Just thought I better post again as I may have confused you regards the Z.
When I said green blob represents X Y Z 0 in machine coords I was actually meaning the way the mill is in the pic. Ie Z fully up is Z Zero in machine coords and when you move it down that is negative values.

Hope I am not making things worse
Hood

Hi. I thought that Z was increasing (becoming more positive) when you cut?

In machine coords Z fully up is normally Zero and thus all moves down are negative.
In Work offset coords the top of material is normally set as Z Zero and thus moving up will be positive numbers and moving down into the work will be negative numbers.
I am probably confusing matters for you as I am talking two different coordinate systems, ie machine coords and work offset coords.
Hood

No I think this exercise is very good! I will remember it now. Cutting makes Z more negative. I just will learn that. Thanks again!