The 3D printer is just a tool though. It's a means to an end. You need the end.
One cool idea would be a service that fixes random broken parts. We've all had the experience where one part of something breaks, glue is useless, and the item is now useless. A service where people send in their broken esoteric parts, and you image them with something, and then send back an intact 3D-printed version of the broken thing, could be a potential play.
One thing I've noticed though is normal 3D printers are just toys: what they produce has no structural integrity whatsoever. So it'd require a real 3D printer. Factory grade stuff.
This is absolutely correct.
I was going to buy a 3-D printer for some projects. The consumer ones are a waste of time. They're slow and the supplies cost too much. Like the inkjet printers back in the day. I don't know what the cartridges are now, but I remember them being $40 or more - and that's what pushed me into laser printers.
That's sometimes referred to as the razor blade business model. Sell the razor cheap, and gouge em for the custom snap on blades. You've now got a life time customer that you can gouge for years. Brilliant!
But, back to 3-D printing. There are business' already set up that do great work. This would be a VERY tough business to compete in. You're going to have to discount yourself to compete.
So the simple answer is no.
But, but but but... doing the design work that the 3-D printers need to produce a part... that's a completely different animal.
CAD is not difficult, but it's sure not something that anyone is going to learn to make one or two parts. You can make a KILLING doing the design work. Then, let your "competition" print it out, and you can do well. NEVER give them the design specs. You designed it, you own it. This will ensure repeat business for parts.
Certain business' pay huge money for parts. Medical, for one example. I know a guy that makes stainless steel couplings for plastic lines - one at a time. They have to be flawless. He spends a lot of time grinding and polishing them to a mirror finish. I throw that out there as just an example to show that side business ventures for parts do exist.
Personally, I'd look into those rare "manufacturing machines" that are very specialized. Call the companies that make a product using these specialized machines and ask them what parts break. Then remake those parts better and stronger. Companies to call might be any mass production food or beverage company. Motorcycle and RV sales. Plumbers. Appliance repair shops. Even manufacturers direct! The list could be huge! I was an office machine tech for many years. I regularly bought custom parts that were made to replace known weak parts of certain machines. They didn't make a fortune from me, but when they came up with something, over 1,000 dealers were buying it. Then we're talking serious cash. I'm sure their markups were well over 100%. Some machines were just made bad. Certain parts would break, and I'd hate to replace it with another weak part, because the cycle would repeat, in time.
Look into doing some custom work, like a gas cap or head light with a 3-D skull on it for Harleys, and you could even launch that nationwide... get things chrome plated, and you will be the man that all bikers will know about - and they'll find your website to buy some cool gotta have it stuff. Harley people are not cheap. They are an ideal market in many ways. There you go - a perfect business model! Clone that concept for other "high-end" products out there, and you'll do well.