This one is going to be long because I am going to try and convince you about the joys of a Zmax for repetitive job milling. *1
Absolutely you can do what you are saying. Some people have a negative zone off the work area for a touch plate to home off the tool. I think this is what you are doing by going to your Xmax and Ymax and keeping a touch plate there. On my setup I probe Zmin with a metal touch plate as well when setting up all my offsets. I also always keep it safely off to the side… I mean i would never accidentally leave it or some other tool in the area and start a job… Which means I would have to drag it out for setup again if my machine lost power or i let the motors idle… maybe because i hit the panic button.
I live in an apartment and I have been told by my wife that me and the mistress… ahem the MPCNC can only make noise at certain times of the day. I do hate to waste active time with setup time. So I usually do all my setup on weekday nights quietly off my global positions. Then on weekends start running jobs. *2
Having a global Z machine position is not a necessary but a creature comfort. Once you have a quick to access global Z from a max Z stop you don’t need to drag out the touch plate all the time if you know you spoil board height with your current tool since you can reference that location for your offsets. My only down time is changing stock while the machine is sitting out of the way homed.
I also love turning on my machine hitting home all which is Z full up till stops, then X and Y and knowing exactly where i am and ready to start the next program. Home All also puts the head up and out of the way and in the back of the box for loading and unloading material on my rig. (Z+, X-, Y+)
I calculate the G92 (offsets) based on my G30 values when I was being quiet the night before. If the job has multiple tools i will go to the top surface of the piece and let my bit slide bring the head down to the correct position and then tighten the collet so again no touch plate needed.
With my current Zmax setup i can change tools doing the offests the night before and be withing 3 thou (using calibrated fingernail check) on surface finish between the two tools.
This is an example of my start Gcode form my leveling excel sheet:
For this example the offset is 0.0,0.0,-8.99.
; Home
G28
; Goto XY of Job (excel input)
G0 X10 Y140
; Goto Z high safe
G0 Z20
; Set work (excel calculated from G30 at work piece middle)
G92 X10.0 Y140.0 Z11.01
; Run job
*1 - This is a hobby machine so all of this is moot. The easiest way to use it is drag it to position with the motors idle and then call that zero with the bit touching the material. I am trying to run it like a real machine so if I move up to industry level CnC my tool process matches. This is extremely useful for PCB milling though where you need to account for board warp and loosing power means you have to probe map the area again if you don’t have some reference plane.
*2 - My shroud is lined with sound dampening towels and foam so honestly the loudest part is the vacuum outside the machine when i close it up. All neighbors have said they can’t hear me maybe the slight hum of a vacuum so they don’t care. I originally tried having the vacuum in the box with the machine and it was super nice and quiet so much i could probably run at night. Only problem it was like running a blow dryer in there. So i had to move the vacuum out and add cool air intake. Was not going to build a heat exchange… I was dangerously close to thinking about it. Also I need the vacuum because with no vacuum the dust gets way too bad in the box and when i open the door it would snow sawdust everywhere…