The development of aluminium window corner jointing techniques

 

Technical Consultant’s comments – Patrick England

Part 1E – Mechanical fixing, strengths and weaknesses

This month I look at the strengths and weaknesses of pinned, sometimes called nailed, corner joints.

As for crimping this method employs ‘L’ shaped cleats, however unlike the cleats for the crimped joints these are usually of cast construction and have hardened steel inserts for the pins/nails owing to the requirement for accuracy of the location of the pin holes and for fitment of the cleat into the profiles.

These cleats typically have a flat surface which when fitted into one or more of the box sections which make up the frame or vent profile fit tight up against the inner face of the outer web of the profile.

The frame profiles are predrilled in accordance with the details supplied by the window systems designers as part of the pre-assembly preparations and provide a very accurate location of the ends of the profiles once the pins/nails have been inserted.

The profiles ends are coated with a small joint sealant and then located on the corner cleats and then ‘interference fit’ pins/nails are driven through the predrilled holes in the profiles and into the holes in the cleats forming a tight joint where the profile ends meet.

The tight fit of the cleats into these box sections and the positive location afforded by the pins/nails provide a greater degree of resistance to the corner joints twisting out of line and the completed frames racking than do the crimped joints.

The problems with this design are the number of 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.
• Using a worn or blunt cutter will result in the edges of the cut ragging which will detriment the appearance of the joint and effectiveness of the joint seal.
• The sharpness and alignment of the pin/nail forming tools is 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.
• The accuracy of the pin/nail forming tool positioning will determine the tightness of the finished joint

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julie.harley@c-a-b.org.uk