No matter what, zoning won't let me have a machine shop in my garage. The thing is that machine shops aren't all the same:
If I'm servicing heavy equipment there will be low-beds delivering equipment, a driveway full of grit, disassembled machines and oil stains along with welding and arc-air gouging, etc. Not something you want next door; nuisance laws stop this.
If I'm making parts for medical instruments, everything is inside, you can't see or hear any of it when I close the door, and traffic consists of me making weekly trips to buy bar stock and the occasional courrier van. What nuisance is that?
Both are machine shops but they are far from having the same impact on a street.