Patent Publication Number: US-6905395-B2

Title: Abrasive flow machining apparatus and method

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention is related to abrasive flow machining and, more particularly, to an abrasive flow machining apparatus capable of processing an orifice within a part by carefully controlling the media flow rate. The invention is also directed to a method for such processing. 
     2. Description of the Related Art 
     Abrasive flow machining is the process of polishing or abrading a workpiece by passing a viscous media having abrasive particles therein under pressure over the workpiece or through an orifice extending through the workpiece. 
     Conventional abrasive flow machining processes are designed to maintain a constant media extrusion pressure which often results in significant changes in media temperature, flow rate and viscosity which adversely impacts the system capability to accurately predict abrasive flow machine (AFM) processing times and, consequently, overall process results. 
     As an example, the media temperature increases as the flow rate of the media increases through an orifice. When the orifice is subjected to media under a constant pressure, the flow rate of the media through the orifice increases as the orifice walls becomes smoother and the orifice diameter increases. As a result, not only does the media temperature increase, but such an increase is localized to the media that passes through the orifice at a higher flow rate. This produces both excessively high temperatures and a non-uniform temperature distribution throughout the media. High temperatures and variations in temperatures throughout the media prevent the media from working in a consistent and effective fashion. Therefore, an apparatus and method that may effectively utilize the media while at the same time maintain the temperature of the media within a relatively narrow temperature band is desired. 
     U.S. Pat. No. 3,634,973, which is assigned to the assignee of the present invention, discloses a reciprocal machining structure utilizing abrasive media but operating in a fashion which does not provide for direct control of the media flow rate through an orifice. While this apparatus is capable of affective abrasive flow machining, such machining would be of a higher quality and the media would last longer if the flow rate were controlled. 
     BRIEF SUMMARY OF THE INVENTION 
     A first embodiment of the subject invention is directed to an abrasive flow machine for moving abrasive media through the orifice of a workpiece comprising a workpiece holder, wherein the holder is adapted to securely retain the workpiece, and wherein one side of the holder defines an upstream side and the other side of the holder defines a downstream side. A first positive displacement pump positioned on the upstream side and connected to the upstream side of the holder for forcing media under a predetermined pressure to the downstream side of the holder. A media opposer is positioned on the downstream side and connected to the downstream side of the holder for opposing the flow of the media to the downstream side, thereby controlling the media flow rate from the upstream side to the downstream side of the holder. 
     In a second embodiment of the subject invention, an abrasive flow machine for moving abrasive media through the orifice of a workpiece comprises a workpiece holder, wherein the holder is adapted to securely retain the workpiece, and wherein one side of the holder defines a first side and the other side of the holder defines a second side. A first positive displacement pump is positioned on the first side and connected to the first side of the holder and a second positive displacement pump positioned on the second side and connected to the second side of the holder. In a first mode the first positive displacement pump forces media from the first side to the second side of the holder while the second displacement pump resists flow thereby controlling flow to the second side of the holder. In a second mode the second positive displacement pump forces media from the second side to the first side of the holder while the first displacement pump resists flow thereby controlling flow to the first side of the holder. 
     A third embodiment of the subject invention is directed to a method for abrasive flow machining using an abrasive media through the orifice of a workpiece, wherein the orifice defines an upstream side and a downstream side. The method comprises the steps of moving media through the orifice from the upstream side to the downstream side at a predetermined constant pressure on a first side and selectively throttling the flow of media to the downstream side to control the flow rate of the media passing through the orifice while maintaining the predetermined constant pressure on a second side. 
     A fourth embodiment of the subject invention is directed to a method for abrasive flow machining using an abrasive media through the orifice of a workpiece, wherein the orifice defines a first side and a second side. The method comprises the steps of moving media through the orifice from the first side to the second side at a predetermined constant pressure selectively throttling the flow of media to the second side to control the flow rate of the media passing through the orifice while maintaining the predetermined constant pressure moving media through the orifice from the second side to the first side at the predetermined constant pressure and selectively throttling the flow of media to the first side to control the flow rate of the media passing through the orifice while maintaining the predetermined constant pressure. 
     A fifth embodiment of the subject invention is directed to a method for abrasive flow machining using an abrasive media through the orifice of a workpiece, wherein the orifice defines an upstream side and a downstream side, comprising the steps of moving media through the orifice from the upstream side to the downstream side at a pressure adjusting the pressure to provide a constant flow rate of the media passing through the orifice. 
     A sixth embodiment of the subject invention is directed to a method for abrasive flow machining using an abrasive media through the orifice of a workpiece, wherein the orifice defines a first side and a second side. The method comprises the steps of moving media through the orifice from the first side to the second side by applying pressure at the first side and relieving pressure at the second side, adjusting the pressure at the first side to provide a constant flow rate of the media passing from the first side through the orifice, moving media through the orifice from the second side to the first side by applying pressure at the second side and relieving pressure at the first side, and adjusting the pressure at the second side to provide a constant flow rate of the media passing from the second side through the orifice. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a simplified sketch illustrating two opposing positive displacement pumps urging abrasive media through the orifice of a workpiece; 
         FIG. 2  is a simplified sketch of a single positive displacement pump displacing media through the orifice of a workpiece and opposing the flow of the media thereafter; 
         FIG. 3  illustrates opposing positive displacement pumps for moving media back and forth through an orifice whereby the drivers of the pumps are linear actuators; 
         FIG. 4  is a simplified sketch of two opposing positive displacement pumps and the control systems which operate them; 
         FIG. 5  is a sketch of an operating system illustrating two opposing positive displacement pumps and the associated hardware; 
         FIG. 6  is a simplified sketch of a single positive displacement pump which provides media through an orifice whereby the media is released to an open environment; and 
         FIG. 7  is a perspective view of an in-line heat exchanger that may be used to control the temperature of the media. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In one embodiment of the subject invention abrasive media is subjected to a constant pressure and forced through an orifice of a workpiece. The flow rate discussed with this embodiment will be equal to or less than the maximum flow rate capability with the downstream side of the orifice open to the atmosphere. In particular, a flow rate of less than this maximum value is obtained by limiting the flow of the media at the down stream side of the orifice. 
     Directing attention to  FIG. 1 , a simplified schematic of an abrasive flow machine  10  for moving abrasive media  15  through the orifice  18  of a workpiece  20  is illustrated. For purposes of this discussion, media will be discussed as having viscosity in the range between 1 to 50 million centipoise. One example of a relatively high viscosity media is a visco-elastic plastic media such as a semisolid polymer composition. One example of a media having a lower viscosity is a liquid abrasive slurry that includes abrasives suspended or slurried in fluid media such as cutting fluids of honing fluids. The fluid may have a rheological additive, and finely divided abrasive particles incorporated therein. The rheological additive creates a thixotropic slurry. The abrasive flow machine  10 , as an entity on its own, will not include the workpiece  20  having the orifice  18  therein but will include a workpiece holder  25  which is adapted to securely retain the workpiece  20  wherein one side  27  of the holder  25  defines an upstream or first side and the other side  29  of the holder  25  defines a downstream or second side. 
     A first positive displacement pump  35  is positioned on the upstream side  27  and connected to the upstream side  27  of the holder  25  for forcing media  15  under a predetermined pressure through the orifice  18  of the workpiece  20  to the downstream side  29  of the holder  25 . 
     Unencumbered flow of the media through the orifice  18  is prevented by a media opposer  45  positioned on the downstream side  29  of the holder  25  for opposing the flow of the media  15  to the downstream side  29 , thereby controlling the media flow rate from the upstream side  27  to the downstream side  29  of the holder  25 . 
     As illustrated in  FIG. 1 , the first positive displacement pump  35  is comprised of a piston  37  within a cylinder  39 , wherein the piston  37  is operable to urge media  15  from the cylinder  39  toward the downstream side  29  of the holder  25 . The piston  37  is moved by a driver  41 . As will be illustrated, the driver  41  for the piston  37  may be a hydraulic actuator ( FIG. 4 ) or as illustrated in  FIG. 3 , the driver  41  may be a linear motor actuator  42  which utilizes for example, a worm gear  43  which engages a mating gear  44  on a rod  38  extending from the piston  37 . It should be appreciated that while only two types of drivers have been mentioned, any number of drivers known to those skilled in the art of hydraulic machinery may be utilized for the positive displacement pumps in accordance with the subject invention. 
     Returning to  FIG. 1 , one method to control both the pressure of the media  15  and the flow rate of the media  15  involves reducing the flow rate through the orifice  18  by restricting the amount of media permitted to travel to the downstream side  29  of the holder  25 . In particular, a second positive displacement pump  55  may be utilized as the media opposer  45  to accomplish this. The second positive displacement pump  55  has a piston  57  within a cylinder  59 . The piston  57  is operable to resist and thereby control the media flow to the downstream side  29  of the holder  25 . 
     Other mechanisms are available to act as a media opposer  45 . Directing attention to  FIG. 2 , an arrangement similar to that in  FIG. 1  is presented, however, the media opposer  45  now takes the form of a relief valve  60 . The media  15  flows directly through the relief valve  60  and the release pressure of the relief valve  60  is controlled based upon the desired media flow rate. 
     In a preferred embodiment, the relief valve  60  is a proportional electric relief valve (PER). A control device monitors the flow rate and decreases a voltage output to the proportional electric relief valve  60  when the actual flow rate is greater than a target flow rate. This causes the relief valve  60  to allow less media  15  to pass through. In the alternative, the voltage output to the valve  60  may be increased which allows more media  15  to pass through when the actual flow rate is less than a target flow rate. Other relief valves described herein may operate in a similar fashion. 
     To accurately determine the media flow rate, a media flow rate measurement device  65  is utilized. One such device is illustrated in FIG.  1 . When the first positive displacement pump  35  is comprised of a piston  37  within a cylinder  39 , the piston  37  may have a rod  38 . An encoder  66  may be used as the flow rate measurement device  65  to measure the linear motion of the rod  38  to determine the media flow rate. Knowing the volume within the cylinder  39 , and the rate of travel of the piston  37 , which is provided by the encoder  66 , the volume flow rate of the media  15  through the orifice  18  may be used to determine the media flow rate and in turn the controller may adjust the media opposer  45  to increase or decrease the flow rate of the media  15  through the orifice  18 . 
     When the media opposer  45  is comprised of the second positive displacement pump  55 , which as previously discussed has a piston  57  within a cylinder  59 , the piston  57  has a rod  58  and under such circumstances the media flow measurement device  65  may be an encoder  67  that measures the linear motion of the rod  58  to determine the media flow rate. It should be apparent therefore that the measurement of the media flow rate may occur at either the upstream side  27  or downstream side  29  of the holder  25 . 
     Encoders  66 ,  67  may each be either a linear encoder or a rotary encoder, both of which are well known to those skilled in the field of measurement equipment. 
     The discussion so far has been limited to flow of media  15  in a single direction from the upstream side  27  of the holder  25  to the downstream side  29  of the holder  25 . In the abrasive flow machine  10  embodiment illustrated in  FIG. 2 , this is the only manner in which the media  15  may flow through the orifice  18  of the workpiece  20 . However, as illustrated in  FIG. 1 , when the media opposer  45  is a second positive displacement pump  55 , the roles of the first displacement pump  35  and second displacement pump  55  may alternate such that in a first mode the first displacement pump  35  may force media  15  through the orifice  18  while the second positive displacement pump  55  acts as the media opposer  45  to control the flow rate of the media  15 . In a second mode of operation, the second positive displacement pump  55  may be used to force the media  15  toward the first positive displacement pump  35  while the first positive displacement pump  35  is used as a media opposer to control flow in the opposite direction. It is apparent from this description that with these alternating modes media  15  may be moved back and forth through the orifice  18  in a reciprocating fashion. 
     Directing attention once again to  FIG. 1 , each of the first positive displacement pump  35  and the second positive displacement pump  55  are comprised of pistons  37 ,  57  within cylinders  39 ,  59  wherein the pistons  37 ,  57  are moved by drivers  41 ,  61 . Just as before, each driver  41 ,  61  may be a hydraulic actuator, which will be described or in the alternative, may be a linear motor actuator as illustrated in FIG.  3 . 
     When the abrasive flow machine  10  is operating such that media  15  is moved only in a single direction through the orifice  18  of the workpiece  20 , the media  15  is moved through the orifice  18  from the upstream side  27  to the downstream side  29  at a predetermined constant pressure. The flow of media  15  to the downstream side  29  is then selectively throttled to control the flow rate of the media  15  passing through the orifice  18  while at the same time maintaining the predetermined constant pressure. 
     In an alternative embodiment when the abrasive flow machine  10  is utilized in a reciprocating fashion, media  15  is moved through the orifice  18  from the upstream side  27 , which is now referred to as the first side  27 , to the downstream side  29 , which is now referred to as the second side  29 , at a predetermined constant pressure. The flow of media  15  to the second side  29  is selectively throttled to control the flow rate of the media  15  passing through the orifice  18  while maintaining the predetermined constant pressure. Thereafter, the media  15  is moved through the orifice  18  from the second side  29  to the first side  27  at a predetermined constant pressure. However, the flow of media  15  to the first side  27  is now selectively throttled to control the flow rate of the media  15  passing through the orifice  18  while maintaining the predetermined constant pressure. Just as before, the amount the media selectively throttled is determined by the media flow rate through the orifice  18 , and this is determined by monitoring the flow rate utilizing one or both of the linear encoders  66 ,  67 . 
       FIG. 4  illustrates a more comprehensive schematic view of the abrasive flow machine  10 , wherein each positive displacement pumps  35 ,  55  has a driver  41 ,  61  and each driver  41 ,  61  may be a hydraulic actuator. 
     In particular,  FIG. 4  includes many elements previously discussed, and the reference numbers for these elements will be retained. However, additional details associated with the driver  41  and the driver  61  in conjunction with the operation of the abrasive flow machine  10  will now be discussed. 
     In the single stroke mode, whereby the first positive displacement pump  35  moves media  15  through the orifice  18  of the workpiece  20  to the media opposer  45 , which is the second positive displacement pump  55 , the driver  41  acts to force the media  15  through the orifice  18  while the driver  61  acts as a media opposer  45  to resist and control such flow. Directing attention to the hydraulic actuator  70  associated with driver  41 , a hydraulic pump  72  moves media through a supply line  74  at which point the hydraulic fluid  76  encounters a poppet valve  78 , which may be a solenoid operated poppet valve (SOP), which for purposes of our discussion is a valve which permits full flow or no flow. The hydraulic fluid  76  also encounters a proportional electric relief valve  80 , which as previously mentioned is capable of adjusting its resistance to flow therethrough. When the hydraulic actuator  70  is being used as a driver  41 , the poppet valve  78  is in the full open position and the relief valve  80  is completely closed. Therefore, the hydraulic cylinder  82  is pressurized with hydraulic fluid  76  at whatever pressure the pump  72  can provide. This may be a predetermined pressure that remains constant throughout the stroke of the first positive displacement pump  35 . A piston  84  in the hydraulic cylinder  82  is acted upon by the pressurized hydraulic fluid  76  such that, through the common piston rod  38 , the piston  37  is advanced against the media  15 , thereby forcing the media  15  through the orifice  18  of the workpiece  20 . 
     When the first positive displacement pump  35 , with the hydraulic actuator  70 , acts as a driver  41 , the second positive displacement pump  55 , with the hydraulic actuator  90 , acts as a media opposer  45 . In particular, the hydraulic actuator  90  has similar components to the hydraulic actuator  70  including a hydraulic pump  92 , supply line  94 , and hydraulic fluid  96 , wherein the hydraulic fluid is directed to a poppet valve  98  and a relief valve  100 . The hydraulic actuator  90  is further comprised of a hydraulic cylinder  102  having a piston  104  therein connected to the piston rod  58  of the positive displacement pump  55 . When the driver  41  urges media  15  through the orifice  18  media  15  is also urged against the piston  57 , thereby transferring a force to the piston  104  which acts against the hydraulic fluid  96  in the hydraulic actuator  90 . When the second positive displacement pump  55  acts as a media opposer  45  the poppet valve  98  is completely closed such that the hydraulic fluid  96  must pass through the relief valve  100 . 
     It should be noted in  FIG. 4  that a single pump utilizing directional valves and a hydraulic fluid reservoir may be used in lieu of the two pumps  72 ,  92 . 
     The media flow rate through the orifice  18  is determined by one of the encoders  66 ,  67  and transmitted to a controller. Utilizing the media flow rate, and comparing it to a target media flow rate, the voltage in the proportional electric relief valve  100  is adjusted to permit hydraulic fluid  96  past the relief valve  100  in such a manner that the retraction of the piston  104  is controlled, thereby controlling the media flow rate. In this manner, when the first positive displacement pump  35  acts as the driver  41 , the poppet valve  78  associated with the hydraulic actuator  70  is fully opened thereby bypassing the relief valve  80 . With respect to the hydraulic actuator  90  of the second positive displacement pump  55 , the poppet valve  78  is fully closed thereby forcing hydraulic fluid  96  through the relief valve  100 , which throttles the hydraulic fluid flow to control the media flow rate. 
     In the second mode the same configuration exists, but in a reversed arrangement. In particular, when the second positive displacement pump  55  acts as a driver  61 , the first positive displacement pump  35  acts as a media opposer. In particular, in this configuration the poppet valve  98  is fully opened such that the full pressure produced upon the hydraulic fluid  96  by the pump  92  is transferred to the piston  104 , which in turn acts upon the piston  57  through the piston rod  58  and forces the media  15  through the orifice  18  toward the first positive displacement pump  35 . Acting as a media opposer, the hydraulic actuator  70  is configured such that the poppet valve  78  is fully closed thereby forcing the hydraulic fluid  76  through the relief valve  80 . The release pressure of the relief valve  80  may be electronically controlled by the controller based upon the media flow rate determined by one of the encoders  66 ,  67 . In this fashion, the operation of the abrasive flow machine may be alternated between the first mode and the second mode to provide a reciprocating motion of the media  15  through the orifice  18  of the workpiece  20 . 
       FIG. 5  represents a sketch of the hardware utilized to implement at least one embodiment of the subject invention described hereto. Just as before, like reference numerals are repeated. However, some additional elements are illustrated in this drawing. In particular, there is a pressure sensor  105  in the form of a pressure transducer associated with the hydraulic supply line  74  to determine the pressure in that line. Additionally, there is a pressure sensor  108  associated with supply line  94  to determine pressure in that line. It should be appreciated the pressure in the supply lines  74 ,  94  will be transmitted to the media  15  by the respective pistons  37 ,  57 . Additionally, a temperature sensor  110  may be utilized to determine the temperature of the media  15 . 
     The pressure of the hydraulic fluid, which translates into the pressure of the media  15 , along with the linear position of each piston  37 ,  57  is processed by a controller  112  which in turn acts to modify the release pressure of the pressure relief valve  80  for the positive displacement pump acting as the media opposer. 
     By more closely controlling the flow rate of the media  15  through the orifice  18 , the temperature may be held within a relatively narrow temperature band in contrast to when the flow rate is not controlled. Nevertheless, it may still be desirable to remove heat from the media  15  during the abrasive flow machining process. For that reason there may be a cooling collar  115  associated with the first positive displacement pump cylinder  39  and a cooling collar  117  associated with the second positive displacement pump cylinder  59 . Each of these cooling collars  115 ,  117  may have a plurality of cooling tubes  116 ,  118  capable of transferring heat from the media  15  when necessary. Under certain circumstances these cooling collars  115 ,  117  may also be utilized to heat the media  15  such as, for example, when the media  15  must begin the abrasive process at a minimum temperature. The cooling collars  115 ,  117  are externally positioned and do not interfere with the flow of media  15 . However, their effectiveness is limited because heat transfers from the media  15  to the collars  115 ,  117  occurs by conduction through the walls of the cylinders  39 ,  59 . 
     It is possible to introduce an in-line heat exchanger directly within the flow path of the media  15 .  FIG. 7  illustrates one such heat exchanger  200  having hollow cooling fins  202  within an internal passageway  205  through which the media  15  flows. Coolant passes through a coolant inlet  207 , enters the hollow fins  202  and exits at the coolant outlet (not shown). Bolts may extend through peripheral holes  209  in the collar  210  to secure the heat exchanger  200 . The heat exchanger  200  may be attached to one or both cylinders  39 ,  59  and may be adjacent to the holder  25 . While this heat exchanger  200  provides a greater heat transfer rate with the media  15 , it also partially obstructs the flow of media  15  such that the cylinder size may need to be increased to accommodate a given flow rate. 
     The controller  112  ( FIG. 5 ) may be a programmable logic controller such as the Micologics 1200 model, which is commercially available from the Allen Bradley Company. Additionally, the proportional electric relief valve may be type TS 10-26, which is commercially available from Hydra Force, Inc. Additionally, the poppet valves may be type SV 10-23 two way normally open valves commercially available from Hydro Force, Inc. 
     The signals from the encoders  66 ,  67  are used by the controller  112  to calculate the actual flow rate of the media  15 . A suitable encoder is the Quadrature type, which is commercially available from Automation Direct, Inc. The use of the encoders  66 ,  67  and the poppet valves  78 ,  98  and the relief valves  80 ,  100  allow the controller  112  to maintain a desired consistent media flow rate. This consistent flow rate allows the media to remain within a narrow temperature band, as measured by the temperature sensor  110 , which in turn maintains consistent media viscosity. By maintaining the media viscosity essentially constant, the controller  112  may more accurately predict the processing time to achieve the desired machining of the orifice  18 . 
     What has so far been described are drivers  41 ,  61  which alternately urge media  15  under constant pressure through the orifice  18  of the workpiece  20  while the flow rate of the media  15  is controlled by the retraction or resistance of the media opposer  50  which may be a pressure relief valve or the other driver. 
     It is possible to eliminate the media opposer  45  and still maintain a constant media flow rate. This is accomplished by varying the pressure provided by the driver  41  to the media  15 . As the abrasive flow machining process proceeds, given a constant media pressure, the flow rate of the media  15  through the orifice  18  tends to increase. Therefore, to maintain the same media flow rate, it is necessary to decrease the pressure imparted to the media  15  by the driver  41 . This may be accomplished in a single direction, or just as before, in a reciprocating motion. 
     Directing attention to  FIG. 6 , in a single direction media  15  is moved through the orifice  18  from the upstream side  27  to the downstream side  29  at a particular pressure. Utilizing encoder  66  the flow rate may be monitored and the pressure provided to the media may be adjusted to provide a constant flow rate of the media  15  passing through the orifice  18 . In particular, the encoder  66  may monitor the linear motion of the piston rod  38  associated with piston  37  to determine the flow rate. The pump  72  delivers hydraulic fluid under pressure to the hydraulic cylinder  82 , where the fluid acts upon the hydraulic piston  84 . With respect to the arrangement illustrated in  FIG. 4 , it is entirely possible for the downstream side  29  of the holder  25  to discharge into the atmosphere as illustrated in FIG.  6 . In the alternative, and again directing attention to  FIG. 4 , it is possible to coordinate the motion of piston  37  with that of piston  57  such that the flow of media  15  applied under pressure at the first side  27  through the orifice  18  is neither hindered nor assisted by piston  57  but that pressure on the second side  29  side is relieved. It is also possible to coordinate the motion of piston  57  with that of piston  37  such that the flow of media  15  applied under pressure at the second side  57  through the orifice  18  is neither hindered nor assisted by piston  37  but that pressure on the first side  37  is relieved. Such an arrangement will permit the abrasive flow machine  10  illustrated in  FIG. 4  to operate in a reciprocating fashion whereby in a first mode that the first positive displacement pump  35  forces the media  15  though the orifice  18  while the second positive displacement pump  55  is passive, and in the second mode the second positive displacement pump  55  forces the media  15  through the orifice  18  while the first positive displacement  35  is passive. 
     The invention has been described with reference to the preferred embodiments. Various modifications and alterations will occur upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.