Where is the machine X-Axis and Y-Axis absolute zero coordinates? Please take a look on the attached image and advice. Is it A, B, C or D?

Thank You.

I'd say C, provided the X axis runs from left to right, but it can be any of them. Just depends on the way you work.

Depends on where you operate and load the machine. At least for me, absolute 0 is always going to be lower left.

Could it be D if you are standing in front of the the stepper motor between C and D?

You can assign the zero for the G53 machine coordinate system at any corner of your choosing. This is typically set by where the homing switches are located. However, when the machine is sitting on the home switches, it is perhaps possible to assign (by parameter) what those coordinates are (other than defaults of zero, I suppose). This option would be totally up to the designer of the machine.

In the old days, we could home the machine on the limits and then assign the coordinates X0 Y0 G92 at that location and this would establish the coordinate system and set the axis displays.

In newer technology, this is circumvented by the use of the G53 coordinate system, which is the basic coordinate system that the machine runs on after homing.

We typically do not worry where the zero coordinates are, because they are easily (and necessarily) shifted with a work offset as soon as you call one in your gcode program.

Suppose you have assigned one corner to be your 'absolute zero' or G53 X0 Y0. If you always do square cornered parts, and they always nest at the exact same corner, then you would never have to alter your X0Y0.

However, this is typically not handy, and the X0Y0 of the part often ends up being handier at some other position. The difference between your machines's G53 X0Y0 and your work piece X0Y0 is the work shift amount and is taken care of in one of your work offsets, typically beginning with G54.

What this means, is that it really is of no importance what you want to call X0Y0 of the machine, because all you need to know is what quadrant you are working in. It is, however, necessary to keep the coordinate system standardized so you can use a cam system, with X- to the left, X+ to the right, Y+ in front of your front, and Y- in front of your back :D

Could it be D if you are standing in front of the the stepper motor between C and D?

Mine is D, because that's where I stand and load my machine.

Mine is D, because that's where I stand and load my machine.

Looks like on the Coordinates / Homing (http://www.artsoftcontrols.com/Videos/Coordinate%20systems/Coordinate%20systems.swf) video tutorial from MACH 3 website
they using lower-left (D on my picture)

If D is lower left, than you should have rotated your pic. :) When I said C, I was assuming C was lower left, as the pic shows.