Bimodal layer shift



  • I am stumped with a "bi-modal" layer shift issue on my BLV Cube at 45 degrees - front right to back left. Hours of research have turned up nothing similar so I am looking for community help.

    PHOTO1: As part of the troubleshooting I printed 4 tubes (solid base, then 0.8mm tubes) at the same time. The front right tube was good - not great. The back left was a little worse, the front left was a little worse and the back right was terrible.
    Tube.png

    I then printed a Moai(Easter Island head) and a shotgun rapid loader widget that both turned out perfect.

    PHOTO2: Feeling confident I printed a 4x7 array of drag chain parts. The front left ones were passible, the back left ones are unusable.
    Array.png

    So how-in-the-world am i getting good and bad parts in the very same print? How is the layer shift finding its way between 2 stable endpoints?

    My setup: BLV Cube; Duet2WIFI, 0.9 Steppers for X-Y, 330x330 bed. Other than water cooling - built to standard BLV design on Thingiverse. 1800 current to steppers. No damage on belts or pulleys. (this printer is essentially new)

    Checked so far: board is cool; steppers are cool; took belt off and verified MGN rails are smooth and no binding. The boden/wire umbilical is not catching on anything, the printhead mount feels solid, replaced the Y motor, then the X motor. Did find 58V being induced by an AC bed heater which I regrounded and is now 0V.!

    Comments WELCOME please.


  • Moderator

    Perhaps some binding in the rails at that position?



  • Looks 100% like a loose pulley on one of the motors , explains why it is at 45° since it is a coreXY and it goes back and forth.

    Make sure the pulleys set screws are set correctly on the detent of the motor shaft and not on the side or so



  • @Phaedrux With the belts off I could not detect any binding - but that could change under belt tension. During one print I did grab the Y bridge and apply considerable rotational pressure to try to force a layer shift. It continued to print fine. Is there anything you have seen or tried to help prove out a binding issue?



  • @NitroFreak I pulled both stepper motors in my initial troubleshooting and verified they are solid. I extended the "D cut" on one of them to make sure the set screw was hitting the flat. At this point I am not discounting anything, but I did pull and verify tight fit on both motors. I will try pulling them again this weekend.



  • @spudaddict said in Bimodal layer shift:

    Try marking the side of the pulley to the stepper shaft with a marker and observe during the print if the line shifts.
    Especially observe the stepper that is responsible of the 45 degree motion that is in the direction of the shift.

    I´m 100% sure that this is not a normal skipped step from the stepper because it is too uniform and back and forth to the exact same space, does not happen.



  • Carefully check the entire length of the belts. I've found stretched spots that are REALLY hard to see, and if they were in "one end" vs. "the other" it could explain the good/bad pattern across the bed.



  • Check that the ribs on the belts are not worn more in one section vs another , look for rubber dust as a sign of wear .
    I had that happen on my delta and it caused all kinds of mysterious issues with travel being ok then not ok .
    I actually saw the belt ribs ( the 3/4 worn ones ) riding on top of the drive pulley teeth rather than in between !



  • Is this a corexy printer? Do the belt tensions vary with the extruder position in the XY plane?



  • @mrehorstdmd

    The BLV cube is a corexy printer....I have built one myself and don't have these issues.

    @spudaddict

    If you have a duet board in your printer(which I assume since you posted here) I would run an M122 command during a print that is having these issues and look to see if any "hiccups" are reported which could translate into missed steps.

    I would post up your config.g file so people here could review it and see if anything sticks out.

    I do agree with the others who have posted that it seems more like a mechanical issue since it is worse in different locations on the print bed.



  • @JamesM Thanks, but I'd still like to hear what @spudaddict has to say. Self built printers are seldom identical, even if they start from the same set of plans/BOM. People make part substitutions and design changes all the time. Print quality varying with bed position is exactly the sort of problem you would expect from belts being improperly laid out, evidence of which is varying belt tension as the extruder carriage moves around.



  • @mrehorstdmd said in Bimodal layer shift:

    @JamesM Thanks, but I'd still like to hear what @spudaddict has to say. Self built printers are seldom identical, even if they start from the same set of plans/BOM. People make part substitutions and design changes all the time. Print quality varying with bed position is exactly the sort of problem you would expect from belts being improperly laid out, evidence of which is varying belt tension as the extruder carriage moves around.

    Thats a good point, thats true.
    However belt tension would have a very sloppy effect on print quality, but just check the first red tower, the sections that are shifted within themselves are very properly aligned



  • @mrehorstdmd As James M said, it is a CoreXY. I am not sure how I would measure tension variation on a moving belt - open to suggestions. IF the x and y rails are orthogonal there should be no tension variation. The strange thing is that in between the red tube print and the drag chain print, the printer made a perfect moai (Thingiverse: 144668). Its at work so I cant pull up a photo. Also, the "good" zones seem to move around the bed.



  • @Danal The belt is new, but I will do as you suggest. There are 2 belts on a CoreXY so I can easily check between them when I tear it down this weekend. I also have a new set of belts I may try as well.



  • @DigiD The belts are new, but I will definitely look them over. I also have a new set of belts I may try this weekend as well.



  • @NitroFreak I absolutely agree, which is why I ruled out most layer shifting "fixes." The pattern varies by location on the bed, is at random heights, but has two endpoints and then reverses back to the first layer origin. Very strange but specific.



  • @JamesM I will run a print tonight and try the M122 command. I too think it is mechanical, but I cant wrap my head around what it could be.



  • @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?



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



  • @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.IMG_0924[1].JPG

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



  • @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



  • Belt tension doesn't have to be checked while it's moving. Simply move the extruder carriage around manually- you'll feel any variations in tension.

    How about some photos or your BLV cube?

    Are you, by any chance, using steel core belts?



  • @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.
    IMG_0940[1].JPG
    IMG_0942[1].JPG
    IMG_0943[1].JPG
    IMG_0941[1].JPG
    IMG_0935[1].JPG



  • @spudaddict That first photo at the top of this thread looks like something is shifting by a mm or two. There has to be something loose. Can you detect any play at all in the bearing blocks on the linear guides? Are the pulley axles solidly mounted? Grab the heater block on the hot-end and try to wiggle it.

    Try pushing on the bed and see if you can make it wiggle.

    The frame has an open front in that design for some reason. Does it flex when you push the two top corners toward each other?



  • Did my eyes deceive me or do you have a 1.8deg stepper on X and a 0.9deg stepper on Y looking at your config.g and the steps/mm??? I'm guessing by the heatsinks on the stepper drivers that it is a clone duet board since the standard duets don't come with them. If that's the case I would put some active cooling on that board since the clones generally have only 1/2 the amount of copper in the pcb compared to the real duet boards.


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