on the first run lets say that the error is 0.5mm too deep, on the second run with no resetting of the datum is the error in depth 0.5mm or 1.0mm

Graham

The error in depth on the second run equals the error in depth in first run.
in other words: 0.5mm

but here is what you just helped me realize:

on the files I do not involve tool changes i don't have errors !
If my code has tool changes i run:

                   G00 Z 50
                   M05
                   M00
                   M03

I manually change the tool and hit "start cycle"
Instead of tool offsets I simply use a small aluminum tube with a brass stop on one end, (think of it like a condom) i used that method for years and had not probs.

Graham i think we are getting somewhere ! Thank you very much for helping me think straight ! With this new information can you make any guesses?

here is a sample of of one of my codes that do introduce the error:
(this one also includes a flip to do top/bottom milling)
G01 X-37.5185 Y13.9874 Z5.1000
G00 X-37.5185 Y13.9874 Z8.5000
G00 X0.0000 Y50.0000 Z50.0000
    A0 F750.
G00 Z 50
M05
M00
M03
G00 X-15.0029 Y8.3547 Z8.5000
F300
G01 X-15.0029 Y8.3547 Z2.4545
F500
G01 X-15.0615 Y8.3566 Z2.4545
G01 X-15.1262 Y8.3596 Z2.4545

I would try using an M1 (optional stop) or an M6 (tool change) an M0 is a 'Stop and Reset' command and it could be fouling things up for you.