Z Touch-off value is incorrect.

To the best I can tell so far is this happens the first time I touch off the Z after starting LinuxCNC. The symptom is that it thinks the stock is about .1 inch positive on the Z more that it really is. I am using a touch-plate that is .9728" thick. I found where Z value is stored in the linuxcnc.var file. If I do the touch-off a second time it is correct. I also think this may happen when I load a new ngc file but have not confirmed that yet. I am running version 2.7.13. Below is the ngc routine that is used for touch-off. Anyone have an idea of what might be happing?

Thanks

Values in the linuxcnc.var
5223 -9.328046 Incorrect
5223 -9.423468 Correct

<pre>
o100 sub
G20
( Set current Z position to 0 so that we will always be moving down )
G10 L20 P0 Z0
( Probe to Z-.5 at F2 [Uses machine units, I work in inch, this is meant to be slow!] )
G38.2 Z-.25 f2
( Set Z0 at point where probe triggers with offset of +.9728 [this is the thickness of my touch plate.] )
(G10 L20 = Set Coordinate System // P - coordinate system 0-9 0 = Not active)
G10 L20 P0 Z1.007
( Rapid up to Z.5 above the material )
(G10 L20 P1)
(Use machine coordinates)
G90
G53 G0
(Incremental mode)
G91
(Rapid Z up 1 inches)
G0 Z1
o100 endsub
</pre>

I'll give that a try and see what happens. My touch plate has a button that causes the o100.ngc to be run. I also have a button on the screen that runs it too.

I tried entering the commands manually and it produces the same result. Pressing the Touch-Off-Z button I created on the screen also yields the same result. I looked at all of the active G codes just before the first Z touch-off and the second one and I didn't see any that were suspicious like a tool offset or something like that. My normal process is to start Axis. Home All. Then press a button that I call "Rapid to Park" which centers the X & Y and brings the Z down two inches. This is done via a G28. This gets the tool closer to where I am going to touch off. I then load the NGC file and touch off the X,Y, & Z. Then cycle start. This is where I see the bit passing above the part when it should be cutting. Stop the program touch off the Z again. Cycle start and all is well.

After some more experimenting I have confirmed it has nothing to do with my touch-plate or the G code that probes the plate. The symptom is also present when using the touch off button on Axis. Right now I am thinking that it has something to do with tool definition or offsets. It seems to happen only after I start a cycle and I am prompted to insert a tool. Right then it cuts air above the part. If I retouch off the Z it works until I load a file that uses a different tool. I haven't really gotten a handle on the tool table yet. I have pretty much ignored the tool table in LinuxCNC and just choose a tool from the Fusion360 library. Since I am just using R8 collets to hold the tools and touch off each new tool I guess that is OK. Except for this issue.

The tool table was the issue. For two tools there were values in the Z column. Once I removed those values the problem went away. I assume that were put there by pressing the tool touch off button. I'll have to learn more about that.

I had mentioned that I had looked for G codes that were in effect that would invoke a tool offset. Maybe there is no such thing. But, while I can take a part from CAD to finished product there are still a lot of holes in my knowledge and using the tool table is one of them.

Thanks Andy