3D model in 3DXML format available here :
http://www.3dvia.com/models/FDB6F2F3C5D7E9FB/log-spirale-3d-xml-file
This proposal is based on the shape of a snail shell.
But how to design it in 3D ?
Simply put :
1-make a flat (logarythmic) spiral
2-project on a cone
3-make a circle slip on this curve with a regular adjustment of the radius.
I don’t know how other guys do but my method to get a clean result is do the design 3 times
First try :
-I made a large model (over 2 meters) because I thought computation would be easier. Wrong : took me few hours to get nowhere.
Second shot :
-Close to the final model but the geometry was too complex to be computed as one solid. And I was not happy with the square shape grid. Garbage !
Finally:
-Sweep on center curve with reference to vertical surface. Angle Law driven.
-every 60 degrees, clockwise, counterclock wise
-manual closing (purple curves)
-trim and split
-parallel on surface +-.75mm
-close contour with line, project line on external skin
-split skin on closed contour, thickness 1.5mm
-boolean add on bodies.
-2 more turns for the top
-finish with a sphere.
(special thanks to J.F.Rameau)
B.