Hi,

I am having trouble modeling this thread.  I've done lots of similar parts, and usually can find a workaround (making multiple helices or sweeping only small sections) but I haven't been able to get this one to work.

I am trying to sweep a thread profile around a helix which follows a straight path for a few revolutions, then tapers to a smaller diameter, than straight out along the tapered path.

The problem I am having is that I need to make my thread profile large enough to cut away all material from my screw blank in the tapered area - but this than causes the cutting path to become self intersecting, and the operation cannot complete. 

I've attached a model with the thread profile and screw blank which I am having trouble with. If you open it up and look at the model, and the attached annotation, it should explain the issue clearly.

Can anyone show me how they would complete this model?  I'm kind of at a loss on this one.