Abstract:
A water table for cutting metallic materials comprises an upper water pool supported above a false vessel bottom. The surface of the upper pool water volume is regulated relative to workpiece support rails secured above the upper pool surface. An actual vessel bottom is positioned below the false bottom and supports a lower pool volume. Bearing directly upon the lower pool surface is an enclosed air pressure volume regulated by an air supply source. The upper and lower pool volumes are hydraulically linked by flow conduits. A frangible rupture disc in a pressure release vent assures an absolute pressure limit within the air volume to avoid over-pressurization or under-pressurization of the water table.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This is a non-provisional U.S. Patent Application, claiming benefit and priority to Provisional U.S. Patent Application No. 62/244,063, filed Oct. 20, 2015, entitled “Plasma Arc Cutting Water Table And Methods Of Use,” and the contents of which are incorporated in their entirety by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to workpiece support tables and platforms having particular utility for cutting processes using plasma arc cutters, laser cutters, oxy-fuel cutters, abrasive water jet cutters, and other Computer Numerical Controlled (CNC) means. More specifically, the invention relates to failsafe apparatuses and methods of use for controlling the pressure and volume of water tables, which are served by a variable surface height water bath for the purpose of fume suppression and slag capture. 
       BACKGROUND OF THE INVENTION 
       [0003]    To control the workplace sanitation and atmosphere of a CNC work environment, workpiece support tables for CNC cutters have been developed to contain a water bath or pool beneath the workpiece support surface. The support surface often comprises a multiplicity of flat bar or angle edges characterized herein as workpiece support rails. 
         [0004]    In the example of plasma arc cutting, the “kerf” of a material cut from a whole is that portion of the material that is removed in fine particles, such as by a saw, or by melting, such as by a cutting torch. At 15,000° C. to 25,000° C., the plasma arc cutting process not only melts the kerf of a metal workpiece, but also partially vaporizes the metal. Consequently, metallic workpieces, especially material removed from within the kerf, become either solid material slag or noxious vapors. Fortunately, some of the metallic vapors produced by plasma arc cutting have an affinity to water solubility. 
         [0005]    In another example, water jet cutters utilize high-pressure nozzles which direct water mixed with fine-grained abrasive particles (e.g., garnet, aloxite) to wear down, shape, and cut a workpiece. This results in wastewater in which the abrasive particles are mixed with fine particles from the kerf, which can often settle to the bottom of the water pool and clog up or otherwise impede circulation. 
         [0006]    There are numerous reasons for changing or regulating the surface level of the water pool relative to the workpiece support rails. One very important reason is to physically remove the accumulated slag or abrasive that falls to the water pool bottom. The water surface level is lowered for physical access to the debris that has accumulated on the pool bottom. However, industrial CNC cutting tables are quite large, having dimensions in the order of 6 meters (m) long and 4.5 m wide. Consequently, the volume of water supported under the workpiece support rails is significant. Moreover, after a period of use, the water may be contaminated with considerable quantities of dissolved metals, abrasives, and other hazardous compounds. Accordingly, regulation of the water table surface level by merely discharging the water into municipal sewers and replacing with fresh water is neither an economic nor environmental option. 
         [0007]    For these and other reasons, workpiece support tables for CNC cutting systems have included closed systems, having two or more water volumes that are isolated, but hydraulically linked, whereby water is transferred between a primary pool beneath the workpiece support rails and a secondary volume. Depending on the surface level required for the primary pool, water is transferred to or from the secondary pool. Obviously, such hydraulic transfers may be effected with bi-directional pumping. However, considering the frequency and volume of such fluid transfer cycles, it has been discovered that a closed air pressure pocket, formed over the secondary volume to push or withdraw fluid to or from the primary volume, is more effective and efficient. The primary volume can be placed at a greater elevation than the secondary volume to gravitationally drain the primary volume through the linking conduits into the secondary volume, when air pressure in the pocket is reduced. Conversely, when a rise in the primary volume surface level is required, pressure in the air pocket can be increased to push water up through the linking conduits into the primary volume, thereby raising the surface level. 
         [0008]    Depending on the elevation difference between the primary and secondary volumes, the magnitude of air pressure in the pocket determines the height of the primary volume. However, if for some reason, such as slag accumulation, flow is obstructed, through the linking conduits between the primary and secondary volumes, then the level control system may increase the pocket pressure beyond reasonable limits in a futile attempt to raise the primary volume surface level. As a result, the equipment may be damaged through leakage, and in extreme cases, workplace accidents may result from radical overpressure leading to the pocket bursting and high-pressure fluid being vented. 
         [0009]    An example of the prior art may be found in U.S. Pat. No. 3,743,260 to Alleman, which discloses one of the earliest surface variable water tables directed specifically to plasma cutting. The Alleman apparatus includes a tank assembly having a pair of false bottom plates beneath the workpiece support surface. These false bottom plates slope from the tank sides toward the tank center but are terminated short of the center to provide a center trough for slag collection. Beneath the false bottom plates is a true tank bottom. A wedge of volumetric space is formed between the underside of the false bottom plate, the tank wall, and the true tank bottom. This wedge of space is open to the water supporting volume above the false bottom plate. When the tank is filled with water, an isolated air chamber may be charged above the water surface within the wedge volume. Depending on the air pressure within the wedge volume, water is displaced from the wedge volume to raise the surface level of the upper tank volume. Control of the air volume within the wedge volume is by means of a manual valve ( 53 ), which requires careful supervision to avoid overpressure. 
         [0010]    U.S. Pat. No. 5,013,884 to Hahn describes a plasma arc cutting system having a water filled chamber beneath the workpiece support surface. An air pressure expansible bladder is secured within the water volume. As the bellows is expanded by air pressure, a corresponding volume of water is displaced within the chamber. Within the fixed volume of the chamber, such displaced water volume may only be accommodated by the open surface adjacent the workpiece support surface. 
         [0011]    The water surface level of a plasma cutting table as described by U.S. Pat. No. 6,387,320 to Poulin is controlled by means of a false bottom plate secured to the true tank bottom by means of a flexible skirt. A fixed water volume in the tank above the false bottom has a controlled surface level by means of air inflated bellows positioned between the underside of the false bottom and the upper face of the true bottom. Air pressure into the bellows expands the bellows length to raise the false bottom against the fixed water volume. 
         [0012]    A need therefore exists for a CNC cutting table in which the pressure level between the fluid volumes is protected by a failsafe mechanism, thereby reducing the considerable safety hazard associated with precisely gauging the primary and secondary fluid volumes to avoid environmental hazards that can be created by overpressure (e.g., structural failure, such as leakage and tank bursts) and under-pressure (e.g., accumulation of toxins in the work environment). The invention disclosed herein meets these needs. 
       SUMMARY OF THE INVENTION 
       [0013]    The CNC cutting water table of the present invention comprises a water holding tank having a stationary false bottom plate, which is spaced closely below the workpiece support rails. The true bottom of the water holding tank is positioned below the false bottom by a distance sufficient to provide a secondary volume space of at least sufficient capacity to accommodate all of the water in the system, which is required of the primary volume between a maximum surface height and a minimum surface height. 
         [0014]    One or more relatively large diameter fluid transfer conduits can be secured to the underside of the false bottom plate with the conduit axes aligned substantially normal to the plane of the false bottom plate. The upper distal ends of these transfer conduits can be secured to the false bottom plate with a fluid-tight seal, such as by welding. A circular junction area of the false bottom plate material, corresponding to an extension of the conduit bore, can be removed to allow fluid to flow through the conduit bore, to and from the upper surface of the false bottom. 
         [0015]    The transfer conduit length is extended below the false bottom plate to proximity with, but not to a junction with, the water holding tank&#39;s true bottom. The bottom edges of the transfer conduits are terminated from the true bottom surface by a distance that corresponds with at least the internal flow area of the transfer conduits. 
         [0016]    The fluid chamber between the false bottom plate and the true bottom is divided between the secondary fluid volume and an air pocket volume. The air pocket volume is served by a two way air supply and exhaust conduit. Air flow into and out of the air pocket is controlled by a primary water volume surface regulator to maintain a desired surface level relative to the workpiece support rails. This air pocket region is also served by a relief flow port that is closed by a calibrated rupture disc, which can provide an absolute guarantee that the air pocket will not be over-pressurized. 
         [0017]    In an embodiment of the present invention, a metal cutting water table comprises a liquid holding vessel, which is positioned beneath a plurality of workpiece support rails that are secured to the liquid holding vessel in parallel alignment against a common support plane. The liquid holding vessel comprises a primary water volume, which is disposed adjacent to the workpiece support rails and above a secondary water volume, wherein the primary water volume and the secondary water volume are hydraulically linked by a plurality of conduits. 
         [0018]    The metal cutting water table can further comprise an air volume, which can bear upon the secondary water volume to support a predetermined surface level for the primary water volume; a regulator that can be usable for controlling additions and reductions of air in the air volume; a pressure relief conduit that can open into the air volume; and a rupture disc that can be usable for closing the pressure relief conduit, or opening the pressure relief conduit at a pressure differential across the rupture disc that is equal to or exceeds a predetermined value. 
         [0019]    The liquid holding vessel can comprise a true bottom below a false bottom, wherein a volume of space can be formed between the false bottom and the true bottom, and wherein the volume of space can be divided between the air volume and the secondary water volume. The false bottom can be penetrated by the hydraulic linking conduits, which can extend downwardly through the secondary water volume to proximity with the true bottom for linking the primary water volume with the secondary water bottom. 
         [0020]    The rupture disc can comprise a calibrated flange plate that can rupture at the predetermined value of the pressure differential to release pressure in that air volume. 
         [0021]    The embodiments of the present invention can include a method of preventing destructive over pressurization in a CNC cutting table, wherein the steps of the method can include securing a primary water volume, which is positioned beneath a plurality of workpiece support rails, to a water holding vessel that is in parallel alignment against a workpiece support plane, and positioning the primary water volume elevationally above a secondary water volume. The primary water volume can be hydraulically linked to the secondary water volume. The steps of the method can further include bearing a pressurized volume of air upon the secondary water volume to sustain a predetermined surface level for the primary water volume, and controlling a supply of air, pumped into said pressurized air volume, relative to a predetermined surface level for the primary water volume. The steps of the method can conclude with closing a pressure relief conduit, which is in communication with the pressurized air volume, by the use of a rupture disc, or using the rupture disc to open the pressure relief conduit at a predetermined pressure differential across the rupture disc. 
         [0022]    The steps of the method for preventing destructive over pressurization in a CNC cutting table can further comprise the step of selecting a rupture disc that is calibrated to fail at the predetermined pressure differential. 
         [0023]    In an embodiment, the step of positioning the primary water volume elevationally above a secondary water volume can be accomplished by the step of bearing a pressurized volume of air upon the secondary water volume. 
         [0024]    In an embodiment, the step of controlling a supply of air pumped into the pressurized air volume can further comprise controlling the supply of air independently of the use of the rupture disc. For example, regulators can be used to regular the flow of air being pumped in and out of the water table system. 
     
    
     
       DESCRIPTION OF THE DRAWINGS 
         [0025]    In the detailed description of various embodiments of the present invention presented below, reference is made to the accompanying drawings, in which: 
           [0026]      FIG. 1  is a schematic of an embodiment of the invention, which is shown as a sectioned elevation. 
           [0027]      FIG. 2A  is a perspective section of an embodiment of the invention. 
           [0028]      FIG. 2B  is a blown-up portion of  FIG. 2A , showing the conduit in more detail. 
           [0029]      FIG. 3  is a perspective outer view of an embodiment of the invention. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0030]    Before describing selected embodiments of the present disclosure in detail, it is to be understood that the present invention is not limited to the particular embodiments described herein. The disclosure and description herein is illustrative and explanatory of one or more presently preferred embodiments and variations thereof, and it will be appreciated by those skilled in the art that various changes in the design, organization, means of operation, structures and location, methodology, and use of mechanical equivalents may be made without departing from the spirit of the invention. 
         [0031]    As well, it should be understood that the drawings are intended to illustrate and plainly disclose presently preferred embodiments to one of skill in the art, but are not intended to be manufacturing level drawings or renditions of final products and may include simplified conceptual views to facilitate understanding or explanation. As well, the relative size and arrangement of the components may differ from that shown and still operate within the spirit of the invention. 
         [0032]    Moreover, it will be understood that various directions such as “upper”, “lower”, “bottom”, “top”, “left”, “right”, “first”, “second” and so forth are made only with respect to explanation in conjunction with the drawings, and that components may be oriented differently, for instance, during transportation and manufacturing as well as operation. Because many varying and different embodiments may be made within the scope of the concept(s) herein taught, and because many modifications may be made in the embodiments described herein, it is to be understood that the details herein are to be interpreted as illustrative and non-limiting. 
         [0033]    The term “air” is used herein to conveniently describe any gas or compressible fluid medium that may be appropriate for a specific cutting operation. Consequently, certain workpiece materials, for reasons of health or safety, may require an inert gas, such as nitrogen. Accordingly, the term “air” is herein defined to include any compressible fluid (a gas, other fluid) suitable to the process. 
         [0034]    Also, for reasons specific to the properties of a workpiece, the term “water” is used herein to describe any liquid or liquid solution suitable to the process for reasons of health, safety, or chemical solubility for released vapors. 
         [0035]    Referring to the schematic of  FIG. 1 , a cutting table surface can be formed by the upper edges of a multiplicity of support rails  52 , which can be secured in a common plane  54  for the purpose of uniformly supporting a workpiece  50  along the underside of the workpiece  50 . Typically, the workpiece  50  is a plate or sheet of metallic material, such as steel, aluminum, copper, brass or alloy. 
         [0036]    The embodiment in  FIG. 1  is shown with a cutting head  30  positioned over the workpiece  50 , which can be secured to a position control carriage  32 . Such attachment of the cutting head  30  to the position control carriage  32  may include a structural member to adjust the vertical position of the cutting head  30  relative to the workpiece  50  surface. However, other methods may be used to position the cutting head  30 . 
         [0037]    The position control carriage  32  can be supported by a transfer beam  34  for controlled positioning of the cutting head  30  transversely of the support rails  52 , for example. The transfer beam  34  can be supported by, for example, tractor rolls  38  riding on parallel guide rails  36  for positioning the cutting head  30  longitudinally of the workpiece support rails  52 . The guide rails  36  are frequently secured along the side walls of a fluid tank structure  10 . 
         [0038]    The parallel workpiece support rails  52 , as shown, are positioned parallel with the parallel guide rails  36  and are secured along the side walls  12  of the tank  10 . The schematic tank embodiment of  FIG. 1  is closed between the side walls  12 , in a fluid-tight manner by a true bottom  14 . 
         [0039]    It can be appreciated that this is only one exemplar method of positioning a cutting head above a workpiece and water table, and other such methods may be available for use without departing from the scope of the claimed invention. For instance, alternative structures could be used to position the cutting head  30  and position control carriage  32 . For example, linear rails could be used to position the control carriage  32 . The rails may be connected to the water tank or not connected. Hydraulic or electric linear actuators could be used to position the cutting head  30 . The tank  10  may be an independent, free standing apparatus as depicted in  FIGS. 2A-3 . In this embodiment, a cutting head  30  and position control carriage  32  could be aligned over the tank  10  via independent railings (not shown), or a gantry operating at a predetermined distance and height from the workpiece support rails  52 . 
         [0040]    Between the workpiece support rails  52  and the true bottom  14  is a false bottom  16 . The volumetric space between the true bottom  14  and the false bottom  16  is divided between a secondary water volume  22  and an air pocket  24 . A plurality of fluid transfer conduits  18 , as shown, can penetrate the false bottom  16  to hydraulically link the top side of the false bottom  16  with the secondary water volume  22 . In this particular embodiment, there is a single air pocket  24  (although not visible in this two-dimensional depiction, this air pocket  24  extends laterally beyond the conduits). However, while air pocket  24  will be referred to in the singular sense for clarity, it should be understood that other embodiments of the invention may comprise multiple air pockets without deviating from the scope of this disclosure. 
         [0041]    In an embodiment, the bottom lip  19  of each of the transfer conduits  18  can be positioned in sufficient proximity to the true bottom to limit the volume of fluid that may be transferred above the false bottom  16 . The limitation occurs when the surface  23  of secondary water volume  22  is driven down by the expansion of the air pocket  24  to the level of transfer of the conduit lip  19 . At that point, the gas comprising the air pocket  24  escapes up the fluid transfer conduits  18  and releases the pressure supporting a primary water volume  20  above the false bottom  16 . Hence, there is a direct relationship between the surface  21  of the primary water volume  20  and the surface  23  of the secondary water volume  22  that is controlled by the volume of the air pocket  24 . 
         [0042]    Normally, the surface  21  of primary water volume  20  is maintained at a level slightly below the workpiece support rails  52 . There are certain cutting tasks, however, that are preferably performed beneath the surface  21 , thereby requiring more of the overall water volume to be displaced from the secondary water volume  22  into the primary water volume  20 . The exact position of the surface  21  of the primary water volume  20  can be maintained by a water level sensor (not shown), such as a float controller or a light beam reflector. 
         [0043]    Electrical signals from the water level sensor (not shown) can be received, for example, by control valves  42 ,  43 , which can be used to regulate a connection  40  between the air pocket  24  and an air supply source  41 , such as a fan or compressor. Gradual air loss from the air pocket  24  may be a normal operational condition. Accordingly, replacement air from the air supply source  41  may be required to simply sustain the surface  21  of the primary water volume surface  20  at a designated position. Obviously, additional air must be added to the air pocket  24  to raise the primary water volume surface  21 , which can be accomplished through control valve  42 . When primary water volume surface  21  is to be lowered, air from the air pocket  24  is released by control valve  43 . 
         [0044]    It can be appreciated that other configurations for adding air to the air pocket  24  may be utilized without departing from the scope of this disclosure. For instance, a single three-way valve might be utilized rather than two valves in series, with the third connection being a vent to release excess air if the air supply source  41  does not have the ability to vent off excess air fed back through the connection  40 . A single two-way valve configuration could be used to add or release air from the air pocket  24 . Alternatively, an embodiment may comprise multiple connections for introducing and removing air from the air pocket  24 , each with an independent valve. 
         [0045]    It should be understood from the level control process for the primary volume described above, the pressure in the air pocket  24 , normally, will have little variation. In most applications, there are but small differences in the hydraulic head between a maximum height and a minimum height of the primary water volume surface  21 . A maximum to minimum height of 1 m, for example, represents only about 10 kilopascals (kPa), while a height of 5 m may be a difference of 70 kPa, for example. Such small changes in the pressure differential range emphasize the fact that the greater air parameter change is in the volume of air required between the maximum and the minimum height of the primary water volume surface  21 . 
         [0046]    As previously explained, cutting operations can produce particles of slag, which are the cooled droplets of molten kerf material. In an embodiment, most of this slag can be collected on screens or by baskets ( 28 , shown in  FIG. 2A ) that can be placed on the surface of the false bottom  16 , under the workpiece  50 , or over transfer conduit openings  26  through the false bottom  16 . Notwithstanding such precautions, slag and other particulate debris may accumulate over time, at the relatively small openings between the transfer conduit bottom lip  19  and the true bottom  14 , to substantially seal the openings and prevent a fluid transfer of water between the primary and secondary water volumes. Should the primary water volume surface  21  of the primary water volume  20  become under-pressured due to evaporation or leakage, the level controller will call for additional air to be provided into the air pocket  24 . However, the volume of the air pocket  24  remains unchanged because of the closed fluid transfer conduits  18 . Consequently, additional air provided into the air pocket  24  can result in an increase in the air pocket  24  pressure. If allowed to continue, the result of such a combination of factors may be to over-pressure the air pocket  24  volume, thereby resulting in damage or destruction of the equipment. 
         [0047]    To prevent such an event of an air pocket  24  over-pressure, the present invention provides a relief conduit  45 , opening into the air pocket  24 , which is normally closed by a rupture disc  47 . In the present embodiment, the rupture disc  47  is a material sheet that is secured across a relief conduit flow bore, such as a thin plate clamped between a pair of flanges. The material sheet or plate (i.e., rupture disc  47 ) is calibrated to structurally fail by rupture at a predetermined pressure differential. 
         [0048]    Turning now to  FIGS. 2A &amp; 2B , another embodiment of the invention is shown, with the tank  10  as an independent structure omitting the cutting head  30  and related elements. In this embodiment, baskets  28  are shown above the false bottom  16  and transfer conduits  18 , for collecting slag and/or abrasive material falling through support rails  52 , in order to slow down the accumulation of material at the true bottom  14  and delay any obstruction of the conduit lip  19  where the water is pushed up in the direction indicated from the secondary water volume  22  by air pocket  24 . 
         [0049]    Also visible in  FIGS. 2A &amp; 2B  is conduit  45 , which is blocked by rupture disc  47 . Rupture disc  47  can comprise any frangible material; for example, in an embodiment, the rupture disc can be made of polytetrafluoroethylene (PTFE) to minimize the risk of corrosion and weakening due to volatile solutes within the secondary water volume  22 . 
         [0050]    In this embodiment, rupture disc  47  is attached to the relief conduit  45  via a flange connection  46  located within side wall  12 . However, it can be appreciated that any other suitable connection, such as a bolted or threaded connection may be used without departing from the scope of this disclosure. Additionally, it should be appreciated that rupture disc  47  may be located both within the side wall  12  as in  FIGS. 2A &amp; 2B , or external to the side wall  12  as in  FIG. 1 , as well as anywhere along the side wall  12  of the tank  10 . 
         [0051]    Turning now to  FIG. 3 , an external perspective is shown of the tank  10  and support rails  52 . Air connection  40  is shown in line with control valves  42 ,  43 . (Air supply source  41  is not shown, although it functions as it does in  FIG. 1 ). Conduit support  45  holds air supply source  41  and control valves  42 ,  43  in place on side wall  14 . While the air supply source  40  and the relief conduit  45  are depicted on opposing longitudinal portions of side wall  14  in this disclosure, it can be appreciated that this is not essential to the invention and that other embodiments may locate these two on the same longitudinal portion of side wall  14 , or on the lateral portions of side wall  14 . 
         [0052]    Although the invention disclosed herein has been described in terms of specified and presently preferred embodiments, which are set forth in detail, it should be understood that this is by illustration only and that the invention is not necessarily limited thereto. Alternative embodiments and operating techniques will become apparent to those of ordinary skill in the art in view of the present disclosure. Accordingly, modifications of the invention are contemplated which may be made without departing from the spirit of the claimed invention.