It occurs to me I may not have been entirely clear as to how scrap do what he do. The parts obviously don't speak to him, he merely projects his own emotions onto them and his thus he crafts items from them his subconscious deems useful in any given situation. For example the bottle cap Dallas dropped that Scrap was seen coddling was his reaction to being surrounded by strange and accusing humans and needing Haylee to reassure him of his place there.

This is also why, thought the reason alludes dear Scrap, the majority of his parts want to hurt others and are subsequently why his expertise lies in weaponry.