AUSTRALIA PATENTS ACT 1990 COMPLETE SPECIFICATION FOR A STANDARD PATENT (Original) APPLICATION NO: LODGED: COMPLETE SPECIFICATION LODGED: ACCEPTED: PUBLISHED: RELATED ART: NAME OF APPLICANT: Norma Beatrice Carmen Bond ACTUAL INVENTOR: Brian Peter Bond ADDRESS FOR SERVICE: LORD & COMPANY, Patent & Trade Mark Attorneys, of 4 Douro Place, West Perth, Western Australia, 6005, AUSTRALIA. INVENTION TITLE: "Security Screen Frame" DETAILS OF ASSOCIATED PROVISIONAL APPLICATION NO'S: AUSTRALIAN PROVISIONAL APPLICATION NUMBER 2002952026 FILED October 14, 2002 The following Statement is a full description of this invention including the best method of performing it known to me/us: TITLE SECURITY SCREEN FRAME FIELD OF THE INVENTION 5 The present invention relates to a security screen frame and a method of construction thereof. BACKGROUND TO THE INVENTION Known security screens generally comprise a frame and a rigid mesh or a grille, the 10 rigid mesh or grille being secured to the frame. Known methods of securing the rigid mesh or grille to the frame include fastening the rigid mesh or grille with screws or rivets, which are inserted through the frame and the rigid mesh or grille. These methods of securing grilles to frames frequently require additional structural support, such as the use of internal corner stakes in the frame. This requires the frames to be 15 relatively wide in order to accommodate the corner stakes. Further, known methods of securing grilles to frames are often aesthetically displeasing, as the screw or rivet can be unsightly. The present invention attempts to overcome at least in part the aforementioned disadvantages of previous security screens. 20 SUMMARY OF THE INVENTION In accordance with one aspect of the present invention there is provided a security screen comprising a frame and an internal grille, the frame being comprised of frame members, each frame member including a side wall and two flange portions extending 2 inwardly of the side wall, the two flange portions combining with the side wall to define a substantially U shaped recess, wherein each flange portion has an inner surface and an outer surface, the inner surface of at least one flange portion including at least one protrusion extending towards the other flange portion, the at least one 5 protrusion being positioned between the side wall and an inner edge of the flange portion remote from the side wall, the at least one protrusion being arranged, in use, to engage with an outer portion of the grille in order to maintain the grille within the frame. 10 Advantageously, this allows the grille to be held within the frame without the need for rivets or screws. Additionally, the frame members can be held in positions relative to each other by the grille, removing the need for internal structural supports such as corner stakes. 15 Preferably, the grille includes at least one protrusion extending from the outer portion, the at least one protrusion of the grill being complementary in shape to the at least one protrusion of the frame member in order to engage therewith. BRIEF DESCRIPTION OF THE DRAWINGS 20 The present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a cross sectional view of a security frame member in accordance with the present invention; 3 Figure 2 is a cross sectional view of a grille used in conjunction with the security frame member of Figure 1; and Figure 3 is a perspective view of the security frame member of Figure 1. 5 DESCRIPTION OF PREFERRED EMBODIMENT Referring to the Figures, there is shown in Figures 1 and 3 a frame member 10 of a security screen frame. The frame member 10 comprises a flexible mesh receiving portion 12 and a grille receiving portion 14. The flexible mesh receiving portion 12 is defined by an outer wall 16, a side wall 18 10 and a base portion 20. The outer wall 16 is substantially parallel to the side wall 18, with the base portion 20 extending between the outer wall 16 and the side wall 18 at a lower end thereof so as to create a U-shaped cross section for the flexible mesh receiving portion 12. 15 Preferably, the outer wall 16 may extend slightly further from the base portion 20 than the side wall 18, so as to allow for a screen or flexible mesh to be secured onto the frame without projecting from an outer plane of the frame. Preferably the flexible mesh receiving portion 12 includes internal projections 13 in order to assist in fixing a flexible mesh or screen within the flexible mesh receiving 20 portion 12 in a known manner. The grille receiving portion 14 is defined by an upper flange portion 22, a lower flange portion 24 and the side wall 18. The flange portions 22, 24 are substantially perpendicular to the side wall 18, and extend inwardly from opposed ends of the side 4 wall 18 so as to define a U-shape in cross section. Preferably, the lower flange portion 24 is an extension of the base portion 20. Each of the flange portions 22, 24 have an inner surface 26 oriented towards the other 5 flange portion 24, 22 and an outer surface 28 oriented away from the other flange portion 24, 22. The inner surface 26 of each of the flange portions 22, 24 includes a plurality of protrusions 30. Each of the protrusions 30 extends along the length of the frame member 10, and is parallel to the side wall 18. The protrusions 30 are spaced between the side wall 18 and an inner edge of the 10 flange portions 22, 24 remote from the side wall 18. The cross section of each of the protrusions 30 is substantially in the shape of a right triangle based on the inner surface 26 of the flange portion 22, 24, and having a first side 32 extending perpendicularly of the inner surface 26 towards the opposed inner surface 26 of the other flange portion 24,22. 15 The second side 34 of the protrusions 30, which acts as the hypotenuse of the protrusions 30, extends rearwardly from the inner surface 26 towards the side wall 18. A cross sectional view of a frame engaging portion 42 of a grille 40 can be seen in 20 Figure 2. The grille 40 is used in conjunction with four frame members 10 and the flexible mesh to create a security screen frame. The frame engaging portion 42 of the grille 40 has substantially the same cross sectional dimension as the grille receiving portion 14 of the frame member 10. The 5 frame engaging portion 42 has a plurality of projections 44 on upper and lower surfaces thereof, which are complementary in size and shape to the projections 30 within the grille engaging portion 14. 5 In use, the grille 40 is inserted into the grille engaging portion 14 of the frame member 10 so that the complementary protrusions 30, 44 engage each other in a ratchet fashion. It will be appreciated that the shape of the protrusions 30, 44 allows relatively easy movement of the grille 40 relative to the frame member 10 in an inwards direction, but provides for great difficulty in effecting outward movement. 10 In a preferred embodiment of the invention, four frame members 10 are engaged with four edge portions 42 of a rectangular grille 40 in order to form a security screen. A flexible mesh to prevent insect ingress may then be inserted within the flexible mesh receiving portion 12. 15 It will be appreciated that the invention may be performed with protrusions on only one side of the frame member or grille, and that as little as one projection may be required to perform the invention. It will be clear to a skilled reader that the inclusion of additional projections will provide a stronger screen. 20 Modifications and variations as would be apparent to a skilled addressee are deemed to be within the scope of the present invention 6