I have a hollow half-cylinder created by extruding a profiledrawn as a 3Dsketch. I have also created a spring shape by usingSwept Boss/Base, and twisting a small circular profile along a path- a line along the axis of the cylinder. nice. the diameter of thesmall circle neatly straddles the edge of the cylinder as it sweepsup to form the spring. :-)

Now I want to subtract the spring from the cylinder to form aspiral groove - looks easy in 'Help'. But I can't. 'Combine' is noton offer, and is grayed out in Insert|Features. Any ideas why?