GB2354734A

GB2354734A - Workpiece positioning machine

Info

Publication number: GB2354734A
Application number: GB0012933A
Authority: GB
Inventors: Richard Caswall Rolt
Original assignee: Individual
Current assignee: Individual
Priority date: 1999-05-28
Filing date: 2000-05-26
Publication date: 2001-04-04
Also published as: GB0012933D0; GB9912611D0

Abstract

A machine in which a workpiece is treated, for example a printing machine or a die cuffing machine, includes a table (18) for supporting the workpiece and a treatment device (4,5) for treating the workpiece. The table is supported on a cushion of air and the machine is provided with means (58) for moving the table relative to the treatment device to ensure exact positioning of the workpiece relative to the treatment device. The air cushion is preferably provided by discreet air bearings (34) mounted between the table and the machine frame (10,12,14). By supplying compressed air to the bearings the machine table (18) may be lifted slightly and readily moved by appropriate actuators (58). Once correctly positioned, air supply to the bearings may be interrupted to lower the table whereupon frictional engagement between the bearing components will prevent movement of the table.

Description

2354734 - 1 WORKPIECE POSITIONING MACHINE This invention relates to a

workpiece positioning machine. Preferred embodiments of the invention include machines in which the workpiece is a web, for example printing machines, particularly but not exclusively screen printing machines, and die cutting machines.

It is well known for a screen printing machine to comprise an ink tray suspended over a printing table upon which a workpiece to be printed is located. A problem associated with the known machines results from the fact that, where automated printing is required, fine adjustment in workpiece positioning relative to the ink tray is best achieved by adjustment of not only the workpiece itself, but also of the printing table. In this way, damage (e.g. creasing) to the workpiece is avoided. Although desirable, smooth and substantially unrestricted movement of large and heavy printing tables has been difficult to achieve in a convenient, reliable and inexpensive manner. A similar problem exists in relation to die cutting machines.

It is an object of the present invention to provide a workpiece positioning machine, for example a printing machine or a die cutting machine, in which the table for the workpiece may be moved in both a smooth and unrestricted manner relative to a treatment device, in order to adjust the position of the workpiece relative to the treatment device.

It is a further object of the present invention to provide a workpiece positioning machine which is convenient and inexpensive to manufacture.

The present invention provides a workpiece positioning machine comprising a treatment device for treating a workpiece and a table for supporting the workpiece to be treated, one of the treatment device and the table bemig moveable on a cushion of air relative to the other of the treatment device and the table.

The treatment device may be of any type and may, for example, comprise the printing head of a printing machine or the die assembly of a die cutting machine.

Preferably, the cushion of air is provided by at least one air bearing. The treatment device and table are ideally moveable relative to one another in any direction within a plane. The relative position of the treatrnent device and table may be fixed after positioning, for example by removal of the air cushion. Relative movement between the treatment device and table is preferably driveable by driving means. The driving means preferably comprises an electric actuator or a hydraulic actuator.

Thus, the present invention has the advantage over the prior art of providing a machine in which the table and workpiece may be moved in a smooth and substantially unrestricted manner relative to a treatment device.

An embodiment of the present invention will now be described with reference to the accompanying drawings, in which:

Figure I is a schematic perspective view of a printing machine according to the invention; Figure 2 is a schematic cross-section view of an air bearing of the printing machine shown in Figure 1; Figure 3 is a schematic perspective view of the air bearing shown in Figure 2; Figure 4 is a schematic plan view of the printing machine shown in Figure 1; Figure 5 is a view of two images generated by two video cameras mounted on the printing machine shown in Figure 1; Figure 6 is a view of a series of video camera frames; and Figure 7 is a schematic view of a video camera of the printing machine shown in Figure 1.

A printing machine 2 according to the present invention is schematically shown in Figure 1. The printmig machine 2 has a mode of operation based on the screen printing principle. Screen printing is a well known technique and forms part of the common general knowledge of persons skilled in the printing art. Accordingly, precise details relating to the mechanics of the screen printing process are not discussed herein.

The printing machine 2 incorporates two major components. These components are a screen frame 4 and a printbed carriage 6. The screen frame 4 consists of an ink tray 8, two supporting walls 10, 12, and a base member 14. The ink tray 8 is rectangular in shape and lined with an appropriate fabric (e.g. silk or nylon) through which ink 16, held by the ink tray 8, is pressed during the printing process. The ink tray 8 is raised from the ground and supported above the base member 14 by means of the supporting walls 10, 12. The base member 14 has substantially the same rectangular shape and is generally of the same size as the ink tray 8. The supporting walls 10, 12 extend between and are attached to the vertically spaced long edges of the ink tray 8 and base member 14. The screen fi-ame 4 thereby defines a tube having a substantially rectangular cross-section.

The printbed carriage 6 comprises a printing table 18, two guidance rollers 20,22 (only one roller 20 is shown in Figure 1), and a work piece supply holder 24. The printing table 18 has a flat printing surface 26 upon which a work piece (i.e. the material to be printed) is supported during the printing operation. The printing table 18 also has sloping surfaces 28,30 located at opposite ends of the printing surface 26 so as to assist with movement of the work piece to and from the printing surface 26. This movement is finther assisted by means of the guidance rollers 20,22. These rollers 20,22 are located at the intersection of the printing surface 26 and the sloping surfaces 28, 30. Means 24 for holding a supply of work piece material is located at one.end of the printing table IS. Work piece material in the context of printing.machinery is typically supplied and stored in rolls. Accordingly, the means 24 is adapted to hold a supply of work piece material in the form of a roll 332. Precise details of such means will be apparent to a person skilled 'in the art.

The printbed carriage 6 extends through the substantially rectangular cross-section of the screen frame 4 and is located so that the printing surface 26 lies adjacent the underside of the ink tray 8 (as shown in Figures 1 and 4). The printbed carriage 6 is movably mounted on the base member 14 of the screen frame 4 by means of three air bearings (not shown in Figure 1). Schematic views of an appropriate air bearing 34 are shown in Figures 2 and 3.

With reference to Figures 2 and 3, each air bearing 34 comprises a fixed base plate 36 and a floating swivel pad 38. These two components 36,38 are not mechanically connected to one another. The fixed base plate 36 is a flat circular member havinp, a centrally located aperture 40 through which highly pressurised air is released. Air is supplied to the aperture 40 from a suitable source through an appropriate pneumatic hose 42. The underside of the floating swivel pad 38 is provided with a circular recess 44 which is located over the aperture 40 of the base plate 36. The underside of the swivel pad 38 is circular in shape and the recess is provided centrally therein. As a result, an annular land surface 46 is provided. This land surface 46 rests on the upper surface of the base plate 36 when highly pressurised air is not supplied to the aperture 40. In these circumstances, friction forces resist relative movement between the base plate 36 and the swivel pad 38. However, when highly pressurised air is supplied to the aperture 40, the pressure of the air within the recess 44 is increased substantially. Indeed, given an adequate air supply, the increase iii pressure is sufficient to lift the swivel pad 38 (and a load provided thereon) from the base plate 36. The annular land surface 46 then acts as a skirt and provides a degree of stability.

With the swivel pad 38 raised above the base plate 36, the two components may be moved relatively to one another without significant resistance from friction forces. Movement of the two components is only limited by the requirement for the aperture 40 to be located below a generally central portion of the recess 44. When a required movement between the two components has been completed, the supply of air to the aperture 40 may be reduced so as to lower the swivel pad 38 and thereby once again locate the two components in abutment with one another. In this way, friction forces between the two components will tend to resist movement from their new relative positions. It will be apparent that the swivel pad 38 may be both rotated (as indicated by arrow Theta) and moved linearly (as indicated by arrows x and y) relative to the base plate 36 - see figure 3.

For the purpose of securing a load (such as the printbed carriage 6 of the'printing machine 2 shown in Figure 1) to the bearing 34, an end portion of a rod 48 is threadedly engaged with the swivel pad 38. The end of the rod 48 opposite to that which is engaged with the swivel pad 38 is provided with screw threads for receiving appropriate nuts 50. The nuts 50 are used to secure the rod 48 and swivel pad 38 to the load in question. The rod 48 is arranged relative to the swivel pad 38 so that the longitudinal axis of the rod 48 extends through the centre of the circular recess 44 and lies perpendicular to the plane of the annular land surface 46.

Referring to Figure 1, the base member 14 of the screen fi-ame 4 is provided with three recesses 52,54,56. Each of these recesses 52,54,56 receives the fixed base plate 36 of an air bearing 34. Through operation of the three air bearings 34, the printbed carriage 6 may be moved small distances relative to- the screen frame 4. This movement may be in the x, y and Theta directions as shown in Figure 4. The printbed carriage 6 is moved by appropriate means 58 mounted on the supporting walls 10, 12. An appropriate type of moving means 58 will be apparent to a reader skilled in the art. Such means may be of any appropriate type.

For example, the moving means may comprise electric actuators, hydraulic actuators or a combination of electric and hydraulic actuators. A suitable control system for operating the moving means is also provided.

During operation of the printing machine 2, the work piece to be printed is moved onto the printing table 18 by appropriate means (not shown). Suitable means in this regard may be a pair of rollers which grip the edges of the work piece and, on rotation, pull the work piece into position. Such rollers may be driven by, for example, an electric motor. Once the work piece is located on the printing table 18, accurate positioning of the work piece prior to printing may be achieved by operation of the air bearings 34 and the moving means 58. In this way, the printbed carriage 6 (together with the work piece) may be moved small distances relative to the ink tray 8. Arrows Z in Figure 4 show an example of how the position of the printbed carriage 6 may be altered. Once the work piece has been correctly positioned, the printbed carriage 6 may, if required, be locked in position by suitable locking means. The locking means may, for example, comprise means for locking the moving means 58 and/or may comprise deactivating the air bearings 34 thereby introducing substantial frictional resistance to movement of the table.

In the printing machine.2 shown in Figure 1, the position of the printbed carriage 6 is adjusted by means of a computer software system which receives input from a CCD camera/photo optic registration system. In the printing machine 2, the registration system comprises two video cameras 60,62 which are mounted on the base member 14 of the screen fra -me 4. The arrangement for mounting the video camera 60,62 is not shown in Figure 1, however a suitable arrangement will be apparent to those skilled in the art. The video cameras 60,62 are located below the ink tray 8 and the printing table 18. The position and orientation of the video cameras 60,62 is such that they may view, through glass or perspex. windows in the printing table 18, the underside of the work piece to be printed. If appropriate, the entire printing table 18 may be manufactured from glass or any other suitably transparent material. Thus, the video cameras 60,62 may monitor the position of a work piece and provide information concerning any required positional corrections. Preferably, the underside of the work piece is previously provided with appropriate registration marks 64 (see Figure 7). Such registration marks 64 are used as a datum for accurate positioning of the work piece.

In a fully automated printing machine, the work piece is pulled from the roll 32 and over the printing table 18 until the video cameras 60,62 view appropriately spaced registration marks 64. At this point, movement of the work piece over the printing table 18 is stopped so that the back edges 66 of the registration marks 64 are in view of the video cameras 60, 62 (see Figure 7). The work piece is thereby roughly positioned for fine adjustment by appropriate movement of the printbed carriage 6. This movement may be controlled automatically, but may alternatively be controlled by an operator. In the latter case, exact positioning may be achieved by aligning the registration marks 64 with datum lines 68 shown on video screens 70 used by the operator to view the images from the video cameras 60,62 (see Figure 5). Once the work piece is correctly positioned, the printing process may be completed by appropriate movement of a paddle 72 across the ink tray 8.

Figures 6 and 7 show how control circuitry can roughly position the work piece so that the back edges 66 of the registration marks 64 are in view of the video cameras 60,,62. Each of the video cameras 60,62 produces a series of frames 74 which, when viewed in rapid succession, may generate a moving picture. Each frame is generated in approximately 40 milliseconds by scanning the field of view 76 of the video camera in question. A number of scanning lines 78 are produced which are compiled by the video camera to give a frame synch pulse. The video camera then begins the scanning process again. However, whilst the video camera is generating the frame synch pulse, it is effectively blind. The exact position of the registration marks is calculated during the scanning phase by analysing previous frames. Each frame is searched for regular marks which could be a registration mark 64. Once the registration mark 64 is found, the amount Delta by which it moves between successive scanning lines is measured. The speed of movement of the registration mark and the point at which the registration mark first appeared in the field of view 76 is determined by interpolation. This information is then processed to stop the back edge 66 of the registration mark 64 in the appropriate position for fine adjustments.

The present invention is not limited to the specific embodiment described above. Alternative arrangements and suitable materials will be apparent to a reader skilled in the art. For example, the screen frame 4, rather than the printbed carriage 6, may be mounted on air bearings 34.

Whilst the invention has been described with particular reference to a printing machine, the adjustment techniques disclosed may be useful in other machines which require fine adjustment of a web relative to a machine frame. For example, the adjustment technique described may be applied to a die-cutting machine in order to provide exact registration between the die and the material to be cut.

Claims

CLAIMS:

1. A workpiece positioning machine comprising: a treatment device for treating a workpiece and a table for supporting the workpiece to be treated, one of the treatment device and the table being moveable on a cushion of air relative to the other of the treatment device and the table.

2. A workpiece positioning machine according to claim I wherein the machine includes a machine frame and the table is mounted on the machine frame by means of a plurality of discreet air bearings which provide the cushion of air.

3. A workpiece positioning machine according to claim 2 wherein each air bearing comprises a base plate, a swivel pad located on the base plate, and means for supplying compressed air to the interface between the base plate and the swivel pad to lift the swivel pad from the base plate to enable easy movement of the swivel pad relative to the base plate.

4. A workpiece positioning machine according to claim 3 wherein a recess is formed in one of the base plate and the swivel pad, the recess being surrounded by a land which engages the other of the base plate and swivel pad in the absence of compressed air in the recess.

5. A workpiece positioning machine according to claim 4 wherein the swivel pad carries a rod and means are provided for adjustably positioning the rod relative to the table whereby the static position of dhe table relative to the machine frame may be vertically adjusted.

6. A workpiece positioning machine according to any preceding claim wherein the table is supported on three discreet spaced apart air bearings.

7. A workpiece positioning machine according to any preceding claim wherein means are provided for locking the position of the table relative to the machine frame after adjustment of the position of the table.

8. A workpiece positioning machine according to claim 7 wherein the locking means comprises means for interrupting the supply of compressed air to the air cushion.

9. A workpiece positioning machine according to any preceding claim wherein driving means are provided for positioning the table relative to the machine frame.

10. A workpiece positioning machine according to claim 9 wherein the driving means are lockable to fix the position to the table relative to the machine frame.

11. A workpiece positioning machine according to any preceding claim wherein the treatment device is a printing device.

12. A workpiece positioning machine according to any preceding claim wherein the treatment device is a die cutting device.