Patent Publication Number: US-4543865-A

Title: Punch press tool for punching small holes in a strip of sheet metal, and obtaining small punched blanks

Description:
The present invention relates to a punch press tool, and more particularly to a punch press tool capable of cutting punched blanks of small diameter, for example a diameter in the order of a few millimeters, for example 2-3 mm. 
     BACKGROUND 
     Various types of punch press tools are known in order to punch holes into a strip of sheet metal, and to derive punched blanks from the strip of sheet metal. Such punch press tools are used frequently in automatic punch presses and punching machines. The usual machines of this type have elements to guide a strip therethrough, and a reciprocating punch press tool is guided over the strip into a die in predetermined position, for example to punch a sequence of holes in the marginal portions of a band or tape of the metal strip. The desired result may be, either, the punching of holes into the metal strip or to obtain punched blanks. 
     Punch presses of this type have the problem that the punched blanks must be reliably carried away from the punch die, for example through a longitudinal bore therein, as soon as the punch press tool reciprocates. This problem arises regardless of whether the punched blanks are to be considered scrap--for example when the purpose of the tool is to punch a sequence of holes into the metal strip--or if the punched blanks themselves are to be the articles desired by the punching operation. Usually, the tool, and hence the punched blanks, will--in cross section--be circular. 
     Punching tools of this type, in which blanks are to be punched in excess of several millimeters diameter, frequently use a pressure element located centrally within the reciprocating punch press which, upon or after the punching operation, pushes the punched blank away from the surface of the punch press tool. This element, for example, is a spring or the like. When the size of the holes, and hence the size of the blanks, becomes very small, for example with circular diameters of 3 mm, 2 mm, or less, difficulties arise since it is no longer possible to place push-away or similar elements within the punch tool itself. As the punched blanks become smaller, however, the tendency of the blanks to adhere to the end face of the punch tool increases, primarily due to their light weight. Adhesion of punched blanks is additionally difficult to control if the surfaces of the strips from which the holes are punched are slightly adhesive, for example by having a thin coating of grease or the like. Various attempts have been made to prevent adhesion of the punched blanks to the punch press tool by special shaping of the cutting surface of the punch press tool. For example, it has been proposed to form the reciprocating punch tool with concave surfaces, or with converging surfaces, primarily in order to shape the punched blanks to have some angled portions, in order to prevent jamming of the punched blanks in the bore of the punching die and, further, to lift off the reciprocating punch press due to the springiness of the punched blank as it snaps back from the angled or bowed position to a flat position. Making the punch die in this shape, however, decreases its stability and, particularly if the punch die is of indented or roof-like shape, causes rapid wear of the cutting edge portions which project beyond other portions. 
     It has been attempted to decrease the cutting gap between the bore of the die and the reciprocating punch press tool. Closer tolerances of the tool-die combination, however, increases the wear on both of the tool elements, and thus decreases the effective operating time of the punch-die combination before re-grinding or re-surfacing of the elements is required. Further, the power requirements to effect punching are increased. 
     THE INVENTION 
     It is an object to improve a punch press-punch die tool combination so that the punched blanks are not held in position in the punched die or against the end face of the punch press tool, while retaining the dimensions which, in accordance with the state of technology, have been determined to be of maximum effectiveness for efficient punching of various metals of given thicknesses and hardness, and which is capable of punching blanks having nominal diameters of only a few millimeters, and, for example, diameters in the range of 3, or 2 mm, and even less. 
     Briefly, the die element of the punch press has a die well, or die opening, which is formed with at least one shallow groove extending in the direction of severing or punching movement of the reciprocating punch element for a length which is at least as long as the depth of projection of the punch element into the punch die. 
     The groove may extend parallel to the direction of punching operation, or it may extend in spiral or inclined form, with small circumferential angle with respect to the axis of the bore or well of the die, and through and into an extending conical portion of the die. 
     It has been found that the punched blank will be formed with a tiny projection due to the presence of the groove which, at the end of the punch stroke of the punch die, has the tendency to jam against the wall of the bore or well of the die, so that, upon upwardly reciprocating movement of the punch press tool, the blank will be retained securely within the bore or well of the die. If the groove extends parallel to the direction of punching movement, for example exactly axially, the punched blank is pushed over its entire length and beyond the longitudinal extent of the groove. If the groove extends spirally, the punched blank will clamp or jam in the groove since, upon return movement of the punch press tool, which is non-rotary, the blank cannot follow the linear return movement of the punch tool, since it cannot rotate with respect to the punch tool due to friction against the walls of the punch die and the groove. 
     The simple provision of a shallow groove, extending axially for the length of the stroke of the punch tool, or longer if spirally, insures that the punched blanks will not follow the punch press or punch stamp upon its return reciprocating movement, so that damage or malfunction is reliably prevented. 
     It has been found that the holes punched from sheet metal element are identical to those which are punched from dies which do not have the groove, since the actual punching operation is determined by the cutting edge of the reciprocating press punch element. 
     It is, of course, possible to form more than one such groove--extending axially or slightly spiralled--around the circumference of the punching well or bore. Preferably, the grooves will be uniformly distributed around the circumference and, for example, two or three are appropriate. 
     If it is desired to utilize a spiral groove, then good results have been achieved if the angle between the spiral and the axis of the bore is in the range of between 10° and 20°. Particularly good results have been obtained with an angle of inclination of about 12°. 
     The lower die element preferably enlarges conically slightly, for example with a cone angle of between 8 minutes to 1°. 
     The dimension of the groove itself is highly dependent on the material from which the punched blanks are to be cut, its hardness, ductility, strength, and material composition, as well as, of course, the thickness thereof. Groove depths in the range of between 0.005 to 0.2 mm have been found suitable for various materials and thicknesses. The relationship of the depth of the groove to the strength of the material to be worked--which includes a thickness factor--can be readily determined experimentally; in thin sheet-metal elements, a shallow groove is sufficient; sheet metal strips which are thicker, or are made of tougher material, are preferably worked with grooves of deeper dimension. The width of the groove is also determined in part by the depth thereof; as the depth increases, the width likewise increases, and a suitable relation of width to depth is in the order of between 1:1 to 5:1, although other dimensions are equally suitable. The depth and width relationships can be readily determined by a few simple try-outs with the respective strips and strip materials to be cut or punched. 
    
    
     DRAWINGS 
     FIG. 1 is a schematic longitudinal view through a punch press in accordance with the invention; 
     FIG. 2 is a top view of the lower punch die of FIG. 1, and 
     FIG. 3 is a view similar to FIG. 1, and showing another embodiment. 
    
    
     The punch press has an upper punch press tool 1, in form of a punch plunger of, for example, circular diameter. The plunger 1 reciprocates as schematically indicated by arrow 2. It has a flat cutting surface 3, to cooperate with a punch die 4. The punch die 4 has a bore or opening or well 5 therein. The opening 5 expands conically, for example by a cone angle of 0.5°. The conical angle is shown exaggerated in the drawing. 
     The conical bore 5 merges into a cylindrical bore 7 of larger diameter, large enough to permit the stamped blanks to fall freely through the opening 7. 
     A sheet-metal strip 6 is placed across the mouth or upper end of the opening 5. Upon reciprocation of the plunger 1, a blank 11 will be cut from the sheet-metal strip 6. 
     In accordance with the invention, the conical bore 5 is formed with two spirally extending inclined grooves 8. The grooves 8 have a depth 9 which depends on the thickness of the sheet metal 6, as well as on the diameter of the punched blanks. The depth of the grooves 8, for extremely small blanks, may be as small 0.001 mm, but may extend up to about 0.2 mm. The lower values are used for thin sheet-metal elements and very small cross-sectional dimensions of the blanks 11. The higher levels are suitable for thicker sheets 6 and larger diameters. The width 10 of the grooves depends, proportionately, on the depth and may, for example, be between about the same to about four to five times the depth. 
     OPERATION 
     The blank 11, which is punched out of strip 6, upon reciprocation of the press plunger or punch plunger 1, will be formed with a small projection or nose. As the plunger 1 is inserted into the bore and continues to push the blank 11, the projection will become wedged at its edge in the groove 8, since the blank 11 cannot follow the inclined or spiral path of the groove 8 by rotating due to the high frictional forces with respect to the cutting plunger 1. Consequently, the blank 11 will be wedged in the die 4, and the plunger 1 can freely reciprocate upwardly without adhesion by the blank 11. Upon a subsequent cutting operation, the blanks 11 which are already within the bore 5 are pushed downwardly by the subsequently cut blanks 11 until, due to the increase in diameter as a result of the conical enlargement of the bore 5, the clamping or wedging effect of the groove and the nose or projection on the blanks will be lost, and the blanks can fall freely downwardly through the enlarged opening 7. The plunger does not rotate. 
     The groove 8 need not extend in a spiral direction. A groove 8&#39; can be cut longitudinally (see FIG. 3), that is, parallel to the axis 12 of the bore 5. If so made, then preferably the length of the groove 8&#39; from the lower surface of the sheet-metal element 6 is so dimensioned that it corresponds to the depth of insertion of the plunger 1 into the bore 5. For example, if the plunger 1 extends into the die 4 by 1 mm, then the longitudinal extent of the groove 8&#39; should be 1 mm, and end there. The operating effect of the groove 8&#39; will be the same as that of the spiral groove 8 of FIG. 1 since the projection formed by the groove 8&#39; will be pushed by the cutting plunger over the edge of the groove 8&#39;, so that the projection will wedge itself in the then smooth wall of the opening 5 of the die 4. As subsequent blanks 11 are cut, the first blank, downwardly, will again be pushed into the region of conical enlargement where it can fall freely through the opening 7. 
     A suitable cone angle of the bore 5 with respect to the axis 12 is in the range of from about 8 minutes to 1°. The exact cone angle will depend on the characteristics of the material 6. It is, of course, desirable to permit the punched parts to become released from the walls of the well or bore 5 as quickly as possible, so that the power requirement to push the blank downwardly by the subsequent blanks may remain as small as possible. The relative dimensions of the cross section of the upper portion of the bore 5, and of the plunger or cutting punch 1, may be selected as well known and as customary in punching technology. 
     The hole formed in the sheet metal element 6 will remain circular and will not be deformed due to the presence of the groove since the shape of the hole is determined by the diameter of the punch or plunger element 1. If a spiral groove is selected, a preferred angle of inclination with respect to axis 12 is 12°, although the angle may vary widely, and, suitably, between 10° to 20°. 
     Various changes and modifications may be made within the scope of the inventive concept. 
     For punching holes of 2,0 mm in a strip of lenouve (metal), having a thickness of 0,5 mm a groove 8 of 0,02 mm width, 3,5 mm depth, is suitable.