Reneanglia is correct if bending any thin walled tube then a steady feed through the mandrels is required, here's a good vid
https://www.youtube.com/watch?v=qqsrsMyBP90 or you can use a manual type as reneanglia link shows.
A good cheap alternative is the type electricians use to bend conduit which is what i used to fabricate the rear wheel arch supports and can be picked up on flee bay for less than £50, you can even bend square tube if you are careful enough with the round mandrels.