Tonight at work I was playing with the Tapered Thread plugin. At the moment I'm using a generic incrimental NPT sub program to do my NPT threadmilling, but I'd really like to do it straight in Gibbs. So far I've played with the 3/8-18 settings that it already had in the Plugin. I created the geometry which ended up being a little helix over the point I want to threadmill. If I can remember correctly the helix went about -.408 deep. Now correct me if I'm wrong, but to try it out I ran a contour on that geometry. I set the depth at zero thinking that the depth would automatically be picked up by the geometry's depth, but when I looked at the code that it posted out, it basically milled above my part (obviously the zero depth was wrong). Any suggestions? I also just used the threadmill in their tool list and filled in the info for it as I normally do for my NPT spreadsheet. Since it's a single thread cutter, it runs the entire length of the helix which would take a pretty long time.
You did everything fine but I think your tool might not be setup correctly. Make sure the length of the 60 deg cutting edge is set properly. Everything else looks ok.