Patent Application
20050188737
Applicants claim the benefit of the following United Kingdom Patent Applications: Ser. No. 0329400.6, filed Dec. 19, 2003, and Ser. No. 0407370.6 filed Mar. 31, 2004.
1. Field of the Invention
This invention concerns a method and apparatus by which very small openings can be formed in sheet material such as metal foil.
Ink-jet printing heads require the formation of very small holes, usually arrays of such holes, in metal foil.
Mechanical drilling is an acceptable method for producing holes having a diameter of 50 μm or greater, but increasing difficulty is experienced below this size. Other processes are available to produce smaller diameter holes, but these present difficulties related to process time, tooling costs etc.
There is a specific requirement for a cost-effective process to manufacture holes in 50 μm thick stainless steel sheet, down to 18μm in diameter.
It is also desirable that such holes taper in one direction, typically at an angle of 15°.
Currently available techniques are described in the paper entitled A Technical Comparison of Micro-electrodischarge machining, Microdrilling and Copper Vapour Laser machining for the Fabrication of Ink Jet Nozzles, David Allen, Heather Almond and Peter Logan, [proceedings of SPIE, Vol. 4019 (2000)]
It is an object of the present invention to provide an improved method and apparatus for making small holes, particularly (but not exclusively) in foil for use in ink jet printing head nozzles.
According to one aspect of the present invention an oversize hole is first formed in the sheet material, and after the hole has been produced the sheet material, at least in the immediate vicinity of the hole, is compressed so as to cause the sheet material to flow at least in the direction of the hole, to reduce the size thereof.
Typically the initial hole is formed by drilling.
The initial oversize hole may be for example up to four times larger than the diameter ultimately required of the hole, but the invention is not limited to this ratio and it is to be understood that the initial hole can be any size—the only criteria being that it must be greater in area than that of the final hole.
The compression may be achieved using a press tool.
Typically the tool has a flat underside and is several times larger in diameter than the pre-formed holes, typically five times larger in diameter.
Preferably the tool is positioned concentrically over the pre-formed hole before it is pressed into the material.
The tool material must be chosen so that the tool is not deformed when pressed into the sheet material.
Hardened tool steel and tungsten carbide are examples of a suitable press tool material if the sheet material is stainless steel.
When using a press tool the sheet material will normally be compressed between it and a base plate and just as the tool should not deform under pressure, so the base plate should not yield under the force applied by the tool.
Similar materials as proposed for the tool may be employed for the base plate.
The material displacement which occurs during the compression and deformation of the sheet material will generally cause the material to flow radially, both outwardly of and inwardly towards, the initial hole. The result will be local thinning of the sheet material around the hole, and a reduction in the diameter of the initial hole.
If unrestricted, material displaced radially outwardly by the compression can produce an annular region of increased thickness around the area subject to compression by the press tool.
According therefore to another aspect of the invention, after the pressing step the sheet material may be reduced in thickness by machining so as to remove any annular regions of increased thickness, so that the whole of the sheet material has the same thickness.
So as not to remove more material than necessary, the machining may be arranged simply to remove material from the region of locally increased thickness around the holes caused by material which has flowed radially outwardly from the area subjected to compression, as by the press tool. However this may still leave a depression around the reduced diameter hole corresponding to the shape and size of the lower end of the press tool.
According to another aspect of the invention the machining may be 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.
Where a press tool is employed, friction between the tool-workpiece (sheet metal) interface can result in the reduced diameter hole assuming a concave barrel shape i.e. its diameter increases from the midpoint of the material thickness towards each surface of the sheet material.
According therefore to a further aspect of the invention further material may be 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 (i.e. one half of the barrel shape), so that the remaining opening tapers in one sense only. The resulting shape can be likened to a venturi shape, which can give a beneficial discharge coefficient when the opening is employed to discharge ink as an ink jet nozzle.
By appropriate choice of initial thickness of the sheet material and size of the initial hole and the precise extent of the machining, the eventual size of the hole and the form of the remaining taper can be controlled, in the case of the size to a diameter typically of the order of 18 μm.
According to a further aspect of the invention instead of the workpiece sheet material being placed on a flat unyielding base plate surface, and a single tool being pressed into the material, two tools can be used to apply equal force on opposite faces of the material, thereby pinching the sheet material between the two tools.
One tool may for example be fixed, and the other tool movable, so that after placing the sheet material on the former, the movable tool may be moved into contact with the sheet material to squeeze it between the faces of the two tools.
The size and end face shape of the two tools may be similar or different to produce different effects on squeezing.
Where the two tools present flat end faces of the same area to the sheet material and are axially aligned so that a generally symmetrical stress pattern will be set up in the sheet material around its opening the risk of bowing of the sheet material (which can occur if the area of the tool (or base plate in the case of a single tool) on one face of the sheet material is different from that of the tool or base plate acting on the other face or the tools or tool and base plate are misaligned) can be reduced.
According to a further aspect of the invention the sheet material may be clamped over an area immediately surrounding the oversize hole, between two plates having aligned openings through which upper and lower tools can protrude to engage the upper and lower faces of the sheet material, the latter being positioned so that the initially formed oversize hole is central of the openings, and therefore of the two tools.
Preferably the tools are a clearance or close slipping fit within the aligned openings, so that outward radial spread of the work-piece material is largely prevented, and the size reduction (brought about by the squeezing of the sheet material) is in fact increased since the squeezed material is only free to move radially inwardly to reduce the size of the hole.
Surface deformation of the sheet material beyond the compressed area will also be reduced when employing such an arrangement, since the sheet material around the area impacted by the two tools is gripped between the two plates and if they rigidly clamp the material therebetween there is little tendency for it to deform (as by increasing in thickness) due to any tendency for material to try and move outwardly from the centrally squeezed region.
Where the end product requires a number of holes in an array in close proximity, oversize holes may be pre-formed in the sheet material as by drilling, each centred on where the final smaller holes are required to be positioned in the array, and the sheet material may then be located between a similar array of tools and a base plate (or array of pairs of upper and lower tools) having the same pitch spacing as the pre-formed holes, and the tools forced into contact with the sheet material so as simultaneously to squeeze the latter around each of the holes to reduce their size as aforesaid, thereby to speed up production.
Alternatively a larger area press tool (or a pair of larger area upper and lower press tools) may be employed so as to encompass an area of sheet material containing a plurality of pre-formed holes so that when the sheet material is correctly located relative thereto and squeezed as aforesaid, the diameters of all the holes encompassed by the tool(s) will be reduced simultaneously.
Further variations of the method allow for the production of holes with differing sections and shapes, many of which can be difficult to produce using other processes. For example, squeezing material around circular holes using flat-faced tools will generally produce holes with a waisted cross section, so that each hole reduces in diameter towards the half-way point and increases therebeyond. Using non-flat differently shaped tool faces, and/or otherwise deliberately altering the coefficient of friction at the tool/material interface and/or between clamping plates and the sheet material around the or each hole, can produce different hole cross section shapes.
Non-circular tools can result in the oversize holes becoming non-circular in their reduced size. Thus for example by using a square section tool a circular oversize hole can when reduced in size become a four lobed hole.
According to a further aspect of the invention composite or integrated tooling may be employed comprising a retractable drill centred in a first press tool and a co-operating opposed second press tool between which sheet material can be squeezed as required with or without clamping plates around the press tools, drive means for advancing and retracting the drill relative to the first press tool and for rotating the drill when required, drive means for advancing and retracting the clamping plates independently of the press tools, and drive means for advancing and retracting at least one of the press tools, whereby in use sheet material can be clamped between the clamping plates, the drill advanced and rotated to drill and oversize hole in the sheet material and thereafter retracted fully into the first press tool and thereafter the press tools advanced to squeeze and compress the sheet material and thereby reduce the size of the oversize hole.
The second press tool may be advanced to engage the opposite face of the sheet material from that engaged by the drill bit to provide a support therefor during drilling, in which event the central region of the second press tool may include a depression or cavity to receive the end of the drill as it penetrates the sheet material.
Where the presence of such a depression or cavity could interfere with the subsequent deformation of the sheet material to reduce the size of the hole, the second press tool may include 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 dive means, which operates for example in synchronism with that for advancing and retracting the drill, operating to retract the piston like closure while the drill is advanced to create a drill receiving cavity but advance to complete the end face of the second tool when the drill is retracted.
In
In
In
Subsequent to the tool pressing process the sheet material workpiece has the form shown in cross section in
The ring of material 24 can be removed by machining along the line 23, leaving a circular depression 22 around each reduced size hole 20.
In a variation of the method, instead of the workpiece being located between a flat unyielding base plate 12, and a tool part 18 which is pressed into the workpiece, two similar tools can be used, pinching the workpiece sheet material therebetween. This is shown in
Tool 30 may be fixed and tool 18 may be driven so that it can be brought down into contact with the sheet material workpiece 14, so that it is sandwiched therebetween.
Bowing of the workpiece 14 is reduced due to the symmetry of the forces applied to the sheet material 14 by the two tool parts 18 and 30 as shown in
In a preferred modification of the
A drill (not shown) may be located in the upper press tool 18 and a piston like closure may be located in a central bore in the lower tool part 30 so that with the lower tool engaging the underside of the workpiece 14 the drill can be advanced through the end of the upper tool 18 to drill the hole 16 in situ. The end of the drill (which protrudes through the workpiece) can be accommodated in the upper end of the bore in the lower tool 30 if the piston like closure is retracted to leave a cavity centrally of the upper end of the lower tool 30 during the drilling. After the hole is formed, the drill can be retracted, and the closure advanced so that the upper face of 30 is flat and continuous again. The upper tool 18 is then driven down to squeeze the workpiece material 14 to reduce the size of the hole.
Where the workpiece requires a number of holes arranged in an array in close proximity, the sheet material may be pre-drilled as before, and an array of upper and lower tool pairs such as 18, 30 with the same pitch spacing as the pre-drilled holes, may be provided, and after placing the pre-drilled sheet therebetween, the tool pairs may be forced into contact with the sheet material workpiece 14 so as to squeeze the latter around each of the holes simultaneously, to speed up the production process. Alternatively larger area tools may be provided so that a plurality of holes are encompassed between the two opposed tool faces, enabling the said plurality of holes to be simultaneously reduced in size by one operation.
A second rigid arm 60 extends from the side of the vertical slide 56 at its upper end and a drilling spindle 62 is rotatably supported in a bearing 64 and extends vertically below the arm 60 through a hollow punch 66, itself carried in a cylindrical housing 68 which extends vertically below the inboard end of a rigid arm 70 the outboard end of which is acted on by a second actuator 72 for raising and lowering the arm 70 and thereby the punch 66.
The housing 68 is slidably supported in a linear bearing 74 carried on the inboard end of a rigid support arm 76, the outboard end of which extends laterally of and is joined to the upright 44.
The actuator 58 is coupled directly to the machine base 40 and the actuator 72 is coupled to the machine base via the upright 44 so that the vertical slide assemblies 56, 60 and 68, 70 respectively, can be raised and lowered relative to the machine base 40 as required.
Although not shown, a third actuator may be provided to move the punch 66 relative to the housing 68 so that the workpiece is gripped between the anvil 42 and both the punch 66 and the lower face of the housing 68, to reduce the material flow in a radial outward sense during compression.
Rotational drive for spindle 62 is provided by motor 78 and drive belt 80 which engages pulley 82.
In use the spindle is driven in rotation to rotate drill 84 on the lower end of the spindle 62 and the actuator 58 is retracted to lower the drill 84 to engage and drill a hole in workpiece 48. In doing so, arm 54 and piston 50 are similarly lowered relative to the fixed part of anvil 42 so that the drill bit can enter the space left above the piston. When the drilling is completed actuator 58 is extended to lift the slide assembly 56, 60 and raise the spindle and drill clear of the workpiece and since the piston 50 travels up with arm 54, so that its upper face is flush with that of the anvil 42, the upper face of the anvil is now continuous and flat once again.
Thereafter actuator 72 is retracted so that the hollow punch 66 is now forced downwardly to squeeze the workpiece between punch and anvil 42, to reduce the size of the hole.
After the squeeze is complete actuator 72 is extended to lift the punch 66 clear of the workpiece and allow the workpiece 48 to be moved (to form another hole) or to be removed and replaced by another.
An alternative arrangement comprises a drilling station where an oversize hole is drilled in the sheet metal. A pressing station is placed alongside the drilling station, the two stations being at a predetermined spacing from one another. A translating device is provided that is capable of moving the work-piece between the two stations by the predetermined distance. Reference tooling for locating the component is used on the translating table, such that the drilled and reduced holes can be positioned relative to a feature on the component such as for example an edge.
In
The platform slide 92 can move from the position shown, completely to the right-hand end of the base. In its first left-hand position the workpiece 96 is positioned below a drill 98 and in the right-hand position, below a punch tool 100.
The drill is carried at the lower end of a spindle and drive unit 102 which is movable vertically relative to a rigid support structure 104 by a vertical drive 105 which includes a spindle 106 and motor 108.
The punch 100 is carried at the lower end of a carrier 110 which is slidable relative to another rigid support structure 112 and is movable by a similar vertical drive 113 comprising a spindle 114 and motor 116.
Power is delivered to the drive motors as required under the control of an operator standing at the console 88, so that after positioning the workpiece 96 and platform slide 92 correctly below the drill 98 the latter is operated to drill a hole in the workpiece. After raising the drill clear, the workpiece support is moved to the right to position it below the punch 100 and after operating drive motor 116 the workpiece is squeezed between the punch 100 and the platform slide 92 to reduce the size of the hole, whereafter the punch is raised clear, to allow the workpiece to be removed.
Having thus described the invention, various modifications and alterations will occur to those skilled in the art, which modifications and alterations are intended to be within the scope of the invention as defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0329400.6 | Dec 2003 | GB | national |
| 0407370.6 | Mar 2004 | GB | national |
