Hi Michael,
I have tried to run the script on a reverse twist helix and it does not seem to work.
The get around was to produce one scroll with a positive bottle rotation 90 degrees
and to produce another scroll ( same helix ) with a negative bottle rotation minus
90 degrees and then mirror the result.
The subtraction from shaft required careful attention as Moi struggled when all bottles
or groups of bottles where selected so as Brian as suggested it would be good if they
could be subtracted one by one as they are produced I think this the way the Autodesk
Feed Screw Generator plugin works.
An way Michael again many thanks for your help much appreciated.

Barry