Patent Publication Number: US-2012036692-A1

Title: Method and apparatus for engine piston installation by use of industrial robots

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional under 35 U.S.C. §120 of U.S. patent application Ser. No. 11/653,716 filed on Jan. 16, 2007, entitled “Method And Apparatus For Engine Piston Installation By Use Of Industrial Robots” the entirety of which is incorporated herein by reference and which application claims the priority of U.S. provisional application Ser. No. 60/759,865 filed on Jan. 18, 2006, entitled “ Method And Apparatus For Engine Piston Installation By Use Of Industrial Robots ” and incorporates therein by reference the entirety of the provisional application. 
    
    
     FIELD OF INVENTION 
     This invention relates to the installation of pistons, also known as stuffing, in an engine and more particularly to the use of robots to perform the piston installation. 
     DESCRIPTION OF THE PRIOR ART 
     Programmable robots are commonly used for a variety of repetitive industrial applications. Painting, welding, dispensing and material handling are examples of typical applications. For processes with high complexity and precision requirements such as engine piston stuffing, manual work and use of dedicated automation equipment are still dominant. 
     In manual engine piston stuffing, two persons normally are needed to cooperate in the process. One person inserts guiding pins through a cylinder bore in the engine block to locate and align the holes on the piston connecting rod; and another person holds the piston with a piston ring compressing device and approaches the cylinder bore to receive the guiding pins with the piston connecting rod. A pneumatically driven pusher is often used for stuffing the piston into the cylinder bore. The connecting rod cap and screws are also placed and then the cap is fastened manually on the connecting rod by the screws with hand tools or a dedicated workstation. Manual piston stuffing work is labor intensive and tedious and prone to worker injury due to the force required and repetitive nature of the tasks. The assembly quality is entirely dependent upon the skill and attention of the workers. 
     Automating of engine piston insertion has been performed by using specially built machines, often called “hard automation”. These dedicated machines are huge, costly, slow and inflexible. Switching between engine models or types to be assembled is difficult, time consuming and costly, making it largely impractical. 
     U.S. Pat. No. 6,047,472 discloses a method and apparatus for use of industrial robots in engine piston stuffing to transport the piston with connecting rod, to put the cap on the connecting rod, and to run down the cap with screws. However, the piston inserting process is still performed with a dedicated automation machine that requires a level of precision and tolerance that limits its application. 
     In both of the prior art piston stuffing techniques described above, there is no active searching action in finding the cylinder bore. Though a passive floating tool or table is sometimes used to align the piston skirt with the cylinder bore, the success of stuffing the piston essentially depends on the skill of the operator if the stuffing is performed manually or the precision of the machine if the stuffing is performed by the dedicated machine, the actual gap between the cylinder bore and the piston skirt, the lead chamfers on both the piston skirt and the cylinder bore. 
     With the increasing demand of reducing the gap between the cylinder bore and the piston skirt and minimizing or eliminating the lead chamfers for the purpose of emission control and engine efficiency improvement, the challenge and difficulty are increasing for both manual and automated piston stuffing processes. It is expected that because of the above requirements, the piston stuffing failure rate will increase due to the limitations of the existing piston stuffing automation technology. Also, the presently available automated piston stuffing processes only work on certain engine types, whereas, the present invention is usable across any engine, or block, configuration i.e. inline, v-block, w-block, etc. 
     SUMMARY 
     An apparatus for stuffing a piston assembly into an associated bore of an engine block. The piston assembly has: 
     a rod cap; and 
     a piston subassembly comprising a connecting rod suitable for coupling to the rod cap and a piston head. The apparatus has: 
     a first robot picking up the piston subassembly and inserting it into the associated bore of the engine block; and 
     one or more tools for inserting the rod cap into the bore and allowing it to be fastened to the connecting rod. 
     A method for stuffing a piston assembly into an associated bore of an engine block. The piston assembly has a rod cap, and a piston subassembly having a connecting rod suitable for coupling to the rod cap piston head. The method is: 
     (a) picking up by a first robot the piston subassembly and inserting it into the associated bore of the engine block; and 
     (b) inserting the rod cap into the bore and allowing it to be fastened to the connecting rod. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows in two views of piston stuffing with three robots. 
         FIG. 2  shows in two views of piston stuffing with two robots and a set of stationary tools. 
         FIG. 3  shows a set of stationary tools for piston stuffing. 
         FIG. 4  shows a process flow diagram for piston stuffing with three robots. 
         FIG. 5  shows a process flow diagram for piston stuffing with two robots and a set of stationary tools. 
         FIG. 6  shows a piston assembly. 
         FIG. 7  shows an engine block and its fixture. 
         FIG. 8  shows the gripper used in robot two to grasp a piston assembly and the grasping of such an assembly by the gripper. 
         FIG. 9  shows the pusher assembly in the gripper of  FIG. 8 . 
         FIG. 10  shows the piston installation process. 
         FIG. 11  shows the apparatus in the jaws of the gripper used to detect the presence or absence of a piston ring in an associated groove of the piston. 
     
    
    
     DETAILED DESCRIPTION 
     Referring to the figures, wherein the numerals indicate the like or corresponding parts throughout the several views, a method and apparatus for engine piston stuffing by use of industrial robots are disclosed. The piston stuffing illustrated here is for a V-6 engine block. A similar sequence can be derived for other types of engines. 
     In order to describe the invention clearly and in detail, the following four subsections are included: 
     1) description of the piston installation process, explaining the robotic piston stuffing solutions with three robots and with two robots and a set of stationary tools, respectively; 
     2) structure of the uniquely designed gripper and its design innovativeness, the gripper jaw configuration, the ring detection and their detailed operational sequence; 
     3) the piston ring detection apparatus and its operation; and 
     4) search of the cylinder bore with the force-controlled robot that is inserting the piston assembly into the associated cylinder bore of the engine block. 
     1) Description of the Piston Stuffing Process 
     A. Three Robot Piston Stuffing 
     In the three robot piston stuffing configuration, shown in the two views of  FIG. 1 , a first robot  100  with its fixture  101 , also shown in more detail as  702  in  FIG. 7 , picks up the engine block  102 , also shown in more detail as  701  in  FIG. 7 , from a pallet. While the pallet is not shown in  FIG. 1  it is similar to pallet  400  shown in  FIG. 2  and can be a pallet on a conveyor system or transported by a robot or other means. Robot  100  moves the picked up engine block  102  to a position close to the second robot  200  and orients the engine block  102  so, as is described below, the second and third robots  200  and  300 , respectively, can perform the piston stuffing. The combination of the engine block  102  and fixture  101  are shown as  1002  in  FIG. 10 . 
     An external axis in the form of a motor  702   a  of  FIG. 7  controlled by the robot controller not shown in  FIG. 1  but well known to those of ordinary skill in the art rotates the crankshaft (not shown) of the engine block  102  to the right orientation for the piston to be stuffed in an associated one of the three cylinder bores  701   c  of  FIG. 7  in the first row  701   a  of  FIG. 7  of cylinder bores in engine block  102 . 
     Referring now to  FIG. 6 , there is shown as typical piston assembly  600 . The assembly  600  has a ring  601 , a piston skirt  602 , a connecting rod  603  with a groove  605 , a connecting rod cap  604  and a piston head  606 . The ring  601 , piston skirt  602 , connecting rod  603  and head  606  are assembled elsewhere prior to the stuffing of the piston in one of the cylinder bores  701   c  and are delivered to the stuffing workstation with an unattached associated connecting rod cap  604 . The stuffing operation consists of stuffing the ring, skirt, connecting rod and head combination, referred to hereinafter as subassembly  607 , into one of the bores  701   c  and then attaching the associated connecting rod cap  604  to subassembly  607 . 
     Robot  200 , which has force control, moves its uniquely designed gripper  201 , which is shown in detail in  FIG. 8  as  800 , onto a piston subassembly  607 , which is shown in detail in  FIG. 6  and just below the gripper in  FIG. 8 . As is described in more detail below, the gripper  201  has jaws  801  to grab the piston subassembly  607 . Before the jaws  801  are closed to grab the piston subassembly  607 , the gripper  201  sucks up the piston subassembly  607  using a suction cup  902  of  FIG. 9  on a pusher  901  of  FIG. 9  against the upper surface of the piston head  606 . 
     At the same time as the operations described above for robot  200 , robot  300  using the guiding pins and cap placing gripper  301  shown in  FIG. 3  picks up the connecting rod cap  604  of piston assembly  600 , moves it under the engine block  102  and protrudes the guiding pins  301  through the crankshaft and the cylinder bore of the engine block  102  to receive the piston connecting rod  604 . Robot  200  moves the gripper  201  with the piston subassembly  607  inserted therein (see  FIGS. 8 and 10  for that combination) above the cylinder bore  701   c  to be stuffed and then moves the piston subassembly  607  into the bore  701   c  to engage the connecting rod  603 , leading the tips of the guiding pins  301  into the screw holes most clearly shown in  FIG. 8  as  806  on the upper half bearing house of the connecting rod  603 . A stabilizing finger  803  in  FIG. 8  is employed to keep the connecting rod  603  in place during transportation of the piston subassembly  607  to the cylinder bore  701   c  from the pallet where the piston subassembly  607  is gripped by the robot  200 . 
     Robot  200  and robot  300  move cooperatively until the lower surface of the piston skirt  602  in  FIG. 6  is close to the upper surface  701   b  of the cylinder bore  701   c  into which the piston subassembly  607  is to be inserted. Then robot  200  enables its active searching function, that is, its force control functionality, to move the subassembly  607  so that the piston skirt  602  finds that cylinder bore  701   c  and the piston subassembly  607  is inserted in that bore until the lower surface  807  in  FIG. 8  of the gripper jaws  801  touch the upper surface  701   b  of the cylinder bore. Next, the piston assembly  600  is pushed further into the cylinder bore. The third robot  300  with its connecting rod cap placing and rundown device  302 , and also shown in more detail as  1001  in  FIG. 10 , places the connecting rod cap  604  of  FIG. 6  and screws on the inserted piston subassembly  607  and fastens the cap  604  onto the connecting rod  603 . The same process is repeated for subsequent cylinder bores in the first row  701   a  of  FIG. 7  of three cylinder bores  701   c  for this V-6 engine block  102 . 
     After the stuffing is finished for the first row  701   a  of cylinder bores  701   c , robot  100  reorients the engine block  102  so that the upper surface of the other row of three cylinder bores (not shown in the figures for the V6 engine) can be stuffed. The piston stuffing procedure described above for the first row  701   a  is repeated to stuff a piston assembly  600  into each of the cylinder bores  701   c  in the second row.  FIG. 4 , which is described below, shows the process flow diagram for the piston stuffing technique of the present invention using three robots. 
     B. Two Robot Piston Stuffing 
     When as is shown in the two views of  FIG. 2 , two robots  100  and  200  and a set of stationary tools  300  are used for piston stuffing, the second robot  200  picks up a piston subassembly  607  consisting of a piston and a connecting rod cap  604 , and puts the cap  604  into the cap feeder  303  in  FIG. 3 . 
     Robot  100  with its tool  702  picks up an engine block  102 / 701  from the pallet  400  in  FIG. 2  which can be on a conveyor or transported by a robot or other means and moves it to above the guiding pins  301  in  FIG. 3  of the stationary tools and orients the engine block so that robot  200  can perform the piston stuffing. After the engine block  102  is oriented, an external axis rotates the crankshaft using crankshaft rotating motor  702   a  to the right orientation for the cylinder bore  701   c  to be stuffed in an associated one of the three cylinder bores  701   c  in the first row  701   a  of cylinder bores  701   c.  The guide pins  301  move up and protrude through the crankshaft and the cylinder bore  701   c  to receive the piston connecting rod  603  while robot  200  moves its gripper with the piston subassembly  607  to above the cylinder bore  701   c  to be stuffed and moves down vertically to engage the connecting rod upper half bearing with the guiding pins  301 , letting the tips of the guiding pins into the screw holes  806  on the connecting rod  603 . The stabilizing finger  803  of  FIG. 8  is employed to keep the connecting rod  603  in place during transportation of the piston subassembly  607  to the cylinder bore  701   c  from the pallet  400  where piston subassembly  607  is gripped by the robot  100 . 
     Robot  200  and the guide pin assembly  301  move down until the lower surface of the piston skirt  602  is above but very close to the cylinder bore upper surface  701   b.  Then robot  200  enables its active searching function, that is, its force control functionality, to move the subassembly  607  so that the piston skirt  602  finds the cylinder bore  701   c  and the piston subassembly  607  is inserted in that bore until the lower surface of the gripper jaws  801  touch the upper surface  701   b  of the cylinder bore  701   c.    
     After that, the piston subassembly  607  is further pushed into the cylinder bore  701   c.  Then the gripper  800  retracts its pusher, leaves the engine block  102  and moves to the pallet  400  to pick up the next piston subassembly  607  and its cap  604  and places the cap  604  onto the cap feeder  303  while robot  100  moves the stuffed cylinder bore to the cap rundown station  302 . A pusher  304  on the stationary tool set  300 , which is shown in  FIG. 3 , comes down to maintain the position of the piston subassembly  607  inside the cylinder bore during the time the tool places the cap  604  on and fastens the screws. 
     When the rundown process is completed, robot  100  moves the second cylinder bore  701   c  to above the guide pins  301 . The same stuffing process described above for the first cylinder bore  701   c  is repeated for the second and third cylinder bores  701   c  in the first row  701   a  of the engine block  102 . The same procedure as that described above, for the first row  701   a  of cylinder bores  701   c  in engine block  102  is used to stuff the pistons in the second row of cylinder bores  701   c  in the engine block  102 .  FIG. 5 , which is described below, illustrates the process flow diagram for piston stuffing with two robots and the set of stationary tools  300  shown in  FIG. 3 . 
     2) Structure of the Uniquely Designed Gripper 
     The piston stuffing gripper  800  shown in  FIGS. 8 and 9 , consists of gripper jaws  801 ; gripper jaw driving unit  805  which is driven by air pressure from a suitable source (not shown) under control of the robot controller; enforcement ring and driving unit  802 ; rod stabilization finger assembly  803  of  FIGS. 8 and 904  of  FIG. 9 ; pushing/suction unit  901  and  902  of  FIG. 9 ; and piston ring detection unit, one embodiment for which is shown in  FIG. 11  and is described below. When picking up a piston subassembly  607 , a built-in pusher  900  with a suction cup  902 , the suction device for which is not shown in  FIG. 9 , in the gripper  800  sucks the piston skirt upper surface against the pusher surface. When the piston subassembly  607  is in a location that has a limited space the pusher cylinder  901  can extend out to pick up the piston subassembly  607  and then return to the start position if necessary. 
     Gripper jaws  801  close and clamp the piston subassembly  607  when the subassembly is in position. An enforcement ring  802  slides down to lock the gripper jaws  801  in position. Then, the rod-stabilizing finger  803 / 904  comes down and its fingertip  803   a / 904   a  will push into the groove  605  on the connecting rod  603  from one side to secure the connecting rod  603 , preventing it from swinging during the transportation of the piston subassembly  607  from the pallet  400  to the cylinder bore  701   c  of the engine block  102 . After the screw holes  806  on the upper half bearing house of the connecting rod  603  are engaged with the guiding pins  301  or before inserting the connecting rod  603  without the guiding pins, the finger  803 / 904  moves up to get out of the way so that the connecting rod  603  can insert further into the cylinder bore  701   c.    
     During the transportation and searching, the jaws  801  grip both the piston rings  601  shown in  FIG. 6  and the skirt  602  to ensure that the piston subassembly  607  is firmly gripped. Before pushing the piston subassembly  607  into the cylinder bore  701   c , the jaws  801  are released. The enforcement ring  802  limits the outward movement of the jaws  801  and leaves a small but necessary gap between the piston skirt outer surface and the jaws&#39; inner surface, letting the piston subassembly  607  be pushed easily through the cylinder bore  701   c.  The pusher  901  will stay at the extended position while the connecting cap  604  is put on and screws are rundown in the three-robot piston stuffing solution. In the two-robot solution which uses a set of fixed tools  300 , the pusher  901  will retract after pushing the piston subassembly  607  in position and a locking pusher  304  in  FIG. 3  on the stationary tools  300  is used to keep the piston  600  in place during cap placing and screw run down. 
     3) Piston Ring Detection 
     To ensure that all piston rings  601  are in place on piston subassembly  607  before piston stuffing, a ring detection unit  1100  of  FIG. 11  is built in the gripper jaws  801 .  FIG. 11  illustrates the ring detection unit  1100  and its principle of operation. A ring-detecting sensor such as a proximity switch  1103  and  1104  is used to detect the presence of a ring  1102 , normally iron based material. 
     When the jaws  801  grip on the piston  1101 , normally aluminum based material, if a ring  1102  exists in that particular ring groove, the associated proximity switch  1103 ,  1104  is triggered and the light associated with that switch is on as is shown in  FIG. 11  by way of example for switch  1103 . If as is shown in  FIG. 11  by way of example for the lowest groove in piston  1101 , a ring does not exist in that groove the proximity switch  1104  remains off and the associated light is off. In this way, the sensors  1103  and  1104  determine the presence of the rings  1102  in the grooves before the piston subassembly  607  is stuffed into bore  701   c.    
     The ring detection circuitry  1100  can be connected into the controller of robot  200  as a digital input to indicate the ring presence. A chart  1105  shows the correlation between sensing distance and size of the sensing object for a typical proximity sensor. 
     4) Piston Stuffing With A Force-Controlled Robot 
     The use of a force controlled robot for assembly is disclosed in allowed U.S. Patent Application Publication No. 2005/0113971 “the &#39;971 Publication”, the disclosure of which is hereby incorporated herein by reference. As is described in the &#39;971 Publication, a torque/force sensor is mounted on the robot wrist to provide a force measurement to the robot controller. In response thereto, a velocity, that is, force, controller which may be part of the robot controller generates an attraction force vector which is superimposed on the measured force in a preferred direction and orientation. 
     The force vector may also be a repulsive force vector as the same may be needed during the mating of the piston subassembly  607  with the associated one of the cylinder bores  701   c  and the force provided by the vector whether it is that of attraction or repulsion need not be constant. 
     The attraction force vector is imposed on the robot drive so that the robot stuffing gripper  800  which holds the piston subassembly  607  is subject to a force which may be constant, that is, the absolute value of the vector. When no contact is established by the gripper  800  with the surface of the engine block  102  that has in it the cylinder bores  701   c,  this attraction force will always drag the gripper  800  toward that location until a proper contact between the piston skirt  602  lower surface and the engine block upper surface  701   b  is established. After the contact with that surface is established, the velocity controller adjusts the robot drive so that the contact force between the piston subassembly  607  and that surface keeps a constant value. 
     If the piston subassembly  607  is in contact with the surface of the engine block  102  that has in it the cylinder bores  701   c,  but the location of the specific bore  701   c  which is to receive the subassembly is not known to the robot, then as is described in the &#39;971 Publication a search velocity force pattern in a plane parallel to that surface is superimposed by the controller with the velocity force command to the robot gripper  800 . An example of the search pattern might be a circular motion or a spiral motion in a plane parallel to the surface to cover the possible location of the bore hole  701   c  that is to receive the subassembly  607 . 
     As long as the uncertainty of the hole location is within the possible range of the search pattern, the piston subassembly  607  will find the bore  701   c  and the attraction force will drag the gripper  800  and thus the subassembly  607  downward so that the subassembly can be inserted in the associated bore  701   c  seamlessly with the gripper jaw&#39;s lower surface  708  against the engine block&#39;s upper surface  701   b.  As is described in the &#39;971 Publication, the search range should be selected to cover the maximum possible uncertainty in the location of the associated bore  701   c  on the surface of the engine block  102  that has in it the cylinder bores  701   c.    
     In the piston stuffing process, force control is activated when the lower surface of the piston skirt  602  is close to but not touching the cylinder bore upper surface  701   b.  A downward retention force valued about 40 N is set and certain search patterns such as spiral and circular can be used. The first search finish condition is set as the piston subassembly  607  is inserted into the cylinder bore  701   c  for a certain distance, such as 3 millimeters. Then the downward retention force increases to a higher value, such as 260 N. This force will “drag” the piston subassembly  607  further into the bore  701   c  until the lower surface of the gripper jaws  801  touch the upper surface  701   b  of the cylinder bore  701   c.  Then the piston subassembly  607  is further pushed by the pusher cylinder  901  into the cylinder bore  701   c  to engage with the crankshaft. During the pushing, the same retention force, 260 N, keeps the lower surface of the gripper jaws  801  in contact with the upper surface  701   b  of the cylinder bore  701   c.  After the connecting rod cap  604  assembling is accomplished, a repulsion force is applied, which moves the gripper  800  away from the engine block  102  smoothly before the force control is deactivated and the robot resumes to its position control mode. 
     The Process Flow Diagrams of FIGS.  4  and  5   
     a) FIG.  4   
       FIG. 4  shows the process flow diagram for the piston stuffing technique of the present invention using three robots. That process starts at  402  where robot  100  loads the engine block  102 . At  404 , robot  200  loads the piston subassembly  607  and robot  300  picks up the connecting rod cap  604  of the piston assembly  600  and the screws for attaching cap  604  to the upper part of the assembly  600 . At  406 , the ring detection unit  1100  built into the gripper jaws  801  determines if any rings are missing from the piston subassembly  607 . If any rings are missing the process stops at  408 . 
     If no rings are missing, then at  410  robot  200  positions the piston subassembly  607  above the bore  701   c  in the engine block  102  into which the subassembly is to be stuffed and receives the guide pins  301 . At the same time, robot  300  at  412  engages the guide pins  301  and moves coordinately with robot  200 . At  413 , robot  100  rotates the crankshaft of the engine block  102  to the right orientation for the piston subassembly  607  to be stuffed in an associated one of the three cylinder bores  701   c  in the first row  701   a  of cylinder bores in the engine block  102 . 
     After robot  200  moves down until the lower surface of the piston skirt  602  is above but very close to the cylinder bore upper surface  701   b,  robot  200  enables at  414  its active searching function, that is its force control functionality, to move the subassembly  607  so that the piston skirt  602  finds the cylinder bore  701   c  and the piston subassembly  607  is inserted in that bore until the lower surface of the gripper jaws  801  touch the upper surface  701   b  of the cylinder bore. At  416 , robot  300  places the cap  604  on the upper portion of the piston assembly  600  and fastens it in place by tightening the screws. 
     At  418 , the method determines if all of the bores  701   c  in the first row  701   a  of bores in the V6 engine block  102  of this example have been stuffed with piston assemblies  600  as described above. If the answer is no, then the method returns to  404  so that a piston subassembly  607  can be stuffed in each of the remaining cylinder bores  701   c  in row  701   a.  If the answer is yes, then at  420  the engine block  102  is reoriented so that the next row of cylinder bores  701   c  can be stuffed with piston subassemblies  607 . At  422 , the method determines if all of the rows in the engine block  102  are stuffed with piston assemblies  600 . If they are, then at  424  the engine block  102  with all of its cylinder bores stuffed with piston assemblies  600  is unloaded by robot  100 . If not, then the method returns to  404  so that a piston subassembly  607  can be stuffed in each of the remaining cylinder bores  701   c  in that row. 
     b) FIG.  5   
       FIG. 5  shows the process flow diagram for the piston stuffing technique of the present invention using two robots  100  and  200  and a set of stationary tools  300 . That process starts at  502  where robot  200  loads the piston subassembly  607  and the connecting rod cap  604  of the piston assembly  600 . At  504 , robot  100  loads the engine block  102 . At  506 , the ring detection unit  1100  built into the gripper jaws  801  determines if any rings are missing from the piston subassembly  607 . If any rings are missing the process stops at  508 . 
     If no rings are missing, then at  510  robot  200  places the cap  604  on the cap feeder  303  and at  512  robot  100  positions the engine block  102  above the station for the guide pins  301  and rotates the crankshaft of the engine block  102  so that to the right orientation for the piston subassembly  607  to be stuffed in an associated one of the cylinder bores  701   c  in the engine block. At  514 , the cap feeder  303  feeds the cap  604  to the rundown station  302 . 
     After robot  200  and guide pin assembly  301  moves down until the lower surface of the piston skirt  602  is above but very close to the cylinder bore upper surface  701   b,  robot  200  enables at  516  its active searching function, that is its force control functionality, to move the subassembly  607  so that the piston skirt  602  finds the cylinder bore  701   c  and the piston subassembly  607  is inserted in that bore until the lower surface of the gripper jaws  801  touch the upper surface  701   b  of the cylinder bore  701   c.  At  518 , robot  100  moves the stuffed cylinder bore  701   c  to the cap rundown station  302 . At  520 , the pusher  304  on the stationary tool set  300  comes down to maintain the position of the piston subassembly  607  inside the cylinder bore during the time the tool places the cap  604  on and fastens the screws. 
     At  522 , the method determines if all of the bores  701   c  in the engine block  102  have been stuffed with piston assemblies  600  as described above. If the answer is no, then the method returns to  502  but robot  100  does not at  504  have to load another engine block  102  as the present engine block  102  does not have all of its cylinders stuffed with piston assemblies  600 . If the answer is yes, then at  524  robot  100  unloads the engine block  102 . 
     As can be appreciated from the above description, the present invention: 1) uses a robot which is force controlled to with the force control search and insert, that is, stuff, a piston subassembly with compressed rings in an cylinder bore of an engine block thereby eliminating the need for extreme precision in the positioning of the block or piston; and put a rod cap and screws on the connecting rod of the subassembly and rundown the screws and may use another robot to position the engine block, and yet another robot may be used to mount a placing gripper for the rod cap and pins which is used to guide the coupling between the rod cap and the connecting rod; 2) has a robot gripper that picks up the piston, compresses the rings, stabilizes the connection rod, and further pushes the piston into its final position; 3) can easily detect the presence or absence of a piston ring by using a detector built into the robot gripper; and 4) can be used on any block configuration, namely in-line, v-block and deep-skirted v-block engines, to accommodate different production rates by addition of robots in the same cell, and to install pistons either on-line or off-line and can be used to stuff one or more or all of the pistons in the block. 
     It is to be understood that the description of the foregoing exemplary embodiments is intended to be only illustrative, rather than exhaustive, of the present invention. Those of ordinary skill will be able to make certain additions, deletions, and/or modifications to the embodiments of the disclosed subject matter without departing from the spirit of the invention or its scope, as defined by the appended claims.