I had originally bought a double flaring tool from a store with HF as the initials.  Like many others, it constantly slipped, I couldn't actually get a double flare, only a single flare.  Tried everything but couldn't keep it from slipping.  Bought this tool, just got it today, and tested it on a scrap piece of brake line, 3/16".  Using the adapter that came with the HF tool, it easily held onto the pipe (same pipe I had tested the HF tool on, but always slipped).  EASILY held on.  Did the first flare, removed the adapter, did the second flare, not going until it stopped, just until I felt it had flared sufficiently.  Did a beautiful job!  Honestly looked better (thicker rim) than the factory flare did on the tubing I was testing on.