This is certainly an excellent project mc.
Thank you for taking the time to post all the details and pictures they are most informative.
I just can't wait for the next installment 

Tweakie.

Well I'll be darned!  Didn't know about the glass scales/Mach3/KStep connection.  That's definitely something to file away for possible future use.  I just bought a 3 axis DRO set for my little X3 manual mill.  If I ever get the cash together to CNC it down the road, I'll have to seriously consider the KStep option.

Do other motion controllers have a similar function?

Tweakie, do I sense a hint of sarcasm in that post?
I'm not sure if there will a next installment as such, as the only things I've got planned to do now is cut a whole for the scale connectos, and put the door on the cabinet.

Dickeybird, the KFlop is a very powerful and highly configurable option.
Provided you can input a suitable signal, you can close the loop with pretty much anything a KFlop output is capable of driving.
Some combinations will require some custom C code, however most common motion control related inputs are handled easily through the standard options. I think the only main requirement for a suitable input, is it has to be quadrature.
Also, it's sampling rate is 180uS on all inputs, so even standard inputs can be used for some pretty time critical stuff.

Hi mc,

No sarcasm intended sir - it is certainly a great project - please keep up the good work.

Tweakie.

Hi mc,

No scarcasim intended sir...

I knew you Brits were known for your cutting humour but I didn't know there was a word for it!;)

 

(now it has been spotted, thought I had better correct the typo).

Tweakie.