In addition to everything else everyone has said I will add one more hint. As has been said, the end must be square. In order to insure this, before I make the flare, I clamp the line up so only a couple or hairs worth are sticking out of the clamp and file the tube flush with the clamp. Then reposition to make the flare. Two things to look out for when you do this. Be sure to get all the filings out of the tube and make sure the clamp does not leave ridges on the part of the tube that will end up being the inner flare. Good luck!
