Has any one tried a set up as shown to true a 7" diameter fly wheel on a SHERLINE mill?
You might get away with using that mill by reversing the setup shown in your photo.
Mount the flywheel (PM-1B1 is it ?) on the mill table with some spacers under the spokes to bring the flywheel as close as possible to "square" with a DTI to minimise the amount of metal you remove. This will ensure that the flywheel is mounted a securely as possible. Clamp onto the spokes with the clamps bearing on the spokes directly over your spacers to ensure that you don't bend or break a spoke. Then make up a purpose built cutting head (or maybe two heads) that will allow you to bring a cutting tool to bear on the face and sides of the wheel rim, and on the face and sides of the hub. The beefier you can make this purpose built cutting head, the more rigid it will be. You'll probably only use that cuttting head once, so keep the design simple/basic.
That will leave the rigidity of the mill column as being a possible limitation, and one way around that (apart from ensuring that the X, Y locks are secured and the Z Gibs are well adjusted,) is to use very light cuts. If you can't slow the mill down enough to be able to use a HSS cutting tool on the rim, you could try using a carbide tool.
Mill the face and sides of teh flywheel, followed by the face and sides of the hub. Then drill and ream the centre hole so you maintain concentricity, before flipping the wheel over.
When you flip the wheel over to machine the other side, you'll be able to sit the machined surfaces of the rim on parallels. If you clamp onto the unsupported spokes, be sure to not over-tighten the clamps. You can use the reamed hole in the centre of the wheel to center the flywheel under the mill, and can then use a DTI on the machined rim to double check that your cuts on the second side are concentric with the reamed centre hole etc.
If you're in doubt about whether your set-up will work, rather than risking ruining your casting - do a trial run using a suitably sized disc of free machining steel (or maybe even a suitably sized piece of brass).
Hope that info helps.