I am trying to model a small auger bit to be used to extrude a thick material, and I've been attempting to make it such that the thread depth (i.e. distance between thread OD and shaft OD) starts large and gradually decreases towards the tip of the bit. If that's not clear, check out the attached image of what I've gotten so far.  In that image, I have the shaft and thread tapering at the same rate towards the tip, but the goal is to have the base of the threading equal in OD to the large end of the shaft (the end with the hex head), and to have the thead OD at the tip to be as shown.

Here are the methods I've tried thus far:

1. a helical sweep of the thread profile.  I used an equation to calculate the OD of the thread relative the shaft center line as a function of distance from the start of the helix.  However, it seems as though solidworks doesn't recalculate the profile dimensions as it does the sweep.
   
2. lofting the threads around the shaft by copying the profile, rotating a 1/4 turn, shifting towards the tip by 1/4 of the pitch, and adjusting the profile dimensions along the way as to gradually decrease the thread OD.  This proved very time consuming and presented an abundance of "zero thickness" and "invalid geometry" errors.
   

I think the solution might be in using the surface tools, but I haven't done any work with surfaces aside from the tutorial SW provides.

Any help/suggestions would be greatly appreciated.

Thanks,

--Chris