I need help how to rotate an object around an local axis in object mode. See attached picture.
How to create a local axis in the same direction as my red drawn line (should be straight…) in the attached picture? I want the "arm to rotate but I still want the top and the bottom stay in its places.
Can anyone help me? 
/Caroline
Options given your unclear image. ALWAYS supply a demo .blend file as a MINIMUM)
Create a custom orientation https://docs.blender.org/manual/de/dev/editors/3dview/object/editing/transform/control/orientations.html#custom-orientations
Add a bone/empty along the axis and parent object to it
I don’t know how to add a .blend file. How can I do that? Im a beginner in blender and new on this site. Can I email it to you?
Thanks for your quick reply but I can’t really follow that link. I have two vertices that I want to connect with an axis. That created axis I want to rotate the object around.
Either I know how to create that customized axis either I know how to rotate the object around it afterwords 
I would really appreciate some step-by-step help 
/Caroline
Hello Caroline and Welcome to BA!
As you are new here, you will be on moderation for your posts - the first 10 will need to be OK-ed by a Moderator, so there may be some delay, however, I still think you may be restricted to post blend files until you are off Moderation.
In the meantime you can upload your blend file to pasteall.org/blends then copy the address for your file from your browser and post it here for us to download. To set a link in the post you use [ u r l ] before and [ / u r l ] after the URL, without the spaces, I put those in so the Forum does not use it as a URL.
When you upload a blend file - cut out everything bar what is needed to show the issues in a copy blend file, we don’t need to see everything else that works fine.
You obviously know how to “Go Advanced” and upload Images - well you can also add blend file there, provided they are not too big!
To rotate about a Local Axis like the Object’s Z axis, you would key R ZZ and an angle or simply move your mouse in the 3D view. ZZ means local Z axis one Z on its own means Global Z axis.
Cheers, Clock.
