Thank you for the replies & sorry about not giving enough information.
-Nut is forged in hex shape, and forged hex shape is not machined after.
-based on these hex flats, threaded hole in the center need to be machined.
CheckerRon, you are right I want to use the forged hex as a datum to locate the internal hole. Also I did check the fig.4-38 in ASME Y14.5M-1994, but did not understand much...
CoryPad, It is 28mm threaded forged nut. Each nut is forged and internal hole and thread and chamfer is machined. I beleive our supplier is machining it on Milling machine, because of that hole is accentric. To control that I have to specify the tolerance.
I am still not clear how will the datum be defined with the outer hex.