Patent Publication Number: US-4317351-A

Title: Methods of and/or apparatus for punching holes in sheet metal

Description:
This invention relates to methods of and/or apparatus for punching holes in sheet metal and has been devised particularly, though not solely, for punching holes in perforated tubs for automatic washing machines. 
     BACKGROUND OF THE INVENTION 
     Hole forming machines using a rotatable member carrying punches and a back up wheel which carries a continuing slug receiving slot have been disclosed in U.S. Pat. No. 3,142,216. 
     In such a machine complete separation of the slug punched from the hole in the parent metal is not always effected because the punch causes the slug to hinge at a hinge point which hinge point is clear of the punch path because of adjacent deformation of the parent metal. 
     It is an object of the present invention to provide a method and/or apparatus for punching holes in sheet metal which will at least provide the public with a useful choice. 
     BRIEF SUMMARY OF THE INVENTION 
     Accordingly in one aspect the invention consists of a method of punching holes in sheet metal using a punching roller having a series of radial punches on a cylindrical surface thereof and a backing roller, the backing roller having for each row of holes to be made two raised lands and a groove therebetween, the groove having a bottom at a predetermined depth, said method comprising the steps of running sheet metal through the nip between the punching roller and the backing roller so that each punch in the punching roller causes a slug of metal to be cracked out of the parent metal until one edge of said slug of metal engages the bottom of the groove at a pivot point whereupon the slug of metal hinges on said pivot point causing complete separation of the slug of metal from the parent metal. 
     In a further aspect the invention comprises apparatus for punching holes in sheet metal, said apparatus comprising a frame, a punching roller mounted on a shaft rotatable in said frame and carrying a plurality of punches on the cylindrical surface thereof, a backing roller mounted on a shaft rotatable in said frame and spaced away from the punching roller by a distance substantially equal to the thickness of metal through which holes are to be punched, said backing roller having for each row of holes to be punched a groove between lands or annular rings thereon said groove having a base disposed relative to the length of said punches such that on a sheet of metal being passed through the nip between said punching roller and said backing roller each punch punches a slug of metal so as to be cracked out of the parent metal until one edge of the slug contacts the base of said groove at a point of contact which acts as a pivot point on which the slug pivots to cause the slug to be removed completely from the parent metal by further action of the punch. 
     To those skilled in the art to which this invention relates many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the scope of the invention as defined in the appended claims. The disclosures and the description herein are purely illustrative and are not intended to be in any sense limiting. 
    
    
     BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
     One preferred form of the invention will now be described with reference to the accompanying drawings in which, 
     FIG. 1 is a diagrammatic part cross section of a pair of rollers having sheet metal therebetween with a punch of one roller passing through the sheet metal, 
     FIG. 2 is a partially diagrammatic side view of apparatus according to the invention and, 
     FIG. 3 is an enlarged view showing the punching action according to the invention. 
    
    
     DETAILED DESCRIPTION ACCORDING TO THE INVENTION 
     Referring to the drawings apparatus is constructed as follows: 
     A frame (not shown) is provided having bearings and shafts 1 and 2 run in these bearings. The shaft 1 carries a punching roller 3 having a plurality of radial punches 4 on a cylindrical surface 5 thereof. A backing roller 6 has a pair of lands 7 arranged either side of a groove 8. The groove 8 has a base 14. The rollers 3 and 6 are spaced apart by a distance sufficient to receive therebetween a sheet of metal 10 in which holes are to be punched as will be described shortly. Although the rollers 3 &amp; 6 are shown as having only one row of punches and only some of the punches on the circumference of roller 3 are shown, it will be apparent that the rollers may have any number of rows of punches arranged longitudinally along the length of the cylindrical roller 3. The rollers 3 &amp; 6 may be driven either by exerting tension on the sheet of metal 10 and depending on the engagement of the rollers 3 &amp; 6 therewith or by driving one or both, preferably the punch roller 3 with any convenient form of drive. 
     The operation of the apparatus is as follows: 
     As the punches 4 are moved over the surface of the piece of metal 10 placed in the nip between the cylindrical surface of the roller 3 and the cylindrical surfaces of the lands 7, roller 3 which as stated is driven by any suitable driving source causes the sheet of metal to pass through the nip and at the same time the punches 4 will be caused to enter the surface of the metal 10. Reffering now to FIG. 3, a punch 4 is shown as having partially cracked out a slug of metal 12 until a point 13 on the slug of metal contacts the surface 14 which is the base of the groove 8 on the backing roller 6. When this occurs the point 13 of the slug 12 acts as a pivot and the rear edge 15 of the punch 4 exerts a force causing the slug to pivot on pivot 13 thus breaking off the cracked out portion and separating the slug 12 from the parent metal at 15, as may be seen at 17 in FIG. 3. The consequence is that the slug of metal is completely free from the parent metal 10 and may fall away as shown at 18 and 19 (FIG. 3). The result is a clean cut hole in a depression 20 (FIG. 1). The depression is due to the lack of support for the metal 10 and to make the depression reasonably symmetrical the groove 8 is made substantially wider than the width of the punches 4. 
     If the engagement of the slug 12 with the base 14 of the groove 8 then complete detachment of the slug 12 from the parent does not always occur because the punch pushes the slug away, the slug 12 hinging at the point 15 and in many cases the hinging metal does not positively break away, the punch 4 passing through the hole 21 while the higher point 15 is to one side of the punch. It is to be noted that the punches 4 may be shaped in cross section to any convenient shape to form e.g. round holes or elongated slots and that the faces 23 of the punches 4 are preferably simple flat faces normal to the punch length simplifying sharpening of the punches. The punches may be screwed into female threaded holes 30 in the roller 3 with fixed nuts 31 provided in recesses 32 in the roller bearing against removable shims (not shown) to adjust the length of the punch projections relative to the base 8. 
     The present invention has been developed particularly though not solely for use in perforating the perforated drum of an automatic clothes washing machine and as a result of the invention such perforations can be made simply and quickly by running a sheet of metal through a machine constructed as above described and having the necessary numbers of rows of punches and backing depressions. 
     Furthermore the invention obviates the requirement of a multiplicity of close fitting punches and dies thus reducing both capital and maintenance costs. 
     Punch lengths, groove depths, and groove widths are selected to obtain complete slug separation having regard to the particular material characteristics and metal thickness being used.