Abstract:
An abrasive jet apparatus is provided which includes an abrasive dispenser defining a compartment for storing a granular abrasive material and at least one metering orifice disposed in open communication therewith for dispensing the granular abrasive material. The apparatus also includes a shutter assembly disposed adjacent the metering orifice, which includes a shutter member angularly displaceable between first and second positions relative to the metering orifice. The shutter member has formed therethrough at least one shutter opening which in the first position is substantially fully aligned with the metering orifice, and in the second position is substantially fully offset therefrom. The apparatus further includes a position actuator operatively coupled to the shutter mechanism for reversibly displacing the shutter member to the first and second positions and a plurality of intermediate positions therebetween for occluding a selective portion of the metering orifice. A flow rate of the abrasive material dispensed through said metering orifice is thereby maintained at a predetermined level.

Description:
REFERENCE TO RELATED APPLICATIONS 
     This Utility Patent Application is based on Provisional Patent Application No. 60/537,036, filed 20 Jan. 2004. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to abrasive jet machines used to cut or otherwise machine process various materials by generating a focused stream of fluid mixed with abrasive particles. The present invention relates in particular, to abrasive jet machines which use a pressurized liquid as the driving fluid to propel the abrasive particles for cutting or other machining operation. 
     The present invention is further related to an abrasive waterjet apparatus with a variable flow rate of an abrasive material to be entrained within the given fluid jet, wherein the flow rate is adaptively modulated to suit a particular operation of such apparatus. 
     Also, the present invention is related to an abrasive jet apparatus having an automatically controlled metering orifice for the abrasive dispenser, whereby flow rate of the abrasive material dispensed therethrough is adaptively regulated over a broad range of applications of the abrasive jet apparatus without operator intervention. 
     2. Prior Art 
     Abrasive waterjet cutting is a machining process where a focused ultrahigh velocity waterjet is used to accelerate abrasive particles which perform cutting. The high velocity waterjet is formed by pumping a fluid, such as for example, water to high pressure through a small diameter orifice. The resulting mixture of abrasive particles and water is discharged through a focusing tube as a high velocity composite jet to perform cutting or milling upon a workpiece. 
     In abrasive waterjet cutting, a water flow orifice restricts and accelerates the flow of high pressure water, typically at approximately 50 KSI to 65 KSI. This high speed jet of water is capable of cutting through various materials with relative ease. For metals, ceramics and other such materials, abrasives are added to the jet to increase the tribologic effect. 
     Abrasive waterjets typically employ a mini-hopper abrasive dispenser that is in turn fed by a large pressurized bulk hopper. Different sizes of abrasive materials (typically garnet having a mesh size within an approximate range of 80 to 220 mesh) are available for use with abrasive waterjets. An operator selects the abrasive size suitable for the material, thickness, finish, and other such parameters of the given workpiece, and sets the appropriate flow rate for the abrasive material which matches the size of the water flow orifice and focusing tube. 
     A typical abrasive waterjet apparatus  10  known in the art is illustrated in  FIG. 1 . The abrasive waterjet apparatus  10  includes a large pressurized bulk hopper  12  supplying an abrasive material to a mini-hopper abrasive dispenser  14 . Presently known mini-hopper dispensers use a fixed or manually selectable metering orifice  16  such as a disk of preselected washer shape, or of manually adjustable aperture. A mechanical member  18  is used with the metering orifice  16  for occlusion against the flow of abrasive material therethrough. Typically, a pneumatic device such as a cylinder that responds to air pressure from a solenoid valve is used to actuate an abrasive valve mechanism  22  in this regard. 
     The abrasive material flow  20 , having been regulated by the metering orifice  16  and admitted by the abrasive valve  22 , freefalls until it meets an air jet  24 . Shortly after passage by the abrasive valve  22 , the abrasive material transitions from a freefall state to one of entrainment within a high speed air jet, forming an air/abrasive flow  26 . The air/abrasive flow  26  is inducted into a waterjet mixing chamber  28  at a partial vacuum, and enters a mixing (or focusing) tube  30  where it contacts and mixes with the high speed waterjet. A highly focused abrasive/waterjet  32  is then expelled from the focusing tube  30  toward the workpiece  36  to be processed. Abrasive material size and flow rate are chosen in light of the specific operation to be performed upon that particular workpiece  36 . 
     Note that the abrasive flow rate used for such machining operations as light material removal may not be sufficient for punching a hole or slicing through a thick section of the workpiece material. The operation may become overly time consuming, among other things. Conversely, a higher abrasive flow rate usually employed for cutting a thick section would not be appropriate for gently forming a delicate, sculptured shape. Presently known abrasive waterjet machines use either a fixed or manually selectable metering orifice that permits very limited control over the flow rate of the abrasive material, thus preventing optimal adaptation of the abrasive material flow rate, much less precise active control thereof. There is a need in the art, therefore, for a system and method whereby the mass flow rate of an abrasive material may be simply yet accurately adapted for assorted abrasive jet machining operations. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide an abrasive jet apparatus in which an abrasive material flow rate exiting an abrasive dispenser is regulated to adaptively suit the particular machining operation intended. 
     It is another object of the present invention to provide an abrasive jet apparatus wherein an opening of a metering orifice of the abrasive dispenser is actively adjusted by electrically driven positioning actuator in a manner corresponding to the type of machining operation intended and the type of a workpiece material to be processed in an accurate and reliable yet efficient manner. 
     It is a further object of the present invention to provide an abrasive jet apparatus which concurrently stores more than one abrasive material in the abrasive dispenser, and selectively dispenses the materials in flow rate controlled manner. 
     These and other objects are attained by a system and method realized in accordance with the present invention. In one exemplary embodiment, the abrasive jet apparatus comprises an abrasive dispenser defining a compartment for storing a granular abrasive material and at least one metering orifice disposed in open communication therewith for dispensing the granular abrasive material. The apparatus also includes a shutter assembly disposed adjacent the metering orifice, which includes a shutter member angularly displaceable between first and second positions relative to the metering orifice. The shutter member has formed therethrough at least one shutter opening that in the first position is substantially fully aligned with the metering orifice, and in the second position is substantially fully offset therefrom. The apparatus further includes a position actuator operatively coupled to the shutter mechanism for reversibly displacing the shutter member to the first and second positions and a plurality of intermediate positions therebetween for occluding a selective portion of the metering orifice. A flow rate of the abrasive material dispensed through said metering orifice is thereby maintained at a predetermined level. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows schematically a block diagram of an abrasive waterjet apparatus of the prior art; 
         FIG. 2A  is a sectional schematic view of one embodiment of an abrasive jet apparatus of the present invention; 
         FIG. 2B  is a sectional assembly schematic view corresponding to the embodiment of  FIG. 2A , viewed from a perspective angularly offset from that of  FIG. 2A , with certain structural and dimensional details illustratively shown; 
         FIG. 2C  shows schematically various examples of alternate embodiments for a portion of an abrasive jet apparatus of the present invention; 
         FIG. 3A  is an exploded perspective view of a portion of the abrasive water jet apparatus shown in  FIGS. 2A–2B ; 
         FIG. 3B  is a sectional view taken also along line  3 — 3  of  FIG. 2A , corresponding in part to the portion of the abrasive water jet apparatus shown in  FIG. 3A ; 
         FIG. 4A  is an exploded perspective view of an alternate embodiment of the portion of the abrasive water jet apparatus shown in  FIG. 3A ; 
         FIG. 4B  is a sectional view analogous to that of  FIG. 3B  for the alternate embodiment shown in  FIG. 4A ; 
         FIG. 5A  is a schematic view illustrating another exemplary configuration of a metering orifice and adaptively displaceable shutter member used therewith, in accordance with another alternate embodiment of the present invention; 
         FIGS. 5B–5C  are schematic views illustrating yet other exemplary configurations of a metering orifice and adaptively displaceable shutter member used therewith, in accordance with other alternate embodiments of the present invention; 
         FIG. 5D  is a graphic representation illustrating the relationship between the degree of occlusion of a metering orifice and the type of machining operation of the apparatus in one embodiment of the present invention; 
         FIG. 6  shows schematically a portion of the abrasive jet apparatus formed in accordance with another alternate embodiment of the present invention, wherein multiple abrasive material compartments are formed; 
         FIG. 7  is a sectional view of the embodiment of  FIG. 6  taken along lines  7 — 7  thereof; 
         FIG. 8  is a graphic representation showing the relationship between the abrasive material flow rate and the degree of occlusion of a metering orifice in one embodiment of the present invention; 
         FIG. 9  is a sectional view in an alternate embodiment of a portion of the abrasive water jet apparatus otherwise shown in  FIG. 2A ; 
         FIG. 10  is a broader sectional schematic view of a dual abrasive chamber embodiment of an abrasive jet apparatus of the present invention illustrated in  FIGS. 6 and 7 ; 
         FIG. 11  is an enlarged view, partially cut away, of a portion of the sectional view shown in  FIG. 10 ; 
         FIG. 12  is a bottom perspective view of a partially disassembled implementation of the embodiment shown schematically in  FIG. 10 ; and, 
         FIG. 13  is another perspective view of the implementation shown in  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to  FIG. 2A , there is schematically shown an abrasive jet apparatus  40  formed in accordance with an exemplary embodiment of the present invention. A corresponding assembly drawing of the apparatus  40  adapted for a particular application, with certain exemplary dimensional parameters indicated for illustrative purposes, is shown in  FIG. 2B . The abrasive jet apparatus  40  includes a hopper compartment  42  which receives and stores one or more abrasive materials  44  in granular form from an outside source (such as a separate bulk hopper) to supply an abrasive dispenser  46  through a passage  48 . The abrasive dispenser  46  is preferably formed within a dispenser housing  50 , the bottom wall portion  52  of which defines one or more metering orifices  54 . 
     An automatically driven shutter assembly  74  is provided adjacent the bottom wall portion to selectively and variably occlude each metering orifice or a portion thereof. In broad concept, then, the rate of granular abrasive material  44  dispensed through a metering orifice  54  is actively controlled—and thereby suitably regulated for the cutting or other machining task at hand—by setting the shutter assembly to occlude a corresponding portion of that metering orifice  54 , obviating the need to replace the orifice with one of another size/configuration, or to repeatedly open and close the orifice to control flow therethrough. As described in greater detail in following paragraphs, feedback control measures are preferably employed to actively monitor and adapt the degree of orifice occlusion, so as to dynamically maintain optimum flow rate for the abrasive material  44 . 
     Though it may be formed in alternate embodiments with various other configurations suitable for the specific application intended, each metering orifice  54  is preferably configured in the exemplary embodiment illustrated as an arcuately contoured opening radially offset from, and extending in substantially concentric manner about, an axial reference  56  defined on the bottom wall  52 , as best illustrated in  FIGS. 3 and 4 . The precise contour and dimensional configuration of the metering orifice  54  may be suitably adapted as required by the requirements of the intended application.  FIG. 2C  illustrates numerous examples wherein the metering orifice is arcuately contoured, and dimensional parameters are varied for different applications. Factors such as linearity of correlation between flow rate and portion of the orifice occluded, shearing of the granular material during shutter assembly operation about the orifice, and the like will bear on the actual choice of overall orifice configuration. 
     During operation, the abrasive material  44  is dispensed effectively in appropriate amounts by release through the non-occluded portion of metering orifice  54 , as illustrated in  FIG. 2A . The dispensed material  44  is then taken up in a high speed air jet  58 , generated and directed about the dispenser housing  50  as shown, within an abrasive valve chamber  60 . An air/abrasive mixture  62  thus forms, to be inducted into a waterjet nozzle  64  of the mixing chamber  66  where such abrasive/air mixture  62  mixes with, preferably, a high speed waterjet  68  for expulsion as a liquid/abrasive cutting jet  70 . The cutting jet  70  exits the mixing chamber  66  for highly focused impingement upon a surface of a workpiece  72  to effect a cutting, milling, or other such machining operation thereon. 
     Depending on the type of operation to be performed on the workpiece  72 , the abrasive material content—in terms of proportional content and granularity—in the liquid/abrasive cutting jet  70  may require variation to maintain optimum efficiency. For instance, a gentle sculpting or surface treating operation would tend to require a lower proportional content (and possibly even a finer grain) of abrasive material  44  in the cutting jet  70 . On the other hand, a more rigorous operation such as punching a hole or slicing through a thick section of the workpiece  72  material would tend to require a higher proportional content (and possibly a courser grain) of abrasive material  44  in the cutting jet  70 . In accordance with the present invention, the optimal flow rate necessary to preserve the desired abrasive material content in the cutting jet  70  is maintained by actively controlling shutter assembly  74  to suitably position a shutter  100  thereof to occlude an appropriate portion of the given metering orifice  54 . 
     In order to increase the efficacy of the abrasive jet apparatus  40  in this manner during assorted abrasive jet machining operations upon the same workpiece  72 , the shutter assembly  74  is automatically actuated by a drive shaft  76  preferably controlled by a servomotor  78 . It is to be understood that instead of the servomotor  78 , a stepper motor, voice coil, or any other suitable type of positioning actuator known in the art may be used in the abrasive jet apparatus  40 . 
     The positioning actuator, further referred to herein simply as motor  78  (for brevity), is preferably positioned above the level of the abrasive material  44 , such that it is safely protected from the particles of the abrasive material. As shown in  FIGS. 2A and 2B , the motor  78  is preferably also disposed within a motor compartment  84  separated from the abrasive material  44  by a separating wall  80 . Optionally, a heat sink  82  may be installed adjacent to the motor  78  within the motor compartment  84  to ensure sufficient heat dissipation. The motor  78  is positioned on a motor plate  86 , mounted on a motor mount  88 . 
     A drive shaft  76  is coupled to the motor  78  by means of a flexible shaft coupling  90  at an upper end  92  thereof, and extends axially through the abrasive dispenser  46 . The drive shaft, too, is protected from potentially damaging contact the particles of the abrasive material  44  by a sleeve-like tubular shaft guide  94  through which it coaxially extends and within which it freely rotates. 
     The tubular shaft guide  94  acts as a loose bearing to support and restrain the drive shaft  76  coaxially along the axial reference  56 . Preferably, the opposing surfaces of the shaft guide  94  and drive shaft  76  maintain sliding contact when the drive shaft  76  is rotated during operation. The shaft guide  94  and drive shaft  76  are, therefore, preferably formed of dissimilar materials particularly suitable for such relative sliding contact. For example, the shaft guide  94  may be formed of such material as stainless steel, with the drive shaft  76  itself being formed of such material as brass or anodized aluminum. Lower weight materials are preferable particularly for the drive shaft  76  to minimize inertial effects and thereby optimize rotational responsiveness to motor actuation (access times, for instance). Various materials known in the art may be employed in accordance with the present invention to best suit the specific requirements of the intended application. 
     The shutter assembly  74  is preferably coupled by a coupling collar  102  to a lower end  96  of the drive shaft  76 . The assembly  74  is formed as shown in  FIGS. 3A and 3B  with a vane, or shutter, member  100  through which one or more shutter openings  101  are formed. Each shutter opening  101  is configured and positioned in a manner corresponding to one or more of the metering orifices  54 . That is, each shutter opening is formed with a contour similar to that of the corresponding metering orifice(s)  54  but with a greater dimensional configuration than that of the corresponding metering orifice(s)  54 . Thus, when the shutter member  100  is set in its angular position to its fully OPEN position, the given shutter opening  101  fully exposes the corresponding metering orifice(s)  54 , the boundaries of the shutter opening  101  remaining safely clear of the orifice periphery to minimize shearing or other such potentially detrimental effects that might otherwise occur as the abrasive material  44  exits through that orifice. When the shutter member  100  is alternatively set to its fully CLOSED position, the given shutter opening  101  is drawn safely away from the corresponding metering orifice(s)  54 , and the shutter member&#39;s solid surface portion fully occludes that orifice. At any of its intermediate settings between these positional extremes, the shutter opening  101  occludes only a portion of the corresponding metering orifice(s)  54 . Such partial occlusion, of course, varies in degree of occlusion with the angular displacement of shutter member  100  about the axial reference  56 . 
     When the motor  78  is turned to an “ON” state, it actuates the drive shaft  76  which, in turn, rotates the shutter member  100  of the shutter assembly  74  in a controlled fashion. This adjusts the overlap between its shutter opening  101  and the corresponding metering orifice(s)  54 , in order to control the flow rate of the abrasive material  44  through that metering orifice  54 . As best shown in  FIGS. 3A–4B , the shutter openings are preferably in the exemplary embodiment shown, with arcuate contour and offset from the shutter member&#39;s central axis. The shutter member may be rotated relative to the metering orifice  54  in both clockwise or counter-clockwise directions. This way, the metering orifice  54  can be quickly closed or opened to the necessary extent, whereby the abrasive material flow rate through the metering orifice  54  is dynamically regulated in accordance with the type of machining operation then being performed by the abrasive jet apparatus  40 . 
     The shutter member  100  is preferably formed of a hard, abrasion-resistant material to withstand repeated frictional contact with the abrasive material  44 . It is preferably formed of a blue-tempered spring steel material, although other suitable materials known in the art may be used. In one exemplary application of the disclosed embodiment, the shutter member  100  is formed, for instance, with its planar portion having a thickness of approximately 1/32 inch, exhibiting a representative hardness of C 49 – 51 . Such parameters will, of course, vary depending on the particularities of the intended application; and, they are set out for illustrative purposes only, the present invention not being limited in any way thereto. 
     Turning next to  FIGS. 5A–5C , there are schematically shown additional examples of numerous other configurations which may be employed for the shutter assembly  74  in accordance with certain other alternate embodiments of the present invention. In the embodiment of  FIG. 5A , the metering orifice  54  is configured with a generally linear contour (an elongate slit), and the shutter member  100  is configured substantially as a pivotally displaceable elongate arm. A shutter opening  101  is provided at an upper edge of the shutter member  100  in the form of an arcuate notch coincident with an end edge contour of the elongate metering orifice  54 . During operation, variable occlusion of the metering orifice  54  is effected by reversibly actuating pivotal movement of the shutter member  100  relative to thereto, along the direction indicated by the arrow  53 . 
     In the embodiment of  FIG. 5B , the metering orifice  54  and shutter member  100  form an intersecting-V configuration. The metering orifice  54  is formed with a substantially triangular-shaped opening oriented relative to the direction of shutter member movement as shown, and the shutter member  100  is configured as a linearly translatable arm member having a corresponding V-shaped notch. During operation, variable occlusion of the metering orifice  54  is effected by reversibly actuating a linear movement of the shutter member  100  relative to thereto, along the direction indicated by the arrow  55 . 
     The embodiment of  FIG. 5C  is similar to that of  FIG. 5B , but with the metering orifice  54  and shutter member  100  contoured differently. Each is formed substantially with a rectangular contour relatively oriented as shown. The shutter member  100  is configured as a linearly translatable rectangular arm member which, during operation, is reversibly actuated to move along the direction indicated by the arrow  55 ′ to effect variable occlusion of the metering orifice  54 . 
     While these represent by example other viable configurations for the shutter assembly  74 , the ease with which linearity between the degree of orifice occlusion and the resulting abrasive material flow rate may be realized varies with the configurations. In this regard, the arcuate metering orifice configuration disclosed herein offers notable advantages, as described in following paragraphs (with reference to  FIG. 8 ). 
       FIG. 5D  illustrates in chart form a representative profile of the degree to which the metering orifice  54  is ideally opened for different types of machining operations to be formed using an exemplary embodiment of the present invention, with a metering orifice configuration similar to that shown in  FIG. 5A . The degree of overlap ideally required between a shutter opening  101  and its corresponding metering orifice  54  varies noticeably with the types of operation represented. 
     In accordance with the present invention, the abrasive jet apparatus  40  may employ a data processor  104  of any suitable type known in the art suitably programmed with a database  106  (or a look-up table, for example) or other known means by which the desired, or ideal, parametric values relating to the type of operation to be performed by the apparatus  40  and the required degree of metering orifice occlusion may be stored for ready access. Data processor  104 , being operationally coupled to the motor  78 , may control the operational parameters of the motor  78  accordingly, so that the opening of the metering orifice  54  is automatically controlled to maintain optimal abrasive material flow rate regulation during operation. 
     In accordance with another aspect of the present invention, the apparatus  40  preferably includes feedback control measures which employ an abrasive flow sensor  108  disposed at or near the mouth of abrasive valve chamber  60  through which the air/abrasive mixture  62  is expelled. Preferably, this sensor  108  a transmit/receive components such as a Light Emitting Diode (LED)  110  and a photo detector  112  optically coupled thereto. In the embodiment shown, these components are positioned at diametrically opposed sides of the abrasive/air mixture  62  flow to monitor for variation. A detection output generated by the photo detector  112  is coupled to the processor  104  for appropriate control processing. Depending on the flow data indicated by this detection output, the processor  104  adjusts the operation of the motor  78  to either open or close the metering orifice  54  to the degree necessary to adjust the flow rate of the abrasive material and thereby maintain optimum cutting conditions. 
     Additionally, or alternatively, another type of the feedback measure may be implemented using an encoder  114  operationally coupled to the shutter assembly  74  to acquire and transmit to the processor  104  data indicating the relative disposition of the shutter member  100  and the metering orifice  54 . Operational parameters of the motor  78  may then be adjusted based upon this data to effect appropriate abrasive material flow rate control, much as described in preceding paragraphs. 
     As shown in  FIGS. 2A–3B , the abrasive dispenser  46  includes a ventilation cap  116  covering the motor compartment  84 . The ventilation cap  116  is connected to an outer casing  118  of the abrasive dispenser  46 , preferably by a case gasket  120 . Additionally, an external spray shield  122  in the embodiment shown encapsulates the entire abrasive dispenser of the apparatus  40 . It is preferably formed with generally a rectangular or other polygonal sectional contour, and preferably dimensioned relative to the outer casing&#39;s tubular periphery, such that it forms against the outer casing a plurality of axially extending paths through which the high speed air jet  58  may be directed to the point of abrasive material release. A coaxial arrangement of the outer casing  118  and spray shield  122  preferably employed in this manner is best illustrated in the sectional  FIG. 3B , wherein the directional arrows  2 A and  2 B respectively indicate the angularly offset viewer perspectives relative to which the corresponding sectional views of  FIGS. 2A and 2B  are taken. 
     As with any structural component that directly contacts the abrasive material  44 , the outer casing  118  is formed of a suitably strong, tough, and wear-resistant material. Examples of suitable materials for components such as the bottom wall  52 , motor mount, and the hopper/dispensing compartment walls include stainless steel, a hard coat anodized aluminum, and the like. For the outer casing  118 , a clear polycarbonate or other such material may be particularly suitable, as the abrasive material  44  contained therein would then remain visible for the operator. There is little practical likelihood of excessive erosive hazing on the interior surfaces when such materials are used, given that the abrasive material&#39;s kinetic energy which typically accounts most for erosive hazing remains minimal prior to dispensing. 
     The outer casing  118  is formed with an abrasive material feed port  124  for introduction of the abrasive material  44  into the hopper compartment  42  from an outside source, and a breather port  126  for the venting necessary to relieve the residual pressure urging the abrasive material  44  into the hopper compartment  42 . The abrasive dispenser will thereby be at ambient pressure, and be released through the non-occluded portion of the metering orifice  54  sufficiently by gravity. 
     In certain alternate embodiments, drive shaft  76  acts both as an aspiration conduit as well as a torque communicating member between the motor  78  and shutter assembly  74 , it concurrently serves as the aspiration conduit through which the high speed air jet  58  is directed. As shown in  FIG. 9 , the drive shaft is formed in this embodiment as a hollow lightweight drive tube  76 ′. The drive tube  76 ′ then defines an axial bore which serves sufficiently to conduct the high speed air jet  58  induced during operation. 
     To minimize destabilizing inertial effects, this drive tube  76  is preferably formed of a material of any suitable type known in the art having minimal weight yet sufficient strength and durability to effectively withstand the torsional forces to be encountered in the intended application. While not shown for clarity, a guide shaft tube  94  is preferably disposed coaxially about the drive shaft/tube  76 ′ much as it is in the embodiment described in preceding paragraphs. The guide shaft tube  94  accordingly serves not only as a protective barrier for the drive tube  76 ′, it acts as a loose bearing to support and restrain the drive tube  76 ′ coaxially along the axial reference  56 . 
     The flexible coupling  90  by which the drive tube  76 ′ is coupled to the drive actuating motor  78  is preferably of a helical coupling type such as made available by MCMASTER-CARR. Typically, such helical couplings are formed with generally cylindrical outer walls in which a plurality of helical cuts (or surface grooves) are provided to accommodate a certain degree of flexion without undue compromise of torsional stiffness. The high speed air jet  58  induced during operation through the drive tube  76 ′ in this embodiment is preferably drawn through the space defined by the helical cuts themselves. 
     A notable advantage provided by this embodiment is that the abrasive material  44  is caused to be entrained in the air jet  58  more immediately upon release from the abrasive dispenser  46 . Another advantage is one of structural simplicity, obviating the need for extraneous structural measures otherwise incorporated in other embodiments (such as a spray shield  122  for defining a path about the outer casing  118 ) for directing the high speed air jet  58  to the point of abrasive material release. 
     In this as well as other embodiments disclosed herein, an advantage inhering in the concentric rotational movement preferably employed to adaptively adjust the shutter member  100  is that both the shutter member  100  and its drive shaft  76  (or drive tube  76 ′) naturally operate to clear themselves of abrasive material particles or other debris. The centrifugal force generated by these components&#39; own concentric rotational movement serves to propel or cast away, in self-clearing manner, any such particles or debris that might collect thereon, otherwise. 
     In the alternate embodiment of  FIG. 4 , a pair of metering orifices  54  are formed and disposed as shown concentrically about the axial reference  56 . Correspondingly, the shutter member  100  is provided with a pair of shutter openings  101  formed therethrough. In its fully open and closed positions, the shutter member  100  alternatively offsets and aligns its shutter openings  101  concurrently with the metering orifices  54 . Where the configurations and relative positions of the metering orifices  54  are such that it becomes difficult to fully open or close one metering orifice  54  without adverse interfering effect upon the other, a plurality of separately displaceable coaxially disposed shutter members  100  may be suitably employed, though reliable yet not overly complex actuation means my be difficult to realize in practice. 
     As also shown in the embodiment of  FIG. 4 , a downwardly swirled aspiration pattern may be employed to facilitate entrainment of the dispensed abrasive material  44  with the high speed air jet  58 . As shown, a plurality of downwardly angled inlet openings  63 ′ may be formed in a sidewall portion  63  of a capsule member enclosing the air/abrasive mixture chamber  62 . The inlet openings  63 ′ are spaced in angularly offset manner one from the other, such that they collectively impart a swirling flow to the high speed air jet  58  passed into the chamber  62  therethrough. 
     Turning now to the alternate embodiment illustrated in  FIGS. 5–6 , the symmetry of the arcuate metering orifice  54  configured as shown lends itself to a two position manually adjusted hopper structure, from which two different abrasive materials  44  may be selectively dispensed. Using a combination of clockwise or counterclockwise rotation and stop movement of the shutter member  100 , either of the two abrasive materials  44  would be dispensed in flow regulated manner, much as described in preceding paragraphs. 
     The dual abrasive hopper structure may be employed to dispense abrasive materials  44  of altogether different type, or more typically to dispense abrasive materials  44  of the same type, but having different granularity. For example, a garnet material having preselected course and fine mesh sizes (typically selected from an approximate range of about 80 to 220 mesh) may be stored and dispensed selectively from the hopper structure. This enables the resulting apparatus  40  to carry out bulk material removal/cuts just as readily as fine feature contouring operations on the same target workpiece  72 , without pause for significant reconfiguration. 
     As shown in  FIGS. 6 and 7 , the dispenser housing in this embodiment  50  is separated by a divider member  132  in two compartments, a coarse abrasive material compartment  134  and a fine abrasive material compartment  136 . Although two metering orifices can be used in this embodiment, e.g., one metering orifice for each compartment  134  or  136 , a single arcuate metering orifice  54  is preferably employed. The metering orifice  54  extends beneath portions of both compartments so as to be effectively bisected by the divider member  132 , as best shown in  FIG. 7 . One portion, or half,  138  of the metering orifice  54  serves as the metering opening for the coarse material compartment  134 , while the other portion, or half,  140  of the metering orifice  54  serves as the metering opening for the fine material compartment  136  of the dispenser housing  50 . The shutter member  100  is formed as shown with a shutter opening correspondingly configured and disposed thereon, such that when it is rotated by the drive shaft  76  in controlled manner clockwise or counter-clockwise, it selectively closes or partially opens to the required degree one or the other of the metering orifice halves  138  and  140 . 
     To guard against inadvertent leakage of abrasive material  44  through the metering orifice  54 , a guard band region  141  is preserved between the metering orifice  54  and the shutter opening  101  of the shutter member  100 . That is, the metering orifice  54  and the shutter opening  101  are so configured and positioned, respectively, that when they are disposed in the maximally offset positions (diametrically opposed positions in the embodiment shown), their nearest peripheral extremities remain at least a preset distance away from one another. For example, a pair of guard band regions  141  of approximately 15° in angular extent (about the axial reference  56 ) define dead zones, by which the shutter member  100  must be angularly displaced at the very least (from the maximally offset position) before any portion of its shutter opening  101  will overlap any portion of the metering orifice  54 . 
     In this embodiment, a secondary metering arc restrictor  146  is shown coaxially disposed between the metering orifice  54  and the shutter member  100 . Formed with any configuration suitable for the given orifice and shutter opening configurations, this secondary metering arc restrictor  146  effectively serves a static adjustment function. Much like the shutter member  100 , the secondary metering arc restrictor  146  is formed with one or more openings  146 ′ which partially occlude the given metering orifice  54  when appropriately aligned therewith. If, for instance, the granularity or material composition of the selected abrasive materials preclude use of the entire metering orifice  54  allocated thereto, the secondary metering arc restrictor  146  may be suitably set in position to restrict the orifice shape or size as needed. Dynamic control of the shutter member  100  may then proceed as in other embodiments, but with the metering orifice  54  so restricted. 
     In certain embodiments, this secondary metering arc restrictor  146  may be configured with an arcuate opening contoured and positioned relative to the axial reference  56  in much the same manner that the metering orifice might be configured and positioned in other embodiments. Angular displacement of the secondary metering arc restrictor  146  relative to the orifice  54  would then variy the position of the residual orifice segment for subsequent use. Use of such secondary metering arc restrictor  146  of this or other suitable configuration need not be limited to the embodiment of  FIGS. 6–7 , though the desirability of such may be particularly apparent in that embodiment. 
     In arrangements where the metering orifice  54  is contoured as an arc, the most significant nonlinearities occur at the ends  142  and  144  of that metering arc. In the middle section of the arc, each increment of the shutter member&#39;s motion (typically within 0.2° in positional accuracy, at an approximate 50 ms seek time) preferably uncovers/occludes substantially the same ratio of edge-to-cross-sectional area. As expected, the abrasive particles experience the most discernible shear forces near the edges  142  and  144  of the fixed arc, such discernible forces being largely absent near the middle of the arc. Therefore, by modifying the ends of the arc (for any given abrasive particle size), by a suitably compensating tapered shape for example, it is possible to diminish or eliminate the discontinuity of the end edge effect. 
     The metering arc  54  may thus be tailored for specific applications in numerous other respects. For example, the metering arc  54  may be configured such that a finer adjustment capability (for a given increment of shutter member motion) at one end of the arc graduates to a courser adjustment capability (i.e., much higher flow rate) at the other. 
     Referring to  FIG. 8 , an example of the flow rate realized by using the embodiment of  FIGS. 2A–2B , with a metering orifice configuration similar to that shown in  FIG. 3A  is graphically illustrated. Over much of at least the intermediate points of operation shown, the relationship between measured flow rate and percentage of shutter open (or degree of occlusion) approaches perfect linearity. 
     It is worth mentioning that the shutter member  100  operates to itself occlude only along one edge of the abrasive flow through the metering orifice  54 , such that shear effects are kept to a minimum. Also, a custom shaft collar  102  is preferably employed as shown in  FIG. 2  to mate the shutter member  100  to the drive shaft  76 . Preferably, a pinned plate arrangement is provided between the shaft collar  102  and the shutter member  100  so as to overcome any thermal problems which may occur in other arrangements. 
     Referring now to  FIGS. 10–11 , the dual abrasive chamber embodiment of the abrasive jet apparatus  40  illustrated in  FIGS. 6 and 7  is more fully shown. While minor variations are evident and some features are more clearly visible, the overall structure of this embodiment is similar to that shown in  FIGS. 2A and 2B ; and, analogous parts/components are denoted by like reference characters. In this embodiment, a coupling  93  is more clearly shown with a bore which accommodates the passage of encoder coupling connections to the motor  78 , for example. 
       FIGS. 12–13  show perspective views of an exemplary implementation of the embodiment shown schematically in  FIGS. 10–11 . The capsule member enclosing the air/abrasive mixture chamber  62  is shown removed in the view of  FIG. 12  to expose the coupling collar  102 , shutter assembly  74 , and respective non-occluded portions of the metering orifice  54 . 
     EXAMPLE 
     In one example, the abrasive jet apparatus  40  formed in accordance with a preferred embodiment of the present invention typically exhibits the following advantages and characteristics, which are listed purely for illustrative, not limiting, purposes: 
     
       
         
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
             
               
               
             
               
               
               
             
               
               
             
               
               
               
             
               
               
               
             
               
             
               
               
               
             
               
             
           
               
                   
                   
               
             
             
               
                   
                 (1) Linear range 
               
             
          
           
               
                   
                 (a) 
                 excellent linearity, enhanced with the arc edges 
               
               
                   
                   
                 modifications; 
               
               
                   
                 (b) 
                 full analog range: from 0 to more than 1000 grams 
               
               
                   
                   
                 per minute; 
               
             
          
           
               
                   
                 (2) CNC controlled servomotor 
               
             
          
           
               
                   
                 (a) 
                 encoder position feedback; 
               
               
                   
                 (b) 
                 instant response (non-pneumatic); 
               
               
                   
                 (c) 
                 single variable throw actuator, (avoiding open/close 
               
             
          
           
               
                 cylinders of prior art abrasive water jet machines); 
               
             
          
           
               
                   
                 (3) Sealed housing 
               
             
          
           
               
                   
                 (a) 
                 internally aspirated vacuum line; 
               
               
                   
                 (b) 
                 IP 67 wash down service; 
               
             
          
           
               
                   
                 (4) Wear resistant coatings and materials; and, 
               
               
                   
                 (5) Abrasive material dispensing valve 
               
             
          
           
               
                   
                 (a) 
                 rotary shutter motion is mechanically simple; 
               
               
                   
                 (b) 
                 self-clearing action; 
               
             
          
           
               
                   
                 i. 
                 abrasive on shutter is centrifugally slung off; 
               
               
                   
                 ii. 
                 no abrasive accumulates near the drive 
               
             
          
           
               
                 shaft/guide tube clearance; 
               
             
          
           
               
                   
                 (c) 
                 drive shaft may have an integral aspiration function 
               
             
          
           
               
                 (provided by the tube 76′, for example). 
               
               
                   
               
             
          
         
       
     
     In one example, the abrasive jet apparatus  40  formed in accordance with a preferred embodiment of the present invention is found to operate sufficiently within the following set of parametric criteria. Such parametric criteria are listed, again, purely for illustrative purposes, and the present invention is not limited thereto. 
     
       
         
               
             
               
               
               
             
               
             
               
               
               
             
               
             
               
               
               
             
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
               
               
               
             
           
               
                   
               
             
             
               
                 1. Environmental 
               
             
          
           
               
                   
                 a. 
                 Must operate in ambient conditions from 32° F. to 165° F. 
               
               
                   
                 b. 
                 Relative Humidity 0–100% over full temperature range 
               
               
                   
                 c. 
                 Dirty industrial area, characterized by abrasive dust and spray 
               
               
                   
                 d. 
                 Unit may be housed in a sealed, air purged canister (1 SCFM) 
               
             
          
           
               
                 2. Physical Envelope 
               
             
          
           
               
                   
                 a. 
                 Present Mini-Hopper Assembly, LAI P/N 901005 
               
               
                   
                 b. 
                 Minimize envelope, especially horizontal section 
               
             
          
           
               
                 3. Power Supply 
               
             
          
           
               
                   
                 a. 
                  24 Vdc 
               
               
                   
                 b. 
                 ±15 Vdc 
               
             
          
           
               
                 4. Abrasive 
               
             
          
           
               
                   
                 a. 
                 Grade 
               
             
          
           
               
                   
                 i. 
                 Maximum: 50 grit, HPX and HPA 
               
               
                   
                 ii. 
                 Minimum: 220 grit, HPX and HPA 
               
             
          
           
               
                   
                 b. 
                 Flow 
               
             
          
           
               
                   
                 i. 
                 Metered flow (lbs/mm) to be linear (preferred) 
               
               
                   
                   
                 over command range 
               
               
                   
                 ii. 
                 Maximum: Equivalent to Ø.375″ metering disk. 
               
               
                   
                 iii. 
                 Minimum: Equivalent to Ø.060″ metering disk. 
               
             
          
           
               
                 5. Construction 
               
             
          
           
               
                   
                 a. 
                 Metering Vanes (Shutter Members) 
               
             
          
           
               
                   
                 i. 
                 The general shape may be intersecting “V” 
               
               
                   
                   
                 shapes (&lt;&gt;) 
               
               
                   
                 ii. 
                 Sections shapes may be modified to provide 
               
               
                   
                   
                 linearity and accuracy 
               
               
                   
                 iii. 
                 Different vane shapes/sizes permissible for 
               
               
                   
                   
                 different grit sizes 
               
               
                   
                 iv. 
                 Throw = .25″ to .75″ (minimize while meeting 
               
               
                   
                   
                 1% repeatability) 
               
             
          
           
               
                   
                 b. 
                 Materials 
               
             
          
           
               
                   
                 i. 
                 Use Commercial Off The Shelf (COTS) 
               
               
                   
                   
                 components wherever possible 
               
               
                   
                 ii. 
                 Control and fixed vane must be abrasion 
               
               
                   
                   
                 resistant/long life 
               
             
          
           
               
                 6. Performance 
               
             
          
           
               
                   
                 a. 
                 Mechanical 
               
             
          
           
               
                   
                 i. 
                 Dynamics 
               
             
          
           
               
                   
                 1. 
                 Access time 
               
             
          
           
               
                   
                 a. 
                 ≦30 ms avg. 
               
               
                   
                 b. 
                 ≦50 ms full scale 
               
             
          
           
               
                   
                 2. 
                 ≦10% overshoot/undershoot during 
               
               
                   
                   
                 position acquisition 
               
             
          
           
               
                   
                 ii. 
                 Failure/default position is closed 
               
               
                   
                 iii. 
                 Reliability: HIGH &gt; ~1000 hr MTBF 
               
               
                   
                 iv. 
                 Prefer actuator in vertical position 
               
               
                   
                 v. 
                 Control vane to be non-binding 
               
             
          
           
               
                   
                 b. 
                 Electrical 
               
             
          
           
               
                   
                 i. 
                 Actuator 
               
             
          
           
               
                   
                 1. 
                 Position sensor feedback: buffered 
               
               
                   
                   
                 output signal available 
               
               
                   
                 2. 
                 Duty Cycle: 
               
             
          
           
               
                   
                 a. 
                 25% active positioning 
               
               
                   
                 b. 
                 Continuous position hold 
               
             
          
           
               
                   
                 ii. 
                 Interfaced with Delta Tau controllers 
               
               
                   
                 iii. 
                 Electrical command 0–10 Vdc 
               
             
          
           
               
                   
                 1. 
                 0 Vdc closed (no power/failure state) 
               
               
                   
                 2. 
                 10 Vdc = maximum commanded 
               
               
                   
                   
                 garnet flow 
               
             
          
           
               
                   
                 iv. 
                 Closed loop operation 
               
             
          
           
               
                   
                 1. 
                 May use Access 28 board within 
               
               
                   
                   
                 Delta Tau controller 
               
               
                   
                 2. 
                 May be “smart sensor” to match 
               
               
                   
                   
                 position sensor output with command 
               
               
                   
                   
                 input voltage. 
               
               
                   
                   
               
             
          
         
       
     
     Although the present invention has been described in connection with specific forms and embodiments thereof, it will be appreciated that various modifications other than those discussed above may be resorted to without departing from the spirit or scope of the invention as defined in the appended claims. For example, equivalent elements may be substituted for those specifically shown and described, certain features may be used independently of other features, and in certain cases, particular locations of elements may be reversed or interposed, all without departing from the spirit or scope of the invention as defined in the appended claims.