problems with Blender 3D model

Unity Engine
1

I have a problem when I import this Blender model in Unity and use it in a prefab:
https://frank-buss.de/tmp/mill.blend
For some rotations it looks right, like this:

491×589 158 KB

But sometimes the cylinder is drawn completely over the box:
528×601 214 KB

Same effect in the editor and in the game. How can I fix this?

2

Sounds like issue with transparency. Check your material in unity - it it’s with transparency support, it can make such effect even if alpha=1. It is pretty common issue.
Just few examples:

https://stackoverflow.com/questions/57292257/models-using-unitys-standard-transparent-shader-are-overlapping-themselves
https://www.reddit.com/r/Unity3D/comments/30zuwp/fbx_models_overlapping_self_with_standard_shader/
So, in your case probably the best solution will be just to change material to opaque, especially if you don’t need transparency, because other solutions usually involve writing shaders. Though it seems that your model require transparency on “wings” (sorry, don’t know how that part of mill is called in English). But they are very transparent already, so using even glitching transparent material just on them should look fine. Also as far as I remember, opaque+transparent material looks completely fine anyways