Re grind the crankpin

Hi All.
The workshop manual says that when you regrind the crankpine you must pay attention to ".090 Rad. Important.".
I understand that Rad means radius.
Could you explain me (please, with easy words because i am not a mechanic!) what is the Rad and why it is important and what it works.
Thank you.
Piero

It is the radius at either side of the journal surface. It prevents there being a sharp corner where the journal meets the cheek. Without the radius, the crank will have a very increased likelihood of breakage there.

An added issue today is that many engineering shops more used to modern car engines are apparently ignorant to this, and some have ground the radius away. Thus rendering the crank scrap.

+1 what Fast Eddie stated. https://image.slidesharecdn.com/tta...-stress-concentration-4-728.jpg?cb=1276760938
The crank flex on a parallel twin is significant enough that if the 90 thou area is removed it WILL fail at these locations.
Salute
Thomas