G92 Homing and Bed Compensation

I am not sure I am understanding this correctly but the way G92 works when bed compensation is in place seems wrong.

If I set up bed compensation, then go to a point and set up the Z height by measuing and using G92 to set the Z position I would expect the Z position at that point to be what I have set !, i.e. that is the current height of Z

So
(Bed compensation set up)
G1 X100 Y100 ; Move to centre

I now move Z down till it touches my feller gauge) .1mm
and send the G92 to set the Z height
>>>g92 z.1
SENDING:G92 Z.1

Then check position
>>>M114
SENDING:M114
X:100.001144 Y:99.864326 Z:-0.053914 E:0.000000

So the Head is really at Z=0.1mm, I told it so with G92
but it thinks the bed is at Z=-.05.

If I now move to Z=0.1 (it's allready there so should not move)
it does move the head.

This seems to mean that with bed compensation set it is not possible to use G92 to set the actual Z height
and to do this you would need to reset bed compensation, set Z height, and reload bed compensation.

I am wondering if the homing uses G92 ?, if it does then Z homing would be wrong with bed compensation was set.



Chris

A reply to my own post !!

As I was explaining the problem in the post I thought I would check if the 'remove compensation/zero Z/replace compensation' would work

Short answer - No,
Longer answer, I think I understand now what is happening, and I do think it is incorrect.

Remove bed compensation M561
Set Z height G92 Z0.0
Replace bed compensation
G30 P0 X60 Y0 Z 0.0
G30 P1 X60 Y180 Z 0.3
G30 P2 X200 Y180 Z-0.1
G30 P3 X200 Y 40 Z 0.4 S

And report position
M114

The resultant Z height reported depends on what the current X,Y setting are
i.e. the G92 parameter is being transformed by the bed compensation before being applied.

My understanding is that it should not be transformed, but applied directly to the Z position
i.e. We are using G92 because we have measured and know the actual Z height at this point.

The only place where G92 is actually usable to set Z height (and presumably home)
is at the points on the compensated plane where Z=0, the obvious one is P0 in the compensation settings
and if P0 of your compensation settings are the same as the place where you do the Z homing then it works
if they are not the same then Z homing gives an incorrect result.

Chris