OMG this topic comes up every dang week!
Ok you want to start a shop and make parts huh....What makes you any better/different than anyone else out there? **ANYONE** can buy a mill and make sum parts in their garage. What can you offer that is different than every tom dick and harry out there?
IMHO you should start a business when you have the knowledge/talent/connections/etc to see how others (possibly your employer) are doing it wrong. You should start when you can see opportunity/efficiencies that others cannot.
So who do you know? What do you know? Can you actually do what you think you know?
As boosted has mentioned this industry is a grind. I mean eat your soul for dinner and then ask for dessert type of grind. Are you prepared to lose everything you have if things don't work out?
I would tell a guy that if I wanted to discourage him. I was told this many times when I was a struggling noob. Most all of the 'friends' I had at the time disappeared when I said I'm going full time with my business. Not really sure why, oh well. I guess maybe they were afraid I'd ask them for help moving or fixing up the building I moved into.
This 'be different, find a niche' thing is a broken record saying that's been preached to new shop wanna-bees for decades. What examples can you give as far as the 'being different' part goes? Nobody ever has suggestions for that because there's not much to be different about. What, weld with a surface grinder? Mill parts on a Lathe? Fact is, there are thousands of shops out there all doing the same thing, with the same 5 axis machines, and now days maybe 3D printing, and all of them are getting their piece of the pie, some bigger some smaller. The WHO do you know, or "who were you able to connect with early on" part is probably the best chance of success a guy can have. An engineer or purchasing person that happens to take a liking to you can be worth gold. In reality about the only way he can be different from the rest is have attractively low pricing, and appeal to his prospects in a way that makes them appreciate his efforts to be self employed. So they give him a chance.
Edit: Another thing that can be a boost, is if you can help a guy out of a jamb. Fix a problem for a purchasing guy or engineer. I walked into a gas well support company's front door, cold call, asked the ditz on the front desk if they send out machine work or welding work. She said they do that, but they do it all in house. I handed her my card and left, oh well.
The next morning at 9am I get a call from the facility manager, and he says "Buddy I'm in trouble".....that was good for 3+ years of work from them at very good rate, and the checks came in the mail in 25 days. Yes he was in trouble, and I was able to get him out of a jamb quickly, using old worn manual machines and a welder. Total investment in said machines, as a guess....less than $5k. Just this customer paid for those machines and others many times over. My business card says "Solving customers problems since 1979" No 5 axis here.