To be fair, I haven't done so (yet) in any of my towns due to still learning the game, and the length of time it takes to train a Smith to the point of forging the Engineer's Wrench.
I need a spare Smith to promote to engineer (but haven't the time to train a new one), because the Engineer can't be used to do smithing work. (much like how the cook can't do farming)
It "only" takes a level 1 smith and a wrench to promote, but needs a level 3 smith to make the wrench. -_-
EDIT: I had the levels wrong, but the 2 level gap was correct. I've fixed it.