Drawing 3d models like this is easy:
- Draw half of the sphere in the middle;
- Draw half of the arm contour (bottom guide curves);
- Put a few curves that should represent the cross sections of the arm;
- Form half of the 3D arm surface using Sweep command and create a full arm using Mirror command;
- Use Pattern (or Array in some 3D CAD software) command to rotate the arm as many times as desired;
- Trim the arms and sphere and delete unnecessary surpluses obtained after trimming.