Patent Publication Number: US-11642750-B2

Title: Automatic wet sanding apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2020-037963 filed on Mar. 5, 2020, incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an automatic wet sanding apparatus. In particular, the disclosure relates to measures for achieving high-accuracy automatic wet sanding while also enhancing the durability of an automatic wet sanding apparatus. 
     2. Description of Related Art 
     An automatic wet sanding apparatus has been hitherto known that performs automatic wet sanding on painted surfaces of vehicle bodies after completion of a painting process in an automobile production line, for example, as disclosed in Japanese Patent Application Publication No. 58-67377. 
     This automatic wet sanding apparatus includes an automatic wet sanding unit that is mounted on an automatic wet sanding robot (e.g., an articulated robot). The automatic wet sanding unit includes a sanding sliding body, such as a sanding brush or sandpaper. In an automatic wet sanding process, the sanding sliding body is pressed against a painted surface, and an automatic wet sanding robot is operated to move the sanding sliding body along the painted surface, with water flowing between the sanding sliding body and the painted surface, to sand down the painted surface. 
     SUMMARY 
     To give a quality finish to a painted surface by automatic wet sanding, it is necessary to adapt a sanding sliding body to the shape of the painted surface with high accuracy. Specifically, it is necessary to perform automatic wet sanding while changing the posture (direction) of the sanding sliding body according to changes in the curvature of the painted surface with high accuracy (e.g., changing the posture of the sanding sliding body so as to be orthogonal to a line normal to the painted surface), as well as maintaining the pressing force exerted by the sanding sliding body on the painted surface at an appropriate level. In particular, painted surfaces of vehicle bodies are often curved surfaces with non-constant curvature (formed by a collection of a plurality of curved surfaces), and a target value of a sanding depth by automatic wet sanding is a few micrometers. Thus, to give a quality finish to a painted surface, it is important to enhance the adaptability of the sanding sliding body to the shape of a painted surface. 
     As a configuration for adapting the sanding sliding body to the shape of a painted surface with high accuracy, it is conceivable to couple a piston rod of an air cylinder to the automatic wet sanding unit having the sanding sliding body mounted thereon, and by controlling this air cylinder, change the posture of the automatic wet sanding unit so as to adapt the sanding sliding body to the shape of the painted surface. 
     When thus using an air cylinder, one may conceive an idea of reducing the diameter of the piston rod to achieve higher adaptability. Specifically, reducing the diameter of the piston rod can increase the pressure of input air for control and enables high-accuracy pressure control. Further, as the area of contact between the piston rod and a part coming into sliding contact therewith (e.g., a seal packing) inside the air cylinder is reduced, the sliding resistance can be reduced. In addition, as the internal volume of the cylinder is reduced, the response speed in adaptation can be increased. Thus, higher adaptability of the sanding sliding body can be achieved. 
     On the other hand, reducing the diameter of the piston rod may cause a decrease in mechanical strength. A conceivable countermeasure is to provide a guide rod that is parallel to the piston rod. This guide rod is slidably inserted into a bush that is provided inside the air cylinder, and a leading end of the guide rod is coupled to the automatic wet sanding unit. As shown in  FIG.  15    (a view of a support structure for a guide rod a of an air cylinder as seen in a cross-section in a direction orthogonal to an extension direction of the guide rod a), a plurality of balls c is interposed between an outer circumferential surface of the guide rod a and an inner circumferential surface of a bush b to allow the guide rod a to slide (move forward and backward) smoothly along the bush b. 
     However, when the guide rod a is simply formed in a columnar shape, each of the balls c comes into point-contact with the outer circumferential surface of the guide rod a, so that locally high stress acts on the guide rod a. In particular, locally high stress may act on the guide rod a, and cause damage thereto, as a result of vibration. Thus, it has been difficult to make two objects compatible with each other: to achieve high-accuracy automatic wet sanding by enhancing the adaptability of the sanding sliding body to the shape of a painted surface through a reduction of the diameter of the piston rod; and to enhance the durability of the automatic wet sanding apparatus. 
     The present disclosure has been contrived in view of this issue, and an object thereof is to provide an automatic wet sanding apparatus that is capable of high-accuracy automatic wet sanding while also having high durability. 
     A solution adopted by the present disclosure to achieve the above object is premised on an automatic wet sanding apparatus that performs automatic wet sanding in which a sanding sliding body is pressed against a painted surface of a painted object that has been painted and the sanding sliding body is moved with water flowing between the sanding sliding body and the painted surface to sand down the painted surface. This automatic wet sanding apparatus includes an automatic wet sanding unit main body on which the sanding sliding body is mounted, and a unit support mechanism that supports the automatic wet sanding unit main body and includes an air cylinder that changes the posture of the automatic wet sanding unit main body. This automatic wet sanding apparatus further includes a guide rod that is slidably supported by a bush provided inside the air cylinder, extends toward the automatic wet sanding unit main body, and is coupled to the automatic wet sanding unit main body. An outer surface of the guide rod has grooves that extend along a shaft centerline of the guide rod and have an arc-shaped cross-section in a direction orthogonal to the shaft centerline, and balls that allow the guide rod to slide along the bush are interposed between a bottom of each of the grooves and an inner surface of the bush. 
     According to these specifications, automatic wet sanding of sanding down a painted surface of a painted object is performed by pressing the sanding sliding body against the painted surface and moving the sanding sliding body with water flowing between the sanding sliding body and the painted surface to sand down the painted surface. To adapt the sanding sliding body to the shape of the painted surface, the posture of the automatic wet sanding unit main body is changed by controlling the air cylinder that supports the automatic wet sanding unit main body. In the present disclosure, the air cylinder is provided with the guide rod. The presence of this guide rod makes it possible to reduce the diameter of the piston rod of the air cylinder without causing a decrease in the mechanical strength of the unit support mechanism. Further, the balls are interposed between the bottom of each groove (having an arc-shaped cross-section) formed in the outer surface of the guide rod and the inner surface of the bush to allow the guide rod to slide along the bush. Therefore, the balls can be brought into line-contact with the guide rod (the grooves of the guide rod), which can mitigate stress due to vibration. Thus, the present disclosure can make two objects compatible with each other: to achieve high-accuracy automatic wet sanding by enhancing the adaptability of the sanding sliding body to the shape of a painted surface through a reduction of the diameter of the piston rod; and to enhance the durability of the automatic wet sanding apparatus. 
     The guide rod may be provided on each side of a piston rod of the air cylinder in a direction orthogonal to an extension direction of the piston rod. 
     This configuration can give sufficient mechanical strength to the unit support mechanism that supports the automatic wet sanding unit main body, making it easy to reduce the diameter of the piston rod of the air cylinder. 
     A leading end of the guide rod may be coupled to a rod end mechanism that turnably supports the automatic wet sanding unit main body. The rod end mechanism may include a rod end to which the leading end of the guide rod is coupled and a bolt that is inserted into a center hole of the rod end and an opening formed in the automatic wet sanding unit main body, and may be configured such that the automatic wet sanding unit main body is supported so as to be able to rotate along with the bolt relatively to the rod end. An outer circumferential surface of the bolt may have, at least at a position corresponding to the center hole of the rod end, a recess that extends along a shaft centerline of the bolt. 
     In this configuration, the recess is formed in the outer circumferential surface of the bolt, which can reduce the area of contact between the center hole of the rod end and the outer circumferential surface of the bolt and thereby reduce the sliding resistance during turning of the automatic wet sanding unit main body. Thus, during automatic wet sanding, the posture of the automatic wet sanding unit can be quickly changed according to changes in the curvature of a painted surface so as to adapt the sanding sliding body to the shape of the painted surface. 
     In the present disclosure, the air cylinder that changes the posture of the automatic wet sanding unit main body having the sanding sliding body mounted thereon is provided with the guide rod. The outer surface of the guide rod has the grooves that extend along the shaft centerline of the guide rod and have an arc-shaped cross-section in a direction orthogonal to the shaft centerline, and the balls are interposed between the bottom of each groove and the inner surface of the bush provided inside the air cylinder. Thus, it is possible to make two objects compatible with each other: to achieve high-accuracy automatic wet sanding by enhancing the adaptability of the sanding sliding body to the shape of a painted surface through a reduction of the diameter of the piston rod; and to enhance the durability of the automatic wet sanding apparatus. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Features, advantages, and technical and industrial significance of exemplary embodiments of the disclosure will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein: 
         FIG.  1    is a schematic configuration view of an automatic wet sanding station in an embodiment; 
         FIG.  2    is a schematic configuration view showing a first automatic wet sanding apparatus; 
         FIG.  3    is a view showing an automatic wet sanding robot; 
         FIG.  4 A  is a vertical sectional view of an automatic wet sanding unit; 
         FIG.  4 B  is a schematic view showing a disc main body; 
         FIG.  5    is a perspective view showing part of a guide rod of an air cylinder; 
         FIG.  6    is a sectional view showing a support structure for the guide rod inside the air cylinder; 
         FIG.  7    is a sectional view showing a structure for supporting a unit main body by a rod end mechanism; 
         FIG.  8    is a schematic configuration view of a pad cleaning unit; 
         FIG.  9    is a schematic configuration view of a pad draining unit; 
         FIG.  10    is a schematic configuration view of a paper checking unit; 
         FIG.  11    is a block diagram illustrating a control system of the automatic wet sanding apparatus; 
         FIG.  12    is a flowchart illustrating an automatic wet sanding operation by the automatic wet sanding apparatus; 
         FIG.  13    is a sectional view illustrating flows of water in the automatic wet sanding unit in a state of performing automatic wet sanding; 
         FIG.  14    is a side view of a vehicle body illustrating moving paths of the automatic wet sanding unit in the automatic wet sanding operation; and 
         FIG.  15    is a sectional view showing a conventional support structure for a guide rod of an air cylinder. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     An embodiment of the present disclosure will be described below based on the drawings. In this embodiment, a case will be described where the disclosure is applied to an automatic wet sanding apparatus that is provided on an automobile production line and performs automatic wet sanding on painted surfaces of vehicle bodies. 
     Schematic Configuration of Automatic Wet Sanding Station 
     First, a schematic configuration of an automatic wet sanding station on an automobile production line in which automatic wet sanding apparatuses are installed will be described.  FIG.  1    is a schematic configuration view of an automatic wet sanding station  1  in this embodiment. The automatic wet sanding station  1  is installed on the automobile production line, on a downstream side of a painting station (not shown). 
     As shown in  FIG.  1   , the automatic wet sanding station  1  has a configuration in which four automatic wet sanding apparatuses  21 ,  22 ,  23 ,  24  are installed two on each side of a conveyor  11  that transfers vehicle bodies V. 
     When the vehicle body V is transferred as indicated by arrow A in  FIG.  1    (when the vehicle body V is transferred on the conveyor  11  from the left side toward the right side in  FIG.  1   ), the automatic wet sanding apparatuses  21 ,  22  located on a downstream side in the transfer direction perform automatic wet sanding on painted surfaces of front doors LFD, RFD and front fenders LFF, RFF of the vehicle body V. Specifically, the automatic wet sanding apparatus  21  (hereinafter referred to as a first automatic wet sanding apparatus  21 ) located on the left side as seen from the transfer direction (the upper side in  FIG.  1   ) performs automatic wet sanding on the painted surfaces of the left front door LFD and the left front fender LFF of the vehicle body V. The automatic wet sanding apparatus  22  (hereinafter referred to as a second automatic wet sanding apparatus  22 ) located on the right side as seen from the transfer direction (the lower side in  FIG.  1   ) performs automatic wet sanding on the painted surfaces of the right front door RFD and the right front fender RFF of the vehicle body V. 
     Meanwhile, the automatic wet sanding apparatuses  23 ,  24  located on an upstream side in the transfer direction perform automatic wet sanding on painted surfaces of rear doors LRD, RRD and rear fenders LRF, RRF of the vehicle body V. Specifically, the automatic wet sanding apparatus  23  (hereinafter referred to as a third automatic wet sanding apparatus  23 ) located on the left side as seen from the transfer direction performs automatic wet sanding on the painted surfaces of the left rear door LRD and the left rear fender LRF of the vehicle body V. The automatic wet sanding apparatus  24  (hereinafter referred to as a fourth automatic wet sanding apparatus  24 ) located on the right side as seen from the transfer direction performs automatic wet sanding on the painted surfaces of the right rear door RRD and the right rear fender RRF of the vehicle body V. 
     As the automatic wet sanding apparatuses  21  to  24  have the same configuration, the first automatic wet sanding apparatus  21  will be described here as a representative. In  FIG.  1   , those of the devices and members composing the automatic wet sanding apparatuses  21  to  24  that are the same are denoted by the same reference signs. 
       FIG.  2    is a schematic configuration view showing the first automatic wet sanding apparatus  21 . As shown in  FIG.  2   , the first automatic wet sanding apparatus  21  includes an automatic wet sanding robot  3  and a changer  4 . The automatic wet sanding robot  3  is formed by an articulated robot, and an automatic wet sanding unit  5  to be described later is mounted on the automatic wet sanding robot  3 . Automatic wet sanding is performed on the painted surfaces of the vehicle body V (in the case of the first automatic wet sanding apparatus  21 , the painted surfaces of the left front door LFD and the left front fender LFF) by the automatic wet sanding unit  5 . The changer  4  replaces sandpaper (the “sanding sliding body” as termed in the present disclosure) that is mounted on the automatic wet sanding unit  5 . In the following, the automatic wet sanding robot  3 , the automatic wet sanding unit  5 , and the changer  4  will be specifically described. 
     Automatic Wet Sanding Robot 
     As shown in  FIG.  3   , the automatic wet sanding robot  3  is formed by an articulated robot. Specifically, the automatic wet sanding robot  3  in this embodiment includes a swivel base  30 , and first to fifth arms  31 ,  32 ,  33 ,  34 ,  35  that are coupled to one another by joints or the like. 
     A rotating mechanism (including a motor) that can rotate around a vertical axis is housed inside the swivel base  30 . A rotating mechanism that can rotate around a horizontal axis is housed at each joint. The swivel base  30  and the first arm  31 , the first arm  31  and the second arm  32 , and the third arm  33  and the fourth arm  34  are coupled to each other by a joint having a rotating mechanism that arrows the arms  31 ,  32 ,  33 ,  34  to turn relatively. The second arm  32  and the third arm  33 , and the fourth arm  34  and the fifth arm  35  are coupled to each other by a rotating mechanism that can rotate relatively around an axis along an extension direction of the arm. Rotational motion of these rotating mechanisms causes the swivel base  30  to rotate or the arms  31  to  35  to swing or rotate, which can in turn move the automatic wet sanding unit  5  to an arbitrary position or change the posture thereof to an arbitrary posture. Rotational motion of each rotating mechanism is performed based on a command signal from a robot controller  83  (see  FIG.  11   ) to be described later. 
     The automatic wet sanding unit  5  is mounted at a leading end of the fifth arm  35 . Specifically, the automatic wet sanding unit  5  is mounted on a frame  36  that is mounted at the leading end of the fifth arm  35 . 
     The configuration of the automatic wet sanding robot  3  is not limited to the above-described one. 
     Automatic Wet Sanding Unit 
     Next, the automatic wet sanding unit  5  will be described.  FIG.  4 A  is a vertical sectional view of the automatic wet sanding unit  5 .  FIG.  4 B  is a schematic view showing a disc main body  54   a  to be described later (a schematic view of the disc main body  54   a  as seen from a direction along a central axis thereof). The vertical sectional view of  FIG.  4 A  shows a section located at a position corresponding to line IV-IV in  FIG.  4 B . 
     The posture of the automatic wet sanding unit  5  (the automatic wet sanding unit  5  in the first automatic wet sanding apparatus  21 ) shown in  FIG.  4 A  is a posture in which the sandpaper  56  mounted on the automatic wet sanding unit  5  faces downward. When automatic wet sanding is being performed, the automatic wet sanding unit  5  is in a posture in which the sandpaper  56  faces the painted surface (the surface extending in a substantially vertical direction) of the left front door LFD or the left front fender LFF of the vehicle body V as shown in  FIG.  3   , i.e., a posture to which the automatic wet sanding unit  5  turns about 90° from the posture shown in  FIG.  4 A  so as to face the vehicle body V. Therefore, when automatic wet sanding is being performed, a downward direction in  FIG.  4 A  is a direction facing the vehicle body and an upward direction in  FIG.  4 A  is a direction facing the opposite side from the vehicle body. In the following description of the automatic wet sanding unit  5  using  FIGS.  4 A and  4 B , a state where the automatic wet sanding unit  5  is in the posture shown in  FIG.  4 A  (the posture in which the sandpaper  56  faces downward) will be taken as an example. 
     As shown in  FIG.  4 A , the automatic wet sanding unit  5  includes a unit main body (automatic wet sanding unit main body)  5 A and a unit support mechanism  5 B that is mounted on the frame  36 . Thus, the unit main body  5 A is supported by the automatic wet sanding robot  3  through the unit support mechanism  5 B and the frame  36  (more specifically, supported at the leading end of the fifth arm  35  of the automatic wet sanding robot  3  through the unit support mechanism  5 B and the frame  36 ). 
     Unit Main Body 
     The unit main body  5 A includes an air motor  50 , a skirt  51 , a water supply pipe  52 , an eccentric head  53 , a disc  54 , a cushion pad  55 , sandpaper (the “sanding sliding body” as termed in the present disclosure)  56 , a hood  57 , a water deflecting member  58 , and a seal member  59 . 
     Air Motor 
     The air motor  50  includes a driving shaft  50   a  that extends downward in the posture shown in  FIG.  4 A . An air supply pipe (not shown) is connected to the air motor  50 , and the driving shaft  50   a  is rotated by the pressure of air supplied through the air supply pipe as an air pump (not shown) is activated. Long dashed short dashed line O 1  in  FIGS.  4 A and  4 B  indicates the center of rotation of the driving shaft  50   a.    
     Skirt 
     The skirt  51  is integrally mounted on a casing  50   b  of the air motor  50 , and an inside of the skirt  51  forms an introduction space  51   a  into which water for automatic wet sanding is introduced. Specifically, the skirt  51  includes a cylindrical mounting part  51   b , a skirt main part  51   c  of which the diameter increases from a lower end edge of the mounting part  51   b  toward a lower side, and a hood mounting part  51   d  that extends cylindrically from a lower end edge of the skirt main part  51   c  toward the lower side. 
     The inside diameter of the mounting part  51   b  is substantially equal to the outside diameter of the casing  50   b  of the air motor  50 . An inner circumferential surface of the mounting part  51   b  is joined to an outer circumferential surface of the casing  50   b  of the air motor  50 . Thus, the skirt  51  is supported by the air motor  50 . Since the diameter of the skirt main part  51   c  increases toward the lower side as mentioned above, the inside diameter of the introduction space  51   a  inside the skirt main part  51   c  also increases toward the lower side. The hood mounting part  51   d  has an annular engaging groove  51   e  that is depressed toward an upper side by a predetermined dimension from a lower end surface of the hood mounting part  51   d . The engaging groove  51   e  is used to fix the hood  57  and the seal member  59  to be described later. 
     Water Supply Pipe 
     The water supply pipe  52  supplies water for automatic wet sanding into the introduction space  51   a  of the skirt  51 . The water supply pipe  52  is connected at an upstream end to a water pump  52   a  (see  FIG.  11   ) and at a downstream end to the skirt main part  51   c  of the skirt  51 , and supplies water for automatic wet sanding into the introduction space  51   a  of the skirt  51  as the water pump  52   a  is activated. 
     Eccentric Head 
     The eccentric head  53  is integrated with the driving shaft  50   a  of the air motor  50 , and is formed so as to have its center offset from the center of rotation O 1  of the driving shaft  50   a .  FIGS.  4 A and  4 B  shows a state where the center of the eccentric head  53  is offset toward the left side in  FIGS.  4 A and  4 B . As indicated by the imaginary line in  FIG.  4 B , the eccentric head  53  is formed by a substantially elliptical disc, and a position in the eccentric head  53  that is located off the center position of the ellipse (in  FIG.  4 B , an off-center position on the right side) is located on the center of rotation O 1  of the driving shaft  50   a . Therefore, when the driving shaft  50   a  rotates (around the center of rotation O 1 ) as the air motor  50  is activated, the eccentric head  53  rotates eccentrically around the center of rotation O 1 . Imaginary line B in  FIG.  4 B  indicates a trajectory of movement of an outer end of the eccentric head  53  (a position at an outer edge thereof on the offset side; point C in  FIG.  4 B ) when the eccentric head  53  rotates eccentrically. As this imaginary line B shows, the outer end (the position at the outer edge on the offset side) of the eccentric head  53  is located on an inner circumferential side relative to outer circumferential ends of disc holes  54   e  to be described later. 
     Disc 
     The disc  54  is composed of a disc main body  54   a  and a disc cover  54   b  that are integrally combined. 
     The disc main body  54   a  is formed by a metal disc that has a larger diameter than the hood mounting part  51   d  of the skirt  51 . An outer circumferential surface  54   c  of the disc main body  54   a  is formed by a sloping surface of which the diameter increases downward. 
     As shown in  FIG.  4 B , the disc main body  54   a  has a disc center hole  54   d , the disc holes  54   e , and communication passages  54   f.    
     The disc center hole  54   d  is formed by a circular opening that is bored at a central portion of the disc main body  54   a . The disc center hole  54   d  extends from an upper surface to a lower surface of the disc main body  54   a.    
     The disc holes  54   e  are formed at three positions on an outer circumferential side, each at a predetermined distance from the center of the disc main body  54   a . The disc holes  54   e  also extend from the upper surface to the lower surface of the disc main body  54   a . The disc holes  54   e  are disposed at positions at regular angular intervals in a circumferential direction (positions at 120° angular intervals). 
     The communication passages  54   f  allow communication between the disc center hole  54   d  and the disc holes  54   e . Specifically, the communication passages  54   f  extend radially from the center of the disc main body  54   a  and each communicate at an inner end with the disc center hole  54   d  and at an outer end with the disc hole  54   e . The communication passages  54   f  also extend from the upper surface to the lower surface of the disc main body  54   a.    
     The disc cover  54   b  is formed by a metal disc that has an outside diameter substantially equal to the outside diameter of the upper surface of the disc main body  54   a . The disc cover  54   b  has a bearing part  54   g  which is a part provided at a central portion and at which the plate thickness of the disc cover  54   b  is increased. The bearing part  54   g  and the eccentric head  53  are connected to each other by a bearing  53   a . Thus, the disc cover  54   b  is rotatably supported by the eccentric head  53 . The disc cover  54   b  is rotatably supported by the eccentric head  53 , for example, as an inner race of the bearing  53   a  is coupled to the eccentric head  53  while an outer race of the bearing  53   a  is coupled to the bearing part  54   g  of the disc cover  54   b.    
     Further, the disc cover  54   b  has openings  54   h  at positions corresponding to the disc holes  54   e  of the disc main body  54   a . The inside diameter of the opening  54   h  is substantially equal to the inside diameter of the disc hole  54   e . With the positions of the openings  54   h  coinciding with the positions of the disc holes  54   e , the disc cover  54   b  is joined to the upper surface of the disc main body  54   a  by means such as screw fastening or welding. This means that the disc center hole  54   d  and the communication passages  54   f  are closed at an upper side by the disc cover  54   b . Thus, in the disc  54 , a water channel  54   i  is formed that continues through the openings  54   h  of the disc cover  54   b  and the disc holes  54   e , the communication passages  54   f , and the disc center hole  54   d  of the disc main body  54   a . Since the disc cover  54   b  is joined to the upper surface of the disc main body  54   a  as mentioned above, the entire disc  54  is rotatably supported by the eccentric head  53  through the bearing  53   a.    
     The center position of the disc main body  54   a , the center position of the disc cover  54   b , the center position of the disc center hole  54   d , and the center of rotation of the bearing  53   a  are located on the same axis (see O 2  in  FIGS.  4 A and  4 B ). In  FIG.  4 B , the positions of the disc  54  when the disc  54  rotates around the center position O 2  by 90° at a time are indicated by the solid line, the dashed line, the long dashed short dashed line, and the long dashed double-short dashed line, respectively. The dimension of offset of the center position O 2  of the disc center hole  54   d  (the center position of the disc  54 ) relative to the center of rotation O 1  of the driving shaft  50   a  of the air motor  50  is set to be smaller than half the inside diameter of the disc center hole  54   d.    
     Cushion Pad 
     The cushion pad  55  is integrally mounted on the lower surface of the disc  54 . The cushion pad  55  is formed by a cushion member made of sponge or the like and has a form of a disc of which the outside diameter is substantially equal to the outside diameter of the disc main body  54   a . An outer circumferential surface  55   a  of the cushion pad  55  is formed by a sloping surface of which the diameter decreases toward the lower side. 
     As shown in  FIG.  4 A , the cushion pad  55  has, at a central portion thereof, a pad center hole  55   b  that is formed by a circular opening. The pad center hole  55   b  extends from an upper surface to a lower surface of the cushion pad  55 . The center position of the pad center hole  55   b  coincides with the center position of the disc center hole  54   d . Thus, the pad center hole  55   b  communicates with the water channel  54   i  formed in the disc  54 . The inside diameter of the pad center hole  55   b  is slightly larger than the inside diameter of the disc center hole  54   d.    
     Sandpaper 
     The sandpaper  56  is detachably mounted on the lower surface of the cushion pad  55 . Specifically, a lower surface  56   a  (a surface that faces the vehicle body V during automatic wet sanding) of the sandpaper  56  is a sanding surface. For example, this sanding surface is composed of resin. On the other hand, an upper surface  56   b  (a surface mounted to the lower surface of the cushion pad  55 ) is mounted to the lower surface of the cushion pad  55  by a touch-and-close fastener, such as Magictape®. 
     The sandpaper  56  has, at a central portion thereof, a paper center hole  56   c  that is formed by a circular opening. In a state where the sandpaper  56  is mounted at a correct position on the lower surface of the cushion pad  55 , the center position of the paper center hole  56   c  coincides with the center position of the pad center hole  55   b . The inside diameter of the paper center hole  56   c  may be set to be equal to the inside diameter of the pad center hole  55   b  or slightly larger than the inside diameter of the pad center hole  55   b.    
     Hood 
     The hood  57  is a member that is mounted at a lower end of the skirt  51  and prevents scattering of water that is released toward an outer periphery of the disc  54  after being introduced into the introduction space  51   a  of the skirt  51 . (This release of water will be described later.) Specifically, the hood  57  includes a cylindrical mounting part  57   a , a hood main part  57   b  of which the diameter increases from a lower end edge of the mounting part  57   a  toward the lower side, and a water deflecting part  57   c  that extends obliquely downward from a lower end edge of the hood main part  57   b.    
     The diameter of the mounting part  57   a  is substantially equal to the diameter of the engaging groove  51   e  formed in the skirt  51 . As the mounting part  57   a  is inserted into the engaging groove  51   e , the hood  57  is supported by the skirt  51 . 
     The outside diameter of the hood main part  57   b  is set to be slightly larger than the outside diameter of the disc  54 . 
     The water deflecting part  57   c  is formed by a part that is slightly bent downward from an outer circumferential end of the hood main part  57   b.    
     Water Deflecting Member 
     The water deflecting member  58  is mounted on the water deflecting part  57   c  of the hood  57  and formed by an annular rubber member that slopes toward an inner circumferential side (such that the diameter decreases) while extending downward from a lower end edge of the water deflecting part  57   c . The water deflecting member  58  is mounted to the water deflecting part  57   c  by means such as bonding or screw fastening. 
     Seal Member 
     Like the hood  57 , the seal member  59  is mounted at a lower end of the skirt  51 . Specifically, the seal member  59  is formed by a flat cylindrical member made of urethane. The diameter of the seal member  59  is substantially equal to the diameter of the engaging groove  51   e  formed in the skirt  51 . The seal member  59  is supported by the skirt  51  as an upper end portion of the seal member  59  is inserted into the engaging groove  51   e  while being overlapped with the mounting part  57   a  of the hood  57 . 
     The height of the seal member  59  is substantially equal to the dimension of a clearance between a ceiling part inside the engaging groove  51   e  and the upper surface of the disc  54 . Therefore, when no external pressure (e.g., water pressure) is acting on the seal member  59 , a lower end of the seal member  59  is in contact with the upper surface of the disc  54  along an entire circumference of the seal member  59  (without clearance) as shown in  FIG.  4 A . Thus, the introduction space  51   a  of the skirt  51  can be turned into a substantially sealed space. When a water pressure acts on an inner side of the seal member  59  and this water pressure exceeds a predetermined value, the seal member  59  deforms elastically and a small clearance is formed between the lower end of the seal member  59  and the upper surface of the disc  54 , and water flows through this clearance. 
     Unit Support Mechanism 
     Next, the unit support mechanism  5 B will be described. As mentioned above, the unit support mechanism  5 B is a mechanism that supports the unit main body  5 A onto the automatic wet sanding robot  3  through the frame  36 . 
     As shown in  FIG.  3    and  FIGS.  4 A and  4 B , the unit support mechanism  5 B includes a pair of air cylinders  60 . As shown in  FIG.  3   , the air cylinders  60  are respectively mounted on both side surfaces (an upper surface and a lower surface in  FIG.  3   ) of the frame  36 . From the air cylinders  60 , one piston rod  61 A and two guide rods  61 B (see  FIG.  2   ) protrude so as to be able to move forward and backward. Specifically, the guide rods  61 B are provided one on each side of the piston rod  61 A (each side in a direction orthogonal to an extension direction of the piston rod  61 A). The configuration of the guide rods  61 B and a support structure therefor will be described later. 
     The automatic wet sanding unit  5  includes a unit case  5 C (see the imaginary line in  FIG.  4 A ) that covers an outer side of the air motor  50  and the skirt  51 . As shown in  FIG.  4 A , lower ends of the piston rod  61 A and the guide rods  61 B are connected to support blocks  62 . One coupling rod  63  extends from a lower surface of each support block  62 . A rod end mechanism  5 D is provided at a lower end of the coupling rod  63 , and the unit main body  5 A is supported by the rod end mechanism  5 D so as to be able to turn around a horizontal axis. The specific configuration of the rod end mechanism  5 D will also be described later. 
     Configuration of Guide Rod and Support Structure therefor 
     Next, the configuration of the guide rods  61 B and the support structure therefor that are the feature of this embodiment will be described. 
       FIG.  5    is a perspective view showing part of the guide rod  61 B of the air cylinder  60 .  FIG.  6    is a sectional view showing the support structure for the guide rod  61 B inside the air cylinder  60  (a view of a cross-section in a direction orthogonal to an extension direction of the guide rod  61 B). 
     As shown in  FIG.  6   , the guide rod  61 B is slidably supported by a bush  60   a  that is provided inside the air cylinder  60 . The bush  60   a  is a cylindrical member and fixed inside the air cylinder  60 . The inside diameter of the bush  60   a  is set to be slightly larger than the outside diameter of the guide rod  61 B. An inner surface of the bush  60   a  has grooves  60   b  that extend along a shaft centerline of the bush  60   a  and have an arc-shaped cross-section. The grooves  60   b  are formed at four positions in a circumferential direction of the bush  60   a . For example, the grooves  60   b  are formed at positions spaced apart at 90° intervals in the circumferential direction. 
     An outer surface (outer circumferential surface) of the guide rod  61 B has grooves  61   a  that extend along a shaft centerline of the guide rod  61 B and have an arc-shaped cross-section in a direction orthogonal to the shaft centerline. The grooves  61   a  are formed at four positions in a circumferential direction of the guide rod  61 B. For example, the grooves  61   a  are formed at positions spaced apart at 90° intervals in the circumferential direction. 
     The guide rod  61 B is inserted into the bush  60   a  such that the grooves  60   b  formed in the inner surface of the bush  60   a  and the grooves  61   a  formed in the outer surface of the guide rod  61 B face each other, and spherical metal balls  61   b  are interposed between the grooves  60   b ,  61   a . While this is not shown, a plurality of balls  61   b  is arrayed in each pair of grooves  60   b ,  61   a  along an extension direction thereof. A bottom plate of the air cylinder  60  has an opening (not shown) of a shape corresponding to the outer shape of the guide rod  61 B, so that the balls  61   b  do not fall from the air cylinder  60  while the guide rod  61 B is allowed to slide. 
     In the above configuration, the guide rod  61 B is supported by the bush  60   a  so as to be able to slide along the extension direction of the guide rod  61 B. The radii of curvature of the grooves  60   b ,  61   a  and the radius of the balls  61   b  are substantially equal. In this configuration, therefore, an outer surface of each ball  61   b  is in line-contact with each of the groove  60   b  of the bush  60   a  and the groove  61   a  of the guide rod  61 B. 
     Configuration of Rod End Mechanism 
     Next, the configuration of the rod end mechanism  5 D will be described. 
       FIG.  7    is a sectional view taken along line VII-VII in  FIG.  4 A , showing the structure for supporting the unit main body  5 A by the rod end mechanism  5 D. As shown in  FIG.  4 A,  4 B  and  FIG.  7   , the rod end mechanism  5 D includes a rod end  64 , a bearing member  67 , and a bearing bolt  66 . 
     The rod end  64  has a cylindrical shape, and the coupling rod  63  is coupled to an upper part of the rod end  64 . The rod end  64  has, at a central portion thereof, a bolt insertion hole  64   a  that extends through the rod end  64  in a horizontal direction. The bearing member (so-called bearing metal)  67  is provided along an inner surface of the bolt insertion hole  64   a . The outside diameter of the bearing member  67  is substantially equal to the inside diameter of the bolt insertion hole  64   a , and the inside diameter of the bearing member  67  is substantially equal to the outside diameter of a threaded part  66   a  of the bearing bolt  66 . 
     A fastening nut  65  is mounted on an outer surface of the unit case  5 C, at a position at which the fastening nut  65  faces the rod end  64  (see  FIGS.  4 A and  4 B ). The bearing bolt  66  is screwed from outside into the bearing member  67  and a screw hole  65   a  of the fastening nut  65 , and the unit case  5 C is thereby turnably supported by the rod end  64 . Thus, during automatic wet sanding, turning the unit case  5 C relatively to the rod ends  64  can turn the entire automatic wet sanding unit  5  and thereby deflect the directions of the disc  54  and the cushion pad  55  to directions along the painted surface of the vehicle body V. As a result, a large area of the sanding surface (lower surface)  56   a  of the sandpaper  56  can be brought into contact with the painted surface of the vehicle body V. While the entire automatic wet sanding unit  5  is thus turned, an inner surface of the bearing member  67  and an outer surface of the threaded part  66   a  of the bearing bolt  66  turn relatively to each other. 
     The rod end mechanism  5 D features recesses  66   b  that are formed in an outer circumferential surface of the threaded part  66   a  of the bearing bolt  66  so as to extend along a shaft centerline of the bearing bolt  66 . As shown in  FIG.  7   , in the outer circumferential surface of the threaded part  66   a  of the bearing bolt  66 , the recesses  66   b  extend along the shaft centerline of the bearing bolt  66  and have an arc-shaped cross-section in a direction orthogonal to the shaft centerline. The recesses  66   b  are formed at eight positions in a circumferential direction of the threaded part  66   a  of the bearing bolt  66 . For example, the recesses  66   b  are formed at positions spaced apart at 45° intervals in the circumferential direction. When the recesses  66   b  are formed, the area of contact between the outer circumferential surface of the threaded part  66   a  of the bearing bolt  66  and the inner circumferential surface of the bearing member  67  becomes smaller than when these recesses are not formed. 
     Changer 
     Next, the changer  4  will be described. As shown in  FIG.  2   , the changer  4  includes a paper peeling unit  41 , a pad cleaning unit  42 , a pad draining unit  43 , a paper mounting unit  44 , and a paper checking unit  45 . 
     Paper Peeling Unit 
     The paper peeling unit  41  peels (removes) the sandpaper  56  of the automatic wet sanding unit  5  from the cushion pad  55  upon completion of automatic wet sanding. If automatic wet sanding is performed on a plurality of vehicle bodies V using the same sandpaper  56  (without replacing the sandpaper  56 ), the sanding efficiency may decrease or paint of the vehicle body V that has previously undergone automatic wet sanding may transfer onto the subsequent vehicle body V. To avoid such a situation, the sandpaper  56  is replaced each time automatic wet sanding on one vehicle body V is completed. The paper peeling unit  41  performs a step of peeling the sandpaper  56  from the cushion pad  55  to replace the sandpaper  56 . 
     The paper peeling unit  41  includes a clamping shaft  41   a  and a clamping hook  41   b . The clamping shaft  41   a  is formed by a metal shaft that is supported by a frame  41   c  so as to be able to rotate around a horizontal axis. The clamping shaft  41   a  is coupled to a clamping shaft motor  41   d  and configured to be able to rotate as the clamping shaft motor  41   d  is activated. The clamping hook  41   b  is provided above and close to the clamping shaft  41   a . Thus, the clamping hook  41   b  can catch the sandpaper  56  between the clamping hook  41   b  and the clamping shaft  41   a.    
     A sandpaper collection box  41   e  is installed under the clamping shaft  41   a , and the sandpaper  56  peeled from the cushion pad  55  drops into the sandpaper collection box  41   e  to be collected. 
     Pad Cleaning Unit 
     The pad cleaning unit  42  cleans the cushion pad  55  from which the sandpaper  56  has been peeled by the paper peeling unit  41 . After automatic wet sanding, paint (paint separated from the vehicle body V by sanding; sanding dust) adheres to the sandpaper  56  and the cushion pad  55 . Therefore, even when the sandpaper  56  is replaced, if automatic wet sanding is performed on the subsequent vehicle body V without cleaning the cushion pad  55 , the paint may transfer onto the vehicle body V. The pad cleaning unit  42  is installed to avoid such a situation. 
     As shown in  FIG.  8   , the pad cleaning unit  42  includes a cleaning tank  42   a , a water supply pipe  42   b , and a circulating circuit  42   c . The cleaning tank  42   a  has an inside diameter that is larger than the outside diameter of the automatic wet sanding unit  5 . A metal mesh  42   d  extending in a horizontal direction is provided inside the cleaning tank  42   a , at an intermediate point in a vertical direction (depth direction). 
     The water supply pipe  42   b  is connected at an upstream end to a water supply pump  42   j  (see  FIG.  11   ) and at a downstream end to the cleaning tank  42   a , and supplies cleaning water (pure water) to the cleaning tank  42   a  as the water supply pump  42   j  is activated. A valve  42   e  for regulating water supply is provided on the water supply pipe  42   b.    
     The circulating circuit  42   c  has a configuration in which a circulating pump  42   g  and a filter  42   h  are provided on the route of a circulating pipe  42   f  The circulating pipe  42   f  is connected at one end (upstream end) to a bottom of the cleaning tank  42   a  and at the other end (downstream end) to a side surface of the cleaning tank  42   a . During cleaning of a pad, water circulating action is performed in which the circulating pump  42   g  is activated to extract water from the bottom of the cleaning tank  42   a  and this water is purified by the filter  42   h  and then returned to the cleaning tank  42   a  through the side surface. A drain valve  42   i  is connected to the filter  42   h . The drain valve  42   i  is opened to discharge water from the cleaning tank  42   a.    
     Pad Draining Unit 
     The pad draining unit  43  drains the cushion pad  55  that has been cleaned by the pad cleaning unit  42 . 
     As shown in  FIG.  9   , the pad draining unit  43  includes a draining table  43   a  and an air blow nozzle  43   b . The draining table  43   a  is composed of a rack frame  43   c  and a mesh-like inclined plate  43   d  mounted thereon. To drain the cushion pad  55 , the automatic wet sanding robot  3  is operated to press the cushion pad  55  against the inclined plate  43   d  of the draining table  43   a , and water is thereby squeezed out from the cushion pad  55 . During draining, air is blown from the air blow nozzle  43   b  toward the cushion pad  55  to increase the draining efficiency. An air blow motor  43   e  (see  FIG.  11   ) is connected to the air blow nozzle  43   b.    
     The cushion pad  55  may be pressed against the inclined plate  43   d  of the draining table  43   a  such that the entire cushion pad  55  is evenly pressed against the inclined plate  43   d . However, it is preferable that the position at which the cushion pad  55  is pressed against the inclined plate  43   d  be changed in a circumferential direction of the cushion pad  55 , as it can further increase the draining efficiency. Specifically, the position at which the cushion pad  55  is pressed against the inclined plate  43   d  is changed in the circumferential direction by moving the center line O 2  (center positions) of the disc  54  and the cushion pad  55  as indicated by the arrows in  FIG.  9   . 
     Paper Mounting Unit 
     The paper mounting unit  44  mounts new sandpaper  56  onto the cushion pad  55  that has been drained by the pad draining unit  43 . 
     As shown in  FIG.  2   , the paper mounting unit  44  includes a paper stand  44   a  and a paper pressing plate  44   b . A plurality of sheets of unused sandpaper  56  is placed on top of one another on the paper stand  44   a . Each sheet of sandpaper  56  is placed on the paper stand  44   a  in such a manner that the surface having a touch-and-close fastener to be mounted to the cushion pad  55  faces upward. 
     An air cylinder  44   c  is connected to the paper pressing plate  44   b . The air cylinder  44   c  is activated to move the paper pressing plate  44   b  between a position at which the paper pressing plate  44   b  presses the upper side of the sandpaper  56  and a position at which the paper pressing plate  44   b  has receded from the sandpaper  56 . The paper pressing plate  44   b  has a U-shaped cutout  44   d , and when the paper pressing plate  44   b  is located at the position at which the paper pressing plate  44   b  presses the upper side of the sandpaper  56  as shown in  FIG.  2   , part of the touch-and-close fastener of the sandpaper  56  is exposed upward. In this state, the cushion pad  55  is pressed against the upper surface of the sandpaper  56 , and then the paper pressing plate  44   b  recedes from the sandpaper  56 , so that the entire touch-and-close fastener of the sandpaper  56  is mounted to the cushion pad  55 . 
     Paper Checking Unit 
     In a state where the sandpaper  56  has been mounted on the cushion pad  55  by the paper mounting unit  44 , the paper checking unit  45  checks whether or not the mounting position of the sandpaper  56  is the correct position. 
     As shown in  FIG.  10   , the paper checking unit  45  includes a stand  45   a  and a camera  45   b . The stand  45   a  includes a pair of plates  45   c  (see  FIG.  2   ) disposed at an interval that is substantially equal to the outside diameter of the cushion pad  55 , and a positioning plate  45   d  that couples the plates  45   c  together at ends on one side. The camera  45   b  is disposed under the stand  45   a  and takes an image of the cushion pad  55  (with the sandpaper  56  mounted thereon) placed on the stand  45   a . The posture of the camera  45   b  is set such that the center line O 2  of the cushion pad  55  in a state of being placed on the stand  45   a  and a center line of the camera  45   b  coincide with each other. Whether or not the mounting position of the sandpaper  56  is the correct position is checked by using data of the image of the cushion pad  55  and the sandpaper  56  taken by the camera  45   b.    
     Control System 
     Next, a control system of the automatic wet sanding apparatuses  21  to  24  will be described.  FIG.  11    is a block diagram illustrating the control system of the automatic wet sanding apparatuses  21  to  24 . 
     As shown in  FIG.  11   , the control system of the automatic wet sanding apparatuses  21  to  24  has a configuration in which a starting switch  81 , a conveyor controller  82 , the robot controller  83 , an automatic wet sanding unit controller  84 , and a changer controller  85  are electrically connected to a central processing unit  8  that comprehensively controls the automatic wet sanding apparatuses  21  to  24 , such that various signals including command signals can be sent and received between the central processing unit  8  and these components. 
     The starting switch  81  sends a command signal for starting the automatic wet sanding apparatuses  21  to  24  to the central processing unit  8  according to operation by a worker. When this start command signal is received, the automatic wet sanding apparatuses  21  to  24  are started (activated) to start an automatic wet sanding operation to be described later. 
     The conveyor controller  82  controls transfer of the vehicle body V by the conveyor  11 . Specifically, the conveyor controller  82  operates the conveyor  11  until the vehicle body V that is an object of automatic wet sanding reaches a predetermined position (the position shown in  FIG.  1   ) in the automatic wet sanding station  1 , and temporarily stops the conveyor  11  at that point. When a predetermined time has elapsed after completion of automatic wet sanding by the automatic wet sanding apparatuses  21  to  24 , the conveyor controller  82  operates the conveyor  11  again to transfer the vehicle body V having undergone automatic wet sanding to the next station, and operates the conveyor  11  until the vehicle body V that is the next object of automatic wet sanding reaches the predetermined position in the automatic wet sanding station  1 . 
     The robot controller  83  controls the automatic wet sanding robots  3  of the respective automatic wet sanding apparatuses  21  to  24 . The robot controller  83  sends command signals to various motors M that are provided in the rotating mechanisms of each automatic wet sanding robot  3  according to information on teaching that is performed on the automatic wet sanding robot  3  in advance. Thus, the robot controller  83  controls the position of the automatic wet sanding unit  5  based on the teaching information. 
     The automatic wet sanding unit controller  84  controls the automatic wet sanding unit  5 . The water pump  52   a , the air motor  50 , and the air cylinders  60  are connected to the automatic wet sanding unit controller  84 . 
     The water pump  52   a  is activated in accordance with a command signal from the automatic wet sanding unit controller  84  and supplies water for automatic wet sanding to the introduction space  51   a  of the skirt  51  through the water supply pipe  52 . The air motor  50  is activated in accordance with a command signal from the automatic wet sanding unit controller  84  and rotates the driving shaft  50   a . The air cylinders  60  are activated in accordance with a command signal from the automatic wet sanding unit controller  84  and move the piston rods  61 A forward and backward. Thus, the automatic wet sanding unit  5  is moved forward and backward and the posture thereof is changed. 
     The changer controller  85  controls the units  41  to  45  of the changer  4 . The clamping shaft motor  41   d , the water supply pump  42   j , the circulating pump  42   g , the drain valve  42   i , the air blow motor  43   e , the air cylinder  44   c , and the camera  45   b  are connected to the changer controller  85 . 
     In the step of peeling the sandpaper  56  from the cushion pad  55  by the paper peeling unit  41 , the clamping shaft motor  41   d  is activated by a command signal from the changer controller  85  and rotates the clamping shaft  41   a . In the step of cleaning the cushion pad  55  by the pad cleaning unit  42 , a water supplying action by the water supply pump  42   j , a water circulating action by the circulating pump  42   g , and a water discharging action by the drain valve  42   i  are performed in accordance with command signals from the changer controller  85 . In the step of draining the cushion pad  55  by the pad draining unit  43 , the air blow motor  43   e  is activated by a command signal from the changer controller  85  and blows air toward the cushion pad  55 . In the step of mounting the sandpaper  56  onto the cushion pad  55  by the paper mounting unit  44 , the air cylinder  44   c  is activated by a command signal from the changer controller  85  and the paper pressing plate  44   b  is moved between the position at which the paper pressing plate  44   b  presses the upper side of the sandpaper  56  and the position at which the paper pressing plate  44   b  has receded from the sandpaper  56 . 
     The changer controller  85  receives imaging data (data of an image of the cushion pad  55  with the sandpaper  56  mounted thereon) from the camera  45   b  provided in the paper checking unit  45  and determines whether or not the sandpaper  56  is mounted at the correct position. 
     Automatic Wet Sanding Operation 
     Next, the automatic wet sanding operation of the vehicle body V in the automatic wet sanding station  1  configured as described above will be described. 
       FIG.  12    is a flowchart illustrating the automatic wet sanding operation by the first automatic wet sanding apparatus  21 . The same automatic wet sanding operation is concurrently performed in the other automatic wet sanding apparatuses  22  to  24 . 
     As shown in  FIG.  12   , in the automatic wet sanding operation by the first automatic wet sanding apparatus  21 , the following steps are sequentially performed after “carrying in vehicle body”: a pad wetting step, front door automatic wet sanding step, front fender automatic wet sanding step, starting to carry out vehicle body, paper peeling step, pad cleaning step, pad draining step, paper mounting step, and paper checking step. 
     Carrying in Vehicle Body 
     In the step of carrying in the vehicle body, the conveyor  11  is activated by a command signal from the conveyor controller  82 , and the vehicle body V that is an object of automatic wet sanding is transferred to the predetermined position (the position shown in  FIG.  1   ) in the automatic wet sanding station  1 . Then, the conveyor  11  stops. The conveyor  11  is kept in the stopped state until a predetermined time elapses that is when automatic wet sanding by each of the automatic wet sanding apparatuses  21  to  24  is completed. 
     Pad Wetting Step 
     In the pad wetting step, the automatic wet sanding robot  3  is operated by a command signal from the robot controller  83 , and the automatic wet sanding unit  5  is immersed in water stored in the cleaning tank  42   a  of the pad cleaning unit  42 . Specifically, the water supply pump  42   j  is activated by a command signal from the changer controller  85  and water is supplied to the cleaning tank  42   a , and with the water thus stored in the cleaning tank  42   a , the automatic wet sanding unit  5  is immersed in the water inside the cleaning tank  42   a . In this way, the sandpaper  56  and the cushion pad  55  are wetted before the automatic wet sanding process is started. 
     Front Door Automatic Wet Sanding Step 
     In the front door automatic wet sanding step, the automatic wet sanding robot  3  is operated to move the automatic wet sanding unit  5  to a position at which it faces the front door (in the case of the first automatic wet sanding apparatus  21 , the left front door LFD) (see  FIG.  3   ). Then, the automatic wet sanding unit  5  is activated by a command signal from the automatic wet sanding unit controller  84 . 
     Specifically, the water pump  52   a  is activated to supply water for automatic wet sanding to the introduction space  51   a  of the skirt  51  through the water supply pipe  52 . 
     Further, the air motor  50  is activated to rotate the driving shaft  50   a . As the driving shaft  50   a  rotates, the eccentric head  53  rotates eccentrically in the introduction space  51   a  of the skirt  51 . The eccentric head  53  rotates eccentrically in the water present in the introduction space Ma. As the water in the introduction space Ma is thus stirred, the pressure of the water in the introduction space  51   a  becomes higher. As described above, the introduction space  51   a  communicates with the water channel  54   i  that continues through the openings  54   h  of the disc cover  54   b  and the disc holes  54   e , the communication passages  54   f , and the disc center hole  54   d  of the disc main body  54   a . Therefore, the water stirred in the introduction space  51   a  is pushed out to the openings  54   h  of the disc cover  54   b .  FIG.  13    is a sectional view illustrating flows of water in the automatic wet sanding unit  5  in a state of performing automatic wet sanding. ( FIG.  13    is a view of a section located at a position corresponding to line XIII-XIII in  FIG.  4 B .) As indicated by arrows W 1  in  FIG.  13   , the water pushed out of the introduction space  51   a  to the openings  54   h  of the disc cover  54   b  flows from the openings  54   h  through the disc holes  54   e , the communication passages  54   f , and the disc center hole  54   d . The water having passed through the disc center hole  54   d  passes through the pad center hole  55   b  of the cushion pad  55  and is pumped toward the painted surface of the vehicle body V through the paper center hole  56   c  of the sandpaper  56 . Then, in the automatic wet sanding process, this water flows into the gap between the sanding surface  56   a  of the sandpaper  56  and the painted surface and is pushed out from the central portion toward the outer circumferential side of the sandpaper  56  between the sanding surface  56   a  and the painted surface. 
     With the water thus flowing, the sanding surface  56   a  of the sandpaper  56  is pressed against the painted surface with a predetermined pressure, and with the water flowing between the sanding surface  56   a  and the painted surface, the automatic wet sanding robot  3  is operated to move the sandpaper  56  along the painted surface of the left front door LFD to sand down the painted surface. 
     During automatic wet sanding, the air cylinder  60  is activated in accordance with a command signal from the automatic wet sanding unit controller  84  to control forward and backward motion of the piston rod  61 A. Thus, the automatic wet sanding unit  5  is moved forward and backward and the posture thereof is changed such that the sandpaper adapts to the shape of the painted surface with high accuracy. Specifically, automatic wet sanding is performed while the posture of the sandpaper is changed according to changes in the curvature of the painted surface with high accuracy (e.g., the posture of the sanding sliding body is changed so as to be orthogonal to a line normal to the painted surface), as well as the pressing force exerted by the sanding sliding body on the painted surface is maintained at an appropriate level. While the piston rod  61 A is thus moved forward and backward, the guide rods  61 B are also moved forward and backward along with the piston rod  61 A. As for forward and backward motion of the guide rods  61 B, since each guide rod  61 B is slidably supported by the bush  60   a  through the balls  61   b  as described above, the guide rods  61 B are smoothly moved forward and backward as the balls  61   b  roll. 
     Since the disc  54  is rotatably supported by the eccentric head  53  as described above, the disc  54 , the cushion pad  55 , and the sandpaper  56  make eccentric motion (motion in which the center point of the disc  54  moves in circles) around the center of rotation O 1  of the driving shaft  50   a , without being forced to rotate when the eccentric head  53  rotates eccentrically. 
       FIG.  14    is a side view of a vehicle body illustrating moving paths of the automatic wet sanding unit  5  in the automatic wet sanding operation. Arrow D 1  in  FIG.  14    is one example of moving paths of the automatic wet sanding unit  5  of the first automatic wet sanding apparatus  21  when the automatic wet sanding unit  5  sands down the painted surface of the left front door LFD. Arrow D 2  is one example of moving paths of the automatic wet sanding unit  5  of the first automatic wet sanding apparatus  21  when the automatic wet sanding unit  5  sands down the painted surface of the left front fender LFF (when the automatic wet sanding unit  5  performs the front fender automatic wet sanding step to be described later). Arrow D 3  is one example of moving paths of the automatic wet sanding unit  5  of the third automatic wet sanding apparatus  23  when the automatic wet sanding unit  5  sands down the painted surface of the left rear fender LRF. Arrow D 4  is one example of moving paths of the automatic wet sanding unit  5  of the third automatic wet sanding apparatus  23  when the automatic wet sanding unit  5  sands down the painted surface of the left rear door LRD. 
     While automatic wet sanding on the painted surface of the left front door LFD is performed by the automatic wet sanding unit  5  of the first automatic wet sanding apparatus  21 , automatic wet sanding on the painted surface of the left rear fender LRF is performed by the automatic wet sanding unit  5  of the third automatic wet sanding apparatus  23 . While automatic wet sanding on the painted surface of the left front fender LFF is performed by the automatic wet sanding unit  5  of the first automatic wet sanding apparatus  21 , automatic wet sanding on the painted surface of the left rear door LRD is performed by the automatic wet sanding unit  5  of the third automatic wet sanding apparatus  23 . This is to prevent the automatic wet sanding robot  3  of the first automatic wet sanding apparatus  21  and the automatic wet sanding robot  3  of the third automatic wet sanding apparatus  23  from coming too close to each other during automatic wet sanding. 
     Since water is pushed out toward the painted surface via the disc center hole  54   d  and the pad center hole  55   b  in automatic wet sanding as described above, automatic wet sanding is performed while water is pushed out from the central portion toward the outer circumferential side of the sandpaper  56  between the sandpaper  56  and the painted surface. Thus, sanding dust resulting from automatic wet sanding is washed away toward the outer circumferential side by water that is pushed out toward the outer circumferential side, so that sanding dust is less likely to remain around the sandpaper  56 . As a result, automatic wet sanding can be performed with the likelihood of clogging due to sanding dust being reduced. 
     The following flow of water also occurs inside the automatic wet sanding unit  5 . As water in the introduction space  51   a  is stirred by eccentric rotation of the eccentric head  53 , the water pressure rises and this water pressure acts on the seal member  59 . As shown in  FIG.  4 A , the upper end portion of the seal member  59  is inserted and supported in the engaging groove  51   e  of the skirt  51 , while a lower end portion thereof is not supported and is in contact with the upper surface of the disc  54  along the entire circumference of the seal member  59 . Therefore, when a water pressure acts on the seal member  59  and this water pressure exceeds a predetermined value, the lower end portion of the seal member  59  deforms elastically toward the outer circumferential side, leaving a small clearance between the lower end of the seal member  59  and the upper surface of the disc  54 . Water flows through this clearance. Arrows W 2  in  FIG.  13    indicate this flow of water. The water thus flowing out toward the outer circumferential side through the clearance between the seal member  59  and the disc  54  collides with the water deflecting part  57   c  of the hood  57  and changes its flow direction to a direction toward the painted surface of the vehicle body V. Then, the water collides with the water deflecting member  58  and changes its flow direction so as to be directed toward the center side (the side toward the cushion pad  55 ) while flowing toward the painted surface of the vehicle body V. Inner surfaces of the hood  57  and the water deflecting member  58  are cleaned by this flow of water, and sanding dust adhering to these inner surfaces, if any, is removed. Then, the water collides with the painted surface of the vehicle body V and is sent (bounced) back by the painted surface, and changes its flow direction so as to be directed toward the center side (the side toward the disc  54 ) while flowing away from the painted surface of the vehicle body V (see arrows W 3  in  FIG.  13   ). As the water thus undergoes changes in the flow direction, the water having flowed out toward the outer circumferential side through the clearance between the seal member  59  and the disc  54  is unlikely to scatter widely in a peripheral part of the automatic wet sanding unit  5 . It is therefore unlikely that paint separated from the vehicle body V by automatic wet sanding adheres to a wide area of the vehicle body V. 
     Front Fender Automatic Wet Sanding Step 
     When the front door automatic wet sanding step is completed, the operation of the automatic wet sanding unit  5  is temporarily stopped, and then the front fender automatic wet sanding step is started. In the front fender automatic wet sanding step, the automatic wet sanding robot  3  is operated to move the automatic wet sanding unit  5  to a position at which it faces the front fender (in the case of the first automatic wet sanding apparatus  21 , the left front fender LFF). Then, the automatic wet sanding unit  5  is activated by a command signal from the automatic wet sanding unit controller  84 . The operation of the automatic wet sanding unit  5  in this step is the same as in the front door automatic wet sanding step described above and therefore will not be described here. 
     Starting to Carry Out Vehicle Body 
     When the front door automatic wet sanding step is completed, the operation of the automatic wet sanding unit  5  is stopped and the vehicle body V starts to be carried out. Specifically, the conveyor  11  is activated to transfer the vehicle body V that has undergone automatic wet sanding toward the next station. 
     Paper Peeling Step 
     As the vehicle body V starts to be carried out, the paper peeling step by the paper peeling unit  41  provided in the changer  4  is performed. In the paper peeling step, the automatic wet sanding robot  3  is operated to move the automatic wet sanding unit  5  to a position at which the sandpaper  56  is caught between the clamping shaft  41   a  and the clamping hook  41   b , and then the automatic wet sanding unit  5  is moved upward to thereby peel the sandpaper  56  from the cushion pad  55 . Thereafter, the clamping shaft motor  41   d  is activated to rotate the clamping shaft  41   a , so that the sandpaper  56  peeled from the cushion pad  55  drops into the sandpaper collection box  41   e  to be collected. 
     Pad Cleaning Step 
     In the pad cleaning step by the pad cleaning unit  42 , cleaning water (pure water) is supplied to the cleaning tank  42   a  as the water supply pump  42   j  is activated, and the water is circulated through the circulating circuit  42   c  as the circulating pump  42   g  is activated. In this state, the automatic wet sanding robot  3  is operated to move the automatic wet sanding unit  5  into the cleaning tank  42   a , and the cushion pad  55  is pressed against the metal mesh  42   d  to squeeze out water (water with paint mixed therein) contained in the cushion pad  55 . Then, the automatic wet sanding unit  5  is slightly raised to separate the cushion pad  55  from the metal mesh  42   d . In this state, the air motor  50  is activated and the cushion pad  55  is rotated (eccentrically rotated) in the water to clean the cushion pad  55 . As the circulating pump  42   g  operates during these actions, water is circulated by being extracted from the bottom of the cleaning tank  42   a  and purified by the filter  42   h  and then returned to the cleaning tank  42   a  through the side surface of the cleaning tank  42   a . Thereafter, the automatic wet sanding unit  5  is further slightly raised to move the cushion pad  55  to above the level of the water in the cleaning tank  42   a , and the air motor  50  is activated again to drain the cushion pad  55  using a centrifugal force. Meanwhile, the drain valve  42   i  is opened to discharge the water from the cleaning tank  42   a.    
     Pad Draining Step 
     In the pad draining step by the pad draining unit  43 , the automatic wet sanding robot  3  is operated to press the cushion pad  55  against the inclined plate  43   d  of the draining table  43   a , and water is thereby squeezed out of the cushion pad  55 . In this process, the center line O 2  of the disc  54  and the cushion pad  55  is moved as indicated by the arrows in  FIG.  9    such that the position at which the cushion pad  55  is pressed against the inclined plate  43   d  is changed in the circumferential direction of the cushion pad  55 . During draining, the air blow motor  43   e  is activated to blow air from the air blow nozzle  43   b  toward the cushion pad  55  and thereby increase the draining efficiency. 
     Paper Mounting Step 
     In the paper mounting step by the paper mounting unit  44 , with the paper pressing plate  44   b  pressing the upper side of the sandpaper  56  as shown in  FIG.  2   , the automatic wet sanding robot  3  is operated to press the cushion pad  55  against the upper surface of the sandpaper  56 . In this state, the air cylinder  44   c  is activated to move the paper pressing plate  44   b  away from the sandpaper  56 , so that the entire touch-and-close fastener of the sandpaper  56  is mounted to the cushion pad  55 . Since the cushion pad  55  is rotatably supported by the bearing  53   a , it is preferable that at a stage preceding the paper mounting step, the cushion pad  55  be pressed against a positioning plate (not shown) to adjust the posture of the cushion pad  55  relative to the center of rotation O 1  of the driving shaft  50   a  (the phase position of the cushion pad  55  in the offset direction) to a correct posture. 
     Paper Checking Step 
     In the paper checking step by the paper checking unit  45 , the automatic wet sanding robot  3  is operated to place the cushion pad  55  (with the sandpaper  56  mounted thereon) on the stand  45   a  as shown in  FIG.  10   , and the outer circumferential surface of the cushion pad  55  is pressed against the plates  45   c  and the positioning plate  45   d . In this state, an image of the cushion pad  55  and the sandpaper  56  is taken from below by the camera  45   b . This imaging data is sent to the central processing unit  8  through the changer controller  85 , and the central processing unit  8  checks whether or not the mounting position of the sandpaper  56  is the correct position. When it is determined that the mounting position of the sandpaper  56  is the correct position, the automatic wet sanding operation starting from the pad wetting step is performed on the next vehicle body V that has been transferred to the predetermined position in the automatic wet sanding station  1  by the step of carrying in the vehicle body. On the other hand, when it is determined that the mounting position of the sandpaper  56  is not the correct position, the action of mounting the sandpaper  56  is redone. To redo the mounting action, for example, the paper peeling step and the paper mounting step are sequentially performed. 
     The actions from “carrying in vehicle body” to the “paper checking step” are repeatedly performed to sequentially perform automatic wet sanding on each of vehicle bodies V transferred to the automatic wet sanding station  1 . 
     Advantages of Embodiment 
     In the embodiment having been described above, the air cylinder  60  that changes the posture of the unit main body  5 A of the automatic wet sanding unit  5  is provided with the guide rods  61 B. The outer surface of each guide rod  61 B has the grooves  61   a  that extend along the shaft centerline of the guide rod  61 B and have an arc-shaped cross-section in a direction orthogonal to the shaft centerline, and the balls  61   b  are interposed between the bottom of each groove  61   a  and the inner surface (each groove  60   b  formed in the inner surface) of the bush  60   a  provided inside the air cylinder  60 . Since providing the guide rods  61 B can enhance the mechanical strength of the unit support mechanism  5 B, the diameter of the piston rod  61 A of the air cylinder  60  can be reduced. As described above, reducing the diameter of the piston rod  61 A can increase the pressure of input air for control and enables high-accuracy pressure control. Further, as the area of contact between the piston rod and a part coming into sliding contact therewith (e.g., a seal packing) inside the air cylinder is reduced, the sliding resistance can be reduced. In addition, as the internal volume of the cylinder is reduced, the response speed in adaptation can be increased. Thus, higher adaptability of the sandpaper  56  can be achieved. Therefore, the configuration of this embodiment can make two objects compatible with each other: to achieve high-accuracy automatic wet sanding by enhancing the adaptability of the sandpaper  56  to the shape of a painted surface through a reduction of the diameter of the piston rod  61 A; and to enhance the durability of the automatic wet sanding apparatuses  21  to  24 . 
     In the embodiment, the guide rod  61 B is provided on each side of the piston rod  61 A of the air cylinder  60  in a direction orthogonal to the extension direction of the piston rod  61 A. This configuration can give sufficient mechanical strength to the unit support mechanism  5 B that supports the unit main body  5 A, making it easy to reduce the diameter of the piston rod  61 A of the air cylinder  60 . 
     In the embodiment, the outer circumferential surface of the threaded part  66   a  of the bearing bolt  66  provided in the rod end mechanism  5 D has the recesses  66   b  that extend along the shaft centerline of the bearing bolt  66 . Thus, the area of contact between the inner circumferential surface of the bearing member  67  disposed inside the rod end  64  and the outer circumferential surface of the threaded part  66   a  of the bearing bolt  66  can be reduced, and thereby the sliding resistance occurring between the bearing member  67  and the bearing bolt  66  when the unit main body  5 A turns along with the bearing bolt  66  can be reduced. Therefore, during automatic wet sanding, the posture of the automatic wet sanding unit  5  can be quickly changed according to changes in the curvature of a painted surface so as to adapt the sandpaper  56  to the shape of the painted surface. 
     OTHER EMBODIMENTS 
     The present disclosure is not limited to the above embodiment and all modifications and applications encompassed by the scope of the claims and an equivalent scope are possible. 
     For example, in the above embodiment, the case has been described in which the present disclosure is applied to the automatic wet sanding apparatuses  21  to  24  for which the painted object is the vehicle body V and which perform automatic wet sanding on the painted surfaces of the vehicle body V. The painted object in the present disclosure is not limited to the vehicle body V, and the disclosure is applicable to automatic wet sanding apparatuses for various painted objects. 
     In the above embodiment, a total of two guide rods  61 B are provided one on each side of one piston rod  61 A. The present disclosure is not particularly limited in terms of the number of the guide rods  61 B and the positions at which they are disposed. In the above embodiment, the grooves  61   a  are formed at four positions in the outer circumferential surface of each guide rod  61 B. The present disclosure is not particularly limited in terms of the number of the grooves  61   a  either. 
     In the above embodiment, the recesses  66   b  formed in the outer circumferential surface of the threaded part  66   a  of the bearing bolt  66  have an arc-shaped cross-section. However, the cross-sectional shape is not limited to an arc shape and the recesses  66   b  may have an arbitrary shape. As to the range of formation of the recesses  66   b  in the outer circumferential surface of the threaded part  66   a , the recesses  66   b  may be formed along the entire threaded part  66   a  in the extension direction thereof, or may be formed only at positions corresponding to the bolt insertion hole (center hole)  64   a  of the rod end  64  (positions corresponding to the bearing member  67 ). 
     The sandpaper  56  is used as a sanding sliding body in the above embodiment, but a sanding brush may instead be used. 
     The air motor  50  is used as a rotation power source in the above embodiment, but an electric motor or the like may instead be used. 
     The present disclosure is applicable to an automatic wet sanding apparatus that performs automatic wet sanding on a painted surface of a vehicle body.