Background to the invention Previously, a self-drilling screw has consisted of a one-piece screw where the tip, shank and head have all been integrally formed. Features of the shank and tip have been altered depending on the sheet material and material of the support that the sheet material needs to be secured to.

It has been common for self-drilling screws, for the securing of metallic of steel sheet materials to metal or steel, to be provided with a threaded shank and a drilling tip that has provided two cutting edges adapted to cut through metal or steel.

To date these screws have proved to be relatively unsuccessful in providing an easy, clean cut through the metal or steel, often damaging the adjacent material around the aperture created by the drilling tip. After time, these self-drilling screws will also rust due to the material they are made from. In order to overcome this problem, the self-drilling screw could be made of a material such as stainless steel which is not susceptible to rust however, the problem which arises is that, stainless steel is not capable of drilling through metal or steel.

Outline of the invention It is an object of this invention to provide an improved cutting screw which is specifically intended for use with metallic or steel sheet materials, and which minimises the problems of damage and rust previously caused by such screws.

The invention provides a two-piece screw, for use in the attachment of metallic or steel sheet material, having a shank, a head and a detachable drilling tip.

It is preferred that the shank of the screw be provided with a hexagonally shaped recessed portion which is adapted to receive the detachable drilling tip.

It is preferred that the drilling tip be similar to that of a standard drill bit having at least one cutting edge extending back a prescribed length from the point of the drilling tip.

It is further preferred that the detachable drilling tip be provided with an outwardly extending portion having a hexagonally shaped perimeter complimentary to, and adapted to fit into, the recessed portion of the shank.

It is preferred that the shank and the head of the screw be integrally formed from a stainless steel material to prevent rusting of the screw and the adjacent sheet material, and the drilling tip be made from a separate material suitable for cutting through metal or steel.

While it is preferred that the head of the screw be of the standard shape of a roofing screw, any relatively broad head would suffice.

In order that the invention be more readily understood we will describe by way of non limiting example a specific embodiment thereof.

Brief description of the Drawing Figure Figure 1 is a perspective view of an embodiment of the two-piece cutting screw according to the invention.

Figure 2 is a perspective view of an embodiment of the two-piece cutting screw showing the drilling tip disconnected from the shank of the screw.

Figure 3 is a side view of an embodiment of the two-piece cutting screw as it is used in the attachment of sheet material to a support.

Figure 4 is a side view of an embodiment of the two-piece cutting screw as it is used in the attachment of sheet material to a support showing the drilling tip becoming dissociated from the shank of the screw once drilled into the support.

Description of an Embodiment of the Invention Figures 1 to 4 show a preferred embodiment of the invention as it relates to a two-piece cutting screw for use in the fastening metallic or steel sheet materials.

In this embodiment of the invention, the screw 1 is provided with a shank 2, a head 3 and a detachable drilling tip 4. The detachable drilling tip 4 is designed to be similar to that of a standard drilling bit. The drilling tip 4 is substantially cylindrical in shape with at least one cutting edge 5 extending back from the point 6 of the drilling tip 4. The or each cutting edge 5 extends from the point 6 of the drilling tip 4 back approximately half the length of the drilling tip 4. The

length of the drilling tip 4 may vary, and is usually dependent on the requirements of different thicknesses and compositions of the materials that is must fasten. The drilling tip 4 may be made of any suitable material that is adapted to cut through metal or steel. The cutting edge may also vary in shape depending on the material to be worked.

The base 7 of the drilling tip 4 is provided with an outwardly extending portion 8 which is adapted to have a width and length smaller than the detachable drilling tip 4. The perimeter 9 of the outwardly extending portion 8 is hexagonal in shape, designed to be similar to that of an allen key, and is adapted to be received by a complimentary recessed portion 10 provided on the shank 2 of the screw 1.

The shank 2 of the screw 1 is cylindrical in shape with its length being dependent on the thickness and composition of the metallic or sheet material and metallic or steel support that it must pass through. The shank 2 has provided a threaded portion 11 which may extend along its entire length, or along a substantial part of the shank 2. The threading may be the same as that of a standard tapping screw which either extends along the entire length of the shank 2, or terminates approximately two thirds of the way along the shank 2. The arrangement of the screw 1 is such that in practice, an aperture will first be cut by the drilling tip with the screw 2 being able to self tap its way through the aperture thereafter.

Provided on the base 12 of the shank 2 is a recessed portion 10 adapted to receive the outwardly extending portion 8 of the detachable drilling tip 4. The depth and width of the recessed portion 10 is dependent on the length and width respectively of the outwardly extending portion 8 of the detachable drilling tip 4, as the screw 1 is designed to have relatively snug fit between these two portions 8,10. The recessed portion 10 is also hexagonally shaped such that is complimentary to the outwardly extending portion 8 of the detachable drilling tip 4 and is thus, adapted to receive the outwardly extending portion 8 therein.

The screw 1 is provided with a standard head 13. The head 13 is hexagonally shaped and has a broad base 14 which is adapted to sit over the aperture created by the screw 1 such that, water is prevented from entering through the aperture. The head 13 is integrally formed with the shank 2 of the screw 1. The head 13 and shank 2 of the screw 1 is made of stainless steel or any other suitable material that is non-rusting such that, the screw and/or the adjacent material around the screw will not rust.

It is customary to drill a smaller hole through which the shank of a self-tapping screw may pass thereby requiring a person to use two tools, one to drill the initial hole, and another to drive the screw through the material and into the support.

By incorporating a drilling bit styled tip into the screw, the screw of the invention is able to fulfil both of these functions at the same time. Therefore, only one tool is required and additionally, the sheet material is protected from damage.

In practice, the appropriate detachable drilling tip 4 is fitted onto the end of the screw 4 if note already done so. The drilling tip 4 os the screw will then"drill"into the sheet material with the remainder of the screw 1 self-tapping through the material and support once the aperture has been created by the drilling tip 4.

The screw 1 is then tightened in place by conventional methods until the sheet material has been secured to the support. Once the detachable drilling tip 4 has "drilled"through the sheet material and support, the drilling tip will become "detached"or"drop off"from the shank 2 of the screw 1, leaving behind only the shank 2 and head 13 of the screw 1 to secure the sheet material to the support.

The screw of the invention provides a simple means of producing an aperture through the sheet material and support, whilst possible damage such as cracking and grazing is minimised. In addition, by having a detachable tip, it eradicates the chance of the tip, screw, or adjacent material forming rust. The screw of the invention thereby acts to both minimise damage to roofing material and to achieve this in an efficient manner.

Whilst we have described herein one particular embodiment of the two-piece cutting screw, and in particular an application as a roofing screw, this type of screw can be used to attach any kind of metallic or steel sheet material to a metallic or steel support or the like.

It is further envisaged that other embodiments of the invention could exhibit variations in the features described. It is however envisaged that other embodiments of the invention will exhibit any number and combination of the features previously described and it is to be understood that variations and modifications in this can be made without departing from the scope thereof.