Plutonium is harder to machine that other metals, as it's density tends to be non-uniform: http://en.wikipedia.org/wiki/Plutonium#Allotropes http://en.wikipedia

Plutonium is harder to machine that other metals, as it's density tends to be non-uniform: http://en.wikipedia.org/wiki/Plutonium#Allotropes http://en.wikipedia.org/wiki/Plutonium#Allotropes You go along milling a hard spot, then hit a soft spot and--oops! You took off too much.