and inexpensive.  
         [0010]     The aforementioned technique produces less vacuum than the high-vacuum and high-flow systems currently in use. This lower level of vacuum results in a reduced force per square inch of part surface holding the part to the worktable. In this case, the total force holding smaller parts may be insufficient to counteract normal cutting forces and the part may move during the cutting process. This deficiency is overcome by the use of a modified cutting technique, which is automatically initiated in areas where the workpiece falls below a predetermined size. The technique comprises a multi-stage cutting cycle, whereby a first cycle leaves material in the bottom of the cut path on parts that fall below a predetermined size. The material remaining at the bottom of the cut path secures the small part to the adjoining parts during the cutting process. The remaining material is cut away in a second pass. Because the thickness of the remaining material is in the order of 10 thousandths of an inch, the cutting force required to remove the material is extremely low and will not move the workpiece, despite the relative low hold-down force. If however, the size of the part falls below yet a lower predetermined value, the speed of the cutting cycle will likewise be reduced by an amount sufficient for preventing movement during the cut cycle.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     Other embodiments, features and advantages of the invention described herein will occur to those skilled in the art from the following description of a preferred embodiment and the accompanying drawings, in which:  
         [0012]      FIG. 1  is a drawing of a moving-table CNC machining center;  
         [0013]      FIG. 2  is a drawing depicting a high-volume blower system;  
         [0014]      FIG. 3  is a drawing depicting an exploded view of a high-volume blower system;  
         [0015]      FIG. 4  is a top view of a moving-table CNC machining center depicting the various components of a vacuum type workpiece hold-down system;  
         [0016]      FIG. 4A  is a cross-sectional view along line A-A of  FIG. 4  depicting the various components of a vacuum type workpiece hold-down system; and  
         [0017]      FIG. 5  is a flow chart depicting the process for cutting one or more workpieces from a material sheet. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0018]     Referring to the drawing in  FIG. 1 , there is illustrated, an embodiment of a computer numeric controlled (CNC) machining system, which generally includes a base member  109 , a worktable  110 , a toolhead support assembly  105 , and an electric spindle,  103 , having a tool  111  mounted therein. The gantry, includes a pair of leg members  108  and  114  rigidly secured at their lower ends to the base member and a transversely disposed section  106  supported on the leg sections and spanning above the worktable. The base member is formed of steel sections welded together to provide a rigid and stable foundation. Worktable  110  is mounted horizontally with the surface parallel to the x-y plane and is adapted to be displaced longitudinally relative to the base member or along a y-axis. The front face of transverse section  106  is provided with a pair of vertically spaced, transversely disposed rails  101  and  102 , on which toolhead support assembly  105  is mounted and displaceable transversely or along an x-axis. Electric spindle  103  is mounted on the bottom of the toolhead support assembly and is adapted to be displaced vertically by the same. Each of worktable  110 , toolhead support assembly  105  and electric spindle  103  is displaceable along its respective axis by a feedscrew arrangement driven by an AC servomotor  104 . The operation of such servomotors is controlled by a programmable computer-numeric controller (CNC) to provide for movement of a tool mounted on the toolhead along a motion path to perform a work function such as routing, shaping, drilling and the like on a workpiece mounted on the worktable. Instead of the worktable being displaceable and the gantry being stationary as described, the worktable can be stationary and the gantry may be displaceable along the Y-axis to provide displacement between the gantry and the worktable.  
         [0019]      FIG. 2  illustrates the embodiment of a vacuum blower assembly  201 .  FIG. 3  is an exploded view of the aforementioned assembly showing a drive motor  303  mounted to a housing  302  and operatively connected to a centrifugal blower impeller  301 . Housing  303  serves as directed exhaust conduit and is shaped in such a manner as to provide for progressive expansion of the exhausted air, augmenting the vacuum produced by the rotating impeller.  
         [0020]     Applicant now refers to  FIGS. 4 and 4 A.  FIG. 4  is a top view of a CNC machining worktable, illustrated and identified as number  110  in  FIG. 1 .  FIG. 4A  is a cross-sectional view of the aforementioned worktable along line A-A. The worktable comprises principally, a rigid support plate  404  horizontally disposed parallel to the x-y plane, and provided with a plurality of spacer blocks  401  and an air inlet port  403 . A porous worktable surface panel  402  is disposed above and parallel to support plate  404 , and is supported by spacer blocks  401 . A plurality of side-members  405  provides a perimeter around the periphery of the horizontally disposed members, forming a sealed plenum chamber.  
         [0021]     Vacuum blower assembly  201  is mounted to the underside of support plate  404  with the center of the inlet of the blower displaced directly in the center of air inlet port  403 . When blower  201  is activated, air is drawn through port  403  and a vacuum is formed in the plenum. Air is drawn through the porous upper board  402  producing a low-pressure zone at the surface, which functions to hold a workpiece when positioned thereon.  
         [0022]     Because the available hold-down force varies in proportion to the surface area of the workpiece, it is conceivable that the hold-down force for relatively small workpieces may be less than adequate. Owing to this theory, the present invention provides a method for altering the workpiece processing program to compensate for any decreased hold-down force that might result from inadequate surface area. The surface area of the workpiece is calculated in the CNC control processor. If the surface area fails to fall within a specific parameter, the control will automatically decrement the z-axis motion, decreasing the cutting depth for the next cut-path. This will result in a very thin skin of material being left in the bottom of the cut-path. Even though the thickness of the remaining material is very thin, it is sufficient for holding the material together as a relatively large, coherent panel, thus preventing any unwanted movement due to cutting forces. The cutting program will then be repeated with the cutting tool penetrating through the remaining skin of material. Because the cutting resistance is many times less than that required to cut through the entire thickness of material, the small workpieces are not disturbed by the cutting force. If however, the size of the part falls below yet a lower predetermined value, the speed of the cutting cycle will be reduced by an amount sufficient for preventing movement during the cut cycle.  
         [0023]     Referring to the flow chart designated as  FIG. 5 , a workpiece is placed on the worktable of a CNC machine  501 . The program cycle is then initiated  502 . The surface area of each part to be cut is calculated  503 . All of the workpieces are then arranged into an ascending cutting order beginning with the smallest piece  504 . The axes are indexed into position for cutting the first part  505 . The CNC control then checks the previously calculated surface area for the part  506 . If the surface area of the part falls below a predetermined value, the z-axis will be decremented to a position that will not allow the cutting tool to penetrate completely through the material, thus leaving a thin skin on the bottom surface  507 . The workpiece will be cut to size  508 , and the machine is directed into position for cutting the next part  509 . When all parts with a surface area that falls below a minimum value have been initially processed, the z-axis is incremented to allow full cutting depth  510 . The axes are indexed back into position for processing the first part in the cutting order  511 . If the workpiece is of sufficient size to ensure adequate hold-down force, the machine is directed to initiate the cutting cycle  514 . If the workpiece is not of sufficient size to ensure adequate hold-down force, the cutting speed is reduced by an amount that will lessen the required cutting force for the part, thus preventing the part from moving during the cutting process  513 . If no additional workpieces are required, the process is ended  517 .  
         [0024]     From the foregoing detailed description, it will be evident that there are a number of changes, adaptations and modifications of the present invention that come within the province of those persons having ordinary skill in the art to which the aforementioned invention pertains. However, it is intended that all such variations not departing from the spirit of the invention be considered as within the scope thereof as limited solely by the appended claims.