Auto bed leveling issues

Hi!

For some reason auto bed leveling isn't working as it should anymore on my reprap i3. I'm using a induction sensor and bilinear leveling on a 3 mm thick aluminium bed with a BuildTak sheet on it. The grid is 4 x 4 points.
The problem is that if I set the z offset at X0, Y0 so that a piece of paper just slides underneath the nozzle, when the print starts, it prints way higher that it's supposed to. I have to set the z probe offset about 0.3-0.5 mm lower than what it should be to get it to print correctly in the middle of the bed. This means that when I home all axes the nozzle digs in to the BuildTak sheet. I can see that the printer is compensating for the uneven bed as the z axis is moving up and down.
I'm not sure exactly when this started as I constantly fiddle with the machine.
The X and Y offset for the probe is correct.

I'm using Slic3r as my slicer.

The latest G29 looked like this:
0 1 2 3
0 +0.067 -0.012 -0.257 -0.630
1 +0.375 +0.197 -0.060 -0.417
2 +0.408 +0.250 -0.017 -0.370
3 +0.140 +0.095 -0.108 -0.558

The Configuration.h from Marlin is attached.

Any ideas would be greatly appreciated!

Regards,
Christian

I have a similar problem, I have set my Z-Offset to -0.5.

And I have enabled baby steps in my Marlin and added a couple of skirts around the model so I can quickly adjust for the best layer height.
I have also put my Proximity sensor on a thumb screw adjustable mount to allow for a quick height adjustment for the nozzle.

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Computer Programmer / Electronics Technician

These proximity sensors are not designed for the way you use them. It is lucky they have such repeatability. If for various reason they shift, nothing strange.
Anyway, a well designed and built cartesian machine doesnt need auto bed leveling (Delta, yes).
If your machine is so bad that it needs it, at least use something better suited, strain gauge, piezo, contact .... More reliable and less bulky.

Edited 3 time(s). Last edit at 02/11/2019 01:33AM by MKSA.

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"A comical prototype doesn't mean a dumb idea is possible" (Thunderf00t)

A warp deviation of more than 0.15mm will make it really hard to get a first layer across the bed, even at a first layer height of 0.3mm regardless if you use auto bed levelling. I'd try to first level the bed as best as you can. Also make sure the bed can expand in the x and y plane. If it's fixed it will warp. Also make sure consecutive probing values are consistent.
I've used shims to level my bed, and I don't use auto bed levelling anymore. I had to bend the bed a little using a glue clamp. It took care of most of the warp. Make sure the bed support structure can handle the clamping force though. Mine is made up from 2040 extrusions, so they can handle the abuse. I used a dial indicator on the printhead to find out how level it was at 3 points in a triangle. After a few attempts it was level enough to start a print at 0.3mm first layer height across the bed.

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Kind regards
Imqqmi

NFAN CoreXY printer:
[reprap.org]

Quote
imqqmi
A warp deviation of more than 0.15mm will make it really hard to get a first layer across the bed, even at a first layer height of 0.3mm regardless if you use auto bed levelling. I'd try to first level the bed as best as you can. Also make sure the bed can expand in the x and y plane. If it's fixed it will warp. Also make sure consecutive probing values are consistent.
I've used shims to level my bed, and I don't use auto bed levelling anymore. I had to bend the bed a little using a glue clamp. It took care of most of the warp. Make sure the bed support structure can handle the clamping force though. Mine is made up from 2040 extrusions, so they can handle the abuse. I used a dial indicator on the printhead to find out how level it was at 3 points in a triangle. After a few attempts it was level enough to start a print at 0.3mm first layer height across the bed.

Hi!

As a test I actually leveled the bed, or the X axis to be precise (I know it's the wrong way to do it. It was only a test), and printed a 150x150 mm cross and the it printed great. So I guess that the automatic bed leveling can't handle the large deviation.
What do you mean by "Also make sure the bed can expand in the x and y plane. If it's fixed it will warp."?

I should also note When Using Marlin Bed Leveling,
When Starting a print G28 will disable the bed leveling data unless you have Bed Leveling Command in the Slicers Startup G-Code.
Using Bi-Linear Bed Leveling you can Save the Bi-linear Bed Leveling Data and Recall this data By Adding a " M420 S1" to the End of Slicers Startup G-Code.

This is a My Startup G-Code

G21        ;metric values
G90        ;absolute positioning
M82        ;set extruder to absolute mode
;M106 S127 ;set Fan half speed
M107       ;start with the fan off
G28 X0 Y0  ;move X/Y to min endstops
G28 Z0     ;move Z to min endstops
G1 Z15.0 F{travel_speed} ;move the platform down 15mm
;Put printing message on LCD screen
;M117 My Print...
G28 X0 Y0
G1 E4.0 ;prime extruder
G92 E0 ;reset extruder length
M420 S1   ; Bed Leveling On
M220 S32.000000  ; Set Startup Start Print Speed.
;M420 V1   ; Print the leveling grid

You can Save the Data simply by selecting the Save Settings on The LCD Display

OR

M500: Save Settings - Marlin Firmware

M420: Enable/Disable Bed Leveling and/or set the Z fade height.
S[bool] Turns leveling on or off
Z[height] Sets the Z fade height (0 or none to disable)
V[bool] Verbose - Print the leveling grid

This Allows you to view bed leveling data.
M420 V

// M851 give a range for adjusting the Z probe offset
#define Z_PROBE_OFFSET_RANGE_MIN -20
#define Z_PROBE_OFFSET_RANGE_MAX 20

Set with M851 Z-5.2 or #define Z_PROBE_OFFSET_FROM_EXTRUDER -5.2.

Edited 3 time(s). Last edit at 02/12/2019 09:09AM by Roberts_Clif.

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Computer Programmer / Electronics Technician

The auto bed leveling is only trying to level the bed by rotation, it's not trying to compensate for uneveness. So a warped bed will remain warped but it's treated as square to the axes on average.

The aluminium will expand due to heating it up and contract when cooling down. This can be as much as 1mm in both directions for 200mm length of aluminium and a delta T of around 100 degrees. If the bed is fixed in all corners the expansion has to go somewhere, which is when the bed will warp up or down (or flex the support structure). It's like a sheet of paper laying on a table. Push from the edges inward, towards each other and the paper will warp since it has nowhere else to go.

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--
Kind regards
Imqqmi

NFAN CoreXY printer:
[reprap.org]

Quote
imqqmi
The auto bed leveling is only trying to level the bed by rotation, it's not trying to compensate for unevenness. So a warped bed will remain warped but it's treated as square to the axes on average.

Not sure I understand you.
When I print a large model without Auto-bed leveling the Z-Axis stay at one location thru the entire level print.
When I print a large model with Auto-bed leveling the Z-Axis moves right then left to compensate for the bed level, for a few layers then finally compensating will fade to height.

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Computer Programmer / Electronics Technician

Quote
Roberts_Clif
I should also note When Using Marlin Bed Leveling,
When Starting a print G28 will disable the bed leveling data unless you have Bed Leveling Command in the Slicers Startup G-Code.
Using Bi-Linear Bed Leveling you can Save the Bi-linear Bed Leveling Data and Recall this data By Adding a " M420 S1" to the End of Slicers Startup G-Code.

This is a My Startup G-Code

G21        ;metric values
G90        ;absolute positioning
M82        ;set extruder to absolute mode
;M106 S127 ;set Fan half speed
M107       ;start with the fan off
G28 X0 Y0  ;move X/Y to min endstops
G28 Z0     ;move Z to min endstops
G1 Z15.0 F{travel_speed} ;move the platform down 15mm
;Put printing message on LCD screen
;M117 My Print...
G28 X0 Y0
G1 E4.0 ;prime extruder
G92 E0 ;reset extruder length
M420 S1   ; Bed Leveling On
M220 S32.000000  ; Set Startup Start Print Speed.
;M420 V1   ; Print the leveling grid

You can Save the Data simply by selecting the Save Settings on The LCD Display

OR

M500: Save Settings - Marlin Firmware

M420: Enable/Disable Bed Leveling and/or set the Z fade height.
S[bool] Turns leveling on or off
Z[height] Sets the Z fade height (0 or none to disable)
V[bool] Verbose - Print the leveling grid

This Allows you to view bed leveling data.
M420 V

// M851 give a range for adjusting the Z probe offset
#define Z_PROBE_OFFSET_RANGE_MIN -20
#define Z_PROBE_OFFSET_RANGE_MAX 20

Set with M851 Z-5.2 or #define Z_PROBE_OFFSET_FROM_EXTRUDER -5.2.

So you have done a G29 and saved the values once, and these values are used with M420 instead of doing a G29 every time you print?

Edited 1 time(s). Last edit at 02/14/2019 01:50AM by chrstrvs.

Quote
imqqmi
A warp deviation of more than 0.15mm will make it really hard to get a first layer across the bed, even at a first layer height of 0.3mm regardless if you use auto bed levelling. I'd try to first level the bed as best as you can. Also make sure the bed can expand in the x and y plane. If it's fixed it will warp. Also make sure consecutive probing values are consistent.
I've used shims to level my bed, and I don't use auto bed levelling anymore. I had to bend the bed a little using a glue clamp. It took care of most of the warp. Make sure the bed support structure can handle the clamping force though. Mine is made up from 2040 extrusions, so they can handle the abuse. I used a dial indicator on the printhead to find out how level it was at 3 points in a triangle. After a few attempts it was level enough to start a print at 0.3mm first layer height across the bed.

Could you post a picture of you glue clamp thing? I have a hard time understanding what you mean.

Quote
chrstrvs
So you have done a G29 and saved the values once, and these values are used with M420 instead of doing a G29 every time you print?

This is correct! Sometimes I only have to level once a month. Unless the bed tension springs screws work themselves loose do to vibrations.
Should add a rubber washer to help stop the Bed tension screws from turning -- Maybe help it from not turning.

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Computer Programmer / Electronics Technician

Quote
Roberts_Clif

Quote
chrstrvs
So you have done a G29 and saved the values once, and these values are used with M420 instead of doing a G29 every time you print?

This is correct! Sometimes I only have to level once a month. Unless the bed tension springs screws work themselves loose do to vibrations.
Should add a rubber washer to help stop the Bed tension screws from turning -- Maybe help it from not turning.

Okay, cool!
I use lock nuts to keep my screws from coming loose.

It's just a c clamp. I measured using a dial indicator and found that it was warped on one side. I used two 2040 extrusions as bed support so it was strong enough to clamp on that and the side/middle of the bed. The rise was clamped down and bowed against the rise. Make sure not to over do it. After (removing the clamp) that the dial indicator gave better measurements.

A clamp like this:


I hope it's a little clearer now.

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--
Kind regards
Imqqmi

NFAN CoreXY printer:
[reprap.org]

Quote
imqqmi
It's just a c clamp.

shopclamps_3.jpg

I hope it's a little clearer now.

You want us to level our bed with a clamp like this, No Thank you!

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Computer Programmer / Electronics Technician

Hehehe, I can't blame you, it worked for me but it's not for everyone It's better to get a bed that's flat to begin with but it can be expensive.

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--
Kind regards
Imqqmi

NFAN CoreXY printer:
[reprap.org]

Quote
imqqmi
It's just a c clamp. I measured using a dial indicator and found that it was warped on one side. I used two 2040 extrusions as bed support so it was strong enough to clamp on that and the side/middle of the bed. The rise was clamped down and bowed against the rise. Make sure not to over do it. After (removing the clamp) that the dial indicator gave better measurements.

A clamp like this:

shopclamps_3.jpg

I hope it's a little clearer now.

Ah, I see. Thank you for that!