I've had a little motorcycle side business for about 15 years. In very good years it turned a very minor profit, in some good years it roughly broke even, and in other good years it let me write off a bunch of stuff I'd have bought anyway and offset my regular income with a loss.
But one of the two major products I sold went out of production a couple of years ago, and the other major line (very niche market technical books on m/c design and construction) is in the dwindling down phase, as it is just too hard to get the word out about the books as advertising costs are pretty prohibitive. Plus, the next batch of books (generally about 180 at a time, shipped in from England) would require me to shell out about $10K, and having that much tied up in what would probably be a 7 or 8 year supply of books is just not in the cards now that I'm retired and don't have the extra income that allowed me to dabble with the side business.
I've got enough industrial-quality metalworking equipment (machining, welding and fabrication) to qualify as a pretty nice small machine shop if I wanted to do that. I wouldn't mind making the odd bit of pocket money helping someone with some interesting project (interesting projects for pocket money aren't work). But the liability aspects for the types of things that interest me are pretty daunting, so I'm unlikely to be willing to do much for other people, and the things I do will be very carefully qualified.
A friend who is in the small-engine modelmaking hobby thinks my CNC equipment might let us make some batches of stuff that could be sold off to dealers servicing that hobby. That's only at the idle speculation point at this time, and I'd have to see just what he has in mind.
But if it doesn't sound like fun going in, or starts to feel like work once I'm doing it, it will come to a screeching halt. I've got plenty of my own projects that I need to build stuff for and I'm NOT going back to work.
cheers,
Michael