How Grade affects a certain weapon?
is it, High Grade = High Dmg?
can someone enlighten me on this?

Grade will affect damage and durability so yes grade will increase damage to some extent, not a lot but some.

http://onigiri.cyberstep.com/_/forum/viewtopic.php?f=10&t=3086
In addition 1 grade = 4 dura and (I'm not sure) 0.5 attack.

hmmm now i know. thank you very much.

To add to that.

The attack seems to scale in a non linear fashion, and is based off of the original weap