How to make a thick, smooth metal coil using plane extruded along spiral? I'm making a metal coil as per the picture below by extruding a plane along a spiral curve. I've added Fill Caps to make it solid

I need the coil to look smooth but the top and bottom to be sharp so I have added an edge split modifier and Subsurf modifier.

This works except for the ends of the coil which Blender tries to smooth as well instead of treating them as 90 degree angles.




Is there any way round this without turning the curve into a mesh?
I was able to create a spiral as you see here by using the spiral addon.


Note that I used 80 steps.  This is where you get a smooth spiral without using edge split or the subdivision surface modifier.


The bevel profile object was a nurbs circle that I subdivided and changed into a square.  Then I stretched out the square to give me the rectangular shape I wanted.  I can give you more details about how I used subdivide on the nurbs circle to make it into the rectangle if you want.