Homing to sides of workpiece with real tool How? Lathe

Can you show us with text and pictures how you home and how you touch off to part.
I suspect you are doing something wrong.

Finally some more time to work on this problem today...
The problem still exists bu the cause may actually be something else.

I may need to start a new thread but I will try here first:
As follows:
1) I mounted a dial gauge on the Z-axis.
2) I move the axis (jog slowly) to the right and stop.
3) I home the axis in this position.
3) I set the dial gauge to 0.
4) I move the axis left with G01 Z-1.000
5) I find the axis has actually moved to -1.1200 (all mm)
6) I move the axis back to 0 with G01 Z0.
7) I find the axis has actually moved to -0.1200 (all mm)

I have a 0.12mm error and it is permanent!
Repeating steps 4-6 many times will not shift anything nor cause creeping. It is permanently off by 0.12mm.


ADDITION:
I just discovered that when I press "Home Axis", The Z axis coordinate does not become 0 as expected but 0.120.
I think this is related to the problem

There is no tool loaded.
Bottom right of window states: Position: Relative Actual

ADDITION2:
I now think this is still a problem with the G54 coordinates being offset by an amount that I cannot explain. I do not know how it got there nor how to get rid of it.

If I can solve this problem, The homing and touch off issues may also be resolved.

Any ideas anyone?

Regards
Bernt

I think that the problem is that you are using "home" when you should be using "touch off"

"Home" sets the G53 {Machine) coordinates and "touch off" sets the G54, 55 etc work coordinates.

You should "home" to a fixed point on the bed, and then "touch off" to the work.

Better still would be to home to physical home switches. Then you can shut down the machine, come back, re-start and re-home and all your G54, G55 etc work coordinates

The machine home should not be the end of the work. Apart from anything else, depending on the axis limits specified you might not be able to then move past that point.

Thanks. I think understand this

What I do not understand is why the touch off offset seems to become incorrect if one repeats the touch off operation

I touched off at a point for Z and entered -7.33. It was verified with the dial gauge.
I then went to 0 with g01 z0.
dial gauge read -0,120.
That would only make sense if the touch off accidently operated on the machine coordinates. But this is not the case since I operated in g54.

Repeating the touch off results in further strange offsets when going to z0.

I have ruined 2 worpieces with this so far and I still have no explanation.

I wanted to video it but my new gopro is not compaible with any android device I own.

Two things I can think of. Tool length offsets are coming into play somehow. Do a T0 M6 first to make sure that the tool table is not affecting the touch off is some way. Then repeat your test , ensuring that you do a touch off on Z, not a tool touch off (sorry can't remember exactly wording). See if you get the same result as before (is dial gauge dose not report z=0). If so, perhaps the Z-axis scale is incorrect, or Linux CNC thinks it is moving 5inch, but the hardware is actually moving more/less than 5 inch?

berntd wrote: I touched off at a point for Z and entered -7.33. It was verified with the dial gauge.
I then went to 0 with g01 z0.
dial gauge read -0,120.

What did the DRO say after the touch-off and the move?

Did you touch-off the tool or the coordinate system?

Is your machine set up for touch-off to workpiece or touch-off to fixture?