In the sentence
Das machst du aber ohne mich.
the word aber isn't a conjuction but a modal particle. Their purpose is to change the mood of the sentence.
The tricky part is, there isn't a rule which particle means what. You have to remember them by phrase.
Das machst du ohne mich.
Do it without me, I don't want to get involved, by any chance.
Das machst du aber ohne mich.
Do it without me, I don't want to get involved, by any chance.
You can do it without my help.
Das machst du doch (aber) ohne mich.
Das machst du aber doch ohne mich.
You can do it without my help
Sadly, you are not going to invite me.
In this example you can see how important modal particles are to understand the meaning of a German sentence. They are used all the time, in speech even more, so listen to Germans speaking to each other.