This invention concerns metal boxes for mounting electrical accessories.

It is well known to manufacture electrical accessories which have a metal mounting box for fixing on a surface. Such mounting boxes are formed with a base and upstanding sides, with the box open to receive an electrical accessory, which when fitted forms an effective lid closing the box. The mounting box is normally finished to match the electrical accessory that will be mounted to it. By way of example such electrical accessories may include switches, sockets, circuit breakers, plain covers for terminal boxes and similar devices. To ensure that a location is provided for a front plate acting as the lid, and that there is no gap between the front plate and the box, known combinations of electrical accessories and metal boxes are made such that the front plate is provided with sides depending from the front surface, where the inside dimensions of the sides is such that they may fit over the outside of the upstanding box sides. The disadvantage of this is that such assemblies require the box dimensions to be smaller than the frontplate of the accessory limits the wiring space available in the box.

It is also known to make assemblies of electrical accessories which do not have a location feature between the box and frontplate, the frontplate of the electrical accessory merely simply sits on the top edge of the upstanding box sides. The advantage of this arrangement is that an exterior surface of the depending side wall of the electrical accessory is flush with an exterior surface of

the mounting box. However the disadvantages of this arrangement is that it is difficult to ensure that there is no gap between the box and the accessory, hence the box has no resistance to the entry of dust etc., and the lack of a positive location between the box and the accessory frontplate means that the assembly is less robust, and the force of any sideways knock to the frontplate is only resisted by the friction between the frontplate and the box and the lateral stiffness of the screws.

It is further known in the art of metal forming to make metal boxes, such as tool boxes, where the side of the box is formed so that a ledge is formed on an external surface of the side wall of the box to provide a location for a lid. However, in such boxes the side wall is bent or formed in such a way that the ledge formed is relatively deep, being several times the thickness of the metal, and has corners with relatively large radii being greater than the thickness of the metal. Hence the ledge has a sloping step surface formed by the blending of the radii, and does not have a flat step surface. The disadvantage of this type of box is that the ledge provides a relatively poor location for the lid, which when closed tends to wedge on the radii forming the ledge. The upstanding box wall above the ledge that is enclosed within the lid when it is shut has a relatively large clearance between it and a corresponding internal surface of the lid. Hence this upstanding box wall above the ledge does not provide an accurate location for the lid.

It is an object of this invention to provide a metal box for mounting on a surface which overcomes the aforementioned disadvantages.

It is a further object of this invention to provide an improved process of making a metal box for mounting on a surface.

According to the present invention a metal electrical accessory mounting box is provided comprising an open box part and a front plate, the open box part having a base and at least one upstanding wall, the wall having an outer edge and a ledge adjacent the outer edge to receive closely and abuttingly locate the front plate on the open box part, the ledge being formed so that a surface of the front plate at an angle to the base is flush with the corresponding external surface of the wall at the abutment of the front plate and wall.

Preferably the outer edge of the wall is displaced towards the inside of the box forming the ledge having a top facing surface and a lateral facing surface, the top facing surface providing an axial location for a lid in a direction substantially at right angles to the plane of the base, and preferably the lateral facing surface providing a lateral location for the lid in a direction substantially parallel to the plane of the base.

According to a further aspect of the present invention a process of making an electrical accessory mounting box comprises the steps of:-

(a) providing a blank sheet of metal;

(b) forming the blank into an open box, comprising a base and at least one upstanding wall having an outer edge;

(c) inserting a mandrel into the box so that an outer surface of the mandrel is spaced from and within the outer edge;

(d) applying a force against a face adjacent the outer edge to deform the wall so as to produce a ledge in the so deformed wall of the outer edge.

Preferably the blank is formed into the box by a deep drawing press operation, and the outer edge is formed by a method such as a pinch trim press operation so that the outer edge is at substantially a constant height above the base.

Preferably the mandrel is constructed with a first edge, and the force is applied by a punch with a second edge, and a closest distance between the first edge and the second edge at their closest position is less than a thickness of the sheet of metal. In an embodiment of the invention the closest distance is less than two thirds of the thickness of the metal.

Electrical devices such as switches, sockets and circuit breakers are commonly mounted in surface mounted boxes or in boxes which are recessed into a surface. The former are known as "surface mounted boxes" and the latter as

"flush mounting boxes". Both these boxes have a base and an open front. Normally in Great Britain the boxes are rectilinear and have four walls, but they can be cylindrical so that they only have a base and a single wall. Mounted to these boxes is usually a fitting such as a front plate on which is mounted an electrical device, the front plate acting as a closure to the box. The comp_ete assembly that is device, box and any intermediate components such as a front plate will be referred to hereafter as an electrical accessory.

For a flush box a plate known as a "grid" or "grid plate" is provided to which a device and front plate is mounted. In order to adjust the front plate which overlaps the walls of the box relative to the box and hence relative to the surface onto which the front plate abuts, the grid plate is adjusted relative to the box by means of screws through the grid plate into the box. Because of the requirement to adjust the grid plate relative to the box the grid plate is smaller in lateral extent than the extent of the inner surface of the side walls of the box. In one early proposal by The British Thomson Co. Ltd. (see GB Patent No. 227,099) a primitive grid plate is provided which is larger in extent than the extent of the inner surface of the box. In this case adjustment is obtained by providing an adjustable bracket either side of the box which can be adjusted relative to the mounting surface so that the front plate abuts the mounting surface. In both cases adjustment is tedious particularly in the latter case because there is no easy way of judging whether or not the front plate is going to be at the correct level after adjusting the grid plate until after the front plate is fitted.

In the case where an electrical device is designed to be fitted to a surface mounting box there is no need to adjust the level of the grid plate which is then frequently screwed directly to the box, the front plate being secured to the grid plate.

A still further object of the invention is to provide a grid plate arrangement for an electrical accessory which overcomes the aforesaid disadvantages and which is also suitable for use in flush or surface mounting boxes.

Accordingly, an electrical accessory according to another aspect of the present invention comprises an electrical device and a mounting grid plate to which the device is mounted, the plate having opposed lateral tongues extending outwardly away from the device so dimensioned and arranged so as to locate the plate on a surface surrounding a standard flush mounting box the tongues being also so dimensioned and arranged so as to locate the plate on a surface if a surface mounted mounting box, the plate having means for securing it to each mounting surface. The invention further comprises a grid plate for use in the electrical accessory of the invention.

By extending the lateral tongues of the grid place well beyond the outer lateral extremity of the standard flush box that is in Great Britain more than 55mm across the extremities of the plate the lateral tongues are easily adjusted to seat on the surrounding surface so that the grid plate, and hence electrical device, are at the correct level relative to the surrounding surface so that it merely remains to screw the front plate directly onto the grid plate where it should automatically abut correctly.

Where a surface mounting box is involved the lateral extent of the grid plate should not be too great. A maximum of 86 mm overall is deemed sufficient. In the preferred embodiments the front plate is sufficiently large in the lateral extent to extend over the tongues and can be of processed or moulded metal or plastics.

The arrangement of a preferred embodiment of the invention is such that when decorating it is easy to remove the front plate without baring any live components of the device

which can remain operative. Also front plates can be easily exchanged if a different decorative effect is preferred.

An embodiment of the invention will now be described by reference to the accompanying drawings in which: -

Figure 1 shows a perspective view of a surface mounting box for an electrical accessory according to the invention, and a perspective view of a corresponding grid and front plate;

Figure 2 shows a cross section through the box and plates shown in Fig. 1 on line AA, with the front plate shown in Fig. 1 positioned ready to be fitted to the box;

Figure 3 shows an enlarged cross sectioned view of an edge of the box shown in Fig. 2; and

Figure 4 shows a perspective view of the grid plate shown in Fig. 1 mounted to a folded metal flush box together with a modular switch connected to wiring and ready for insertion into the grid plate.

Figure 5 shows a partly sectioned view of a press tool for deforming the wall of the box of Figs. 1 to 3 to produce the ledge shown in Fig. 3; and

Figure 6 shows a perspective view of the tool (shown in section in Fig. 5) for deforming the wall of the box to produce a ledge.

In Figure 1 can be seen a metal surface mounting box at 1 and a grid plate 310 (hereafter referred to as a "grid") and a front plate or cover 2. The box 1 comprises a base 10 with four upstanding side walls 11, 12, 13, 14. Each side wall is provided with a knockout 15, 16, 17, 18 one or more of which the installer may remove to provide an entry hole for electrical cable or an electrical conduit. Two of the side walls are fitted with fixing lugs 20, 22 which protrude into the interior of the box. Each fixing lug has a threaded hole 24, 26 to receive a pan head fixing screw 312 and 313 to retain the grid 310 onto the box. A continuous ledge 100 is formed around the perimeter of the upstanding side walls 11, 12, 13, and 14. A fillet radius 30, 31, 32, 33 (see Fig. 2) is provided between the upstanding sidewalls 11 and 13 and the base 10. The fillet radius 30, 31, 32, 33 is continuous around the box base 10, and is provided to facilitate the manufacture of the box by deep drawing on a press tool without significant thinning or creasing of the material. For the same reason a similar radius is provided at each box corner 34, 35, 36, 37. The box is rectangular with box side first dimension 40 and second dimension 42. For a one gang (or substantially square) box these dimensions will normally be the same, but for a two gang (or rectangular) box the second dimension will be longer than the first dimension.

The enlarged scrap view in Fig. 3 of the ledge 100 clearly shows a flat step 102 formed by a displacement 106 of a periphery 104 of the side wall 13. Displacement 106 is slightly greater than the material thickness 108 of the side wall 13. The partially sheared step thickness 112 is significantly less than the material thickness 108. Internal corners 114 and 116 have small radii while the

externally visible corner 118 has a larger radius. The interior corner 120 also has a larger radius with a dimension approximately half or one times the material thickness 108. The ledge 100 has a minimum substantially flat width 122 which is greater than half the material thickness 108. A top edge 110 of the periphery 104 is illustrated as having a smoothly curved surface, however since this edge is formed by a pinch-trim operation on a press tool and is not subsequently finished its shape after the creation of the displacement of the periphery 104 may be different from that shown. The precise shape obtained depends on the degree of planishing done during the creation of the displacement 106.

From Figure 1 it may be seen that the cover 2 comprises a top 200 and depending sides 201, 202, 203, 204. A bottom edge 214 extends around the depending sides. A chamfer 206 is provided on the cover to give a distinctive styling feature. The top 200 has two countersunk holes 210, 212 to receive countersunk fixing screws 208 (only one shown) and an aperture 232 to receive an electrical accessory such as a modular switch 334 shown in Fig. 4. The switch module is arranged so that a protruding shroud part 333 is shaped to fit closely to an aperture wall 234 of aperture 232. It is intended that a shroud face 335 will be flush with the top 200 of the cover 2 when it is correctly positioned.

A grid 310 for mounting to the box 1 using pan head fixing screws 312, 313 passing through hole 314 and slot 316. Mounting grid 310 also has threaded holes 318 and 320 for accepting the countersunk fixing screws 208 to retain the cover 2. Electrical devices such as the switch 334 may be fitted to the grid 310 in the aperture 340 and may be

retained by resilient snap fits 380, 382 (shown in Fig. 4) which co-operate with the features 342, 344 of the aperture 340. An earth terminal 346 is permanently attached to the grid. On two opposing sides the grid has low walls 348, 350. These low walls act to strengthen the grid and they are sufficiently high to prevent the dangerous practice of electricians inserting a screwdriver into terminal screws 336 of the electrical module while the electrical module is fitted to the grid which if correctly installed will be connected to an earth wire. The grid also has four tongues 352, 354, 356, 358 which extend on the two opposing sides that do not have the low walls 348, 350. Tongue 352 is narrower that the other three tongues to provide sufficient space to mount the earth terminal 346. Likewise low wall 348 does not extend around the corner adjacent to the earth terminal, so that access to the earth terminal is not restricted. The three wider tongues 354, 356, 358 are provided with square holes 360, 362, 364 which facilitate the accurate forming of the tongues.

The grid 310 is arranged to be suitable for mounting to either one of surface mounting box 1 or a flush box such as folded metal flush mounting box 370 or a plastic flush mounting box (not shown) . The flush mounting box 370 is arranged for mounting in a wall so that when the box is mounted a top edge 372 will be substantially flush with an external facing wall surface.

The tongues 352, 354, 356, and 358 are cranked adjacent to the square holes to ensure that their rear faces 353, 355, 357, 359 respectively are at a suitable height so that when the grid is mounted directly to an external facing wall surface in the flush mounting box 370 and the cover 2 is

fitted to the grid 310 and screwed back so that the bottom edge 214 is touching the external facing wall surface, the top 200 of the cover 2 is correctly positioned flush relative to shroud face 335 of switch 334.

The arrangement of the tongues 352, 354, 356, and 358 is necessary as in use the grid is mounted to the flush mounting box 370 with pan head screws 312, 313 before the switch 334 is connected to wiring 377, 378. The switch is inserted into the grid aperture 340 and then the cover 2 is mounted to the grid using countersunk fixing screws 208. The benefit of connecting the wiring to the switch module after the grid is mounted is that maintenance and alterations to an installation is easier, particularly for larger grids where there are a number of switch modules are mounted to the one grid. To ensure an acceptable installation, without placing restrictions on the installer as to the type of flush mounting box that he uses, it is necessary that the tongues should extend outwards as far as possible while still being covered by the cover when it is mounted to the grid. Hence in some installations it is possible that the installer does not use a mounting box, and the tongues rest only on the externally facing wall surface. The benefit of the arrangement of this embodiment is that the bottom edge 214 of the frontplate or cover when mounted will always be touching the externally facing wall surface. The distance between lateral extremities of the tongues is 78 mm which adequately overlaps sides of flush box 370 yet is within the 80 mm interior width of a plastic surface mounting box. The distance apart of screws 313 is

60.3 mm so that each tongue extends about 9 mm beyond each adjacent screw.

In a similar manner, when the grid 310 is mounted to a surface box 1, the rear faces 353, 355, 357, 359 of the lugs 352, 354, 356, and 358 will rest on the grid locations 300, 302, 304, and 306. In this way when the cover is mounted to the grid, the bottom edge will rest against the flat step 102 of the ledge 100 ensuring a minimum gap and the top 200 of the cover 2 is correctly positioned flush relative to the shroud face 335. The installer does not have to make any adjustment or alteration to the cover or the grid to use then with either a flush box or a surface box. Neither is the installer required to adjust the relative heights of the grid, the module or the cover to achieve a satisfactory installation.

A press tool 400 for forming the ledge on a deep drawn box 3 is shown in Figure 5. The press tool 400 comprises a massive anvil 402 that is rigidly clamped to a bed of a suitable press (not shown) , and a moving carrier 404 that is held in a punch or tool holder of the press. The massive anvil 402 has an adjustable height stop 406 against which the deep drawn box 3 may be rested, a clamping anvil 408 which has had the sharp edge 410 "killed" to produce a small radius, and a planishing insert 412. The moving carrier 404 has slidably mounted a clamping jaw 414 and a shear punch 418, both being retained by retaining pin 422, which passes through slot 424 in the shear punch and slot 423 in the carrier. The clamping jaw 414 is urged in the direction of arrow M by a urethane spring 416 that is mounted to a pocket 432 reces' ad in rear face 426 and acts against a pressure face 428 o the carrier 404. The shear punch 418 has a rear face 430 and a forming profile 432, which has a shearing edge 434 and end ramps 436, 438 leading to set back face 444 and 446 respectively. The

forming profile has working lengths 440 and 442 that are at least slightly longer than half and preferably less than three quarters of the box dimensions 40 and 42. Set back faces 444, 446 are parallel to the forming faces 448 and 450, but set back behind them by set back dimension 452.

In operation the press tool is preferably mounted horizontally as shown, and a deep drawn box is inserted into the tool as shown, so that a base 10' rests against the adjustable stop 406, and a first radiused box corner 34' is positioned as shown. The adjustable stop 406 has previously been adjusted to suit the depth of the box to be formed so that the desired height 105 of the periphery 104 (see Fig. 3) will be obtained. The moving carrier 404 is moved in the direction of arrow M so that clamping jaw 414 clamps the first radiused box corner 34' against the clamping anvil 408. As the moving carrier continues to move the urethane spring 416 is compressed until eventually the rear face 426 contacts the carrier face 428. Meanwhile the movement of the carrier has brought the shear punch 418 into contact with the periphery of the box, and as the carrier continues to move in the direction of arrow M the shear punch deforms the box, creating the ledge. The suitable press is capable of resisting any forces perpendicular to the direction of arrow M without a significant deflection in the perpendicular direction. Hence as the shear punch deforms the box side, due to the closeness of the killed sharp edge 410 and the punch corner 420, the deformation is a combination of shear and bending. To ensure that the periphery is formed with the desired smooth finish, free from creases, as the carrier approaches the end of its movement in the direction of arrow M the metal of the box is squeezed solidly between the shear

punch 418 and the planishing insert 412, and also between the clamping jaw 414 and the clamping anvil 408 to perform a planishing operation. Hence the periphery of the box is produced with a well defined flat step 102, and the size of the corners 114 and 116, and 118 and 120 are controlled within acceptable limits. By adjusting the sharpness of the killed sharp edge 410 and the punch corner 420 and the amount the box is squeezed in the planishing operation the shape of the top edge may be influenced.

The carrier then reverses its movement and moves in a direction opposite to that of arrow M. When the box is free it may be removed and rotated through 90 degrees, and re-inserted into the tool. The press operation is then repeated to form a ledge on the second corner, and likewise for the subsequent corners.

For a rectangular box with a substantial difference in lengths of the long sides and the short sides it will be necessary to use a press tool with the correct working lengths 440 and 442 that correspond to the lengths of the sides being formed.

To form the grid locations 300, 302, 304 and 306 the box is then inserted into another press tool which operates in a similar manner to that shown and described in Fig. 5, except that the box is mounted with side wall 12 or 14 perpendicular to the direction of arrow M, and the planishing anvil 412 is replaced by a shaped anvil which has radiused corners corresponding to radiused corners

280-3 and 284-7 adjacent to the grid locations 300, 302 and 304, 306 respectively. The shear punch 418 is replaced by a flattening punch which is shaped to flatten the periphery 104 firstly at 300 and 302 and then at 304 and 306.

With care, the planishing operation will leave the top edge 110 providing a lead in for the cover. It is important to note that the box 1 is finished with paint, preferably of a hard durable material, with a significant thickness. This paint film is advantageously applied by a method such as electrostatic application where good coverage is provided on edges. Such a film ensures that any hazardous sharpness of the top edge is covered by the paint film and the hazard is removed. The paint thickness has to be allowed for in deciding on an appropriate dimension for the displacement 106 of the periphery 104, since when fitted to the box the sides 201, 202, 203, 204 of the cover must be flush with the box sides. In addition the cover must be a clearance fit over the periphery to ensure easy mounting, but if the clearance is too great the benefit of the location provided by the periphery will be lost.

Since there is an increase in material thickness at the box corners as the ledge is formed, it is necessary to ensure that an external planishing radius on the planishing insert

412 that corresponds to the internal forming radius 456 is sufficiently large to ensure that the increased material thickness can be accommodated. If such allowance is not made, then the periphery height 105 will increase unacceptably at the corners.

Although the specific embodiment described above is for a substantially square box co-operating with a grid and cover to accept one modular electrical device, the invention applies equally to rectangular or other shaped - for example circular - boxes, and the invention applies equally where the box, grid and cover are adapted to accept more than one modular electrical device.