according to the picture, “D” does not seem to be measured from a point on the Z-Axis travel but from the bottom of the Z-Slider. The Z-Axis mechanism as a whole is itself located on the Z-Slider, the latter seems to be fixed with screws onto the X-Axis Gantry Block to a choice of 2 (or 3?) mounting positions (see “mounting holes” on page 12 of the Assembly Instructions) with a resulting choice of distance “D” measered from the bottom of the Z-Slider (4 3/32″ – 5 7/16″).
However, the measurement that is missing in that diagram (and that I’d like to know too) is:
a) What is the distance between the bottom of the Z-Slider and the bottom of the Spindle Mount, when Z-Axis is moved into the lowest position ? (seems to be a few millimeters)
If you have that measurement, then you can proceed to find the answer to your initial question (“how low can I go?”) by adding:
b) the amount with which you insert your Spindle into the Spindle mount (or: how much your Spindle protrudes the bottom of the Spindle mount)
c) how long your routing bit is.
Can someone provide the measurement a) above?