These studies look great!
One thing i recommend though with cubes is when you are drawing lines that converge, no matter where you start, pick one line to base all your other lines off of.
Here's an example:
When i am drawing a cube i think of all the planes i'm drawing and try to make them as parallel to each other as possible.
So with this one, the green arrows point to the lines i drew, the blue arrows point to the first line i look at to make sure it was drawn in the right direction and the purple arrows point at the lines i used to double check that my interpretation of the direction i thought the lines the blue arrows point at are correct. sometimes you think something is going one way but its actually not so i double check it against the other lines. after some practice you probably can do this quite easily yourself as well 
and its actually quite fun!
Hope that helps 
