Many situations arise in day-to-day engineering practice which require a metal plate to be fastened in position.

In order to achieve this, a wide variety of fasteners has been developed over the decades, some of widespread and general application and others of a more specialist nature.

Among the former, a variety of threaded nuts have been produced which, for example, may be welded to a sheet or located in or adjacent an aperture in the sheet, the nut in each case having an internally threaded portion in which a mating thread may be screwed to locate the nut and surrounding plate. Such weld nuts, or nuts provided with e.g. a spring steel cage adapted to engage in a pre-punched aperture in the sheet, are available in a wide variety of shapes, sizes and materials and their use

is widespread. However, the known types do not always possess the necessary desirable combination of properties for many high demand engineering applications. In purely engineering terms, it is clearly desirable to have a fastening which is as strong as possible. Spring clip cages tend not to be very strong, while welding tends to be variable in its effect and, in any event, to lead to damage to the structure of both nut and plate which can promote crack formation under strain and eventual failure. Additionally, it is always desired to provide the specified fastening effect in inexpensive fashion while maintaining uniformity and reliability.

One aspect of providing such fastenings inexpensively is to avoid the need to preform the aperture in the plate, for example as disclosed in US-A-5423645.

Analogous considerations apply to studs which are also widely used to provide fastenings on sheets. The term "stud" as used herein means a fastener having a head destined to be attached to the sheet and a shaft or shank on to which a cooperating fastener member may be fixed.

The most widespread means for effecting such fixing is to thread the shaft, though other fixing techniques may be used, such as, for example, riveting or engagement of a spring clip with, e.g. a groove formation on the shaft.

The head of the stud may be fixed to the plate, either by its face remote from the shaft, or by its face surrounding the shaft, by welding or other means.

A wide variety of stud fixing systems is known in the patent literature, for example as disclosed in US-A- 4966512 and 4114670.

Particular problems arise in the case of fixtures which must remain fixed even when subjected to very high transient loads, for example such as might be caused by impact on a fabricated assembly. A particular application area which places very substantial demands on engineering fasteners is that of fixing in place components which are to be subject to very sudden, very high stress. A specific example of this is the fixture in place of an inflator cylinder designed to inflate a vehicle safety airbag. When the airbag is to be deployed in order to fulfil its design function of saving life, or reducing injury, it is necessary that it be inflated extremely rapidly. It is conventional, for this purpose, to provide as the source of inflation a cylinder of compressed gas provided with some form of rupturable seal. When the release of the airbag is triggered, the seal is ruptured and effectively a controlled explosion of the compressed gas takes place. Such an explosion can place very severe strains on the fastener or fasteners holding the compressed gas cylinder in place. Failure to hold the compressed gas cylinder in place may convert it essentially into a which can itself cause damage or injury.

The problem underlying the invention is to develop a method of securing a metal plate using a press-in stud fastener which, when inserted, is capable of withstanding high mechanical stress without separation of the plate and the fastener. This is of particular concern in the field of press-in stud fasteners for holding airbag inflation canisters in position when inflation is triggered.

The present invention provides an improved method of

securing a fastener consisting of a head and an elongate shank in a plate, as more particularly set out in the appended claims. Further developments of the invention are evident from the sub-claims.

It is highly preferred that the annular recess includes a plurality of generally radial ribs or splines, most conveniently six equi-angularly spaced relatively stubby ribs or splines.

Such a stud may be used to fasten a sheet metal component in a desired position. For this, the stud must first be firmly connected with the sheet metal, and this is effected by providing in the sheet metal a hole substantially of internal shape and size matching that of the first portion of the shank and then deforming the first portion generally axially, preferably by a cold- forming process, so that the first portion of the shank pushes the edges of the sheet material into the annular slot in the face of the head adjacent the shank and, at the same time, deforms the first portion of the shank to engage in firmly interlocked fashion with the hole in the sheet. The second portion of the shank acts as a guide for a cold-forming tool during this process. The third portion of the shank is, as indicated above, provided with a fastening configuration. Preferably and most conveniently, this is a screw thread, but other fastening configurations may be envisaged, for example a deformable rivetable head or some other form of engageable configuration, for example a cylindrical shank with an annular groove into which a spring clip may be fitted, or a cylindrical shank with a transverse bore through which a retainer pin may be fitted. The first and second portions of the shank may be cylindrical or may have a

non-circular configuration.

As indicated above, the fastener and sheet metal are connected together by cold-forming parts of the fastener.

If the annulus in the face of the head of the fastener directed towards the shank contains one or more ribs or splines, this materially improves the resistance to rotation of the fastener about the axis of its shank following insertion into the sheet metal.

It is to be understood throughout the specification and the appendant claims that the term "sheet" is used generically to indicate any generally sheet-like article, or to indicate part of an article, which part consists of a roughly flat metallic sheet. In order that the fastener may be attached to the sheet, an aperture of internal shape and size the same as, or only slightly greater than, the external shape and size of the first portion of the shank must be formed in the sheet. The component itself may be a substantial extending sheet of metal or may be, for example, the wall of a metallic component.

By way of example, a preferred way of putting the invention into effect is described below with reference to the accompanying drawings in which: Figure 1 is a drawing of a fastener in perspective, Figure 2 is a cross-section through the fastener of Figure 1, Figure 3 is a cross-section through an assembly consisting of a fastener in accordance with Figure 1 or -2

cold formed into the end of a deflector cap located on an airbag inflation canister, and Figure 4 is a perspective view of an alternate form of fastener for use in the method according to the invention.

Referring to the drawings, the fastener shown in Figure 1 consists of a wide head 1 and a shank extending therefrom upwards as shown in the drawing. The portion of the shank remote from the head 1 is externally threaded at 2, and between it and the head are successively larger diameter sections 3 and 4. As more clearly seen in the cross-section shown in Figure 2, the head 1 has surrounding the section 4 an annular groove 7 in which are located a set of equally spaced ribs 9.

Figure 3 shows the fastener of Figure 1 following its insertion into a deflector cap 10 which, in use, is welded at 11 to an airbag inflation canister 12, only the top portion of which is schematically shown. The cap 10 has a number of apertures through which gas ejected from canister 12 may inflate an airbag.

In order to hold the assembly of canister 12 and cap 10 in position during inflation, cap 10 is held to a strong portion of the airbag module which is installed by suitable fixture to the bodywork of the vehicle. The cap is attached to the strong portion by means of a fastener in accordance with the invention, for example by threading shank 2 firmly into a threaded bore in the strong portion of the airbag module.

As can be seen from Figure 3, the generally flat end of

cap 10, horizontal in the Figure as drawn, carries the fastener of the invention which has been united with the cap 10 by cold forming. During this process, the cylindrical section 4 of the fastener is compressed axially by a suitable tool which is located in close fitting fashion about portion 3 of the shank. As is clearly evident from a comparison of Figures 2 and 3, the axial extent of the narrower diameter section 3 is increased and that of section 4 decreased by such cold forming. In addition, the deformation of section 4 causes the inner edge surrounding the circular aperture in the end of cap 10 to be deflected downwards into annular groove 7 with the edge being deformed partially to fit round each of the ribs 8.

We have found that fasteners of this type have exceptionally high resistance to being pulled out of the cap 10 in the upward direction as shown in Figure 3.

Accordingly, when the airbag is installed, if inflation commences, the chances of the explosive escape of gases from canister 12 pulling cap 10 off the head of the fastener are very much reduced. By suitable dimensioning and choice of materials, the resistance to pulling apart in the axial direction of the shank may be raised to a desired safety factor multiple of the highest force likely to be encountered in practice.

Figure 4 shows a fastener analogous to Figure 1 but in which the resistance of the fastener, once set by cold forming into the sheet, against turning about its axis is enhanced not by ribs in the annular groove, but by a non- circular configuration of the sections of the shank intermediate the threaded section 20 as shown in Figure 4 and the head, i.e the guide section 23 and the deformable section 24 are of non-circular cross-section.

With the exception that the cold-setting tool must clearly fit over section 3, setting of this fastener by cold forming into a metal sheet pre-punched with an aperture matching the cross-section of portion 24 is as described above with reference to Figure 1.