Bending a square pipe

[steven alex]

In the company I work for we make ( amongst other things ) square down pipes bent from strips of plastisol coated steel 3 metres long by 418 wide. The sizes are such that we can make 3 pipes from one standard sheet of 3m x 1.254 m x 0.7mm thick. The method we use is to mark out by hand the places where the operative must make a ninety degree bend so as the pipe forms a square with the one of the 20mm bends inside the pipe to offer a support edge for the sealant prior to riveting.

The marks on the flat plate looks a little like this ( forgive the drawing ) as viewed end on ie the 418 side.

20 94 94 95 95 20 ( the marks are in mm and follow on from each other )
--l---------l---------l---------l---------l--

Now forming this pipe does present problems namely the gripping of the piece in the jaws of the machine but it tends to be a simple twist operation just to free it and as we have made hundreds of these pipes they are now a matter of routine to us. However, they never present themselves as totally square! They look square and we have had no complaints but as a matter of principal I would like the pipes to come out as near perfect as possible. When the sides are measured they always seem to come out at different sizes sometimes three or four mm different. This is not simply down to one of the operatives missing a line it seems that a bend allowance formula needs to be employed but I cant understand it.

[Geof]

I had a similar thing bending U shapes and I experimented to find the right spacing between my bend marks. Just vary the spacing on some test pieces and see what it does to the squareness. Eventually you should be able to get to square by trial and error

[steven alex]

Originally Posted by Geof I had a similar thing bending U shapes and I experimented to find the right spacing between my bend marks. Just vary the spacing on some test pieces and see what it does to the squareness. Eventually you should be able to get to square by trial and error

Cheers Geof I reckon thats the only way to do this trial and error