GB2088283A

GB2088283A - Alignment Device

Info

Publication number: GB2088283A
Application number: GB8101411A
Authority: GB
Original assignee: UK Secretary of State for Defence
Current assignee: UK Secretary of State for Defence
Priority date: 1980-11-05
Filing date: 1981-01-16
Publication date: 1982-06-09
Also published as: GB2088283B

Abstract

A method of aligning a first member 1 to a second translucent or transparent member 20 includes the steps of: providing a multiplicity of light sources 10 mounted in the first member and arranged to be visible substantially as points of light and applying a multiplicity of markings to the second member 20 in such a way that when the first and second members are correctly aligned each light source projects a light beam which coacts visibly in a predetermined manner with a corresponding one of the markings. The method finds use particularly in screen printing where a workpiece 8 is located over dowels 9 formed in the machine bed 1 the dowels containing optical fibres; screen 20 is fixed in position and bed 1 adjusted until desired alignment effected. <IMAGE>

Description

SPECIFICATION Improvements in or Relating to Alignment Means This invention relates to the alignment in relation to a first member, of a second member of partially translucent or transparent material, particularly by means of light sources mounted in the first member and arranged to project at least part of their light emission through the second member. In industrial processes and manufacturing it is often necessary to achieve a high degree of accuracy when aligning a member to another member. One such process is screen printing in which a screen must be accurately positioned over an item to be printed on prior to the printing operation. When printing is to be carried out then the screen may be located in a frame, which must then be aligned with the item to be printed, hereafter referred to as the workpiece.The workpiece has locating holes which enable it to be located on a series of dowels. The procedure has been to align the screen approximately by eye and make a test sample which will show how much correction, if any, is necessary to achieve the desired accuracy.

This method is costly both in time and material consumption.

The present invention seeks to provide a method of aligning the second member accurately in relation to the first member which obviates the use of a trial and error method.

According to the invention a method of aligning in relation to a first member a second translucent or transparent member includes the steps of: first providing a multiplicity of light sources mounted in the first member and arranged to be visible substantially as points of light, by projecting at least part of their light emission through the second member; and second applying a multiplicity of markings to the second member in such a way that when the first and second members are correctly aligned each light source projects a light beam which coact visibly in a predetermined manner with a corresponding one of the markings.

Each light source may conveniently be coaxially mounted in a corresponding dowel.

The first member desirably includes a flat plate through which the dowels are arranged.

The light sources are advantageously the ends of optical fibres remote from an optical fibre illuminating means.

Preferably the second member comprises a masking means which provides the markings for the first member to be aligned to. Most preferably the first and second members are elements in a screen printing process, the first member comprising an adjustable dowelled machine bed on which a workpiece is located and the second member comprising a silk screen bearing an emulsion having the said markings therein.

The invention will now be described, by way of example only, with reference to the accompanying drawings of which: Figure 1 is a plan view of a machine bed with a workpiece positioned on it; Figure 2 is a plan view of a silk screen showing the images to be printed and the cross images; Figure 3 is a three dimensional view of a cross image; Figure 4 is a cross-sectional side elevation of a silk-scren printer modified to use an optical fibre alignment system; Figure 5 is an enlarged cross-sectional side elevation of a modified silk screen printer showing an optical fibre; and Figure 6 is a fragmentary plane view of a correctly aligned cross image.

In Figure 1 a machine bed 1 is movable in the directions indicated by arrows 2, 3, 4, 5, 6, 7, any movement being restricted to the plane in which the machine bed lays. A workpiece 8 having three holes in an accurately determined L configuration is shown located over three dowels 9 protruding perpendicular to the machine bed 1. The dowels are made of hardened steel and enable each workpiece to be positioned on the machine bed at an identical location. The degree of protrusion of the dowels above the machine bed is approximately equal to and not greater than the thickness of the workpiece. Each dowel has a coaxial optical fibre 10 mounted within, which when illuminated is visible as a point of light to an operator looking down onto the machine bed.

Figure 2 shows a silkscreen 20 with an opaque hardened emulsion coating on one side having formed on it images 21 to be printed onto the workpiece and three cross images 22, one of which serves as a datum point from which all measurements are taken.

Figure 3 shows an enlarged cross image formed in the emulsion 30 which is 10 to 15 microns thick. The cross image is filled with a transparent material 31 to prevent the passage of printing ink through the screen mesh 32 at this point.

Figure 4 shows the positions of the various elements prior to printing; the adjustable machine bed 1, the silkscren 20 which is emulsion side down, a squeegee 40 mounted on a squeegee carrier 41, a workpiece 8 located over dowels 9 which have coaxially mounted optical fibres 10 within them connected to a common light source 42. The region immediately around the dowel 9 is shown in detail in Figure 5, the silkscreen 20 with emulsion 30 is brought into register so that the transparent filled cross image 22 is aligned with the optical fibre 10 to an operator viewing the optical fibre through the screen mesh 32 from above. The view that an operator would see when the workpiece and silkscreen is aligned correctly is shown in Figure 6. The optical fibre 10 shows through the screen mesh 32 as a point of light coacting with the cross image 22. In use a silkscreen is clamped into position after a workpiece has been layed over the machine bed dowels. The position of the machine bed is then adjusted until all through cross images are in the correct alignment relative to the optical fibres.

Printing ink is then placed on top of the silkscreen and the squeegee traverses the length of the silkscreen along the squeegee carrier forcing ink through the images in the emulsion and pressing the emulsion layer onto the workpiece thereby printing the images onto the workpiece.

Claims

1. A method of aligning a first member to a second translucent or transparent member includes the steps of: first providing a multiplicity of light sources mounted in the first member and arranged to be visible substantially as points of light, by projecting at least part of their emission through the second member; and second applying a multiplicity of markings to the second member in such a way that when the first and second members are correctly aligned each light source projects a light beam which coacts visible in a predetermined manner with a corresponding one of the markings.

2. A method according to Claim 1 in which each light source is coaxially mounted in a corresponding dowel.

3. A method according to Claim 2 in which the first member includes a flat plate through which the said dowels are arranged.

4. A method according to any one of the preceding claims in which each light source is an end of an optical fibre remote from an optical fibre illuminating means.

5. A method according to any one of the preceding claims in which the second member comprises a masking means which provides the markings for the first member to be aligned to.

6. A method according to any one of the preceding claims in which the first and second members are elements in a screen printing process, the first member comprising an adjustable dowelled machine bed on which a workpiece is located and the second member comprising a silkscreen bearing an emulsion having the said marking therein.

7. A method according to Claim 6 in which the said markings are cruciform.

8 An alignment method substantially as hereinbefore described with reference to the accompanying drawings.

9. An alignment apparatus substantially as hereinbefore described with reference to the accompanying drawings.