you might try this, to see how it sounds:
take a empty tin can (can of soup or something)
clean the can off. Sand the edge, so you're less likely to cut yourself.
use a can opener to cut most of the other end off, do not fully remove the end. You want to leave it hanging by a few hairs (not so weak it'll fall off)
Talk into the open end. The lid at the other end will rattle and baffle the sound, it should give a metal robot flavor to your voice.
Also, try to speak in a bassier voice (you're a metal robot, don't sound whiny).
I picture any of the "robot" races as being inhuman, possibly devoid of feeling or emotion. Kinda like playing a vulcan. Certainly doing things like the 5 second pause will make the PC seem alien.
To play up the robot aspect, making recommendations on "resource expenditures" and such, dehumanizing any other character can go a long way. When one PC says, "hey let's take these guys prisoners", you could reply, "Negative. It is a waste of resources to maintain these biologicals." or similar kinda responses.
A robot could have no regard for human life, exemplified by not wanting to spend resources helping them or keeping them alive. He wouldn't kill the humans out of spite, he'd kill a downed opponent because he has no further use for it, and does not want to spend resources maintaining its existance. The downed opponent has no further use, because to the robot, it is an inferior entity, and can provide no further value.