The development of aluminium window corner jointing techniques

 

Technical Consultant’s comments – Patrick England

Part 1D – Mechanical fixing, strengths and weaknesses

This method employs ‘L’ shaped cleats, usually cut from extruded profiles, which fit into one or more of the box sections which make up the frame or vent profile. The fit of the cleats into these box sections has to be quite tight as this will be a factor in preventing the corner joints twisting out of line and the completed frames racking.

The cleats typically have a profiled recess on the outer face of the cleat into which an area of the outer web of the box section is pressed. A predetermined width of the outer web of the profile box is pressed, and in some cases sheared at the contact end, into the cleat recess. This process is carried out for each corner, with both profiles positioned on the corner cleat and with the crimping tools aligned symmetrically in order to achieve a tight and true 90º joint.

The design of the profile of the recesses in the cleat provides a ramp which pulls the profiles together at the mitre as the frame web is forced into it. To achieve a water-resistant joint it is usual to apply a small joint sealant.

The problems with this design are that there are many variables which can result in a poor joint, for example:

• If the ends of the profiles are not cut to a true 45º then the resulting joint will not be 90º. uUing a worn or blunt cutter will result in the edges of the cut ragging which will detriment the appearance of the joint.
• The sharpness and alignment of the crimping tools are also critical in achieving a tight joint.
• The tolerances on the cleat and the profiles will also have a bearing on the tightness of the fit of the cleat.

If the cleat is not a tight fit in the profiles box section then the profile can twist on the cleat opening the joint up. This is a particular problem where the box section is disposed towards one side of the profile. This effect is reduced with profiles that have two box sections disposed with one towards the outer edge of the profile and the other towards the inner edge of the profile.

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