Patent Publication Number: US-11642711-B2

Title: Drawing machine and drawing method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a U.S. national stage application filed under 35 U.S.C. 371 of International Patent Application No. PCT/JP2016/086275 filed Dec. 6, 2016, the content of which is incorporated herein by reference in its entirety for all purposes. 
     TECHNICAL FIELD 
     The present invention relates to a drawing machine and a drawing method for drawing a metal tube. 
     BACKGROUND ART 
     One example of a conventional wire drawing machine is described in JP2003-053418 A (Patent Document 1), and is referred to as a slip-type wire drawing machine. Such conventional slip-type wire drawing machine sets the rotation speed of a capstan higher than the speed of the metal wire so as to cause a slip between the capstan and the metal wire and, using the capstan, the metal wire is pulled through a wire drawing die, thereby drawing the metal wire. 
     CITATION LIST 
     Patent Document 
     Patent Document 1: JP2003-053418 A 
     SUMMARY 
     Technical Problem 
     However, in a situation where the above-mentioned conventional wire drawing machine draws a pipe-shaped metal tube having a hollow structure such as a narrow tube, it is not capable of controlling the outer diameter and the inner diameter of the metal tube. 
     Solution to Problem 
     In order to solve the problem above, an aspect of the present invention provides a drawing machine including: a first drawing unit that includes a first drawing die that reduces at least an outer diameter of a metal tube passing therethrough, thereby drawing the metal tube, a first upstream capstan that is provided before the first drawing die so as to deliver the metal tube to the first drawing die, and a first downstream capstan that is provided after the first drawing die so as to draw the metal tube from the first drawing die; a second drawing unit that includes: a second drawing die that reduces at least the outer diameter of the metal tube delivered from the first drawing unit, thereby drawing the metal tube, a second upstream capstan that is provided before the second drawing die so as to deliver the metal tube to the second drawing die, and a second downstream capstan that is provided after the second drawing die so as to draw the metal tube from the second drawing die; and a tension applying section that applies a predetermined tension to the metal tube between the first drawing unit and the second drawing unit. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a schematic view showing a configuration of a drawing machine  100  according to an embodiment of the invention. 
         FIG.  2    is a block diagram illustrating a configuration of a control unit  200  that controls the drawing machine  100 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The invention will now be described by means of its embodiment with reference to the attached drawings. However, the following embodiment is not intended to limit the invention set forth in the claims and all the combinations of features described in the embodiment are not necessarily indispensable for the solution according to the invention. 
     In the present embodiment, a metal tube  10  has a pipe-shaped structure. In other words, the metal tube  10  has, in a cross-section perpendicular to the extending direction of the metal tube, a predetermined outer diameter (hereinafter also referred to as the “outer diameter of the metal tube  10 ”) and, on the inner side thereof (e.g., at the center of the cross-section), a circular or elliptical space having a predetermined inner diameter (hereinafter also referred to as the “inner diameter of the metal tube  10 ”). It should be noted that a drawing machine  1  according to the present embodiment may be a drawing machine that draws, in addition to the metal tube  10  having the pipe-shaped structure, a metal wire made of materials having different physical properties (e.g., hardness) at an inner side and an outer side in a cross-section perpendicular to the extending direction of the metal tube. 
     Configuration of Drawing Machine  100   
       FIG.  1    is a schematic view showing a configuration of a drawing machine  100  according to an embodiment of the invention.  FIG.  2    is a block diagram illustrating a configuration of a control unit  200  that controls the drawing machine  100  in the above embodiment. 
     In the present embodiment, the drawing machine  100  is configured so as to include a housing  12 , an unwinding unit  20 , drawing units  40 , a winding unit  60  and a control unit  200  that controls each unit. In the drawing machine  100 , the unwinding unit  20 , the drawing unit  40  and the winding unit  60  are arranged in this order along the route through which a metal tube  10  is delivered out, drawn and wound (hereinafter referred to as the “passage”), from upstream to downstream (from left to right in  FIG.  1   ). The drawing machine  100  draws the metal tube delivered from the unwinding unit  20  sequentially at the respective drawing units  40  by reducing the diameter thereof, and winds up the metal tube  10  with the reduced diameter at the winding unit  60 . 
     The control unit  200  controls the operation of the drawing machine  100 . The control unit  200  is configured so as to have a system controller  110 , an unwinding unit controller  120 , a drawing unit controller  140  and a winding unit controller  160 . The system controller  110  is connected to the unwinding unit controller  120 , the drawing unit controller  140  and the winding unit controller  160 , and performs overall control of each unit controller. 
     The unwinding unit controller  120 , the drawing unit controller  140  and the winding unit controller  160  are connected to various components provided in the unwinding unit  20 , the drawing units  40  and dancer sections  52 , and the winding unit  60 , respectively, and control the respective units. Although only one drawing unit controller  140  is shown in  FIG.  2   , a drawing unit controller  140  is provided for each of the n stages of drawing units  40 - 1  to  40 - n  (n being a positive integer). 
     The system controller  110 , and the unwinding unit controller  120 , the drawing unit controller  140  and the winding unit controller  160  provided in the control unit  200  control the unwinding unit  20 , the drawing unit  40  and the dancer section  52 , and the winding unit  60  as stated above, so as to deliver the metal tube  10  from the unwinding unit  20 , draw it by causing it to pass through the respective drawing units  40 - 1  to  40 - n , and wind it up at the winding unit  60 . 
     Each structure in the drawing machine  100  will be described below, referring to  FIGS.  1  and  2   . In the present embodiment, as illustrated in  FIG.  1   , n stages of drawing units  40  are serially arranged between the unwinding unit  20  and the winding unit  60  along the route through which the metal tube  10  is delivered, so as to sequentially draw the metal tube  10 . In the below description, the drawing units  40  are respectively referred to as drawing units  40 - 1  to  40 - n  along the direction extending from the unwinding unit  20  toward the winding unit  60 . The drawing machine  100  also has n−1 stages of dancer sections  52  and each dancer section  52  is provided between the adjacent drawing units  40 . 
     Unwinding Unit  20   
     The unwinding unit  20  is configured so as to have an unwinding bobbin  22 , guide rollers  24 ,  26  and  28 , and a dancer section  32 . In the unwinding unit  20 , the metal tube  10  is provided so as to run across the unwinding bobbin  22 , the guide roller  24 , the guide roller  26 , a dancer roller  34 , the guide roller  26  and the guide roller  28 , in this order, with a predetermined tension being applied thereto (hereinafter, the metal tube provided in such manner will be described as being “provided in a tensioned state”). 
     The unwinding bobbin  22  is rotatably mounted in the housing  12  of the drawing machine  100 . The unwinding bobbin  22  is connected to an unwinding motor  122  and rotates when driven by the motor. With that rotation, the metal tube  10  wound around the unwinding bobbin  22  is pulled out therefrom and delivered to the passage. In the present embodiment, the unwinding bobbin  22  is driven by the unwinding motor  122  at a controlled speed. That is, the unwinding unit controller  120  controls the drive of the unwinding motor  122  so that the unwinding bobbin  22  rotates at a predetermined speed. The unwinding unit controller  120  controls the rotation speed of the unwinding motor  122  based on the angle of a dancer arm  36  detected by a potentiometer  138 . 
     The guide rollers  24 ,  26  and  28  are rotatably mounted in the housing  12  of the drawing machine  100 . The metal tube  10  is wound around each of the guide rollers  24 ,  26  and  28  a predetermined number of times so as to allow the metal tube  10  to be delivered in a non-slip manner. The guide rollers  24 ,  26  and  28  rotate due to tension applied to the metal tube  10  by the drawing unit  40 , so that the metal tube  10  delivered from the unwinding bobbin  22  is sequentially delivered in a non-slip manner along the passage. 
     The dancer section  32  is configured so as to include the dancer roller  34 , a dancer arm  36  and a torque motor  38 , and applies a desired tension to the metal tube  10  delivered from the unwinding bobbin  22 . 
     The dancer roller  34  is rotatably supported at one end of the rod-shaped dancer arm  36 . The metal tube  10  is provided in a tensioned state across the guide roller  24 , the guide roller  26 , the dancer roller  34 , the guide roller  26  and the guide roller  28 , in this order, and a predetermined tension is applied to the metal tube  10  by the dancer roller  34  in a downward direction in  FIG.  1   . 
     The dancer arm  36  is arranged approximately horizontally in  FIG.  1   , i.e. in a direction approximately perpendicular to the direction in which tension is applied to the metal tube  10  by the dancer roller  34 . This horizontal arrangement is regarded as a reference position for the dancer arm  36 . The other end of the dancer arm  36  is supported so as to be fixed to a drive shaft of the torque motor  38  and the drive shaft of the torque motor  38  acts as the pivot point of the dancer arm  36 . 
     The potentiometer  138  ( FIG.  2   ) is provided at the drive shaft of the torque motor  38  and it detects a pivot angle of the dancer arm  36 . The potentiometer  138  is connected to the unwinding unit controller  120  and provides the pivot angle detected by the potentiometer  138  to the drawing unit controller  140 . It should be noted here that, although the potentiometer  138  detects a pivot angle of the dancer arm  36  in the present embodiment, the potentiometer  138  may detect a position or displacement of the dancer roller  34 , for example, a position or displacement of the dancer roller  34  in a direction in which tension is applied to the metal tube  10  by the dancer roller  34 . In that case, tension may be applied to the metal tube  10  by vertically moving the dancer roller  34  (linearly moving it in a direction of applying tension to the metal tube), instead of rotating the dancer roller  34 . 
     The torque motor  38  applies a predetermined tension to the metal tube  10  through the dancer arm  36  and the dancer roller  34 . That is, the torque motor  38  transmits its rotation torque to the metal tube  10  through the dancer arm  36  and the dancer roller  34 , thereby applying tension to the metal tube  10 . The torque motor  38  is connected to the unwinding unit controller  120  and generates a predetermined rotation torque based on the commands (torque commands) from the unwinding unit controller  120 . 
     The dancer section  32  may have, instead of the torque motor  38 , an actuator such as a servomotor (which is used in, for example, a torque control mode), a rotary solenoid (which is used, for example, to generate a rotation torque according to a supplied current), an air cylinder (which is used, for example, to adjust the thrust of the dancer arm  36 ) and a DC motor (which is used to generate a rotation torque according to a supplied current). The dance section  32  may have, instead of an actuator such as the torque motor  38 , a weight (for adding the weight of such weight to the dancer arm  36 ), a spring (a tension spring or compression spring connected to the dancer arm  36  and used to adjust the tension position or compression position thereof), a spiral spring (which is arranged about the rotation axis of the dancer arm  36  and wound for use), etc. Each of the means indicated above as examples is used for controlling the tension applied to the metal tube  10  so as to make such tension have a predetermined value. 
     Although the dancer section  32  applies a tension to the metal tube  10  via the pivoting dancer arm  36 , the configuration is not limited thereto. The dancer section  32  may have a dancer roll that moves, for example, vertically or horizontally and may apply a tension to the metal tube  10  via such dancer roll. In such case, the position of the dancer roll may be detected by using, for example, a linear encoder, a position-proportional-output-type position sensor, an ultrasonic ranging sensor, a laser range finder, etc. to control the tension applied to the metal tube  10 . 
     By transmitting the predetermined torque generated by the torque motor  38  to the metal tube  10  through the dancer arm  36  and the dancer roller  34  with the above configuration, the dancer section  32  applies a particular set tension to the metal tube  10 . In other words, the tension applied to the metal tube  10  delivered from the unwinding unit  20  is determined according to the rotation torque of the torque motor  38 . 
     As described above, in the unwinding unit  20  of the present embodiment, the dancer section  32  applies a predetermined tension to the metal tube  10  delivered from the unwinding bobbin  22  at a constant speed and this allows the metal tube  10  to be delivered to the drawing unit  40 - 1  with a desired tension being applied. 
     Drawing Unit  40   
     In the present embodiment, the drawing machine  100  is configured so as to have n stages of drawing units  40 - 1  to  40 - n  between the unwinding unit  20  and the winding unit  60 . The unwinding unit  20 , the drawing units  40 - 1  to  40 - n  and the winding unit  60  are provided so as to be connected to each other. The metal tube  10  delivered from the unwinding unit  20  sequentially passes through the drawing units  40 - 1 ,  40 - 2 , . . .  40 - n  in this order and is thereby drawn. The metal tube  10  drawn by the drawing unit  40 - n  is then delivered to the winding unit  60 . Since the drawing units  40 - 1  to  40 - n  each have the same configuration in the present embodiment, the drawing units  40 - 1  to  40 - n  will hereinafter be collectively referred to as a “drawing unit  40 ” unless the drawing units  40 - 1  to  40 - n  are individually specified. Further, each of the structures included in the drawing units  40 - 1  to  40 - n  will also be referred to collectively. 
     The drawing unit  40  is configured so as to have a drawing die  42 , guide rollers  44  and  46 , and drive capstans  48  and  50 . In the drawing unit  40 , the metal tube  10  is provided in a tensioned state across the guide roller  44 , the drawing die  42 , the guide roller  46  and the drive capstan  50 . 
     The drawing die  42  is disposed between the guide roller  44  and the guide roller  46 . The drawing die  42  has a die hole extending along the direction in which the metal tube  10  is provided in a tensioned state. When the metal tube  10  passes and is drawn through the die hole, the outer diameter of the metal tube  10  is reduced and the metal tube  10  is accordingly drawn. Here, the reduction rate of the diameter (the reduction rate of the cross-section) of the metal tube  10  is determined according to the diameter of the die hole provided in the drawing die  42 , and the metal tube  10  is drawn according to the reduction rate. In each stage of the drawing units  40 - 1  to  40 - n , the die hole diameter of the drawing die  42  is selected as appropriate so that the metal tube  10  drawn at the drawing unit  40 - n , as the last stage, will have a desired diameter. 
     In the present embodiment, the drawing units  40 - 1  to  40 - n  gradually reduce the diameter of the metal tube  10  that passes therethrough. Accordingly, the die hole formed in the drawing die  42 - n  has a smaller diameter than that of the die hole formed in the drawing die  42 - 1 . Further, the die hole formed in the drawing die  42 - 2  has a smaller diameter than that of the die hole formed in the drawing die  42 - 1 . 
     In the present embodiment, the drawing die  42  is stored in a die holder fixed to the housing  12 . The drawing machine  100  may have means for measuring a force of the drawing when the metal tube  10  is drawn through the drawing die  42 . Such measuring means may be, for example, means for detecting a force with which the drawing die  42  presses the die holder, which thereby measures the drawing force, and may alternatively be means for detecting a distortion of the die holder fixed to the housing  12 , which thereby measures the drawing force. 
     Here, if the metal tube  10  and/or the drawing die  42  are immersed with lubricating oil, vibration, etc., of the metal tube  10  passing through the drawing die  42  can be prevented, thereby resulting in improved stability. Accordingly, an oil tank for immersing the metal tube  10  and/or the drawing die  42  with lubricating oil may be provided. For example, an oil tank may be arranged between the drawing die  42  and the guide roller  44  and the metal tube  10  may be configured so as to pass through the oil tank. In that case, it is preferable to provide means for supplying lubricating oil to the oil tank such that the lubricating oil flows over the oil tank during the drawing operation. Alternatively, an oil tank may be arranged so as to contain the drawing die  42  therein, and the metal tube  10  may be configured so as to pass through the oil tank in a vertical or horizontal manner. It should be noted, however, that a seal is needed at the portion through which the metal tube extends. 
     Immersing the metal tube  10  and/or the drawing die  42  with lubricating oil provides the following advantages. An optimum lubricating oil can be used for the drawing performed at the drawing die  42  in each drawing unit  40 . The composition of the lubricating oil greatly affects the wearing of the drawing die  42 , and the above configuration enables stable supply of a lubricating oil having a composition specialized for drawing. Furthermore, it is possible to simplify a circulation and cleaning system for the lubricating oil, which would be necessary to reduce the effects of contamination in the oil caused by abrasion between the metal tube  10  and the drive capstan  50 , and this leads to reduced manufacturing costs. 
     The guide rollers  44  and  46  are rotatably mounted in the housing  12  of the drawing machine  100 . The guide rollers  44  and  46  rotate due to tension applied to the metal tube  10  by the rotation of the drive capstan  48  or  50  and sequentially deliver the metal tube  10  along the passage in a non-slip manner. 
     The drive capstans  48  and  50  (which is an example of an upstream capstan and a downstream capstan) are rotatably mounted in the housing  12  of the drawing machine  100 . A drive motor  150  (see  FIG.  2   ) is connected to the drive capstans  48  and  50  and the drive capstans  48  and  50  rotate at a predetermined torque based on the commands from the drawing unit controller  140 . The drive capstans  48  and  50  respectively deliver the metal tube  10  to the drawing die  42  and draw the metal tube  10  from the drawing die  42 . 
     The outer surfaces of the drive capstans  48  and  50  are thermal-sprayed so as to increase the hardness of the surface and enhance durability and also to prevent a slip from occurring between the surfaces (surfaces in contact with the metal tube  10 ) of the drive capstans  48  and  50  and the metal tube  10 . The outer surface of the drive capstan  50  may alternatively be coated with an elastic body having a large coefficient of friction (e.g., resins such as urethane and rubber). Such surface-treated drive capstan  50  allows the metal tube  10  to be drawn through the drawing die  42  in a non-slip manner and to be delivered to the next stage. 
     Dancer Section  52   
     The dancer section  52  is configured so as to include a dancer roller  54 , a dancer arm  56  and a torque motor  58 , and applies a tension to the drawn metal tube  10  in the drawing unit  40 . 
     The dancer roller  54  is rotatably supported at one end of the rod-shaped dancer arm  56 . The metal tube  10  is wound around the dancer roller  54  in a non-slip manner, and a tension is applied to the metal tube  10  by the dancer roller  54  in a downward direction in  FIG.  1   . 
     The dancer arm  56  is arranged approximately horizontally in  FIG.  1   , i.e., in a direction approximately perpendicular to the direction in which tension is applied to the metal tube  10  by the dancer roller  54 . This horizontal arrangement is regarded as a reference position for the dancer arm  56 . The other end of the dancer arm  56  is supported so as to be fixed to a drive shaft of the torque motor  58  and the drive shaft of the torque motor  58  acts as the pivot point of the dancer arm  56 . 
     A potentiometer  158  ( FIG.  2   ) is provided at the drive shaft of the torque motor  58  and it detects a pivot angle of the dancer arm  56 . The potentiometer  158  is connected to the drawing unit controller  140  and provides the pivot angle detected by the potentiometer  158  to the drawing unit controller  140 . 
     The torque motor  58  applies a predetermined tension to the metal tube  10  through the dancer arm  56  and the dancer roller  54 . That is, the torque motor  58  transmits its rotation torque to the metal tube  10  through the dancer arm  56  and the dancer roller  54 , thereby applying tension to the metal tube  10 . The torque motor  58  is connected to the drawing unit controller  140  and generates a predetermined rotation torque based on the commands (torque commands) from the drawing unit controller  140 . 
     Winding Unit  60   
     The winding unit  60  is configured so as to have guide rollers  66 ,  68  and  70 , a dancer section  72  and a winding bobbin  80 . In the winding unit  60 , the metal tube  10  is provided in a tensioned state across the guide roller  66 , a dancer roller  74 , the guide roller  66 , the guide roller  68 , the guide roller  70  and the winding bobbin  80 , in this order. 
     The guide rollers  66 ,  68  and  70  are rotatably mounted in the housing  12  of the drawing machine  100 . The metal tube  10  is wound around each of the guide rollers  66 ,  68  and  70  a predetermined number of times so as to allow the metal tube  10  to be delivered in a non-slip manner. The guide rollers  66 ,  68  and  70  rotate due to tension applied to the metal tube  10  by the rotation of the winding bobbin  80 , so that the drawn metal tube  10  in the drawing unit  40 - n  is sequentially delivered along the passage in a non-slip manner. 
     The dancer section  72  is configured so as to include the dancer roller  74 , a dancer arm  76  and a torque motor  78 , and applies a desired tension to the drawn metal tube  10  in the drawing unit  40 - n.    
     The dancer roller  74  is rotatably supported at one end of the rod-shaped dancer arm  76 . The metal tube  10  is provided in a tensioned state across the guide roller  66 , the dancer roller  74 , the guide roller  66 , the guide roller  68  and the guide roller  70 , in this order, and a predetermined tension is applied to the metal tube  10  by the dancer roller  74  in a downward direction in  FIG.  1   . 
     The dancer arm  76  is arranged approximately horizontally in  FIG.  1   , i.e., in a direction approximately perpendicular to the direction in which tension is applied to the metal tube  10  by the dancer roller  74 . This horizontal arrangement is regarded as a reference position for the dancer arm  76 . The other end of the dancer arm  76  is supported so as to be fixed to a drive shaft of the torque motor  78 , and the drive shaft of the torque motor  78  acts as the pivot point of the dancer arm  76 . 
     A potentiometer  178  ( FIG.  2   ) is provided at the drive shaft of the torque motor  78  and it detects a pivot angle of the dancer arm  76 . The potentiometer  178  is connected to the winding unit controller  160  and provides the pivot angle detected by the potentiometer  178  to the winding unit controller  160 . 
     The torque motor  78  applies a predetermined tension to the metal tube  10  through the dancer arm  76  and the dancer roller  74 . That is, the torque motor  78  transmits its rotation torque to the metal tube  10  through the dancer arm  76  and the dancer roller  74 , thereby applying tension to the metal tube  10 . The torque motor  78  is connected to the winding unit controller  160  and generates a predetermined rotation torque based on the commands (torque commands) from the winding unit controller  160 . 
     The winding bobbin  80  is rotatably mounted in the housing  12  of the drawing machine  100 . The winding bobbin  80  is connected to a winding motor  180  and rotates when driven by the motor. With that rotation, the drawn metal tube  10  in the drawing unit  40 - n  is wound around the winding bobbin  80 . In the present embodiment, the winding bobbin  80  is driven by the winding motor  180  at a controlled speed. That is, the winding unit controller  160  controls the drive of the winding motor  180  so that the winding bobbin  80  rotates at a predetermined speed. More specifically, the winding unit controller  160  controls the drive of the winding motor  180  based on the circumferential speed of the drive capstan  50 - n  and the pivot angle of the dancer arm  76 . 
     As described above, in the winding unit  60  according to the present embodiment, while the dancer section  72  applies a predetermined tension to the metal tube  10  delivered from the drawing unit  40 - n , the winding bobbin  80  winds up the metal tube  10  at a constant speed. 
     Drawing Operation of Drawing Machine  100   
     Next, the operation of the drawing machine  100  having the above-described configuration, in order to draw the pipe-shaped metal tube  10 , will be described with reference to  FIGS.  1  and  2   . 
     Setting of Circumferential Speeds of Drive Capstans  48  and  50   
     In each stage of the drawing unit  40 , the outer diameter of the metal tube  10  which has passed through the drawing die  42  is determined by the hole diameter of the drawing die  42 . In other words, the outer diameter of the metal tube  10  that is wound in the winding unit  60  is controlled by the outer diameter of the metal tube  10  delivered from the unwinding unit  20  and the hole diameter of each drawing die  42 . 
     On the other hand, in the present embodiment, the inner diameter of the metal tube  10  which has passed through the drawing die  42  of a particular drawing unit  40  is controlled by the ratio between the circumferential speed of the drive capstan  48  provided before the drawing die  42  and the circumferential speed of the drive capstan  50  provided after the drawing die  42  in the particular drawing unit  40 . 
     Since the volume of the metal tube  10  (including the inner space) passing through the drawing die  42  per unit time is constant, the following equation is established (wherein, in the drawing unit  40 , the outer diameter and the inner diameter of the metal tube  10  before passing through the drawing die  42  are D1 and d1, respectively, the outer diameter [namely, the hole diameter of the drawing die  42 ] and the inner diameter of the metal tube  10  after passing through the drawing die  42  are D2 and d2, respectively, the circumferential speed of the drive capstan  48  is V1, and the circumferential speed of the drive capstan  50  is V2):
 
 V 1(π D 1 2 /4−π d 1 2 /4)= V 2(π D 2 2 /4−π d 2 2 /4)  (Equation 1).
 
     That is to say, the following equation is established:
 
 V 1( D 1 2   −d 1 2 )= V 2( D 2 2   −d 2 2 )  (Equation 2).
 
     In Equation 2, since D1, d1 and D2 are known constants, d2 (the inner diameter of the metal tube  10  after passing through the drawing die  42 ) can be controlled to a desired size by controlling the ratio between V1 (the circumferential speed of the drive capstan  48 ) and V2 (the circumferential speed of the drive capstan  50 ). The ratio between the rotation speed of the drive capstan  48  and the rotation speed of the drive capstan  50  may be controlled based on the circumference of the drive capstan  48  and the circumference of the drive capstan  50 , instead of the ratio between the circumferential speed of the drive capstan  48  and the circumferential speed of the drive capstan  50 . For example, if the circumference of the drive capstan  48  is equal to the circumference of the drive capstan  50 , the ratio between the circumferential speed of the drive capstan  48  and the circumferential speed of the drive capstan  50  is equal to the ratio between their rotation speeds. 
     In the present embodiment, the circumferential speeds of the drive capstan  48  and the drive capstan  50  are set in each drawing unit  40  with reference to the circumferential speed of the drive capstan  50 - n  in the drawing unit  40 - n  of the last stage. 
     First, when the reduction rate of the outer diameter of the metal tube  10  (the ratio between the outer diameters of the metal tube  10  before and after each drawing die  42 ), the reduction rate of the inner diameter of the metal tube  10  (the ratio between the inner diameters of the metal tube  10  before and after each drawing die  42 ) in each drawing unit  40 , and the circumferential speed V2 of the drive capstan  50 - n  in the drawing unit  40 - n  are set, the circumferential speed V1 of the drive capstan  48 - n  is determined based on Equation 2. 
     The circumferential speed V2 of the drive capstan  50 -( n −1) in the drawing unit  40 -( n −1) before the drive capstan  48 - n  is approximately the same as the circumferential speed V1 of the drive capstan  48 - n . The circumferential speed V1 of the drive capstan  48 -( n −1) can be determined based on Equation 2. In this way, the circumferential speed V1 of the drive capstan  48  and the circumferential speed V2 of the drive capstan  50  in each drawing unit  40  can be determined so as to obtain the metal tube  10  having desired outer and inner diameters. 
     Operation of Unwinding Unit  20   
     The unwinding unit  20  causes the metal tube  10  to be delivered out from the unwinding unit  20  at an approximately constant speed. Specifically, the unwinding unit controller  120  controls the speed of rotation of the unwinding motor  122  so that the speed at which the metal tube  10  is delivered from the unwinding unit  20  (hereinafter the speed at which the metal tube  10  is delivered at a particular point of the passage will be referred to as a “wire speed”) is maintained at an approximately constant value according to the circumferential speed of the drive capstan  50 - n . The wire speed of the metal tube  10  delivered from the unwinding unit  20  is approximately equal to the circumferential speed of the drive capstan  48 - 1  which is determined based on the circumferential speed of the drive capstan  50 - n.    
     In the present embodiment, the unwinding unit controller  120  controls the speed of rotation of the unwinding motor  122  by using the wire speed of the metal tube  10  at the guide roller  44  as a feed-forward signal and the pivot angle of the dancer arm  36  as a feedback signal. More specifically, the circumferential speed of the drive capstan  48 - 1  is fed to the drawing unit controller  140  as the speed at which the metal tube  10  is delivered out from the unwinding unit  20 , namely, the wire speed of the metal tube  10  passing through the drive capstan  48 - 1 . The circumferential speed of the drive capstan  48 - 1  may be detected by, for example, an encoder provided at the drive capstan  48 - 1 . Then, the unwinding unit controller  120  provides a speed signal indicative of the detected wire speed to the unwinding motor  122  as a feed-forward signal, thereby controlling the rotation of the unwinding motor  122 . 
     Meanwhile, when the dancer arm  36  pivots and a predetermined tension is accordingly applied to the metal tube  10  delivered from the unwinding bobbin  22 , a certain difference is created between the wire speed of the metal tube  10  delivered from the unwinding bobbin  22  and the wire speed of the metal tube  10  passing through the guide roller  44 . The unwinding unit controller  120  generates a feedback signal based on the pivot angle detected by the potentiometer  138  and controls the rotation of the unwinding motor  122  so as to correct a gap in wire speed due to the above difference, thereby maintaining the wire speed of the metal tube  10  delivered out from the unwinding unit  20  at an approximately constant value. 
     More specifically, the unwinding unit controller  120  calculates a pivot angle deviation between the pivot angle of the dancer arm  36  detected by the potentiometer  138  and the pivot angle of the dancer arm  36  at the reference position. Then, the unwinding unit controller  120  determines the rotation speed of the unwinding motor  122  so as to approximate the calculated pivot angle deviation to zero, and provides a rotation speed command to the unwinding motor  122  based on the determined rotation speed. Using the pivot angle deviation as a feedback signal, the unwinding unit controller  120  controls the rotation speed of the unwinding motor  122  through P control, PI control, PID control, etc. 
     Operation of Drawing Units  40   
     Next, the operation of each drawing unit  40  drawing the metal tube  10  delivered from the unwinding unit  20  will be described. 
     The drawing unit controller  140  controls the rotation speed of each drive motor  150  connected to each drive capstan  48  and drive capstan  50  based on the circumferential speed which has been set based on Equation 2. For example, in a situation where: the drawing machine  100  is provided with five stages of drawing units  40 ; the metal tube  10  delivered from the unwinding unit  20  (i.e., the metal tube  10  serving as a base material) has an outer diameter of 1.5 mm, a thickness of 0.075 mm and an inner diameter of 1.35 mm; and such metal tube  10  is wound such that the metal tube  10  to be wound by the winding unit  60  has a target outer diameter of 1.0 mm and a target inner diameter of 0.9 mm, with the ratio between the outer diameter and the inner diameter of the metal tube  10  being constantly maintained, the hole diameter of the drawing die  42 , as well as the circumferential speeds of the drive capstans  48  and  50 , in each drawing unit  40 , will be set as shown in the table below. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                 Ratio of 
               
               
                   
                   
                   
                   
                   
                   
                   
                 circumferential 
               
               
                   
                 Wire 
                 Metal 
                 Metal 
                   
                   
                   
                 speed of drive 
               
               
                   
                 drawing 
                 tube 
                 tube 
                   
                 Circumferential 
                 Circumferential 
                 capstan 48 to 
               
               
                 Wire 
                 die 
                 outer 
                 inner 
                   
                 speed of drive 
                 speed of drive 
                 circumferential 
               
               
                 drawing 
                 diameter 
                 diameter 
                 diameter 
                 Thickness 
                 capstan 48 
                 capstan 50 
                 speed of drive 
               
               
                 unit 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm/min.) 
                 (mm/min.) 
                 capstan 50 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 40-1 
                 1.40 
                 1.40 
                 1.26 
                 0.07 
                 44.44 
                 51.02 
                 0.8711 
               
               
                 40-2 
                 1.30 
                 1.30 
                 1.17 
                 0.65 
                 51.02 
                 59.17 
                 0.8622 
               
               
                 40-3 
                 1.20 
                 1.20 
                 1.08 
                 0.60 
                 59.17 
                 69.44 
                 0.8521 
               
               
                 40-4 
                 1.10 
                 1.10 
                 0.99 
                 0.55 
                 69.44 
                 82.64 
                 0.8403 
               
               
                 40-5 
                 1.00 
                 1.00 
                 0.90 
                 0.50 
                 82.64 
                 100.00 
                 0.8264 
               
               
                   
               
            
           
         
       
     
     In a situation where the metal tube  10  delivered from the unwinding unit  20  (i.e., the base material) has an outer diameter of 1.5 mm, a thickness of 0.075 mm and an inner diameter of 1.35 mm, and the metal tube  10  is drawn such that the metal tube  10  to be wound by the winding unit  60  has a target outer diameter of 1.0 mm and a target inner diameter of 0.85 mm while having a constant thickness of 0.075 mm, the hole diameter of the drawing die  42 , as well as the circumferential speeds of the drive capstans  48  and  50 , in each drawing unit  40 , will be set as shown in the table below. 
     
       
         
           
               
               
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                 Ratio of 
               
               
                   
                   
                   
                   
                   
                   
                   
                 circumferential 
               
               
                   
                 Wire 
                 Metal 
                 Metal 
                   
                   
                   
                 speed of drive 
               
               
                   
                 drawing 
                 tube 
                 tube 
                   
                 Circumferential 
                 Circumferential 
                 capstan 48 to 
               
               
                 Wire 
                 die 
                 outer 
                 inner 
                   
                 speed of drive 
                 speed of drive 
                 circumferential 
               
               
                 drawing 
                 diameter 
                 diameter 
                 diameter 
                 Thickness 
                 capstan 48 
                 capstan 50 
                 speed of drive 
               
               
                 unit 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm) 
                 (mm/min.) 
                 (mm/min.) 
                 capstan 50 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 40-1 
                 1.40 
                 1.40 
                 1.25 
                 0.075 
                 64.91 
                 69.81 
                 0.9298 
               
               
                 40-2 
                 1.30 
                 1.30 
                 1.15 
                 0.075 
                 69.81 
                 75.51 
                 0.9245 
               
               
                 40-3 
                 1.20 
                 1.20 
                 1.05 
                 0.075 
                 75.51 
                 82.22 
                 0.9184 
               
               
                 40-4 
                 1.10 
                 1.10 
                 0.95 
                 0.075 
                 82.22 
                 90.24 
                 0.9111 
               
               
                 40-5 
                 1.00 
                 1.00 
                 0.85 
                 0.075 
                 90.24 
                 100.00 
                 0.9024 
               
               
                   
               
            
           
         
       
     
     The drawing unit controller  140  further controls, based on the pivot angle of each dancer arm  56  detected by the potentiometer  158 , the rotation speed of the drive capstan  50  provided in the drawing unit  40  located before each dancer arm  56 . 
     Specifically, when a difference in the circumferential speed occurs between the drive capstan  50  and the drive capstan  48  located before and after a dancer section  52 , the dancer arm  56  of such dancer section  52  pivots based on such difference. For example, if the circumferential speed of the drive capstan  50  is slower than the circumferential speed of the drive capstan  48 , the dancer arm  56  pivots upward in  FIG.  1   . The potentiometer  158  (an example of a speed difference detecting section) detects the difference (pivot angle), and the drawing unit controller  140  then controls the rotation speed of the drive capstan  50  located before the dancer section  52  based on the detected difference. 
     With such configuration, since the circumferential speeds of the drive capstans  48  and  50  located before and after the dancer section  52  can be maintained so as to be substantially constant, the inner diameter of the metal tube  10  passing through each drawing die  42  can be controlled to a desired size. 
     The dancer section  52  (an example of a tension applying section) may control the tension of the metal tube  10  between the two adjacent drawing units  40 . With such configuration, the tension applied to the metal tube  10  by the previous drawing unit  40  can be reset and the metal tube  10  can be delivered to the next drawing unit  40  with a predetermined tension applied thereto. 
     Operation of Winding Unit  60   
     The winding unit  60  winds up the metal tube  10  such that the metal tube  10  which has been delivered from the unwinding unit  20  and then drawn in each drawing unit  40  has an approximately constant wire speed. That is, the winding unit controller  160  controls the speed of rotation of the winding motor  180  so that the wire speed of the metal tube  10  supplied to the winding unit  60  is maintained at an approximately constant value. 
     In the present embodiment, the winding unit controller  160  controls the speed of rotation of the winding motor  180 , for example, by using the speed of rotation of (i.e., the wire speed of the metal tube  10  at) the drive capstan  50 - n  which is located before the winding unit  60  as a feed-forward signal, and the pivot angle of the dancer arm  76  as a feedback signal. More specifically, the wire speed of the metal tube  10  passing through the drive capstan  50 - n  is detected by an encoder provided at the drive capstan  50 - n  and is fed to the drawing unit controller  140 . Then, the winding unit controller  160  generates a speed signal indicative of the detected wire speed as a feed-forward signal and supplies it to the winding motor  180  to control the rotation of the winding motor  180 . 
     Further, the winding unit controller  160  generates a feedback signal based on the pivot angle detected by the potentiometer  178  and controls the rotation of the winding motor  180 . 
     More specifically, the winding unit controller  160  calculates a pivot angle deviation between the pivot angle of the dancer arm  76  detected by the potentiometer  178  and the pivot angle of the dancer arm  76  at the reference position. Then, the winding unit controller  160  determines the rotation speed of the winding motor  180  so as to approximate the pivot angle deviation to zero, and provides a rotation speed command to the winding motor  180  based on the determined rotation speed. Using the pivot angle deviation as a feedback signal, the winding unit controller  160  controls the rotation speed of the winding motor  180  through P control, PI control, PID control, etc. 
     By way of the above operation, the winding unit  60  can maintain the wire speed of the metal tube  10  delivered from the drawing unit  40 - n  (in other words, the wire speed of the metal tube  10  at the drive capstan  50 - n ) at an approximately constant value regardless of the amount of metal tube  10  already wound around the winding bobbin  80 , and at the same time, wind up the metal tube  10  around the winding bobbin  80  so that there is no gap between the above wire speed and the wire speed of the metal tube  10  passing through the guide rollers  68  and  70 . 
     The drawing machine  100  according to the present embodiment is capable of producing a metal tube  10  having a desired outer diameter and inner diameter due to the above-mentioned configurations and operations. 
     The examples and applications described above through the embodiment of the invention can be combined as appropriate depending on the intended purpose of use, or used by making various modifications or improvements, and the invention is not limited to the above-described embodiment. It will be apparent from the description of the attached claims that such combinations or embodiments in which such modifications or improvements are made can also fall within the technical scope of the invention. 
     REFERENCE SIGNS LIST 
       10  . . . metal tube;  20  . . . unwinding unit;  22  . . . unwinding bobbin;  24 ,  26 ,  28  . . . guide roller;  32  . . . dancer section;  34  . . . dancer roller;  36  . . . dancer arm;  38  . . . torque motor;  40  . . . drawing unit;  42  . . . drawing die;  44 ,  46  . . . guide roller;  50  . . . drive capstan;  60  . . . winding unit;  66 ,  68 ,  70  . . . guide roller;  72  . . . dancer section;  74  . . . dancer roller;  76  . . . dancer arm;  78  . . . torque motor;  80  . . . winding bobbin;  100  . . . drawing machine;  110  . . . system controller;  120  . . . unwinding unit controller;  122  . . . unwinding motor;  138  . . . potentiometer;  140  . . . drawing unit controller;  150  . . . drive motor;  160  . . . winding unit controller;  178  . . . potentiometer;  180  . . . winding motor;  200  . . . control unit.