XXL LCD Controller and the Z-Offset

I'm not entirely sure if there is already a thread for this as I can't search for either XXL or LCD, but. . .

I finally hooked up the XXL LCD I got. The controller itself works fine in that it boots up, controls the printer and seems to be a solid item.

The problem I'm having is the Z-Offset used by the XXL. Initially I set it to 0.95, which seems to have been the good offset for my previous prints, but when I start the print it just runs along with the nozzle on the bed.

"Ok," I thought, "perhaps it just interprets the value differently for some reason." And so I raised it to 1.5 to see if that made a difference and still the nozzle ran on the bed.

Alright, time to make sure it's actually accepting the offset. In that interest I just cranked the offset up to 10 and started a print.

It went through the homing process and then went in to the heating process for the extruder. Meanwhile I take a look at the printer while waiting and notice that it actually HAS included the offset. Unfortunately, it included it in the wrong direction by running the nuts down 10mm past the plate point.

This wouldn't be a major issue if I could set a negative offset, but that doesn't seem to be an option.
Is there a reason, perhaps in the Robo firmware that it would interpret an offset added by the controller as a negative value?

Maybe something to do with something Mike mentioned at one point about how the z-axis switches are viewed in a normally closed vs normally open state?

I know I can add offsets in repetier or mattercontrol, but I'd like to figure out why it's doing what it's doing.

 

Can you try setting the offset to negative? I'm getting similar results as you.

 

It didn't seem like it would let me set a negative offset. Once I entered the offset portion of the menu the lowest it would allow me to go was 0.50.

I'll try again tonight though.

 

Alright so I tinkered with it over lunch and I came to a few conclusions, most notable:

Z_Probe_Offset is not what we want.

The z probe offset is making the assumption that our nozzle tip is a certain distance away from the measuring probe. Since we're using the same sensing method for our home and probe it's adding this offset into the probe points.

For grins I added a 1.5mm offset from my normal. I then looked at the z probe values: 2, 1.6, 1.5, 1.6 etc

What this means is that it thinks the bed is actually 2mm below where it is, so it pushed the nozzle into the bed.

I think we need to remove the Z_Probe_Offset completely and run it through a G-code offset, if necessary.

It may be possible to work the auto level without a offset, but I find that unlikely. Rather this:

Essentially how I'm thinking of doing it is homing the system, after it homes z you tell it to raise the nozzle

G1 Z0.5

Once you get the nozzle resting in the correct z position you then force a new z home.

G92 Z0

With the new Z home defined you can run

G29

To start the auto level.

My other school of thought is, does the G29 need to be before or after the new definition of Z0?

Were you to define the Z0 as the original, offset, value, you would see probe values be +-0.5mm typically. Then once you finish G29 tell it to raise to the correct Z0 position. In principle I believe these values are adjusted to account for the new home, so an offset value of .6 and a probe reading of -.7 the actually deviation from 0 would be -.1mm.

If you were instead to put it before the G29 you would see vaues like -1.2, etc. This could make the nozzle be far above the bed. Though it might also work fine.


I would think adjusting the Z0 value after G29 is best practice, and I know this used to work. Though with it I had my fair share of high starts, but certainly more perfect starts.

 

I tried this with the rise and zero after the G29 command it seems to have worked well. I added it to the beginning G-Code in Cura. I think this will be the go-to solution for using the offset & G29 in Cura as there is currently no other reliable way to add an offset. For me this is important because I prefer the line paths Cura chooses over Slic3r.