On my last build I fabbed up the pipes and flanges out off stainless.
The pipes were just cut and welded so that was real straight forward. The flanges were rough cut, drilled, ground and finish ground by hand. Not a biggie, but knowing this wasn't the first time I did it and wouldn't be the last made me wish there was an easier way. My solution was building a fixture that I could use on my CNC's.
Used 3/16" thick 316SS for it's corrosion resistence and welding attributes.
Clamped it up and drilled outside bolting holes to final size. Exhaust hole was left small so that it could be milled to final size.
Took the drilled blank, and built a fixture. Here it is in the vise with the blank bolted down.
A little programming on the big Mazak mill.