A man is sitting on the ground welding something.
A welder works on a new piece outside.
Just because a man is welding something doesn't mean that he is a welder. Sitting on the ground doesn't necessarily mean that the man is outside. The man in sentence 1 may have been working on an old project, so we can't know if it was a new piece or not.
exists e1 e2 x1 x2 x3 x4.(n_ground(x2) & on(e1,x2) & Theme(e2,x4) & Actor(e2,x3) & v_weld(e2) & n_thing(x4) & (x2 = x3) & Actor(e1,x1) & v_sit(e1) & n_man(x1))
exists e1 s1 x1 x2 x3.(Time(e1,x3) & n_outside(x3) & on(e1,x2) & n_piece(x2) & a_new(s1) & Theme(s1,x2) & Actor(e1,x1) & v_work(e1) & n_welder(x1))