@spudaddict OK, so smoking gun found. I went to swap out the other stepper motor and noticed three case screws (on the bottom) were missing. I replace the screws and two stripped out and two were loose. I pulled the T20 driver gear off, and there is grease speckled underneath.

So, I am guessing without the case screws the internal bearings got skewed and that the motor is likely unrecoverable.

I tapped the motor case and got 2 screws to hold - luckily they are diagonally opposite each other. Just for grins I started my "tube test" design as 3 of the 4 tubes consistently turn out terrible. Here is what I got:

Comparatively perfect minus some relatively minor tuning.

New stepper motor is on order and this is one to put in the troubleshooting books - if you ever get a repeatable bi-stable layer shift on a 45degree angle, check your stepper motor.

THANK YOU everyone for your input. I am implementing pretty much every suggestion I got as there are definitely other items that I can improve on. The improved belt alignment and stiffer front plane has been integrated into my new print head design. (and now I can even print it!!) Other improvements will continue. Thanks again - especially JamesM, mrehorstdmd, Danal who stuck it out to the end.

@Haggan90
Its mechanically interesting that your wall variations match each other on the x face and the y face. Do the indents and outdents match each other on the positive and negative x face as well? In other words, if you turn your cube to see the side profile of both X walls, do the indents look like

\ \
/ /
\ \ indicating some kind of wobble

OR

\ /
/ \
\ / indicating some kind of compression or extrusion issue

What kind of lead screws are you using and at what pitch?

What is the infill % and type you used on this print?

What power setting do you have on your Z stepper motors?

Are you using bed mesh leveling and have you updated it recently?

@spudaddict OK, so smoking gun found. I went to swap out the other stepper motor and noticed three case screws (on the bottom) were missing. I replace the screws and two stripped out and two were loose. I pulled the T20 driver gear off, and there is grease speckled underneath.

So, I am guessing without the case screws the internal bearings got skewed and that the motor is likely unrecoverable.

I tapped the motor case and got 2 screws to hold - luckily they are diagonally opposite each other. Just for grins I started my "tube test" design as 3 of the 4 tubes consistently turn out terrible. Here is what I got:

Comparatively perfect minus some relatively minor tuning.

New stepper motor is on order and this is one to put in the troubleshooting books - if you ever get a repeatable bi-stable layer shift on a 45degree angle, check your stepper motor.

THANK YOU everyone for your input. I am implementing pretty much every suggestion I got as there are definitely other items that I can improve on. The improved belt alignment and stiffer front plane has been integrated into my new print head design. (and now I can even print it!!) Other improvements will continue. Thanks again - especially JamesM, mrehorstdmd, Danal who stuck it out to the end.

@Danal lol... troubleshooting was fun at first, but im getting to the point i just want to print.

Both items are very true. Working through the many great suggestions I have gotten so far. Will definitely post if I find the smoking gun.

@Danal Morning Danal. I am not ruling anything out at this point, but a couple considerations:

There are two cross bars near the bottom (not ideal) that are rigid - so any flexing would require the vertical 2040 extrusions to actually bend against the 40mm width. Im not sure there is anything that happens during normal operation of the printer that can generate that much force.

Also, the Y bridge works to hold the width steady since it cannot get longer/shorter as it travels front to back... as long as the MGN rail alignment is correct.

I am going to put a temp brace on as you suggested, but I suspect what I will get is confirmation that my MGN alignment is either right or wrong. If it is wrong and the top is now absolutely rigid - the Y bridge should bind up. Badly.

@JamesM lol... yes. Sharp eyes.

My only spare 2A stepper motor is for the 2nd filament driver(1.8deg). I swapped it into both the X and Y positions to see if there was a motor issue and need to pull it back out. I tried the M121 command near the end of the gray tube print and there were zero errors and zero hiccups - so likely a mechanical issue.

When the back cover is on, the 4x120 fans move a lot of air across the Duet board, but I have had the cover off since I started troubleshooting. I will put a fan on and under it next.

Thank you for your help and suggestions.

@mrehorstdmd Yes, this last round seems to have much less shift.

I can detect some rotating on the "Y" bridge, but takes some force. (pushing forward on the left end, pulling on the right at the same time - and visa versa) I tried doing that during a print with long lines (a large box) and it didn't seem to have an effect. The open front has bothered me since before I began the build. They will flex a little if the Y bridge is near the back, less flex when it is near the front.

I DO get some wiggling on the nozzle in the Y direction. Not sure how that can create a diagonal offset, but I plan on reprinting the head assembly today/tomorrow with stiffer supports. The X and Y MGN carriages seem solid.

Thank you for your continued suggestions. I will see what reducing the print head wiggle brings.

@mrehorstdmd I tried steel belts on a modified Anet A8, they were terrible. Currently have fiberglass backed belts on it and have a coupe nylon corded rubber ones to try out.

I reprinted my tubes design. The offsets are much smaller. I was able to force the front right tube to offset by pressing and pulling the print head. Still troubleshooting...

My BLV is a bit hacked up since I have been swapping out motors, moving wires, inspecting belts and cannibalizing my A8 bed parts... but here ya go.

@spudaddict Config.g

; Configuration file for Duet WiFi (firmware version 2.03)
; executed by the firmware on start-up
;
; generated by RepRapFirmware Configuration Tool v2.0.4 on Wed Oct 02 2019 19:29:38 GMT-0600 (Mountain Daylight Time)

; General preferences
G90 ; send absolute coordinates...
M83 ; ...but relative extruder moves
M550 P"Wyrm Forge" ; set printer name

M667 S1 ; select CoreXY mode

; Network
M552 S1 ; enable network
M586 P0 S1 ; enable HTTP
M586 P1 S0 ; disable FTP
M586 P2 S0 ; disable Telnet

; Drives
M569 P0 S0 ; physical drive 0 goes 0=backwards, 1=forwards
M569 P1 S0 ; physical drive 1 goes
M569 P2 S1 ; physical drive 2 goes
M569 P3 S1 ; physical drive 3 goes
M584 X0 Y1 Z2 E3 ; set drive mapping
M350 X16 Y16 Z16 E16 I1 ; configure microstepping with interpolation
M92 X79 Y158.88 Z809.21 E108.90 ; set steps per mm X is 158.88 with 0.9 stepper replaced
M566 X900.00 Y900.00 Z12.00 E120.00 ; set maximum instantaneous speed changes (mm/min)
M203 X18000.00 Y18000.00 Z180.00 E1200.00 ; set maximum speeds (mm/min)
M201 X3000.00 Y3000.00 Z100.00 E9000.00 ; set accelerations (mm/s^2)
M906 X1700 Y1700 Z800 E1400 I30 ; set motor currents (mA) and motor idle factor in per cent
M84 S30 ; Set idle timeout

; Axis Limits
M208 X-28.0 Y-6.0 Z0 S1 ; set axis minima
M208 X320 Y320 Z460 S0 ; set axis maxima

; Endstops
M574 X1 Y1 S1 ; set active high and disabled endstops

; Z-Probe
M574 Z1 S2 ; set endstops controlled by probe
M307 H3 A-1 C-1 D-1 ; disable heater on PWM channel for BLTouch
M558 P9 H4 F120 T9000 ; set Z probe type to bltouch and the dive height + speeds
G31 P500 X8.1 Y-61.1 Z1.36 ; set Z probe trigger value, offset and trigger height
M557 X30:250 Y30:250 P7
;M557 X10:320 Y10:360 P7 ; define mesh grid

; Heaters
M305 P0 T100000 B4138 R4700 ; set thermistor + ADC parameters for heater 0
M143 H0 S140 ; set temperature limit for heater 0 to 140C
M305 P1 T100000 B4138 R4700 ; set thermistor + ADC parameters for heater 1
M143 H1 S280 ; set temperature limit for heater 1 to 280C
M307 H1 A987.5 C204.5 D8.4 S0.85 V24.1 B0 ; set A=gain, C=heat time, D=dead time, S=power %, V=voltage, B0=bang bang

; Fans
M106 P0 S0 I0 F500 H-1 ; set fan 0 value, PWM signal inversion and frequency. Thermostatic control is turned off
M106 P1 S1 I0 F500 H1 T45 ; set fan 1 value, PWM signal inversion and frequency. Thermostatic control is turned on

; Tools
M563 P0 S"Right" D0 H1 F0 ; define tool 0
G10 P0 X0 Y0 Z0 ; set tool 0 axis offsets
G10 P0 R0 S0 ; set initial tool 0 active and standby temperatures to 0C

; Custom settings are not defined

; Miscellaneous
M911 S10 R11 P"M913 X0 Y0 G91 M83 G1 Z3 E-5 F1000" ; set voltage thresholds and actions to run on power loss

@spudaddict Here is a photo of the best and the worst pieces from the drag chain print. They were in diagonally opposite corners... which seems to indicate a mechanical issue due to the CoreXY configuration.

These were printed at the same time just 150mms or so apart.

@NitroFreak Thats what is killing me! How is it so perfectly sloppy?

@mrehorstdmd See my reply to you above. A fellow 3D enthusiast from work built the same printer in tandem as I built mine. His BLV cube does not have this issue either. I'm not sure how to measure belt tension - maybe put a spring on the end and measure an elongation?