Patent Publication Number: US-6698324-B2

Title: Punch unit for punching a hole in a soft metal sheet

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a punch unit for punching a hole (hereinafter referred to as “punch hole”) in a soft metal sheet such as an aluminum sheet and the like (hereinafter also referred to as “workpiece”), wherein the punch hole assumes a circular shape or any other suitable shape, and the soft metal sheet having been punched is used in preparing a printing plate in the field of printing plate making. 
     2. Description of the Related Art 
     Heretofore, in order to punch a soft metal sheet such as an aluminum sheet and the like used in preparing a printing plate in the field of lithographic printing plate making, a punch unit of a reciprocating type has been used as is in the case where a piece of paper is punched as a workpiece. 
     However, in case that the soft metal sheet such as the aluminum sheet and the like is punched by using a conventional punch unit heretofore having been used in punching a piece of paper, the conventional punch unit has its punch and its die remarkably shortened in service life. Further, in the case of the conventional punch unit, the soft metal sheet tends to stick to the punch when punched, which impairs the quality of a punch hole of the soft metal sheet thus punched. This is a problem inherent in the conventional punch unit. 
     In order to solve this problem, it is necessary and important to set a radial clearance between the punch and the die at an appropriate value. However, in the conventional punch unit, a punch guide and a die block thereof are formed into two different components, and then assembled into a die assembly by using suitable fastening means such as rivets, screws and the like. Due to this, it is difficult for the conventional punch unit to precisely control the radial clearance between the punch and the die. When the punch is deviated from its correct position with respect to the corresponding die, a punch hole of the soft metal sheet thus punched becomes poor in quality. 
     SUMMARY OF THE INVENTION 
     Consequently, it is an object of the present invention to provide a punch unit for punching a precise high-quality punch hole in a soft metal sheet in a steady manner. 
     In accordance with a first aspect of the present invention, the above object of the present invention is accomplished by providing: 
     In a punch unit of a reciprocating type provided with a punch and a die both adapted to punch a punch hole in a soft metal sheet, wherein the punch is slidably guided by a punch guide and reciprocally driven in a condition in which the die cooperates with the punch, the improvement wherein: the punch guide and the die are integrally formed into a block. 
     As described above, in the conventional punch unit, since the die unit is constructed of two different components, that is, the punch guide and the die block which are assembled into the die assembly by using suitable fastening means such as rivets, screws and the like, it is difficult to precisely control the radial clearance between the punch and the die when the soft metal sheet is punched. Further, in the conventional punch unit, there is a fear that the punch deviates from its correct position with respect to the die since the die assembly assembled from two different components is relatively poor in rigidity. 
     In contrast with this, in the punch unit of the present invention, such two different components, that is, the punch guide and the die are integrally formed into a single block. Due to this, the punch unit of the present invention is excellent in rigidity, which solves the above fear inherent in the conventional punch unit. 
     In accordance with a second aspect of the present invention, the above object of the present invention is accomplished by providing: 
     the punch unit as set forth in the first aspect of the present invention, wherein the punch guide comprises: 
     a lower guide for guiding the punch in the side of the die, wherein the lower guide is provided with a through-hole window, the through-hole window extending in a direction perpendicular to a longitudinal axis of a bore of the lower guide, whereby an upper portion of the through-hole window forms the lower guide and a lower portion of the through-hole window forms a gate, the gate serving as a part of the cleaning means for removing the fragment of the soft metal sheet from the punch in the punching operation; and 
     an upper guide for guiding the punch in its driven side. 
     In the punch unit of the present invention has the above construction, when the punch is reciprocally driven in a vertical plane to punch a punch hole in the soft metal sheet, the fragment of the soft metal sheet having stuck to the punch is scraped off the punch by the cutting edge portion of the gate and discharged outward through a workpiece passage and the through-hole window. Consequently, in the punching operation, it is possible to reduce a frictional drag of the punch caused by such fragment of the soft metal sheet. This also improves the punch hole in quality. Since the gate serves as a cleaning blade during the punching operation, it is possible to reduce the gate in thickness. 
     Preferably: the soft metal sheet has a thickness of less than or equal to 0.5 mm; and, a radial clearance between the punch and the die is set at a value of from 4 to 15% of a thickness of the soft metal sheet. 
     In the conventional punch unit for punching a piece of paper and the like, a radial clearance between the punch and the die is set at a value of from 5 to 10 μm. Consequently, when a soft metal sheet such as an aluminum sheet and the like, has a thickness less than 0.5 mm is punched by using the conventional punch unit, a secondary shearing action occurs in the punch hole to have the soft metal sheet often stick to the punch, which prevents the punch from producing a high-quality punch hole and considerably reduces the service life of each of the punch and the die. 
     In view of the above problems, in the punch unit of the present invention, when the sheet has a thickness less than 0.5 mm is punched, a radial clearance between the punch and the die is set at a value of from 4 to 15% of a thickness of the soft metal sheet, which enables the punch unit of the present invention to punch a high-quality punch hole in the soft metal sheet under its shearing action. The reason why the radial clearance between the punch and the die is set at the above value of from 4 to 15% of a thickness of the soft metal sheet is as follow: namely, when the radial clearance is less than or equal to 4% of the thickness of the soft metal sheet, the punch tends to gall the die. On the other hand, when the radial clearance is more than 15% of the thickness of the soft metal sheet, burrs are produced in the punch hole. Preferably, the radial clearance between the punch and the die should be set at a value of from 6 to 12% of the thickness of the soft metal sheet. In case that the soft metal sheet has a thickness of 0.3 mm for example, the radial clearance should be set at a value of 30±5 μm. 
     Preferably, the punch unit of the present invention is further provided with a sheet hold-down means for holding down the soft metal sheet when the soft metal sheet is punched, which improves the punch hole in quality. 
     After the soft metal sheet is punched, the soft metal sheet thus punched tends to stick to the punch due to its softness when the punch is retracted from the soft metal sheet. Such sticking action of the soft metal sheet to the punch often results in formation of burrs, wherein such burrs assume a lampshade-like shape as a whole and project outward from a peripheral portion of the punch hole. This prevents the punch hole from being improved in quality and produced in a steady manner. 
     In view of the above problem, the sheet hold-down means is introduced in the punch unit of the present invention for holding down the soft metal sheet when the sheet is punched, so that the soft metal sheet thus punched is prevented from being lifted by the punch after completion of the punching operation thereof, whereby the punch hole is improved in quality. 
     Preferably, in the punch unit of the present invention, the punch is provided with a cutting edge portion assuming a circularly-curved shape in its font end portion for improving the quality of the punch hole. 
     Heretofore, as shown in an upper portion of FIG. 7A, a conventional punch has been provided with a wedge-shaped cutting edge portion  2   a ′ in its front end portion in order to reduce a punching load imposed on the punch in a punching operation of a workpiece. However, when the workpiece is punched by using such a conventional punch to form a punch hole, a pair of notches “g” are often formed in a peripheral portion of the punch hole to impair the quality of the punch hole, as shown in a lower portion of FIG.  7 A. The cause found by the inventor of the present invention for such formation of the notches “g” in the peripheral portion of the punch hole is the presence of a wedge-shaped concave bottom portion “e” in a front end portion of the conventional punch, which concave bottom portion “e” of the conventional punch locally hits the workpiece at the end of the punching operation of the workpiece to produce these defective notches “g”. In view of the above, in the punch unit of the present invention, as shown in an upper portion of FIG. 7B, a circularly-curved cutting edge portion  2   a  (a radius of which is denoted by the reference letter “R”) is provided in a front end portion of its punch. Since the punch of the punch unit of the present invention has such circularly-curved cutting edge portion  2   a  in its front end, it is possible for the punch unit of the present invention to prevent the punch from locally hitting the workpiece (i.e., the soft metal sheet). This prevents the notches “g” from being formed in the peripheral portion of the punch hole formed in the soft metal sheet by using the punch unit of the present invention. Consequently, it is possible for the punch unit of the present invention to solve the above problem inherent in the conventional punch. 
     The punch unit of the present invention is most effectively used when the workpiece or soft metal sheet to be punched is made of aluminum or aluminum alloys. 
     As described above, the punch unit of the present invention is capable of punching a high-quality punch hole in the soft metal sheet, and also capable of remarkably increasing the service life of each of its punch and its die. Consequently, it is possible for the punch unit of the present invention to remarkably improve a punching operation of an aluminum sheet and the like in accuracy and productivity, which punching operation is required to produce a printing plate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, advantages and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a front view of an embodiment of the punch unit of the present invention for punching a punch hole in the soft metal sheet, wherein a sheet hold-down means of the punch unit is removed; 
     FIG. 2 is a cross-sectional view of the punch unit of the present invention, taken along the line of X—X of FIG. 1; 
     FIG. 3 is a front view of the punch unit of the present invention shown in FIG. 1, illustrating the sheet hold-down means of the punch unit; 
     FIG. 4 is a cross-sectional view of the punch unit of the present invention, taken along the line Y—Y of FIG. 3; 
     FIG. 5 is a partially enlarged front view of a die portion of the punch unit of the present invention shown in FIG. 1; 
     FIG. 6 is a cross-sectional view of another embodiment of the die portion of the punch unit of the present invention shown in FIG. 5; 
     FIG. 7A is a front view of a conventional punch, illustrating the shape of a cutting edge portion of the conventional punch and its effect; and 
     FIG. 7B is a front view of the punch of the punch unit of the present invention, illustrating the shape of a cutting edge portion of the punch and its effect. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The best modes for carrying out the present invention will be described in detail using embodiments of the present invention with reference to the accompanying drawings. 
     The present invention may, however, be embodied in various different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present invention to those skilled in the art. 
     In the accompanying drawings, like reference numerals refer to like parts throughout the drawings. 
     First Embodiment 
     A first embodiment of a punch unit  1  of the present invention for punching a punch hole in a soft metal sheet will be described with reference to the drawings. 
     With reference to FIG. 1, a main frame of the punch unit  1  will be first described in construction. As shown in FIG. 1, the punch unit  1  is constructed of: a punch block  11  which is integrally formed with an upper punch guide  12 , a lower punch guide  13   a  and a die block  14 ; and, a motor bracket  21  on which a motor  22 , a drive portion  23  and like components are mounted. As is clear from FIG. 1, the motor bracket  21  is fixedly mounted on the punch block  11  by using of a plurality of screws  28  to form the main frame of the punch unit  1 . 
     The punch block  11  assumes a substantially hollow square block shape, and is provided with a horizontal workpiece passage  16  in a left corner portion of its lower section, as viewed in FIG.  1 . As is clear from FIG. 1, this horizontal workpiece passage  16  has a workpiece-receiving opening formed in its left end portion. Further, preferably, the workpiece-receiving opening of the passage  16  is chamfered in a manner shown in FIG. 1 to facilitate entrance of the workpiece (i.e., a soft metal sheet) to the passage  16 . 
     The upper punch guide  12  and the lower punch guide  13   a  represent an upper guide bore and a lower guide bore, respectively. Consequently, hereinafter, the upper guide bore and the lower guide bore will be referred to as the guide bores  12  and  13   a , respectively. These guide bores  12 ,  13   a  are formed as through-holes passing through the punch block  11  in a manner such that the guide bores  12 ,  13   a  extend in a direction perpendicular to a plane in which the workpiece (i.e., soft metal sheet) extends. As is clear from FIG. 1, the lower guide bore  13   a  is disposed adjacent to an upper portion of the workpiece passage  16 . On the other hand, the upper guide bore  12  is disposed above and spaced apart from the lower guide bore  13   a  in the punch block  11 , so that these guide bores  12 ,  13   a  function to slidably receive therein and guide the punch  2  in the punching operation thereof. 
     In the vicinity of the workpiece passage  16 , there is provided a through-hole window  15 . This window  15  extends in a direction perpendicular to a longitudinal axis of the lower guide bore  13   a . In other words, an upper portion of this window  15  forms the lower guide bore  13   a . On the other hand, a lower portion of the window  15  is disposed adjacent to the workpiece passage  16  to form a gate  13   b . The gate  13   b  has a bore diameter which is slightly larger or substantially equal to a diameter of a cutting edge portion of the punch  2 . Due to the presence of the gate  13   b , it is possible for the punch unit  1  to remove therefrom any fragment of the workpiece having stucked to the punch  2  by using an edge portion of the gate  13   b  when the punch is moved up and down in the punching operation of the workpiece. The thus removed fragment of the workpiece is then discharged out of the punch unit  1  through both the workpiece passage  16  and the through-hole window  15 , so that frictional drag of the punch  2  due to the presence of the fragment stuck thereto is remarkably reduced in the punch unit  1 . Incidentally, since the gate  13   b  serves as a cleaning blade, it is possible for the gate  13   b  to have its wall thickness be small. 
     In the punch unit  1  having the above construction, the punch  2  has its upper and its lower portion slidably received in the upper guide bore  12  and the lower guide bore  13   a , respectively. Consequently, it is possible for the punch unit  1  to move the punch  2  up and down with high accuracy in the punching operation with respect to a radial clearance between the punch  2  and the die  3 , which ensures that the punch unit  1  forms a high-quality punch hole in the workpiece, i.e., soft metal sheet. 
     As shown in FIG. 1, a bearing unit  17  is provided in an upper section of the punch block  11  to pivotally support a swing arm  31  through a pivot  32  therein. 
     The die block  14  is integrally formed with a lower portion of the workpiece passage  16  to form the die  3 . 
     In contrast with this, in the conventional punch unit for punching a punch hole in a piece of paper and the like, in general, its punch guides and its die block are formed into different components. Such different components are then assembled and used in the conventional punch unit. 
     On the other hand, in the punch unit  1  of the present invention, since the punch guides  12 ,  13   a  and die block  14  are integrally formed into a single piece of the punch block  11 , it is possible to have the punch  2  and the die  3  aligned in center with each other with high accuracy, which permits the radial clearance between the punch  2  and the die  3  to be set at an appropriate value. Further, such one-piece punch block  11  is excellent in rigidity, which effectively prevents the punch guides  12 ,  13   a  from deviating in center with respect to the bore of the die  3  and ensures that a high-quality punch hole is formed in the workpiece (i.e., soft metal sheet). 
     As shown in FIG. 5, the punch  2  assumes a stepped-shape in its lower portion in which: the cutting edge portion  2   a  of the punch  2  has a diameter of “d 1 ”; and, a shaft portion  2   b  of the punch  2  has a diameter of “d 2 ” which is larger than the diameter of the cutting edge portion  2   a  of the punch  2 . The shaft portion  2   b  of the punch  2  is slidably received in both the upper punch guide  12  and the lower punch guide  13   a . As shown in FIG. 4, a through-hole  2   c  is formed in an upper portion of the shaft portion  2   b  of the punch  2  to extend in a direction perpendicular to the longitudinal axis of the punch  2 . On the other hand, the swing arm  31  is provided with a pair of elongated holes  31   a  together with a plurality of through-holes  31   b ,  31   c , and has the elongated holes  31   a  aligned in center with the through-hole  2   c  of the punch  2  to permit a punch pin  33  to pass therethrough, as shown in FIG.  4 . Through this punch pin  33 , the punch  2  is rotatably connected with a front end portion (i.e., follower end portion) of the swing arm  33 . 
     In order to align the punch  2  in center with the die  3 , preferably, both the punch guide bores  12 ,  13   a  and the bore of the die  3  are simultaneously formed. Due to this, in the stepped-shape of the punch  2  shown in FIG. 5, the diameter “d 1 ” of the cutting edge portion  2   a  of the punch  2  is preferably smaller than the diameter “d 2 ” of the shaft portion  2   b  of the punch  2 . By using the punch  2  assuming such stepped-shape, it is possible for the punch unit  1  of the present invention to stabilize engagement between the punch guide bores  12 ,  13   a  and the shaft portion  2   b  of the punch  2  with high accuracy, and therefore possible to set the radial clearance between the punch  2  and the die  3  at an appropriate value, which ensures that a high-quality punch hole is formed in the workpiece or soft metal sheet by using the punch unit  1 . 
     Further, as shown in FIGS. 5 and 7B, in the punch unit  1 , since the cutting edge portion  2   a  of the punch  2  assumes the circularly-curved shape (a radius of which is represented by “R” shown in FIG.  7 B), it is possible for the punch unit  1  to prevent the defective notches from being formed in the peripheral portion of the punch hole, in which peripheral portion the punch  2  hits the soft metal sheet (i.e., workpiece) at the end of the punching operation. 
     FIG. 5 shows the details of the die block  14 , provided in which is the die  3  having a cutting edge portion  3   a . This cutting edge portion  3   a  is coaxially arranged with the punch guide bores  12 ,  13   a , and has a diameter of “d 3 ”. Disposed under the cutting edge portion  3   a  of the die  3  is an undercut portion  3   b  having a diameter of “d 4 ” which is larger than the diameter “d 3 ” of the cutting edge portion  3   a  of the die  3 . In the punch unit  1 , the radial clearance between an outer peripheral surface of the punch  2  and an inner peripheral surface of the bore of the die  3  is set at a value of from 4 to 15% of a thickness of the soft metal sheet (workpiece) In other words, the relationship between the outer diameter “d 1 ” of the cutting edge portion  2   a  of the punch  2  and the inner diameter “d 3 ” of the bore of the die  3  may be defined in the following equation: 
     
       
         ( d   3   −d   1 )/2=( T ×a value of from 4 to 15)/100 
       
     
     where: 
     T is a thickness of the soft metal sheet (workpiece) 
     Preferably, the radial clearance between the punch  2  and the die  3  is set at a value of from 6 to 12% of the thickness of the soft metal sheet. 
     In contrast with this, in the conventional punch unit for punching a punch hole in a piece of paper and the like, such radial clearance between the punch and the die is set at a value of from 5 to 10 μm. However, when the conventional punch unit is used to punch the soft metal sheet, a secondary shearing action occurs in the punch hole to have a fragment of the soft metal sheet often stick to the punch, which prevents the punch from producing a high-quality punch hole and considerably reduces the service life of each of the punch and the die. In the conventional punch unit, due to the presence of such fragment stuck to the punch, the punch suffers from a considerable frictional drag during the punching operation of the soft metal sheet, which often stops the operation of the punch. However, it is possible to solve this problem of the frictional drag of the punch by using the punch unit  1  of the present invention. 
     In the punch unit  1  of the present invention, a thickness of the cutting edge portion  3   a  of the die  3  is represented by the reference letter and numeral “t 1 ”. Although the thickness “t 1 ” of the cutting edge portion  3   a  of the die  3  is preferably set at the smallest possible value in order to reduce its interference area (i.e., frictional contact area) with the punch  2 , it is necessary for the cutting edge portion  3   a  of the die  3  to withstand a punching load imposed thereon during the punching operation. Due to this necessity, the punch unit  1  of the present invention uses a value of from 0.5 to 1.2 mm as the thickness of the cutting edge portion  3   a  of the die  3 . The diameter “d 3 ” of the undercut portion  3   b  disposed under the lower portion of the cutting edge portion  3   a  of the die  3  is set at a value slightly larger than the inner diameter “d 1 ” of the bore of the die  3  to permit a chad (i.e., a small metal piece formed when the punch hole is punched in the soft metal sheet, i.e., workpiece) to freely drop out of the undercut portion  3   b.    
     In the first embodiment, though the die  3  assumes a two-stepped bore shape, it is also possible for the die  3  to assume any other shape, for example such as one shown in FIG. 6, in which one: a land portion “a” and an undercut angle portion “b” are provided. 
     On the other hand, the motor bracket  21  is formed of a metal plate, and assumes a U-shaped form. The motor bracket  21  and the punch block  11  are assembled together using a plurality of screws  28 . The motor bracket  21  is provided with an end plate portion  21   a  on which the motor  22  is fixedly mounted using four pieces of screws  29 . 
     A thickness of the undercut portion  3   b  is represented by the reference letter and numeral “t 2 ”. 
     An eccentric cam  23  is fixedly mounted on a rotating shaft of a motor  22 . On the other hand, a link  24  is rotatably mounted on an outer peripheral surface of this eccentric cam  23 . The link  24  is provided with a link opening  24   a . This link opening  24   a  is aligned in center with the through-holes  31   c  of a drive end portion of the swing arm  31  to permit a link pin  34  to pass therethrough, so that the link  24  is rotatably engaged with the swing arm  31  through the link pin  34 . 
     As shown in FIG. 2, the swing arm  31  assumes a channel-like shape in cross section, and has its follower end portion disposed in the side of the punch  2 . The swing arm  31  has an intermediate portion of this follower end portion rotatably mounted on the pivot  32 . This pivot  32  passes through the bearing unit  17  disposed in the upper portion of the punch block  11 . The follower end portion of the swing arm  31  is provided with the elongated holes  31   a  which are aligned in center with the through-hole  2   c  of the punch  2  to rotatably receive therein the punch pin  33  in a manner such that the punch  2  is capable of moving up and down. On the other hand, as described above, the drive end portion of the swing arm  31  is rotatably engaged with the link  24 . Further, as is clear from FIG. 4, the follower end portion of the swing arm  31  is provided with a pair of fin portions  31   d . Each of these fin portions  31   d  is used to suspend therefrom a hanging rod  7  through which the sheet hold-down means  5  is supported so as to be movable up and down. 
     On the other hand, the motor  22  is energized and controlled by a motor drive and control means or circuit  25  (shown in FIG.  1 ). When the eccentric cam  23  is rotatably driven by the motor  22 , the swing arm  31  is swingably driven on the pivot  32  by the eccentric cam  23  through the link  24  and the link pin  34 . As a result, the punch  2  is reciprocally driven by the swing arm  31  through the elongated holes  31   a  and the punch pin  33  so as to be moved up and down. Due to this arrangement, it is possible for the punch unit  1  to punch a high-quality punch hole in the workpiece or soft metal sheet (not shown) having been inserted into the workpiece passage  16 . After completion of the punching operation, a detection means (not shown) detects a predetermined angular position of the eccentric cam  23 . According to detection of such predetermined angular position of the eccentric cam  23 , the motor stops in operation to have the punch  2  stopped at its upper dead point corresponding to a standby position of the punch  2 . 
     Further, the punch drive and control circuit  25  is also capable of energizing the motor  22  in a manner such that the motor  22  permits the punch  2  to perform its lost motion at predetermined time intervals. For example, after every 10 times of punching operations of the workpiece or soft metal sheet, the punch  2  performs its lost motion three times to remove the fragment of the soft metal sheet having stuck to the punch  2  by using the upper and the lower edge of the gate  13   b , which enables the punch  2  to perform its cleaning operation. Alternatively, it is also possible for the punch unit  1  to permit an operator to turn on a switch (not shown) at the beginning of the punching operation or to turn off the switch (not shown) at the end of the punching operation, so that the punch  2  performs its automatic lost motion to clean itself. 
     In the punch unit  1  having the above construction, when the eccentric cam  23  is rotatably driven by the motor  22 , the swing arm  31  is swingably driven on the pivot  32  by the eccentric cam  23  through the link  24  and the link pin  34 . As a result, the swing arm  31  drives the punch  2  through the elongated holes  31   a  and the punch pin  33  in a manner such that the punch  2  moves up and down. Due to this, the workpiece or soft metal sheet (not shown) having inserted into the workpiece passage  16  is punched by the punch  2 . After completion of the punching operation, the detection means (not shown) detects the predetermined angular position of the eccentric cam  23  to stop the motor  22  in a manner such that the punch  2  is stopped at its upper dead point corresponding to the standby position of the punch  2 . 
     Now, the sheet hold-down means  5  of the punch unit  1  will be described in detail with reference to FIGS. 3 and 4. FIG. 3 is a front view of the punch unit  1 , illustrating the sheet hold-down means  5 . On the other hand, FIG. 4 is a cross-sectional view of the punch unit  1 , taken along the line Y—Y of FIG.  3 . 
     The sheet hold-down means  5  is capable of holding down the soft metal sheet when the punch  2  is pulled upward after completion of the punching operation of the soft metal sheet, which enables the punch  2  to remove the fragment of the soft metal sheet therefrom, whereby the punch hole thus formed in the soft metal sheet is remarkably improved in quality. 
     As shown in FIGS. 3 and 4, the sheet hold-down means  5  is provided with a hold-down plate  6 . The hold-down plate  6  is constructed of: a bottom hold-down portion  6   a ; a pair of upright portions adjacent to opposite end portions of the bottom hold-down portion  6   a ; and, a pair of horizontal support portions  6   b  each of which has its inner end portion integrally connected with an upper end portion of each of the upright portions. As a result, the hold-down plate  6  assumes a dish-like shape as a whole, and is provided with a through-hole in its central portion, through which through-hole the punch  2  is capable of freely passing through the hold-down plate  6 . Further, as is clear from FIG. 4, the hold-down plate  6  has each of its support portions  6   b  supported by each of the hanging rods  7  in a manner such that the hold-down plate  6  is vertically movable along the lengths of the hanging rods  7 . On the other hand, each of the hanging rods  7  is connected with each of the fin portions  31   d  of the follower end portion of the swing arm  31 . 
     As shown in FIG. 4, the sheet hold-down plate  6  has its upper surface disposed adjacent to a lower surface of the gate  13   b,  and is provided with a vertically elongated hole  6   c  (shown in FIG. 3) in each of its upright portions. Passed through such vertically elongated holes  6   c  of the sheet hold-down plate  6  are a pair of pins  9 . These pins  9  are embedded in opposite side surfaces of the gate  13   b,  horizontally extend outward therefrom, and pass through the vertically elongated holes  6   c  of the sheet hold-down plate  6  to serve as guide means for guiding the hold-down plate  6  when the hold-down late  6  moves up and down during the punching operation. Movably mounted between the fin portions  31   d  of the swing arm  31  and the support portions  6   a  of the sheet hold-down plate  6  are pair of compression coil springs  8 . The compression coil springs  8  force the sheet hold-down plate  6   b  to be resiliently separated from the fin portions  31   d  of the swing arm  31 . 
     In the punch unit  1  of the present invention having the above construction, at the beginning of the punching operation, the swing arm  31  is swung on the pivot  32  counterclockwise to drive the punch  2  downward. As a result, the sheet hold-down plate  6  is also moved downward through the compression coil springs  8  to hold down the soft metal sheet (i.e., workpiece). After that, the punch  2 , is further moved downward to punch a punch hole in the soft metal sheet. During this punching operation, the sheet hold-down plate  6  continues to hold down the soft metal sheet. 
     After completion of the punching operation, the swing arm  31  begins to swing on the pivot  32  clockwise, so that the punch  2  also begins to move upward. However, the compression coil springs  8  continue to exert their resilient forces on the sheet hold-down plate  6  until the soft metal sheet (workpiece) is separate from the punch  2 . Due to this, the soft metal sheet thus punched is still urged downward by the sheet hold-down plate  6  even after completion of the punching operation. In other words, the sheet hold-down plate  6  forces the soft metal sheet to be separated from the punch  2  when the punch  2  is moved upward after completion of the punching operation. 
     As a result, the soft metal sheet thus punched is separated from the punch  2 . When the soft metal sheet is separated from the punch  2 , the sheet hold-down plate  6  is also moved upward by the hanging rods  7  as the swing  31  continues swinging on the pivot  32  clockwise. Consequently, when the punch  2  is moved upward, the soft metal sheet (not shown) having stuck to the punch  2  is removed from the punch  2 . Due to this, in the punch unit  1 , there is no fear that the peripheral portion of the punch hole is partially extended upward to produce defective burrs in the peripheral portion of the punch hole. This makes it possible for the punch unit  1  to punch a high-quality punch hole in the soft metal sheet (i.e., workpiece). 
     In the above embodiments, though the punch hole assumes a circular shape, it is also possible for the punch hole to assume any other shape, for example such as a square shape, a rectangular shape, an oval shape, an elongated round shape, a keyhole shape and the like. 
     EXAMPLE 
     The punch unit of the present invention for punching a punch hole in a soft metal sheet was used to punch a punch hole having a diameter of 6 mm in each of a first and a second sheet both made of pure aluminum (hereinafter referred to as the first and the second aluminum sheet) having a thickness of 0.2 mm and a thickness of 0.3 mm, respectively. A radial clearance between the punch and the die in the punch unit of the present invention was 15 μm (which corresponds to 7.5% of the thickness of the first aluminum sheet) when the first aluminum sheet having a thickness of 0.2 mm was punched. On the other hand, when the second aluminum sheet having a thickness of 0.3 mm was punched, such radial clearance was 30 μm (which corresponds to 10.0% of the thickness of the second aluminum sheet). 
     In contrast with this, when the conventional punch unit for punching a piece of paper and the like was used for punching these aluminum sheets, both the punch and the die of the conventional punch unit were considerably worn out after 500 pieces of punch holes had been formed in these aluminum sheets. Further, the thus worn-out punch and die of the conventional punch unit stuck to the aluminum sheets to fail to continue its further punching operation. 
     On the other hand, the punch unit of the present invention was found to be capable of continuing its further punching operation even after 30,000 pieces of punch holes had been formed. 
     As described above, in the punch unit  1  of the present invention, since the punch guides  12 ,  13   a  and the die  3  are integrally formed into a single punch block  11 , the die block  14  of the punch unit is excellent in rigidity. This makes it possible for the punch unit  1  of the present invention to precisely set the radial clearance between the punch  2  and the die  3  at an appropriate value, and also makes it possible to prevent the punch  2  from deviating in position from its correct center line with respect to the center line of the die  3 , which ensures that a high-quality punch hole is formed by the punch unit  1  of the present invention. 
     Further, in the punch unit  1  of the present invention, since the sheet hold-down means  5  is provided therein, there is no fear that the soft metal sheet (i.e., workpiece) stuck to the punch  2  is lifted together with the punch  2  after the punch hole is formed in the soft metal sheet. Due to this, it is possible for the punch unit  1  of the present invention to form a high-quality punch hole in the soft metal sheet. 
     Further, even when the punch unit  1  of the present invention is used to form a notch in the soft metal sheet, there is no fear that the punch  2  deviates from its center line in the punching operation due to the provision of the punch guide means  12 ,  13   a  in the punch unit  1 , wherein the punch guide means  12 ,  13   a  functions to keep constant the radial clearance between the punch  2  and the die  3  in the punching operation of the soft metal sheet so that the high-quality punch hole is formed in the soft metal sheet. 
     Still further, in the punch unit  1  of the present invention, since the punch  2  is provided with the circularly-curved cutting edge portion  2   a  in its front end portion, there is no fear that the defective notches are formed in the peripheral portion of the punch hole of the soft metal sheet in contrast with the case of the conventional punch unit which is provided with the punch having a V-shaped cutting edge portion in its front end portion as shown in FIG.  7 A. 
     Although the punch unit  1  of the present invention for punching the soft metal sheet is capable of punching a sheet (i.e., workpiece) of pure copper and the like, the punch unit of the present invention may be most effectively used when it punches the aluminum sheets or aluminum-alloy sheets, used make a printing plate. 
     Finally, the present application claims the Convention Priority based on Japanese Patent Application No. 2000-016880 filed on Jan. 26, 2000, which is herein incorporated by reference.