Grinding a phoenix crank

[Zetomagneto]

Phoenix are doing me a billet crank at the moment, IIRC Marc said that the nitriding on it would be 0.020" deep

Are they doing you a special? Normally their A 7 cranks are from forging, not billet

Yes,  it is a special. Current Phoenix A7 cranks are from forgings as you say; Some earlier A7 Phoenix cranks were investment cast and then HIP treated. I have a 1 1/2" version of those here.

My understanding of a billet crank is that it is machined from a solid billet.

If I understand your reply correctly yours is from a casting , in which case it cannot be a billet crank.
A forged crank is better than a cast one, interested to know why you would be using a casting.