Relating to the production of small openings in sheet material

Abstract

In order to form a tapered hole in sheet material, an oversize hole is first formed typically by drilling. The sheet material is then compressed or squeezed in the vicinity of the hole using a press tool to cause the material to flow in the direction of the hole to reduce the size of the hole. The compression produces a hole that tapers from both surfaces of the sheet material to the midpoint of the hole, and an annular region of increased thickness around the area subjected to compression. Machining may be used to remove any annular region of increased thickness so that the remaining sheet material has the same thickness. Further material may be removed by machining to reduce the thickness of the sheet material in the vicinity of the hole so the remaining opening tapers from one surface of the sheet to the other.

1 . A method of forming a hole of a particular size in sheet material comprising the steps of: forming an initial oversize hole larger than the said particular size in the sheet material, and compressing the sheet material after the hole has been formed, at least in the immediate vicinity of the hole, to cause the sheet material to flow at least in the direction of the hole, whereby the size of the opening is reduced to form an end product.  
   
   
       2 . A method as claimed in  claim 1  wherein the oversize hole is formed by drilling.  
   
   
       3 . A method as claimed in  claim 2  wherein the oversize hole is on the order of four times larger than the diameter ultimately required of the hole.  
   
   
       4 . A method as claimed in  claim 1  wherein the compression is achieved using a press tool and the sheet material is metal foil.  
   
   
       5 . A method as claimed in  claim 4  wherein the tool has a flat underside and is several times larger in diameter than the pre-formed hole.  
   
   
       6 . A method as claimed in  claim 5  wherein the tool is positioned concentrically over the pre-formed hole before it is pressed into the material.  
   
   
       7 . A method as claimed in  claim 5  wherein the tool material is selected so that the tool is not deformed when pressed into the sheet material.  
   
   
       8 . A method as claimed in  claim 7  wherein the sheet material is stainless steel and the tool is formed from hardened tool steel or tungsten carbide.  
   
   
       9 . A method as claimed in  claim 4  wherein the sheet material is compressed between the tool and a base plate the material for which is selected so that the base plate will not yield under the force applied by the tool.  
   
   
       10 . A method as claimed in  claim 9  wherein the tool and base plate are formed from similar material.  
   
   
       11 . A method as claimed in  claim 4  wherein the material displacement which occurs during the compression and deformation of the sheet material causes the material generally to flow radially, both outwardly of and inwardly towards, the initial hole with the result that there is a local thinning of the sheet material around the hole, and a reduction in the diameter of the initial hole.  
   
   
       12 . A method as claimed in  claim 11  wherein the sheet material which is not compressed is unrestricted and wherein material displaced radially outwardly by the compression produces an annular region of increased thickness around the area subject to compression.  
   
   
       13 . A method as claimed in  claim 12  wherein after the compression the sheet material is reduced in thickness by machining so as to remove any annular region of increased thickness so that the whole of the sheet material has the same thickness.  
   
   
       14 . A method as claimed in  claim 12  wherein after compression the sheet material is machined to remove material only from the region of locally increased thickness around the hole caused by unrestrained material which has flowed radially outwardly from the area subjected to compression.  
   
   
       15 . A method as claimed in  claim 14  wherein after machining a depression is left around the reduced diameter hole corresponding to the shape and size of the press tool.  
   
   
       16 . A method as claimed in  claim 15  wherein further machining is performed so as to reduce the thickness of the whole of the sheet material workpiece to the same thickness as that in the depression surrounding the reduced diameter hole.  
   
   
       17 . A method as claimed in  claim 16  wherein the machining is performed as a single step to remove the ring of increased thickness and thereafter further material from around any depression created by the compression so that the sheet is of uniform thickness.  
   
   
       18 . A method as claimed in  claim 4  wherein friction at the interface between the tool and sheet metal results in the reduced diameter hole assuming a concave barrel shape such that its diameter increases from the midpoint of the material thickness towards each surface of the sheet material.  
   
   
       19 . A method as claimed in  claim 18  wherein further material is removed by machining so as to reduce the thickness of the sheet material to approximately one half the thickness of the material in the region of the depression left by the press tool, so that one part of the double flared concave barrel shaped opening is removed leaving only one flare and the remaining opening tapers in one direction only.  
   
   
       20 . A method as claimed in  claim 19  wherein the resulting hole corresponds to a venturi shape, which can give a beneficial discharge coefficient when the opening is employed to discharge ink as an ink jet nozzle.  
   
   
       21 . A method as claimed in  claim 19  wherein the initial thickness of the sheet material and size of the initial hole and the extent of the machining, are selected so that the eventual size of the hole is of the order of 18 μm in diameter.  
   
   
       22 . A method as claimed in any of  claim 4  wherein the compression is obtained using two press tools working in opposition and the two tools are used to apply equal force on opposite faces of the material, thereby pinching the sheet material therebetween.  
   
   
       23 . A method as claimed in  claim 22  wherein one tool is fixed, and the other tool is movable, so that in use after placing the sheet material on the fixed tool, the movable tool is moved into contact with the sheet material to squeeze it between the faces of the two tools.  
   
   
       24 . A method as claimed in  claim 23  wherein the end face shape of the two tools is similar.  
   
   
       25 . A method as claimed in  claim 24  wherein the two tools present flat end faces of the same area to the sheet material and are axially aligned so that bowing of the sheet material is reduced due to the symmetry of the forces applied to opposite faces of the material.  
   
   
       26 . A method as claimed in  claim 23  wherein the end face shape of the two tools is different to produce different effects on squeezing.  
   
   
       27 . A method as claimed in  claim 1  further comprising the steps of clamping the sheet material over an area surrounding the oversize hole between two plates having aligned openings through which upper and lower press tools can protrude to engage the upper and lower faces of the sheet material, positioning the lower press tool so that the initially formed oversize hole is central of the openings, and therefore of the two press tools, whereby the outward radial spread of the sheet material is largely prevented.  
   
   
       28 . A method as claimed in  claim 27  wherein each of the tools is dimensioned to be a clearance or close slip fit within the openings, and the hole size reduction is increased since the squeezed material is only free to move radially inwardly thereby to reduce the size of the hole.  
   
   
       29 . A method as claimed in  claim 28  wherein the sheet material is rigidly clamped between the two plates, and surface deformation of the sheet material beyond the compressed area is reduced, since the sheet material around the area impacted by the two tools is prevented from being deformed as by increasing in thickness due to the outward flow of material from the centrally squeezed region.  
   
   
       30 . A method of forming an array of holes in close proximity to one another in sheet material comprising the steps of pre-forming oversize holes in the sheet material, centering each pre-formed hole in a position where a final smaller hole is required, locating the sheet material between an array of tools and a base plate having the same pitch spacing as the pre-formed holes, and forcing the tools into contact with the sheet material so as to simultaneously squeeze the sheet material around each of the holes to reduce their size.  
   
   
       31 . A method as claimed in  claim 30  wherein each initial hole is circular in cross section and a square cross section tool is employed to reduce the size of each hole so that the cross sectional shape of each final hole becomes a four lobed hole.  
   
   
       32 . Apparatus for performing the method of  claim 1  comprising means for compressing sheet material between two opposed substantially flat faces of two members at least one of which is movable towards and away from the other, drive means for effecting the said movement, and control means for controlling the operation of the drive means.  
   
   
       33 . Apparatus as claimed in  claim 32  wherein both members are movable by drive means.  
   
   
       34 . Apparatus as claimed in  claim 32  wherein at least one of the members is a tool which is surrounded by a separate clamping member which is movable relative to the other clamping member to grip sheet material therebetween around the area which is to be compressed by the tool, and drive means for effecting the clamping is provided and the two drive means are controlled as required to clamp and compress the sheet material.  
   
   
       35 . Apparatus for performing the method of  claim 1  comprising a retractable drill centered in a first press tool and a co-operating opposed second press tool between which sheet material can be squeezed as required to reduce the size of a drilled hole, drive means for advancing and retracting the clamping plates independently of the press tools, further drive means for advancing and retracting the drill relative to the first press tool and for rotating the drill when required, separate drive means for advancing and retracting at least one of the press tools, and control means for controlling the drives and the operation of the drill, whereby in use sheet material can be clamped between the clamping plates, the drill can be advanced and rotated to drill an oversize hole in the sheet material and thereafter retracted fully into the first press tool, and thereafter the press tools can be advanced to squeeze and compress the sheet material and thereby reduce the size of the oversize hole, whereafter the drive means are operated to allow the sheet material to be moved.  
   
   
       36 . Apparatus as claimed in  claim 35  wherein in use the second press tool is advanced to engage the opposite face of the sheet material from that engaged by the drill bit to provide a support therefor during drilling and the central region of the second press tool includes a depression or cavity to receive the end of the drill as it penetrates the sheet material.  
   
   
       37 . Apparatus as claimed in  claim 36  wherein the second press tool includes a central cylindrical bore within which is slidable a piston like closure having a flat upper surface which corresponds to that of the end face of the second tool, and drive means which operates in use to retract the piston like closure while the drill is advanced to create a drill receiving cavity and to advance the closure so as to be flush with the end face of the second tool when the drill is retracted.  
   
   
       38 . Apparatus for performing the method of  claim 1  comprising a drilling station, where the oversize hole is drilled in a sheet material component, a pressing station alongside the drilling station, the two stations being at a predetermined spacing, a translating device capable of moving the component work-piece between the two stations by the predetermined distance, and reference tooling for locating the component on the translating table, such that the drilled and reduced hole will be positioned relative to a feature on the component such as an edge thereof.