"As defined by section 921(a)(21)(A), the term “engaged in the business” means, as applied to a manufacturer of firearms, “a person who devotes time, attention, and labor to manufacturing firearms as a regular course of trade or business with the principal objective of livelihood and profit through the sale or distribution of the firearms manufactured.”"

The way I read the above definition of "engaged in the business", in the context of the document, allowing a friend to borrow your jig and router, without charging them to do so, is legal. I'm not a lawyer so I may be way off.