There are good and bad sides to starting your own forum.
The good part of running your own forum is that you can focus on exactly the topic you want to discuss, and really zero in on aspects of that topic. You can control who and what gets onto the forum, and nobody can ban you or lock your posts.
I have never locked a post, and never will, but I may delete a post if it goes south, which sometimes happens due to human nature.
The bad part about a forum with too narrow a scope is that there may be insufficient people intersted in such a narrow topic.
You also get inundated with spammers from all over the world if you are not vigilant every day. Keeping the spammers out of the memberships takes up a lot of time.
If you run your own forum, you also have to pay for your own forum, unless you can get donations. People aren't happy when the forum goes down.
Machining topics seem to be widely popular across multiple forums on the internet.
More narrow topics may not be as popular, and you may end up with a very small group, which is ok if your main interest is focusing on a specific topic.
I guess it all depends on what you are trying to do with the forum you create, and who you are trying to interact with.
This forum has wide appeal, lots of members, and a variety of topics, but still maintains focus pretty well on engine building. I read this forum for machinging specific topics, and read other forums to fill in the gaps in my particular interests.