I have a mesh of a section of conveyor belt that gets extended into a complete belt extending along the Y-axis with an Array modifier and wrapped around the X-axis with a Curve modifier, and I want the edges along that curve to be smooth-shaded. The belt also has a 45° bevel on the sides, and I want that bevel to be flat-shaded. The Auto Smooth feature sounds like it should do what I want, but when I activate it, it does not behave as described in Blender's documentation.
At the default value of 30°, the 45° bevel gets smooth-shaded along the entire length of the belt, and there are visible seams between some of the Array-added belt segments.
Dropping the Auto Smooth angle down to 19° causes a small portion of the belt's bevel at the center of the curved sections to become flat-shaded while the rest remains smooth-shaded. The seams between sections remain.
Lowering the angle further causes the flat-shaded regions to gradually extend along the length of the curved sections of the belt...
but I have to drop the angle all the way down to 2.2° before the flat-shaded regions reach the straight sections of the belt and join up, simultaneously removing the seams between sections.
Why is Auto Smooth behaving like this?
I tried recalculating normals, merged vertices by distance to ensure there are no doubles, and made sure that the belt mesh has no other geometry data applied, but the strange behavior persists. As far as I can the bevel should be flat-shaded at an Auto Smooth Angle of 30°, but it isn't. What am I missing?