Marlin Autobed Level vs. Z probe?

First post out of what I assume will be many. Hope you guys can help me out. I can't seem to figure out auto-bed leveling on my machine/firmware. I guess I don't understand the relationship between Z probe settings and Auto-Bed settings. First my info
RepRap Guru Prusa I3 Acrylic
Marlin Firmware
Repetier Host- Slic3r built in for slicing.

The firmware that came with the machine was marlin pre-bed leveling. Desiring bed leveling I downloaded the most recent marlin, and ported over all the relevant config.h settings from the version supplied to me. It works fine, prints fine etc. I'm using a z prove mounted on an servo setup, and I have that functioning perfectly. I set my offsets perfectly. When I home all using the probe then move the Z down to 0 manually in repetier host I get just the right friction on a piece of paper (at x/y middle). My problem is I don't understand the interplay between autobed leveling and my Z probe levels. I'm currently editing my G code to home all (safe homing enabled) with g28, then run auto bed with g29, then print. The Z is set perfectly after g28, but after the autobed g29 it is set too low, jamming the extruder. Does the g29 command disregard the results of the g28 operation? Do I need to set an additional offset somewhere to get my g29 height set as well as my g28 height? I don't get it. The bed leveling function probes four locations, then extrapolates the supposed plane of the bed,you would think lowering to 0 at any given x/y would bring you to the perfect height. I know its working because although it sets Z too low, I can watch the Z motors spin as it moves across the first layer. Obviously the auto bed level isn't truly integrated deeply with the system I'm using, because theoretically moving around X and Y manually should change the displayed Z height (as far as that number is representing the distance to the bed) but it doesn't.

TLDR; After G28 lowering Z to 0 gives me perfect clearance, after G29 its too low, what gives?


Thanks!

If you have a Z-Probe to do Auto Bed Leveling... Ignore what happens after a G28. Instead... Tweak your Z_PROBE_OFFSET_FROM_EXTRUDER up or down a little bit at a time to get it exactly where you want it after a G29.

G28 and G29 have always reported different heights. That is in the process of getting cleaned up. The point is, if you are going to use ABL, set your numbers based on that. Soon, G28 and G29 should be consistent.

And don't forget to do a M502 & M500 every time you change a number in your Configuration.h file.

Great. In order to make that process precise I need to understand a little more about how autobed works. So it zips around the bed and does its thing calculating a plane, mine stops after its 4th probing somewhere near the far right corner. It then raises itself to 15 (my height setting)+- the Z offset. so it'll read something like Z=18.5 at that height. My question is to which X/Y location does Z=18.5 apply. Obviously the Z is different locations around the bed, thats the whole point. I have to then lower or raise the Z axis from what it thinks Z=0 is to a proper height, then factor that change into my Z access. I don't know what X/Y to do that process at, is the math Z=18.5 at the last probe location? At the center point of the Bed?

Thanks in advance!

Quote:

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Originally Posted by clearlynotstefan

It then raises itself to 15 (my height setting)+- the Z offset. so it'll read something like Z=18.5 at that height. My question is to which X/Y location does Z=18.5 apply.

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At the end of the Grid Probing, it will do the calculations and set the nozzle height. It will perform the Z_RAISE_AFTER_PROBING (now called Z_HEIGHT_AFTER_PROBING) function. If it is reporting Z=18.5mm, you should be able to lower the nozzle at the current location 18.5mm and have the nozzle just barely touch the glass.

As you move back and forth across the bed, you should see the Z-Axis go up and down to compensate for any bed tilt.