Hi,

The seat back diagonals SB5 and SB6 have caused some problems, I'll try to explain how to fabricate these tubes. As usual we assume that the 25 x 25mm tube is a perfect square - refer to the front frame tutorial.

If SB3 and SB2 we're at the same angle and paralell then the cuts at the ends of SB5/SB6 would be two simple 52 degree cuts, like this.




A section through rail SB2 would look like this




SB2 is rotated from this position around "Point A" into it's vertical position, by 17 degrees, From the red position to the green position. This means that the cut at the bottom of the tubes SB5/SB6 is a simple 52 degree cut and the top is a 52/17 degree mitre as viewed.




Line A - B is constant

Measure the front face as if it were a flat plane, the 27 degrees is the true angle on the surface of the inside face of the tube which occurs because of the compound mitre.




If you think of the underside of the SB2 tube as a flat horizontal plane and SB5 or SB6 as a tube penetrating that plane it might be easier to visuallise.

Hope this helps

cheers

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