GB2178684A - A component assembly system - Google Patents

Abstract

A method of joining detail elements on to the base elements, for example welding nuts (detail elements) 32, 34 etc on to metal brackets (base elements) 30 etc for use in the automobile industry. The method is automatic and includes loading the base elements on to a platen which is codified to identify the base elements and the detail elements eg by positioning pegs 25 in holes 23; moving the platen to an identification station where the code is read to identify the type of platen and automatically controlling detail element placement means and joining means to position and secure the detail elements on to the base elements. The invention also comprises apparatus for carrying out the above described method and a projection welding method utilising a conductor of higher electrical conductivity than the base element engaging the opposite side of the base element to that engaged by the electrodes, to deflect the electrical flux path from the side of the base element on which the detail element is located. <IMAGE>

Description

SPECIFICATION A component assembly system This invention relates broadly to a method and apparatus for the automatic joining together of elements to form composite components. In particular, the invention is applicable to the fastening, for example by welding, of detail elements (for example: nuts, studs, bosses and even small assemblies) on to base elements.

There are many engineering components, especially in the automobile industry, wherein it is required to provide a tapped hole for the reception of an externally threaded co-operating component such as a bolt or setscrew. A well known economic way of manufacturing such a component is to form the base element in sheet metal (usually mild steel) with clearance holes at the locations where the tapped holes are required and then to weld a nut to the base element over the clearance hole, so that the nut provides an effective tapped boss on the component. In terms of this invention, the nut then provides a detail element.

Similar techniques are used to weld studs on to various base elements, although in the case of a stud the clearance hole in the base element is not required. Whilst in general the two detail elements which have been most commonly welded to base elements are nuts and studs, the method may have been used to secure other detail elements and in any case, it is to be understood that the present invention is not restricted to the securing of nuts and studs and therefore the term "detail element" is to be widely construed.

Moreover, it is not essential that the base element should be made of sheet material. Indeed, in some of its aspects, the invention is not restricted to the manufacture of metal components since those aspects could be used for instance in the manufacture of welded or bonded plastics components and the expression "welding" is therefore intended to include bonding techniques. Moreover, the invention can be used with joining techniques other than welding or bonding, such as joining by screws or rivets.

It is an object of the invention in one aspect to provide an effective and economic method of automatically manufacturing components comprising joined base elements and detail elements, and it is a particular object to provide a method and apparatus which is flexible so that a variety of components can be manufactured without having constantly to re-set the machine.

It is an object of another aspect of the invention to provide an improved method and apparatus for the resistance welding of metal detail components on to base components.

Where nuts for example have been welded by resistance welding techniques on to base elements, it is known to provide small dimples on the underside of the nut near to its corners, so that these dimples engage with the base element, and concentrate the electrical current into very small areas, for the purpose of effecting welds near to each corner of the nut. Resistance welding generally involves passing the current through the nut, and thence through the base element, and out at the other side of the base element. However, projection welding is known, in which two electrical contacts are established, one with the nut, and the other with the base element adjacent to the nut, but on the same side of the base element as the nut itself.Whilst this technique has obvious advantages, in that all the electrical equipment can be arranged at one side of the base element (and incidentally this lends itself to the use of programmable manipulators-industrial robots-for welding) it has been found that the welds produced by projection welding techniques are not always satisfactory.The aspect of the invention which is concerned with improving the welding technique provides an improved method of projection welding,which has all the advantages of that method, without the known disadvantages.

According to a first aspect of the invention, a method of automatic joining of detail elements to base elements comprises the steps of: loading one or more base elements to a corresponding number of prepared locations on a platen; codifying the platen to identify the type of base element(s) proper to that platen and the number and type of detail elements proper to that proper base element or elements; moving the platen to an identification station; automatically identifying the type of platen using the codifying means at the identification station and controlling detail element placement means and joining means in response to the identified platen to place in position on the base element or elements and to join thereto detail elements correct in number, type and location.

The codification of the platen may be provided by an encoding array of holes formed through the platen or a part attached to the platen. Alternatively, the coding may be provided by the number, location, shape or other physical characteristic of one or more of the base elements or detail elements. Either visual or tactile detection systems may be employed to identify the type of platen, reading off the encoding physical characteristic of the base element(s) or detail element(s).

According to a preferred form of this aspect of the invention the automatic identification of the platen is achieved by detection of a light pattern projected through an encoding array of holes formed through the platen. Preferably the identification is carried out by a matrix of photoelectric detectors each corresponding to a possible encoding hole or location hole on the platen. The detectors may be located on the same side of the platen as that from which light is projected, the detectors being arranged to receive light- reflected back through the holes.

Preferably the presence or absence of a base element is also sensed at the identification station and the placement means and joining means are further controlled to ensure that detail elements are only placed and joined at any prepared location if a base element is present at that location. In the preferred method, utilising the matrix of photoelectric detectors, these detectors are also used to detect the presence or absence of a base element. Preferably this is achieved by detecting a light beam which will be obscured by the presence of a base element at the prepared location.

It is further preferred that after leaving the identification station, the platen is moved first to a detail component placement station at which the correct number and type of detail elements is or are fed successively to an automatic placement head programmed to traverse the locations for detail elements on that platen, and to place a detail element on the base element(s) at each of these locations, the platen then being moved to a joining station at which an automatic joining head is programmed to traverse the detail element locations on that platen and to carry out a joining operation on each of the placed detail elements. Preferably the platen is arrested at each of the placement and joining stations long enough for the automatic placement and joining heads to carry out their functions.

In one method of carrying out the invention, the joining step comprises welding the detail element(s) to the base element(s) and in the preferred process the automatic joining head is a welding head.

In the preferred method, the platen is moved on a conveyor from a loading station through the identification, placement and welding stations and back to the loading station.

According to a second aspect of the invention apparatus for the automatic joining of detail elements to base elements comprises a conveyor for carrying platens supporting the base element(s) at prepared locations through a series of stations, there being platen identifying means at one station; detail element placement means and joining means at subsequent stations, and programmable logic control means adapted to receive information from the identification means and to control the placement means and the joining means in response to the received information to ensure that detail elements of the type and number proper to the base elements on the platen are placed in the correct locations on the base element(s) and joined to the base element(s) in these locations.

Preferably the identification means comprises a light source and a matrix of photoelectric detectors at the identification station, the arrangement permitting encoding of the platens by providing holes through the platens, each hole permitting a light beam to pass to one of the matrix of photoelectric detectors, whereby the coding is achieved by the selection of the pattern of holes through the platens. The light beam may be arranged to be reflected in its passage from the source to the detectors.

According to another preferred feature of this aspect of the invention, the detail element placement means incorporates an industrial robot fitted with a placement head and able to carry that head to any location over the entire effective area of a platen at the placement station. Preferably the placement head comprises articulated means for gripping the detail element and power operated actuator means for operating the gripping means to release the detail element. There may also be a locating pin adapted to be projected through a bore of the detail element for accurate location of the detail element. It is further preferred that the actuator means is adapted to operate the locating pin, there being a lost motion arrangement between the actuator means and the gripping means, so that the locating pin is caused to operate before release of the detail element.It is still further preferred that the actuator means is pneumatically operated.

According to a preferred construction, the joining station incorporates an industrial robot fitted with a joining head and able to carry that head to any location over the entire effective area of a platen at the joining station.

In one arrangement, the joining head is a welding head. Preferably the welding head comprises a pair of electrically conductive blocks, one connected electrically to a first welding contact and the other connected electrically to a second welding contact,the two blocks being attached to each other by electrically insulating means arranged to permit limited motion between the two blocks in the direction towards and away from the location at which a detail element is to be welded. In the preferred construction, the two blocks are substantially coaxial and are connected by an electrically insulating sleeve which is secured to one of the blocks but slidable on or in the other. It is a preferred feature that a resilient or rubber or like sleeve is fitted between the two blocks to provide a resilient connection between them which also isolates one block electrically from the other.

In the preferred arrangement, which is particularly adapted for the welding of nuts on to base elements, one of the welding contacts comprises a cap engageable with the top surface of a nut, and the other welding contact comprises a boss, concentric with the welding cap, and adapted to engage with the base element around the outside of the nut. The concentric arrangement of the two welding elements has been found advantageous in the projection welding of nuts on to base elements.

According to a third aspect of the invention a platen for use in automatic joining of detail elements on to base elements comprises a carrier plate having a series of prepared locations for a set of base elements, and encoding means identifying the platen and signalling the number and type of detail elements required for the base elements for which there are prepared positions. In a preferred construction, the or part of the prepared locations comprises a button of high electrical conductivity flush with or proud of the surface of the platen at each location where a detail element is to be secured to a base element.

One method of encoding the platen is to form a pattern of holes through the platen to enable light beams to pass through the holes, whereby the number and dispostion of the light beams will give an indication of the type of platen.

A preferred method of providing prepared locations is to provide locating legs projecting from the top surface of the platen for engagement in corresponding holes formed in the base elements. Where the detail elements are themselves bored (as in the case where the detail elements are nuts for example) pegs may be arranged at the detail element locations so that they form projections on which the detail elements can themselves be located.

Another method of providing prepared locations is to provide upstanding edge locations means for the base elements. Such edge location means may comprise dowels.

According to a fourth aspect of the invention a method of welding a detail element on to a base element comprises: engaging the detail element with a first electrode; engaging the base element with a second electrode on the same side of the base element as the detail element, and engaging the opposite side of the base element in the area opposite to the detail element with a conductor of higher electrical conductivity than the base element, so that the electrical flux path through the base element is deflected from the side of the base element on which the detail element is located.

Preferably the conductor has an area such that it is directly opposite both the electrodes.

In the preferred method, the base element is engaged by the second electrode all round the detail element, and the conductor has an area at least equal to the area bounded by the engagement of the second electrode with the base element. The conductor may conveni ently be made of copper.

According to a fifth aspect of the invention apparatus for welding a detail element on to a base element comprises a platen adapted to receive the base element, the part of the platen on which the base element rests being made of electrical insulating material; a conductor button on the platen in a location aligned with the position which the detail element will occupy on the base element and having an operative surface level with or proud of the surface of the electrical insulation part of the platen; a first electrode adapted to engage with the detail element on the opposite side of the base element to that which rests on the platen, and a second electrode adapted to engage with the base element on the opposite side of the base element to that which rests on the platen.

Preferably the first electrode comprises a cap, and the second electrode comprises a boss surrounding but spaced from the first electrode.

It is further preferred, that the first and second electrodes are mounted on a welding head in accordance with the second aspect of the invention.

Indeed, it will be appreciated, that a single apparatus may incorporate all the apparatus aspects of the invention, and therefore features of one aspect of the invention may be used in combination with features of another aspect of the invention.

The construction and arrangement of one form of apparatus in accordance with the invention for welding nuts to generally planar metal elements, to provide nutted components, will now be described by way of example only, with reference to the accompanying drawings. in which: Figure 1 is a plan view of the layout of the apparatus, Figure 2 is a diagrammatic elevation of a nut placement station, Figure 3 is a plan view of one type of platen used to convey a set of components through the apparatus, Figure 4 is a section on the line Ill-Ill in Figure 3, Figure 5 is a section through a nut place ment head, Figure 6 is a side view of part of the nut placement head looking in the direction of the arrow VI in Figure 5, Figure 7 is a view partly in section, of the nut placement head in the direction of the ar row Vil in Figure 5, Figure 8 is a vertical section through a welding head, Figure 9 is a horizontal cross-section through the welding head taken on the line IX IX in Figure 8, Figure 10 is an elevation looking in the di rection of the arrow X in Figure 9, Figure 11 is a detail section on the line Xl-Xl in Figure 8, Figure 12 is a plan similar to Figure 3, but showing an alternative layout, and Figure 13 is a detail section on the line XIII XIII in Figure 12.

In Figure 1 there is illustrated the layout of the complete apparatus for welding nuts on to components. There are many engineering components, especially in the automobile industry, which comprise essentially a plate-like (planar) element or a bracket with one or more nuts welded on to it.The nuts are aligned with clearance holes in the planar element and provide the component with a ready made tapped boss to receive an externally screw-threaded device, such as a setscrew or bolt. Whilst it is envisaged that the apparatus in accordance with the invention will normally be used in the manufacture of components which include a generally planar element, it is to be understood, that the apparatus hereinafter described is not restricted in its use to the manufacture of planar components.Nor is the use of the apparatus restricted to the welding on of nuts, since it could also be used for welding in place other items. However, for simplicity of description, reference will be made hereinafter to the welding on of nuts.

Heretofor, each nut has been welded to the planar element by a manually controlled resistance welding device. The apparatus according to the invention which is included in the layout of Figure 1 is able to perform the nut welding operation automatically, and on a set of components at each operation. The apparatus includes a horizontal roller conveyor 12, which is operated continuously and traverses a continuous loop, passing through a series of five operation stations, designated: STATION 1; STATION 2; STATION 3A: STATION 3B and STATION 4. Also, the entire collection of apparatus which is referred to as a nutting cell is protected by a fixed fence 14 with gates 16 and 18 to prevent unauthorised personnel gaining ready access to the apparatus, in order to minimise the danger of accidents.At STATION 4, which is the manual loading and unloading station, part of the conveyor 12 is outside the fence 14, so that an operator (indicated at 20) is able to gain free access to the conveyor to carry out the loading and unloading operations.

For the purpose of carrying the planar elements on the conveyor through the series of operating stations, there are provided platens such as that shown at 22, in Figures 3 and 4.

Essentially, this platen comprises a metal bed 24 on to which is secured a large rectangular carrier plate 26 made of electrically insulating material-for example a resinous material.

Each platen is made to carry a predetermined set of planar elements, and in Figure 3, the particular set comprises: a relatively long rectangular shallow channel element 30 to which are to be secured two square nuts 32 and 34; a short shallow channel element 36 to which are to be secured two square nuts 38 and 40; a somewhat more square element 42 to which are to be secured three square nuts 44, 46 and 48; a short angle element 50 to which are to be secured two square nuts 52 and 54; a short rectangular plate 56 to which a single square nut 58 is to be secured and a short shallow channel element 60 to which a single square but 62 is to be secured.

The locations of the elements 30; 36; 42; 50; 56 and 60 may be identified on the carrier plate 26 by markings on the plate such as outlines of the shapes of the elements, because it will be appreciated that the operative is required to select a set of elements from stock bins (not shown) adjacent to STATION 4 and to place them in the correct locations on the platen 22. However, there are two other means of ensuring correct location of the elements on the platen. Firstly, at each position where a nut is to be fitted, there is a location peg 70 (see Figure 4) which projects from the surface of the carrier plate 26, and this projection is received in a clearance hole of the element which has to align with each nut.Secondly, there are dowels 72 which project upwardly from the carrier plate 26 to provide edge locations for some of the elements (particularly those with only a single location peg 70, since these elements can only be orientated on the platen by engagement with edge location devices). It is also to be noted, that at each location for an element on the platen, there are one or more holes 74 which extend through the complete platen.

A series of five platen identification holes 23 is formed through the bed 24 of the platen at a position clear of the carrier plate 26 and adjacent to the leading edge of the platen e.g. the lefthand edge as seen in Figure 3.

Pegs 25 are provided which can be fitted selectively into the holes 23 (in Figure 3, pegs are shown in only two holes). This provides a means whereby the platen can be encoded according to the presence or absence of a peg. In this particular arrangement, the top two holes 23 (as seen in Figure 3) are used to indicate a particular size of nut to be attached to the base elements on the platen. As will appear hereinafter, this particular apparatus is adapted to handle two sizes of nut and by fitting pegs into one or other of the top two holes 23 the platen is codified as to which of these two types of nut is to be used. In fact, two holes gives four possible codes so that it would be possible to have four different types of nut. It should be mentioned that all the nuts on one platen are of the same size, and this determines the kind of component which can be made on that platen.

The lower three holes 23 provide a means of indicating what components should be on the platen and thus codify the platen in respect of the set of base elements. In Figure 3 a single peg 25 is shown in the middle one of the three holes indicating a platen to carry elements 30, 36, 42, 50, 56 and 60.

At each nut position, the platen is provided with a copper button 90, which is a disc fitting into a hole 92 in the carrier plate 26, and being of substantially the same thickness as the plate 26, so that the top surface of each button 90 is flush with or stands just proud of the top surface of the plate 26. It is important that the buttons 90 are made of a material of good electrical conductivity, and copper has been chosen for this purpose. Each button is secured in the platen by setscrews 94, and it will be appreciated, that it is possible to change the buttons 90 is they become worn.

Each location peg 70 comprises a centre portion 75 which is slidable in a hole in the button 90, and which has an outside diameter such that it fits closely in one of the clearance holes in the planar elements, such as 30; an upper reduced diameter portion 76 which is a sliding fit in the screw-threaded bore of a nut, such as the nut 34, and a collar 78 at the lower end, which is a sliding fit in a bore 80 in the bed 24 and which normally abuts against the underside of the button 90 to limit upward movement of the peg. A compression spring 82 is positioned in the bottom of the bore 80, and the collar 78 rests on this spring. It will be appreciated that if downward pressure is applied to the portion 76 of one of the pegs 70, it is possible to push that peg into the platen, compressing the spring 82, but the normal condition is as illustrated in Figure 4.

Immediately following STATION 4 (loading and unloading) the conveyor 12 is provided with an accelerating section 17 of known construction, so that when a platen 22 arrives at this section, it moves rapidly forwards on the conveyor, separating it from the following platens. When a platen arrives at one of the operating stations, it is arrested by platen stops which hold it long enough for the operation appertaining to that station to be carried out, and then release it, so that it is carried forwardly on the conveyor 12, to the next operation station. Platen stop systems of this kind are well known and do not require detailed description here.

At STATION 1, platen and nut type identification is executed by a light sensing system.

For this purpose, a bank of five electric lamps of photo emitters (not shown) is provided immediately above the path of the platen and reflectors (not shown) are positioned immediately below the path of the platen. The lamps with their associated reflectors are positioned so that each is in alignment with one of the holes 23 in the platen. The lamps are continuously illuminated when the nutting cell is in operation and consequently light beams pass downwardly through any unplugged holes in the platen and are reflected back and are sensed by photo-electric detectors located adjacent to the emitters. (The bank of photoelectric detectors is gated electrically, so that they are only active when a plate is arrested at STATION 1).Therefore, the beams of light passing through the coding holes 23 at the leading end of the platen provide an indication of the components to be nutted on that platen and also the size of the nut to be used. This information is fed from the matrix of photoelectric detectors to a programmable logic controller 100 (see Figure 1) which comprises a number of micro-processors arranged to control the automatic operations of the apparatus. It will be appreciated that the information given the controller 100 from the light sensing arrangement at STATION 1 can be used to control the feed of nuts, the placing of the nuts and the subsequent welding of the nuts to ensure correct completion of the components from the planar elements 30, 36, 42, 50, 56 and 60 which are on the particular platen.

Another feature of the light sensing system at STATION 1 is the provision of a further matrix of lamps or photo emitters and reflectors which extends over the full area of the carrier plate 26 to provide a means of checking that the correct elements have been placed on the platen in the required locations.

This is because the lamps of the further set are aligned with the holes 74 (there may be more of these holes than those illustrated in Figure 1) and each element covers at least one of the holes 74 and thereby prevents a light beam passing through that hole. Once the light sensing system has identified the type of platen, it will indicate to a selection of the photo detectors corresponding to the location of the holes 74 which should be covered by elements, that no light should be detected. If light is detected at any one of this selection of photo-electric detectors, that will be an indication that the hole 74 is not obscured and that in turn means that the required element is not present. A signal can then be given to the controller 100 which will issue the necessary control signals to ensure that no subsequent operations are carried out on that particular location of the platen.On the other hand, the controller can be arranged to issue signals which will ensure that the remaining operations are carried out on the platen. For example, supposing that in the case of the platen 22, the operator had for some reason omitted to place the element 42 in position, but had placed all the other elements to be nutted on the platen. At STATION 1, light beams would pass through and be reflected back through the two holes 74 which should have been obscured by the element 42, and the photo-electric matrix will receive an indication that the element 42 is absent.This will be signalled to the controller 100, which will then issue the necessary control signals to the subsequent operating sta tions, to ensure that nuts are fed to the locations for the elements 30, 36, 50, 56 and 60, and that those nuts are then welded to the planar elements, but at the same time, it will ensure that the nuts 44, 46 and 48 are not fed to the location where the element 42 should have been placed, and that no welding operations are carried out at that location. The sensing system at STATION 1 and the method whereby it controls the subsequent operations of the apparatus forms a significant feature of the invention.

At STATION 2, there is provided an industrial robot 110 mounted on a column plinth 112 alongside the path of the conveyor 12 (see Figurn 2). The robot 110 has a turret 114 and two articulated arms 116 and 118 and is generally of a known construction, and is referred to as a five axis machine. It is fitted with a nut placing head 120 at the free end of the arm 118 and the robot is able to move this head 120 to any position over the area occupied by a platen 22 arrested at STATION 2. The operation of the robot 110 is controlled from a first controller 122 located within the nutting cell and in turn controlled by signals from the programmable logic controller 100. Immediately in front of STA TION 2 there are two large bowls 124 and 126 situated above the path of the conveyor 12.These bowls are intended to receive supplies of nuts for use in the apparatus and it is intended that the apparatus can deal with two nut sizes (e.g. M8 and M6) and nuts of one size only are placed in each bowl. The bowls are fitted with vibratory feed mechanisms so that a stream of nuts flows from each bowl along a respective nut feed tube 128 or 130 towards the placing head 120 of the robot 110. The type of nut to be placed is selected by an operator who removes one type of nut placing head 120 (i.e. for M8 nuts) and replaces it with another type of head (i.e. that for M6 nuts). The identification of the platen indicates the type of nut required for that platen as previously described, and this serves as a cross-check by the programmer 100. The placement head is illustrated in detail in Figures 5, 6 and 7.It comprises a mounting plate 132 which is secured to the robot arm 118 and a depending mounting block 134. A cover plate 136 is secured to, but spaced from a lower face 138 of the block 134 to provide a space within which there is a nut gripping and release mechanism. The latter comprises a centre stationary boss 140 on opposite sides of which are arms 142 and 144 pivoted about fixed axes 146. Each arm has an inturned neb 148 at its lower end and is bevelled as shown at 150 at its upper end; further, there are small compression springs 152 arranged between the boss i4Q and each arm 142 and 144, urging the arms into the position illustrated in Figure 7, in which the arms are substantially parallel with each other and the nebs 148 are spaced apart by a distance less than the width of a nut (such as 34).When a nut arrives in the lower end of the placement head through the feed tube 128 or i30 (see Figure 5) it slides into the space between the face 138 and the cover plate 136 and is supported there on the nebs 148.

This is illustrated in Figure 7.

An actuating member 154 is also slidable in the space between the face 138 and the cover plate 136 and is carried by a piston 156 of a pneumatic cylinder 158 secured to the mounting plate 132. Operation of the pneumatic cylinder 158 therefore causes vertical reciprocation of the actuating member 154. Moreover, there is an extension pin 160 depending from and movable with the actuating member 154, and being in axial continuation of the piston 156. The pin 160 is slidable in a bore of the boss 140, by a shoulder 161 of the pin located above the pin co-operating with the bore. A nut 159 on the piston 156 abuts on the upper side of the actuating member 154, and the portion of the piston 156 beneath the member 154 is of enlarged diameter and threaded to abut with the lower side of the actuating member to cause the piston 156 and actuating member to move together.In Figure 7 the actuating member is shown in a raised, inactive position. In this raised position, it will be noted that the lower rounded end of the pin 160 is level with the bottom of the boss 140, and clear of the top side of the nut 34 which is held by the nebs 148. This represents the position of the parts of the nut gripping and releasing mechanism, at the commencement of a nut placing operation.

When the nut 34 is to be placed over a peg 70 through a clearance hole in one of the base elements such as the element 30 illustrated in Figures 3 and 4, the robot 110 is operated under the control of the signals received from the controller 100 and the controller 122, to bring the placement head 120 directly over the location where the nut 30 has to be placed.

As soon as the head 120 arrives over the required location above the platen 22, the head 120 is first lowered to bring the nebs 144 closely adjacent to the base element 30 on the platen. Air is admitted to the pneumatic cylinder 158, and this causes the piston 156 to be projected downwardly, carrying the actuating member 154 with it. During the first part of the downard motion of the actuating member, the pin 160 will slide through the bore of the nut 34 (and the outside diameter of the pin 160 is such that it will pass through the screw-threaded bore of the nut without rotation) until it encounters the reduced diameter portion 76 of the location peg 70.However, the pin 160 under the driving force applied by the pneumatic cylinder 158 is able to push the peg 70 downwardly by a small amount against the action of the compression spring 82 until the nut is in engagement with both the upper end of the peg 70 and the lower end of tbe pin 160. In other words, at this stage the upper part of the peg 70 and the lower end of the pin 160 form an effective continuous guide rod for the nut.

Downward motion of the actuating member 154, causes the bevelled inside faces 164 and 166 on the actuating iiead to engage with the bevelled upper ends 150 of the arms 142 and 144, and this has the effect of drawing the upper end of the arms inwardly, thereby compressing the springs 152, and moving the nebs 148 apart. Eventually, the position is arrived at where the nut 34 is released from the arms 142 and 144, and the shoulder 161 of the pin pushes the nut downwardly to allow the nut to slide downwardly on the pin 160 and the peg 70, into contact with the top surface of the base element 30, to which it is to be secured.It will be appreciated therefore, that the actuating member 154 has a lost motion with respect to the arms 142 and 144, during the first part of its downward motion, wherein it ensures correct location of the nut 34 but during the latter part of its downward motion, it carries out the nut releasing operation. As a result of the operations just described, the nut 34 is correctly placed on the base element ready to be welded thereto.

This operation is repeated at STATION 2, for each of the nuts which are to be fitted to the base components on the platen, whilst that particular platen is stationary at STATION 2.

In the case of the platen illustrated in Figure 3, this will involve placing a total 11 nuts in their correct locations.

When the platens 22 leave STATION 2, they travel along a straight run of the conveyor 12, which passes STATION 3B and ends opposite STATION 3A. STATIONS 3A and 3B are identical with each other, and therefore it is only necessary to describe one in detail. Each of these stations is a welding station for achieving the welding of the nuts to their base elements, and the reason why there are two stations is that the welding operation at each of these stations will take longer than either the identification which takes place at STATION 1 or the nut placing which takes place at STATION 2. Therefore, the run of the conveyor 12 between STATION 2 and STATIONS 3B and 3A is controlled from the programmable logic controller 100, so that the platens 22 are fed alternately to STATION 3A and STATION 3B.In any case, it will be noted, that at each of these stations, the platen moves at right angles over a short section of conveyor 13 or 15, and there is a known type of platen control arrangement for initiating movement of the platen on to either of the conveyors 13 and 15. At the exit side of STATIONS 3A and 3B, the conveyors 13 and 15 lead on to another section of the conveyor 12 parallel with the section leading from STA TION 2, and passing through STATION 4 at which the unloading operation can be carried out.

The construction of the apparatus at one of the welding stations will now be described in detail reference being had in particular to Figures 1 and 8 to 11. Each welding station includes an industrial robot 170, which need not be described in detail, since it is of a known construction but it will be mentioned that it is a three axis robot. At the free end of the robot arm, there is a welding head 172, and the control system includes robot controllers 174 and 176 associated respectively with the robots 170 at the welding stations, and controlled in turn by signals from the programmable logic controller 100. Again, the arrangement is such, that the welding head 172 can be carried by the robot to any position over the platen located in one of the welding stations.In practice of course, the signals received from the robot controller 174 or 176 carry the welding head 172 to a series of locations corresponding to the positions occupied by the nuts which have been placed on the base elements at STATION 2.

Essentially, the robot head 172 comprises an outer electrode system 178 and an inner electrode system 180 for applying mechanical pressure and supplying an electric voltage to an outer hollow cylindrical electrode 182 and an inner solid electrode 184.

An upper, square cross-section block 186 is carried at the free end of the arm of the robot 170 and conventional hydraulic or pneumatic means (not shown) are provided for pressing the block 186 downwardly. Although the robot lowers the head 172 into an operative, welding position, the mechanical pressure is exerted between the block 186 and a reaction plate (not shown) which is secured to the apparatus in a fixed location over the area of operation of the robot head 172. This ensures that the mechanical pressure for welding is not transmitted through the arms of the robot.

This arrangement is entirely conventional and therefore need not be described in detail. The block 186 forms part of the outer electrode system 178. A middle block 188 forms part of the inner electrode system 180 and has a section 190 of spider cross-section (i.e. a central boss with four radial arms) and a lower square cross-section part which provides corner platforms 192 (see especially Figure 11). A central circular cross-section boss 194 projects below the platform part of the middle block 188.

A lower block 196 forms part of the outer electrode system 178. In its top part there is a large bore 198, and a rubber insulation bush 200 located in this bore receives the boss 194 on the middle block and has a lip at its top end resting on a top surface of the lower block. This provides location of the middle block 188 in the lower block, whilst at the same time permitting limited downward motion of the middle block relatively to the lower block and electrical insulation between the middle block (inner electrode system 180) and the lower block (outer electrode system 178).

The lower block is attached to the upper block 186 through a set of four spring systems 202 located one in each corner of the head 172 (see Figure 9) one of these systems being shown in detail in Figure 11. A pin 204 has a flange 206 resting on one of the platforms 192 of the middle block, and the lower end of the pin is received in a bore 208 in the lower block 196 and is secured to that block by a setscrew 210. Thus, the pin 204 is movable with the lower block.

The upper end of the pin 204 passes through a bush 212 and into a hole 214 in the upper block 186 where it is secured by a setscrew 216. Thus the pin 204 is secured to the block 186 and therefore the upper and lower blocks 186 and 196 are secured together by the four pins 204 and form a rigid system.

Where the pin 204 passes through the platform 190 of the middle block, it is slidable in a rubber insulation bush 218 and this also forms part of the insulation between the outer electrode system and the inner electrode system. A short helical compression spring 220 surrounds the pin 204 and extends between a washer resting on the top end of the bush 128 and a washer engaging with the bottom end of the bush 212. The latter is screwthreaded externally and engages in a screwthreaded hole 221 in the upper block 186. By turning the bush 213 it is therefore possible to adjust the axial location of the bush and therefore the force exerted by the spring. A locking screw 222 is provided for locking the bush 212 in any selected axial position.

It will be appreciated, that the four springs 220 provide a force resisting downward movement of the outer electrode system 178 relatively to the inner electrode system 180.

A terminal 224 (see Figures 9 and 10) is provided on the lower block 196 and a terminal 226 is provided on the middle block 188.

The line conductor 221 and the neutral conductor 223, separated by an insulator 225, are mounted on an insulation sleeve 227 on a terminal bolt 229. This bolt clamps the neutral conductor 223 on to the terminal 226, and a shunt cable 229 has one end clamped to the line conductor 221 and its other end clamped by a bolt 231 on to the terminal 224. This arrangement provides a means whereby electric power is connected to the two electrode systems. Each of the blocks 196 and 188 is made of electrically conductive material such as copper.

A lower portion 228 of the lower block 196 is cylindrical, and the outer cylindrical electrode 182 is attached to the lower end of this lower portion 228 by a nut 230. The bore of the electrode 182 is iarger than the distance across the corners of the nut, such as 34, with which the welding head 172 is to be used. Therefore, when the welding head is lowered-see the illustration in chain-dotted lines at the foot of Figure 8~the nut 34 is received within this electrode and the lower end of the electrode engages with the base component.

A piston 232 made of non-conductive material for example, nylon, is positioned on top of the middle block 188 and is slidable in a bore 234 in the upper block 186. An inlet port 236 leads into the bore 234 and is adapted to receive a gland (not shown) on a pneumatic supply pipe. The admission of air under pressure into the bore 234 through this supply pipe is controlled by the robot controller 174. It will be appreciated that when pressure is applied to the piston 232, this tends to move the middle block 188 downwardly relatively to the upper block 186.

The boss 194 on the middle block has a tapered bore 238 within which is received the upper, tapered end of a sleeve 240 made of electrically conductive material. The sleeve 240 extends down into the bore of the outer electrode 182 and the inner electrode 184 is attached to its lower end by a nut 242. A cylindrical liner 244 is fitted into the lower block 196 and an insulation bush 246 in the liner provides a location for the sleeve 240 and the nut 242 are spaced from and therefore electrically isolated from the outer electrode 182.

In use, the welding head heats up and therefore provision is made for passing cooling fluid through it. This comprises an inlet port 248 leading to a vertical bore 250; a vertical coolant pipe 252 leading from the bore 250 into the lower, closed, end of the sleeve 240; an outlet port 254 leading from the top end of the bore in which the sleeve 240 is located; a pipe (not shown) leading from the outlet port 254 to an inlet port 256 in the lower block 196, the port 256 communicating with the annular space 258 in the lower block surrounding the liner 244 and finally an outlet port 260 from this annular space. Cooling fluid pumped into the inlet port 248 therefore passes down through the pipe 252 into the lower end of the sleeve 240 adjacent to the inner electrode 184, up the inside of the sleeve, through the ports 254 and 256 and into the lower part of the lower block 196 adjacent to the outer electrode 182, before returning via the port 260 to the cooling fluid reservoir.

When a nut 34 is to be welded, the robot 170 lowers the welding head until the inner electrode 184 is resting on the top of the nut.

At this stage, the outer electrode 182 is just clear of the surface of the base element 30.

Pressure is then applied through the hydrau lic apparatus to the upper block 178. The first effect of this is to apply downward pressure through the entire welding head 172, thus pressing the inner electrode 184 on to the nut 34. Thereafter, as pressure is exerted on the upper block, that block moves downwardly relatively to the middle block 188, by compressing the springs 220. In other words, the outer electrode system 178 moves down relatively to the inner electrode system 180 and this brings the outer electrode 182 into pressure engagement with the base element 30.

The electrical power is then switched on, and this causes current to flow via the outer electrode, through the base element 30, through the nut 34 and to the inner electrode 184.

This causes the welding of the nut to the base element.

When adequate time has elapsed for welding to be completed, the electrical current is switched off, and air pressure is admitted above the piston 232 to hold the inner electrode system in the lowered condition. The hydraulic system is then operated to raise the welding head 172. Initially, the outer electrode system rises under the action of the springs 220, lifting the outer electrode 182 off the base element. After that, the inner electrode is released from the nut.

It has been found desirable to release the outer electrode from the base element before the inner electrode is released from the nut to ensure that the outer electrode does not stick to the base element.

The welding apparatus carries out what is known as projection welding, which is a form of resistance welding, and in order to assist in projection welding, each nut is formed with very small dimples on its underside, near to each corner. Consequently, it is these dimples which actually rest on the top surface of the base element. When electrical current is applied through the power cables, it passes through the outer electrode 182, thence through the base element itself, up through the dimples and the nut and thence through the inner electrode 184. Now it will be noted, that the current is not required to travel from one side of the base component to the other, as is required in the more common resistance welding techniques. Instead, the current traverses the base element, entering and leaving it at the same top surface.

Projection welding has been attempted previously, but one of its drawbacks is that there is a tendency for current "tracking": that is for the major part of the current load to travel across the top surface of the base element.

This some times scores the top surface, and in any case, tends to produce defective welds at the four corners of the nut. However, it will be recalled that there is a substantial copper button 90 located below each position at which a nut is to be welded. Since the copper button 90 provides a much lower resistance to electrical current than the base element itself, there is a tendency for the current to be drawn down through the base element, and thence through the copper button, before flowing back up through the dimples on the undersides of the nut. This creates a better distribution of the electrical current producing the welds, which in turn enhances the welds themselves. The exact theory of operation is not fully understood, but it has been found that the presence of an electrically conductive button such as that shown at 90 in Figure 4 enhances the quality of the welds.

When the welding is completed, the welding head 172 is allowed to retract upwardly away from engagement with the nut and base element. It can then be traversed by the robot 170, to put it in position over the next nut, to repeat the process to carry out a welding operation on that nut. It is to be understood however, that an appreciable downward force is applied to the welding head 172 by the hydraulic system, in order to ensure that an effective weld is produced. When downward pressure is released, the welding head 172 tends to spring upwardly. The reaction plate (not shown) prevents uncontrolled "bounce" of the welding head 172 and it has been found advisable to provide bearings (not shown) on each welding head, for engagement with the reaction plates.

It is now possible to describe a typical operation utilising the nutting cell which has been described with reference to the drawings. The operator. sited at STATION 4 first loads a series of platens 22 on to the conveyor 12 at STATION 4, and then proceeds to load each platen in turn with the correct number of base elements. When a platen has been filled with the correct number and type of base elements, it is released by the operator, and it begins to travel around the conveyor 12. At any one time, the operator may have a number of platens arrested at STATION 4, for the purpose of unloading and loading.

When the platen 22 arrives at STATION 1, the identification system is operated, and this first signals to the controller 100 the type of platen which has been identified. This in turn indicates to the controller the number, and type of nut which will be required for that particular platen. Moreover, there is also a detection of whether or not base elements have actually been positioned at each of the prepared locations on the platen, and this information is also given to the controller 100, to ensure that nuts are only supplied, placed and welded at positions where a base element is actually located.

Following identification at STATION 1, the platen is released, and allowed to move to STATION 2. At STATION 2, the robot 110 is operated under the control of the programmable logic controller 100 and the robot controller 122, and is traversed around the various nut locations, at each such location placing a nut in position on the base element in register with the clearance hole through that element.

When the correct number of nuts has been placed in position at STATION 2, the platen is released, and travels along the conveyor 12, to one of the short conveyors 13 and 15, where it is carried into one of the welding STATIONS 3A and 3B. At the welding station, the robot 170 traverses the welding head 172 over the series of nut locations, and at each location, the head descends and carries out the welding operation.

Finally, the platen leaves the welding station and travels to the run of the conveyor 12, which leads it to STATION 4 where the operative unloads the finished components from the platen. The platen is then ready to be reloaded for a next journey around the apparatus.

Turning now to Figures 12 and 13, there is illustrated an alternative platen 324 for use with the nutting cell previously described, in place of the platen 22. In general construction, the platen 324 is similar to the platen 22, and therefore it is unnecessary to describe it in great detail. There is a series of five identification holes 323 fulfilling the same purpose as the holes 23 previously described.

Also, the platen has a carrier plate 326 on which the base components are located.

In Figure 12, there are illustrated a pair of generally square base components 330 and 332, each of which is required to have welded to it three nuts 334, 336 and 338. A hole 374 is formed through each of the components 330 and 332 to provide a means of ascertaining that the component has actually been placed at the prepared location on the carrier plate 326.

There are also two elongate channel base elements 340 and 342, each of which has to be fitted with nuts 344 and 346. Each of these channel elements 340 and 342 has a hole 374 to provide a means of identifying that the component has actually been placed in position.

Everything which has been described so far with reference to Figure 12 is very similar to that described with reference to Figures 3 and 4, excepting for the actual shape of the components. Indeed, the components 330 and 332 are very similar to the component 42 illustrated in Figure 3. However, the platen 324 is also adapted to accept bracket type components two of which are illustrated at 350 and 352. Reference to Figure 13 shows that the bracket component 350 has a longitudinally extending web 354, with angled flanges 356 and 358, and a large flange 360, to which a nut 362 has to be welded.

A slot 364 is formed in the platen 324, and this slot is shaped to accept both the longitudinally extending webs 354 of the two components 350 and 352 in an overlapping relationship as illustrated in Figure 12.

The bottom of the slot 364 is closed by a closure plate 366, which is attached by setscrews to the body of the platen 324.

The arrangement is such, that with the webs 354 located in the slot 364, the flanges 360 are able to overlie buttons 90 let into the platen 324, at positions where the nuts 362 are to be welded to those flanges.

The platen 324 illustrated in Figures 12 and 13 is used in exactly the same way as the platen 22 illustrated in Figures 3 and 4, and this alternative platen has only been described to illustrate that it is possible by slotting the platen, to receive components other than planar components on or in the platen for the purpose of having nuts welded to them.

The nutting apparatus described above with reference to the drawings represents a very simple form of apparatus in accordance with the invention. A more complex apparatus also in accordance with the invention will now be described by way of example.

This apparatus is used to manufacture components for electrical equipment comprising circuit boards (the base elements) and various electrical components such as transformers, capacitors and the like (the detail elements).

The detail elements have to be secured to the circuit board in intimate contact with the printed circuit by riveting.

A platen in this arrangement has location for several printed circuit boards-the- locations comprising pegs such as the peg 70, for engagement in holes in the boards. The detail elements are available in containers at STA TION 2 and are fed to the placement head which places the required items on the circuit boards in accordance with the type of platen identified at STATION 1.

At STATION 3, the welding head is replaced by a riveting head adapted to both feed rivets and effect the riveting operation.

This example illustrates the breadth of joining operations which may be carried out by using the invention.

Claims (47)

1. A method of automatic joining of detail elements to base elements comprising the steps of: loading one or more base elements to a corresponding number of prepared locations on a platen; codifying the platen to identify the type of base element(s) proper to that platen and the number and type of detail elements proper to that proper base element or elements; moving the platen to an identification station; automatically identifying the type of platen using the codifying means at the identification station and controlling detail element placement means and joining means in response to the identified platen to place in position on the base element or elements and join thereto detail elements correct in number type and location.

2. A method of automatic joining of detail elements to base elements as claimed in Claim 1, in which the codification of the platen is provided by an encoding array of holes formed through the platen or a part attached to the platen.

3. A method of automatic joining of detail elements to base elements as claimed in Claim 1, in which the coding of the platen is provided by the number, location, shape or other physical characteristic of one or more of the base elements or detail elements.

4. A method of automatic joining of detail elements to base elements as claimed in any one of Claims 1 to 3, in which a visual or tactile detection system is employed to identify the type of platen, reading off the encoding physical characteristic of the base element(s) or detail element(s).

5. A method of automatic joining of detail elements to base elements as claimed in Claim 2, in which the automatic identification of the platen is achieved by detection of a light pattern projected through the encoding array of holes.

6. A method of automatic joining of detail elements to base elements as claimed in Claim 5, in which the identification is carried out by a matrix of photoelectric detectors each corresponding to a possible encoding hole or location hole on the platen.

7. A method of automatic joining of detail elements to base elements as claimed in Claim 6, in which the detectors are located on the same side of the platen as that from which light is projected, the detectors being arranged to receive light reflected back through the holes.

8. A method of automatic joining of detail elements to base elements as claimed in any one of Claims 1 to 7, in which the presence or absence of a base element is also sensed at the identification station and the placement means and joining means are further controlled to ensure that the detail elements are only placed and joined at any prepared location if a base element is present at that location.

9. A method of automatic joining of detail elements to base elements as claimed in Claim 6, in which the matrix of photo-electric detectors is also used to detect the presence or absence of a base element.

10. A method of automatic joining of detail elements to base elements as claimed in Claim 9, in which the absence of a base element is detected by detecting a light beam which will be obscured by the presence of a base element at the prepared location.

11. A method of automatic joining of detail elements to base elements as claimed in any one of Claims 1 to 10, in which, after leaving the identification station, the platen is moved first to a detail component placement station at which the correct number and type of detail elements is or are fed successively to an automatic placement head programmed to traverse the locations for detail elements on that platen, and to place a deta element on the base element(s) at each of these locations, the platen then being moved to a joining station at which an automatic joining head is programmed to traverse the detail element locations on that platen and to carry out a joining operation on each of the played detail elements.

12. A method of automatic joining of detail elements to base elements as claimed in Claim 11, in which the platen is arrested at each of the placement and joining stations long enough for the automatic placement and joining heads to carry out their functions.

13. A method of automatic joining of detail elements to base elements as claimed in any one of Claims 1 to 12, in which the joining step comprises welding the detail element(s) to the base element(s).

14. A method of automatic joining of detail elements to base elements as claimed in Claim 11, in which the automatic joining head is a welding head.

15. A method of automatic joining of detail elements to base elements as claimed in any one of Claims 1 to 14, in which the platen is moved on a conveyor from a loading station through the identification, placement and welding stations and back to the loading station.

16. Apparatus for the automatic joining of detail elements to base elements comprising: a conveyor for carrying platens supporting the base element(s) at prepared locations through a series of stations, there being platen identifying means at one station; detail element placement means and joining means at subsequent stations, and programmable logic control means adapted to receive information from the identification means and to control the placement means and the joining means in response to the received information to ensure that detail elements of the type and number proper to the base elements on the platen are placed in the correct locations on the base element(s) and joined to the base element(s) in these locations.

17. Apparatus for the automatic joining of detail elements to base elements as claimed in Claim 16, in which the identification means comprises a light source and a matrix of photo-electric detectors at the identification station, the arrangement permitting encoding of the platens by providing holes through the platens, each hole permitting a light beam to pass to one of the matrix of photo-electric detectors, whereby the coding is achieved by the selection of the pattern of holes through the platens.

18. Apparatus for the automatic joining of detail elements to base elements as claimed in Claim 17, in which the light beam is arranged to be reflected in its passage from the source to the detectors.

19. Apparatus for the automatic joining of detail elements to base elements as claimed in Claim 16 or Claim 17, in which the detail element placement means incorporates a programmable manipulator fitted with a placement head and able to carry that head to any location over the entire effective area of a platen at the placement station.

20. Apparatus for the automatic joining of detail elements to base elements as claimed in Claim 19, in which the placement head comprises articulated means for gripping the detail element and power operated actuator means for operating the gripping means to release the detail element.

21. Apparatus for the automatic joining of detail elements to base elements as claimed in Claim 19 or Claim 20, in which a locating pin is adapted to be projected through a bore of the detail element for accurate location of the detail element.

22. Apparatus for the automatic joining of detail elements to base elements as claimed in Claim 21, in which the actuator means is adapted to operate the locating pin, there being a lost motion arrangement between the actuator means and the gripping means, so that the locating pin is caused to operate before release of the detail element.

23. Apparatus for the automatic joining of detail elements to base elements as claimed in any one of Claims 20 to 22, in which the actuator means is pneumatically operated.

24. Apparatus for the automatic joining of detail elements to base elements as claimed in any one of Claims 16 to 23, in which the joining station incorporates a programmable manipulator fitted with a joining head and able to carry that head to any location over the entire effective area of a platen at the joining station.

25. Apparatus for the automatic joining of detail elements to base elements as claimed in Claim 24, in which the joining head is a welding head.

26. Apparatus for the automatic joining of detail elements to base elements as claimed in Claim 25, in which the welding head comprises a pair of electrically conductive blocks, one connected electrically to a first welding contact and the other connected electrically to a second welding contact, the two blocks being attached to each other by electrically insulating means arranged to permit limited motion between the two blocks in the direction towards and away from the location at which a detail element is to be welded.

27. Apparatus for the automatic joining of detail elements to base elements as claimed in Claim 26, in which the two blocks are substantially coaxial and are connected by an electrically insulating sleeve which is secured to one of the blocks but slidable on or in the other.

28. Apparatus for the automatic joining of detail elements to base elements as claimed in Claim 27, in which a rubber or like resilient sleeve is fitted between the two blocks to provide a resilient connection between them which also isolates one block electrically from the other.

29. Apparatus for the automatic joining of detail elements to base elements as claimed in any one of Claims 26 to 28, in which one of the welding contacts comprises a cap engageable with the top surface of a nut, and the other welding contact comprises a boss, concentric with the welding cap, and adapted to engage with the base element around the outside of the nut.

30. A platen for use in the automatic joining of detail elements to base elements comprising a carrier plate having a series of prepared locations for a set of base elements, and encoding means identifying the platen and signalling the number and type of detail elements required for the base elements for which there are prepared positions.

31. A platen as claimed in Claim 30, in which the or part of the prepared locations comprises a button of high electrical conductivity flush with or proud of the surface of the platen at each location where a detail element is to be secured to a base element.

32. A platen as claimed in Claim 30 or Claim 31, which is encoded by forming a pattern of holes through the platen to enable light beams to pass through the holes, whereby the number and disposition of the light beams will give an indication of the type of platen.

33. A platen as claimed in any one of Claims 30 to 32, in which the prepared locations are provided by providing locating pegs projecting from the top surface of the platen for engagement in corresponding holes formed in the base elements.

34. A platen as claimed in any one of Claims 30 to 32, in which the prepared locations are provided by upstanding edge location means for the base elements.

35. A platen as claimed in Claim 34, in which the edge location means comprises dowels.

36. A method of welding a detail element on to a base element comprising: engaging the detail element with a first electrode; engaging the base element with a second electrode on the same side of the base element as the detail element, and engaging the opposite side of the base element in the area opposite to the detail element with a conductor of higher electrical conductivity than the base element, so that the electrical flux path through the base element is deflected from the side of the base element on which the detail element is located.

37. A method of welding a detail element on to a base element as claimed in Claim 36, in which the conductor has an area such that it is directly opposite both the electrodes.

38. A method of welding a detail element on to a base element as claimed in Claim 37, in which the base element is engaged by the second electrode all round the detail element, and the conductor has an area at least equal to the area bounded by the engagement of the second electrode with the base element.

39. A method of welding a detail element on to a base element as claimed in any one of Claims 36 to 38, in which the conductor is made of copper.

40. Apparatus for welding a detail element on to a base element comprising: a platen adapted to receive the base element, the part of the platen on which the base element rests being made of electrical insulating material; a conductor button on the platen in a location aligned with the position which the detail element will occupy on the base element and having an operative surface level with or proud of the surface of the electrical insulation part of the platen; a first electrode adapted to engage with the detail element on the opposite side of the base element to that which rests on the platen, and a second electrode adapted to engage with the base element on the opposite side of the base element to that which rests on the platen.

41. Apparatus for welding a detail element on to a base element as claimed in Claim 40, in which the first electrode comprises a cap, and the second electrode comprises a boss surrounding but spaced from the first electrode.

42. Apparatus for welding a detail element on to a base element as claimed in Claim 40 or Claim 41, in which the first and second electrodes are mounted on a welding head as claimed in any one of Claims 25 to 29.

43. A method of automatic joining of detail elements to base elements substantially as herein described with reference to the accompanying drawings.

44. Apparatus for the automatic joining of detail elements to base elements constructed and arranged substantially as herein described with reference to the accompanying drawings.

45. A platen for use in the automatic joining of detail elements to base elements, constructed and arranged substantially as herein described with reference to Figures 3 and 4 or 12 and 13 of the accompanying drawings.

46. A method of welding a detail element on to a base element substantially as herein described with reference to Figures 8 to 11 of the accompanying drawings.

47. Apparatus for welding a detail element on to a base element constructed and arranged substantially as herein described with reference to Figures 8 to 11 of the accompanying drawings.