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Z Probe v. Auto bed leveling???

Posted by clearlynotstefan 
Z Probe v. Auto bed leveling???
August 17, 2016 08:42PM
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?


Edited 1 time(s). Last edit at 08/17/2016 08:44PM by clearlynotstefan.
Re: Z Probe v. Auto bed leveling???
August 17, 2016 10:43PM
Auto level uses Z probe setting in the firmware - probe offset (X, Y coordinate of the probe), and trigger height.

If the machine is built well, bed level calibration is done either manually or by firmware feature, you hardly need to run auto level in each printing session. I got rid of Z probe on the effector.

G28 resets previous auto level data.
G28 uses maximum Z height. G29 uses probe Z height.

For instance my Bulbul Junior has 178.72 mm Z height.
Running G28 moves the effector to 178.72 mm off the print bed.
When I used to run Auto level, I set probe Z height at 10 mm off the print bed when the probe - allen key - is extended.

Edited 1 time(s). Last edit at 08/17/2016 10:54PM by janpenguin.
Re: Z Probe v. Auto bed leveling???
August 17, 2016 10:55PM
I assumed so, but it doesn't seem to work that way in practice. I calculated my Z offset in repetier, set it as my firmware z offset. When I home the bed (with the probe in the center [100x100], then move the nozzle to the same point and lower it to 0 it is a perfect clearance. Then I run G29, it probes its 4 points, then if I move the nozzle to the center and bring it to z0 it'd smash into the bed!
Re: Z Probe v. Auto bed leveling???
August 18, 2016 03:15AM
Not sure if I can help but I'll try. I use Duet hardware and DC42s branch or reprap firmware on a modified RRP Mendel so I'm not familiar with your particular setup. Also, I don't use Repetier. I think first we need to be clear on the terminology. Auto bed levelling is not the same as auto bed compensation. Auto bed levelling would be used in a situation where you have the means to physically adjust the bed supports. For example, on a Cartesian printer with 3 lead screws, each with it's own stepper motor. As you have a Prusa Mendel, I suspect you are referring to Auto bed compensation. In this case, the bed is probed in a number of places, usually the 4 corners, then the firmware applies a compensation for the fact that the bed might not be level. This compensation is used throughout the print run to constantly adjust the Z position, unlike auto bed levelling which would not perform any firmware compensation. I'm not trying to be pedantic, just clarifying the issue.

If my assumptions are correct, my suggestion would be to probe the bed first i.e set the bed compensation algorithm, then home the bed after. As I said, I'm not familiar with Marlin firmware or Repetier host so someone more knowledgable may have a better suggestion.

Edit. There is also this thread, [forums.reprap.org]. It's 23 pages but you may find the answer you need buried in there somewhere.

Edited 1 time(s). Last edit at 08/18/2016 03:21AM by deckingman.
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