Basically the title, you need to use the skills you have now and be a productive member of society.
I don’t mean go back and show the wheel or try invent germ theory etc.
For example I’m a mechanic i think I could go back to the late 1800s and still fix and repair engines and steam engines.
Maybe even take that knowledge further back and work on the first industrial machines in the late 1700s but that’s about it.
Well then you’d probably be fine all the way back to premodern times, assuming you can convince clients to trust you with their mine water pump or whatever. As long as you could get along without devoted replacement parts.
Once you reach that point, the modern lathe thing becomes an issue, a commercial foundry might not be around for cast parts, and the technology to cast ferrous metals at all isn’t guarenteed. The ability to perfectly eyeball things and use relatively primitive materials becomes a major constraint. If you master that, you can probably hack it all the way back to early civilisation building crossbows or animal-powered pumps.