I like that mechanical stuff!
Something seems weird with yesterday's perspective (the left drawing), the arm is round and the hand look really flat in comparison.
and some other parts are quite off in terms of perspective as well, especially the knuckle-plate.
It's hard for me to get the initial quick concept sketches, when I am initially exploring the form details right.
Maybe that's just how it is, maybe my skill level or the level of my imagination needs more care, as most things I do it's always getting better with the second drawing, means once the idea was already clearly nailed down -
For the pencil drawings on paper there is no quick copy paste resize shortcut (I love digital drawing for that aspect), but that's also a point why I am drawing with pencil and paper for practice purposes, because most lines are absolute once you drew them (except if I am really off I am using an eraser)
nice portrait, seems to be in line with your avatar =)
