unified bed leveling resulting in Z-height issue

Before I used Unified Bed Leveling (UBL), I was using bilinear leveling with BLTouch. everything was fine and calibrated. a 20mm calibration cube would come out ~20mm.
But now that i implemented UBL, all my models have errors in Z height. a 7mm print became 8.5mm. a 20mm cube became 22mm, etc etc.
Is this an issue with fade height?

I am at a total loss here.

Yes... It is very possibly a side effect of the fade height. How large is your Z_PROBE_OFFSET_FROM_EXTRUDER number? Or what is the G29 P5 number? If you have a large number (say greater than 1 mm) you are going to see the fade height impact things.

-0.956070 is my value

I should to make it zero?

Ideally, the mean of the mesh points should be close to 0.000 mm. I think things work out best homing the Z Axis at the center of the bed.
If you home in the center of the bed, you can look at the G29 T map to see which corners are too high (or low) and adjust them.

How do I get the mean closer to 0? Do I shift my probe higher? Should my Z-probe offset be input before G29 P1 or should the offset be zero before G29P1.

You should adjust your Z_PROBE_OFFSET_FROM_EXTRUDER number such that when you home, you can lower the nozzle and when it just barely touches the bed, the LCD is saying Z is equal to 0.00 mm.

If you set your printer up to home in the center of the bed, and post a G29 T we will have a much better idea what is happening. Probably, you should post a G29 W also.

Yes my Z_PROBE_OFFSET_FROM_EXTRUDER is at a number which when I do G1 Z0, my nozzle barely touches the bed. However after I do a G29 P1. my mean mesh is around -0.9

G29 T gives
Bed Topography Report:
(0,7) (7,7)
(35,173) (192,173)
-0.844 -0.805 -0.764 -0.815 -0.887 -0.964 -0.943 -0.984

-0.941 -0.855 -0.874 -0.900 -0.940 -0.991 -0.960 -1.018

-0.944 [-0.902] -0.906 -0.975 -0.990 -1.017 -0.979 -0.962

-0.968 -0.931 -0.971 -1.020 -1.018 -1.063 -1.003 -0.935

-0.996 -0.982 -0.960 -1.027 -1.033 -1.071 -1.017 -0.972

-1.015 -0.997 -0.984 -1.042 -1.039 -1.085 -1.024 -0.981

-1.068 -1.032 -0.980 -1.034 -1.064 -1.082 -1.068 -1.044

-1.241 -1.081 -1.024 -1.061 -1.055 -1.085 -1.108 -1.099
(35,8) (192,8)
(0,0) (7,0)

and G29 W gives
Unified Bed Leveling System v1.01 active.
Mesh 1 Loaded.
UBL object count: 1
planner.z_fade_height : 10.0000
# of samples: 64
Mean Mesh Height: -0.990893
Standard Deviation: 0.079207
zprobe_zoffset: -1.3799999
MESH_MIN_X ((((((200) / 2) - (200) / 2) + 8)>(0 + 35)?((((200) / 2) - (200) / 2) + 8)0 + 35)))=35
MESH_MIN_Y ((((((200) / 2) - (200) / 2) + 8)>(0 + -27)?((((200) / 2) - (200) / 2) + 8)0 + -27)))=8
MESH_MAX_X ((((((200) / 2) + (200) / 2) - (8))<(200 + 35)?((((200) / 2) + (200) / 2) - (8))200 + 35)))=192
MESH_MAX_Y ((((((200) / 2) + (200) / 2) - (8))<(200 + -27)?((((200) / 2) + (200) / 2) - (8))200 + -27)))=173
GRID_MAX_POINTS_X 8
GRID_MAX_POINTS_Y 8
MESH_X_DIST 22.43
MESH_Y_DIST 23.57
X-Axis Mesh Points at: 35.000 57.429 79.857 102.286 124.714 147.143 169.571 192.000
Y-Axis Mesh Points at: 8.000 31.571 55.143 78.714 102.286 125.857 149.429 173.000
Kill pin on :41 state:1
Unified Bed Leveling sanity checks passed.

Edited 1 time(s). Last edit at 01/21/2018 08:45AM by auggie246.

Quote
auggie246
Yes my Z_PROBE_OFFSET_FROM_EXTRUDER is at a number which when I do G1 Z0, my nozzle barely touches the bed. However after I do a G29 P1. my mean mesh is around -0.9

G29 T gives
Bed Topography Report:

Yes. But if that was true... There should be some mesh point on that Topography map that is close to 0.00 mm. Your Z-Probe_Offset_From-Extruder is not right. All of the bed leveling systems use the exact same low level probing system. This part of the code probably works...

Hmm then why does my G1 Z0 shows that the nozzle is right above the bed. weird. I will play around with it for now

i tried changing my probe offset to some other value. my mean mesh still shows ~-0.9....
This is just weird

Besides changing the Z Probe Offset number in configuration.h, you also need to do a M502 and M500 to get the value updated in the EEPROM.