Can someone help me understand MESH LEVELING?

So I have it enabled and have gone through the procedure and it is less than complicated. All of that I understand. What I am confused about is the display. Why do I have this line set as is:

Code:

#define MESH_HOME_SEARCH_Z 4  //Z after Home, bed somewhere below but above 0.0.

Shouldn't it be set like this:

Code:

#define MESH_HOME_SEARCH_Z 0

When the process is complete and you then save the settings, if you issue a G28 command to home all the axes you end up at X=10, Y=10, Z=4. Now physically the head is at X=10, Y=10 but the nozzle is actually barely touching the bed so technically Z=0 and not 4 like is says. Do I just ignore that or do I need to do something else in firmware?

I should also mention that I only made three edits in the leveling section. One to enable MESH, second to change from 3 x 3 to 4 x 4 grid, third to make sure that Auto Bed Leveling is turned off.

 

No Mike, it doesn't. I played with it all afternoon trying different parameters the head never moves up until you actually turn the encoder. So the 4 is an arbitrary number. I also tried 0 and it works exactly the same way.

I believe the 4 is there so they don't have to do math on negative values, it appears to have no other function.

I am revising my first sentence. The number 4 is what the LCD says the Z height is before each point, but it doesn't actually move the hot end in the Z axis at all, just in the Y or X depending on which spot is next.

 

Yeah, @daniel871 I left it at 0 and it made no difference to the math. My first print came out perfect, except for the first layer offset. It was just a few hundredths too low. I am going to play with M851 to see what effect it really has.

I really think the 4 is there because most probes are above the level of the nozzle and may need to go less than 4 if the bed is warped. I'm going to go with 0, since on the Robo R1 series it can never be negative.

 

@Mike Kelly Here is the behaviour of Mesh leveling based on Marlin's implementation and specific to the Robo's mechanics.

1. It homes all Axis.
2. It moves to X10, Y10 at Z Home and reports whatever number you have entered in firmware.
   Apparently any arbitrary number will work as long as when you are done with setting the current touch point your result isn't negative.
3. You turn the encoder, in the case of the Robo lifting the nozzle from the bed to slip your gauge between the bed and the nozzle.
4. You press the encoder knob to set that location and the hot end moves to the next location. At the same time, again Robo specific, lowers the nozzle back to Z home and displays the same firmware provided number again.
5. It repeats this for all touch points

As you know Z home is actually below the surface of the glass bed. It will exhibit precisely the behaviour that @daniel871 describes.

 

We might as well redesign the entire R1. I am sure we here can come up with a much more robust design and probably come in with a BOM that is about the same.

 

now that would be a hell of a printer!