KJN and Hepco do a series of self tapping 'core screws' for their extrusions. http://www.aluminium-profile.co.uk/a...ore_Screw.html For the 45 X 45 I used, the central 'hole' is 10mm but it is not a 'plain hole' ... more like a star washer. I used 12mm dia and about 30mm long with a size 50 Torx head. They are designed to join profiles together at 90 deg. To build a T, you would drill a 10-11 mm hole through the centre of the cross piece, the screw is slid along the lower slot of the cross piece until the head is visible through the drilled hole, bring up the vertical extrusion and the screw is screwed into the central hole. The Torx bit being inserted through the drilled hole. A very secure fixing but if the end of the extrusion is not absolutely square then the 'joint' will not be.