That is no good for bending tube the dies are all the wrong shape, its more than capable as a 'pipe' bender but you are trying to bend 'tube' with it and I'm sure the bends in the 5-6mm walled scaffold pipes will come just fine.
The reason you are getting kinks in the tube with the pipe 'shoes' is that the inner radious of the bend is not held in place, that and you are trying to break it's back rather form a curve. The Rotary draw bender shown (black one) below pulls the tube around the former so is not trying to break it's back.
This is a tube die note the high sided faces to contain the tube.
What you need is one of these:
http://www.ebay.co.uk/itm/Pro-Tools-...item3cc954922a
And now you know why people don't make their own roll bars or cages.