If the plutonium core of a nuclear warhead is so ready to go critical, how is it safely manipulated during assembly?

Just curious. I imagine robotic equipment with very little vibration...

EDIT: I stand corrected. Plutonium doesn't go critical until a huge amount of energy has been transferred into it, as it would when a shell of shaped charges that surround it explode all at once. I will delete this post once I am sufficiently satiated by its engagement and insights.

EDIT 2: Every day is a new lesson learned. The process of inducing criticality is very very complex. Could someone please write an accurate, succinct summary of how plutonium criticality is achieved so I could put it here?

EDIT 3: I would like to sincerely thank everyone for their insight. From everything I'm gathering in the comments (and please feel free to correct me as necessary):

Tl;dr plutonium is compressed into a very compact density by a precise configuration of explosive "lenses" (shaped charges) in a nuke. Once this density is achieved, the mass achieves criticality. The plutonium itself doesn't go critical until it's either been compacted to such a density that its own radiation will energize it to a point of no return, or as with the "dragon's tail" experiment when a core was made critical by reflecting its own radiation back into it, which in turn caused a chain reaction.