Why spot welding robot not used in arc welding?
A spot welding robot has generally 5 axis opposed to an arc welding robot that needs 6 axis.. The controls and communication channels with the welding power supplies are conf
igured differently as well.
Star Wars Movies
What is the robot in the ARC-170 Starfighter?
I'm not quite sure which ARC-170 starfighter you are talking about but the droid you saw was most likely an astromech droid.