Welcome to AAC.

I would take the pipe to a job shop that has a lathe big enough and have them cut it off and chamfer the edges for you. In the end, it will probably be cheaper, definitely be safer, and will not be a roadblock to finishing your project.

There are plenty of shops that turn large motors and the like, you shouldn’t have much trouble finding one.

But, failing that, I would probably try to use a mechanical transmission for speed control, it is less likely to cause extra smoke.

 

My experience with Job Shop Pricing here in Michigan is that to get a piece of 16 inch pipe put into a lathe of adequate size will cost quite a bit. Much more than purchasing a good quality fire ring from a decent supplier. Based on prices from 25 years ago.
How precise does a fire ring need to be?? 0.1 inch is simple and easy, 0.01 inch takes some effort and patience. But the disk grinder is less than $20USD at Harbor Freight, and a package of 6 cutoff disks is about $5 USD. And then you have an OK tool that can also spin a wire wheel for removing rust.