I was under the impression that you cut all your parts with your first machine. It's impressive to see that you made it so accurately by hand.
Here is a link to some 1/2 inch drill rod that you can use for your z-axis. I've used drill rod like this for all the axis's on my machine and it works very well. A little 600 grit wet sanding will polish the drill rod up very easily.
1/2 inch drill rod