Poor man's rigid tapping?

[cncsnw]

Centroid uses the special label/symbol '@' for manually-controlled Z positions.

Label your manual axis '@', and post CNC codes that use '@' in place of 'Z'.

Note that this is for a 2-axis machine with no servo-controlled Z; not for the type of setup you first inquired about. I have never tried running a machine with both a servo-controlled Z axis, and also a manual '@' axis. Since they are independent labels and functions, it might work just fine. Try it and let us know what you find out.

[CRM]

I don't yet have a glass scale for the quill. That is in the plans. When I do get one and install it, I intended to "sum" the quill axis (W axis) with the Ram (Z axis). The original ProtoTrak control did this.
Years ago, when I first started this retrofit, I inquired if that was possible and was told Centroid could support that functionality. The intent is to know where the tool is relative to the work coordinate system regardless of the individual positions of the quill and ram. (coincident axes). I guess I never thought any further as to how that would be displayed on the screen. Does the Z position automatically do the math between the main Z axis (the servo controlled ram) and the W axis (manually controlled quill) and display it as Z position, or does it display both separately and compensate the moves while executing a program?
We have a knee type mill at work with a Fagor 8055 control that has a servo controlled quill (the Z axis) and a glass scale on the knee (the W axis). The control sums these axes and displays the result as the Z position on the screen. Below it is a W axis position to indicate where the knee is. So, if you move the knee, both the W axis and the Z axis show a position change. Moving the quill (either by CNC control or manually with the servo power shut off) will only show up as a position change of the Z axis display.
Unfortunately, while executing a CNC program, the control does not recognize a change in the knee position, even though the display shows that change. The real world effect of this is that we are unable to run programs where tool lengths vary considerably and require a change in knee height. So, wherever the knee was positioned when the work coordinate system was set is where the entire program must be run from. That is NOT a limitation we were told we'd have when we bought the machine, and it is a SERIOUS limitation on a CNC knee mill with the quill being the CNC controlled Z axis.
A bed mill where the ram is the primary z axis and the manual quill is the secondary z axis would not be as significant a problem, since you still have essentially the full envelope of machine Z axis travel under CNC control.