Bed not levelled between X-Y towers

Hi there, I am currently trying to calibrate my custom Kossel but I have ran into some problems with the Z-levelling. My prints all stick well to the bed and are rather levelled at all three towers (X, Y and Z). However, I noticed from printing that the effector dips at the point between the X-Y towers (at the opposite end of the Z tower), causing the filament extruded there to be extremely thin. I have checked my bed and I seems rather flat. I have tried recalibrating multiple times but dont seem to be getting much luck. Other then adjusting the endstop offsets, I am also adjusting the diagonal rod lengths through the EEPROM, to tweek the dimensions of my prints (since printing a circle gives me a slight oval).

My current EEPROM settings are as followed:

start
Printer is now online.
Marlin 1.1.0-RC7
Last Updated: 2016-07-31 12:00 | Author: (oxivanisher)
Compiled: Sep  7 2016
Free Memory: 4507  PlannerBufferBytes: 1232
V24 stored settings retrieved (398 bytes)
Steps per unit:
M92 X100.00 Y100.00 Z100.00 E140.25
Maximum feedrates (mm/s):
M203 X200.00 Y200.00 Z200.00 E25.00
Maximum Acceleration (mm/s2):
M201 X9000 Y9000 Z9000 E10000
Accelerations: P=printing, R=retract and T=travel
M204 P2000.00 R3000.00 T3000.00
Advanced variables: S=Min feedrate (mm/s), T=Min travel feedrate (mm/s), B=minimum segment time (ms), X=maximum XY jerk (mm/s),  Z=maximum Z jerk (mm/s),  E=maximum E jerk (mm/s)
M205 S0.00 T0.00 B20000 X20.00 Z20.00 E20.00
Home offset (mm)
M206 X0.00 Y0.00 Z0.00
Endstop adjustment (mm):
M666 X-2.35 Y-4.57 Z-0.06
Delta settings: L=diagonal_rod, R=radius, S=segments_per_second, ABC=diagonal_rod_trim_tower_[123]
M665 L327.53 R159.21 S160.00 A0.89 B-0.34 C1.86
Material heatup parameters:
M145 S0 H180 B70 F255
M145 S1 H240 B100 F255
PID settings:
M301 P30.21 I4.15 D54.95
Filament settings: Disabled
M200 D3.00
M200 D0
Z-Probe Offset (mm):
M851 Z0.30

My electronics are running RAMPS 1.4 with DVR8825 steppper drivers.

Thanks in advanced for any help that I can get!

The dip may be caused by your adjusting the diagonal rod lengths. Did you calibrated the tower positions first?

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Quote
dc42
The dip may be caused by your adjusting the diagonal rod lengths. Did you calibrated the tower positions first?

Yes I did. There was already such an issue when I left the M665 A,B and C values as 0.00
I first levelled the bed, then tweaked the A,B,C values in M665 and then recalibrated the endstops and this is the current result that i ended up with..

I suspect you have either leaning towers or a geometrical error that is causing effector tilt, which is causing the scale error.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Alright thank you DC for the reply. I will try reprinting the roller mechanism with a more accurate printer and try again. Else I may need to look at alternate methods of the linear guide system

Roller mechanisms can be accurate enough (I use them), but you need to make sure that the carriages are not rotated about either the Z axis or about their faces. When I rebuilt my delta, I had one too many washers between one of the carriage wheels and the carriage truck, and this caused the carriage to be rotated about the Z axis, which threw the calibration out. Also make sure that in each pair of parallel rods, the spacing between the bearings is the same top and bottom.

I found it useful to mount a bulls-eye spirit level on the effector, so that I can see if it tilts when I move it in the XY plane.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Quote
dc42
Roller mechanisms can be accurate enough (I use them), but you need to make sure that the carriages are not rotated about either the Z axis or about their faces. When I rebuilt my delta, I had one too many washers between one of the carriage wheels and the carriage truck, and this caused the carriage to be rotated about the Z axis, which threw the calibration out. Also make sure that in each pair of parallel rods, the spacing between the bearings is the same top and bottom.

I found it useful to mount a bulls-eye spirit level on the effector, so that I can see if it tilts when I move it in the XY plane.

I suspect that for mine, the rollers cause the two parallel rods to not be centered with respect to the tower extrusions. One rod was probably nearer to the tower extrusion than the other rod. DC, may I ask if you got your model for the roller wheel holder anywhere online? or did you design them yourself. I am using the v-slot compatible large wheels, which I got off a chinese site.

I got the wheeled carriage truck design from think3dprint3d's fork of Johann's GitHub repo originally, then I modified it for 2020 extrusion and published it in my own repo and on Thingiverse. The angle about the Z axis can be adjusted by adding shims between the wheels and the printed part.

---

Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].