January 20, 2012

Multiple radius bend on 2" 16 ga. square tube

This project required bending some 2" square steel tube for a couple of free-form trusses that hold up elements of a museum display. I wasn't really sure I could do this, but it worked out pretty well. Hand bending large tube with a very thin wall thickness is hard to do without kinking, but I've had some success with using thin jigs centered on the tube that push the wall of the tube in. It can lead to some extreme distortion, but it allows you to get a much tighter bend without kinking.


The other trick here is that the radii were variable; from 150" to 18". I was only able to get it down to about 34", but I was able to cheat the design a bit to make it look good enough.

I followed the same process I use to bend a large ellipse. This involves breaking the curve down into arcs, and then transferring the drawing to a full scale pattern. I used the old theater trick of a brown paper grid and charcoal. Having a full scale pattern is essential, because this always involves a fair amount of tweaking to get it to look right. Besides getting the overall shape correct, you have to make sure there isn't a sudden shift in the radius that would make it look lumpy. Progressing from the longest radii to the shortest seems to prevent this issue.


It was a lot of work, but they ended up looking pretty darn good. No kinks, no welds and no blood spilled. I keep remembering my physical therapist's wise words "your body is not a machine". Well, sometimes it is. At least I didn't break it this time around.

For a more comprehensive article on hand bending, see this.

the jig setup


2 foot long, 1/4" thick steel jig

inner wall distortion

outer wall distortion

the finished bends and a test piece

Posted by Hal Eckhart at January 20, 2012 09:53 AM
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