My son is machining an AR-15 lower receiver out of billet alluminum. I've spent over an hour trying to research the legal aspects on this and I have not found an answer to my question of what is actually needed to legally make one. Is permission from the ATF needed before manufacturing begins? How much manufacturing tax is there? I've seen the BATF FAQ and there is no specifics on this.

It is legal to manufacture your own semi-automatic firearms, for your own personal use.  No serial number is required.

It is illegal to manufacture firearms for the purpose of selling them.

usually the term manufacture refers to making for nonpersonal use. the term make would be a better one to use here.

that is how most documents will use the terms.