747 auto leveling seems to be off

I have a 747 that seemed to be printing fine and now it seems to have some sort of problem. It is a printer in a school and anything is possible - kid bumping into it - jogging it into a wall etc.

The build plate had a pretty bad tilt along the X axis sloping from high onthe left to low on the right. I thought that the slope was so bad that somehow autolevel could not compensate for it. I shimmed the carrage on the right side and it seems to help with the slope but the auto level is still too high onthe right.

When I jog the printer from left to right I get a distance between the nozzle and build plate from zero all the way to about .4mm. This is enough to where prints that are large enough don’t stick on the right side because the nozzle is too high off the bed. I can see the z motors acutally moving as I jog left to right so I know that the printer thinks that it correctly compensating using autolevel but its actually raising the nozzle too high.

When i print a bed level test it does not stick on the right and is too low on the left.

Has anyone experienced anything similar?
Does anyone know how to turn auto level off so I can test this?
Does anyone what kind of auto bed leveling is standard in the 747 marlin build?

Is the gantry just out of alignment? Sending a G34 “align Z” command as part of your startup GCODE may fix it. My Mini 2 loses Z alignment whenever the power goes off, and it does the G34 so fast that I just include it in my startup gcode in case one side’s motor droops at all while inactive. I do not have a 747 to test if that command is active, but it works well with the Mini 2.

If the command doesn’t do anything, you may have to manually adjust gantry height (WITH MOTORS OFF!) to get it close to parallel to the bed.

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I will try this in added in the header gcode. I reset the bltouch and had it run a bed auto level through the GLCD - one print is coming out OK.

If the gantry were slightly out of align - wouldnt the auto level just pick it up as a bed level being out of alighn and still account for it though?

It depends how out of alignment it is. There’s simple co-planar alignment, which is correctable with re-aligning the gantry to the bed, but if the frame isn’t square, due to being on an unstable surface, then movement of the bed forward/back can change the distance between the gantry and bed as the weight of the bed shifts.

Over the last 6 months i’ve spent with our taz pro here at work, I’ve experienced this issue many times & unfortunately this series of printer inherently just does not have a workable bed-leveling solution either in the firmware or software. Hardware fixes work but take very long & require printed or purchased additional parts such as replacement shims.

Here’s an easy quick fix you can do anytime you notice this issue has occurred, see attached image.

Sorry I should have added to this picture, befoer you even begin with step #1 press “menu” on the LCD screen then press the “motors off” button THEN perform step 1

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Thanks.

In step one do you mean " z " axis up? Are your thumbs on the belt?

Also what do you suppose this does?

I believe I have found the problem:

I believe I have fixed the problem.
Some of the default movements to max z and -x direction caused the filament guide tube to wrap around the left side belt tensioner. Then when the printer would would print - the aut level would be accurate because the BL touch is on the back of the extruder and would not get pulled, but when the printer went to print the tension of the tube would increase only as it approached the +x axis lifting the nozzle up slightly. There is a strange amount of “wiggle” in the toolhead - it seems very easy to raise up and down compared to the TAZ5, and 6. I suppose this is because it is on V wheels on a single piece of extrusion. You can see in the two picture

s how one is wrapped around the belt tensioner. I would often unwrap it but some part of the homing/ startup process would rewrap it - I have found that a zip tie will keep it from getting wrapped again.

Thumbs are on the metal part of the axis, all this does for us at least is guarantee that the axis is snug up as high as it will go and is flush with the predetermined end point that it has. While the motors are off, if you can snug the axis up on both ends & while holding it snug hit the autohome button

It’s hit and miss for our printer here at work whether or not homing the Z will square up the x axis to the rest of the frame. The easy workaround I came up with was to simply hit Home Z, then after the axis finishes homing - press the “Motors Off” button on the LCD screen which will unlock the X axis to travel up and down freely. Homing initially just brings it up top close enough to do the next step, which is; once the motors are off, grab the X axis with both hands - then while holding the x axis from below with just your thumbs on each hand, grab the top of the printer frame directly above & behind the X axis rods with your other four fingers, then squeeze both hands slowly to manually position the X axis as high as it will go up which, once it hits the end stops, should bring it perfectly square and parallel to the upper horizontal frame extrusions. While squeezing your hands & holding the X axis snug against the top most position it will go to, touching the endstops, slide your right hand over to the left slowly while maintaining tension & keeping your grip such that neither side of the X axis drops while you slowly move your right hand to the MIDDLE of the horizontal x axis top rod. Once you have your thumb on the upper X axis rod & your other four fingers on that hand holding the top horizontal frame extrusion, contuing to squeeze the whole time so that you maintain complete tension on the x axis & it stays completely snug up against the upper Z axis endstops/axis limits, take your left hand off -if you keep your grip with the right hand on the middle of the x axis, neither side will drop. Once your left hand is free, press the “home Z” button on the LCD screen with it & then as soon as you feel the pressure of the X axis falling due to gravity released, release your grip & the motors will take over & finish the homing but this time starting & anding the Z axis home from a perfectly parallel x axis position.

Sometimes we have to do this, sometimes we don’t - just depends on if the x axis sags on one side or the other while powered off. We have the incredibly heavy dual extruder tool head on our x axis so wherever it stops when the power is off has a tendency to slowly droop on that side of the x axis downwards with gravity. This technique mitigates that deficiency entirely.

The cantilever load from such a heavy dual extruder on 10/12mm round rods however, is a whole nother nightmare & has prompted us to buy the bearings that this printer should have came with from the factory. We got some knockoff hiwin square profile rails, 500mm & we are converting our printer at work’s X axis to the square rails setup in order to mitigate the excessive droop caused by having such a heavy toolhead so far away from the center point of the rotation of the X axis, which only gets worse as the plastic bushings wear as the printer is used. Tightening up the plastic bushing holders worked at first until they were completely tightened all the way & then it didn’t do anything beyond that. We’re looking to get away from the poor initial equipment choice lulzbot made when they used round rod & plastic bushings for all three axes on a $5000 printer. It’s unfortunate but we have it already so the best we can do is just upgrade it to what is should have been from the beginning, despite the high initial investment & the subsequent upgrade costs, in the end each upgrade we’ve done has improved print quality significantly & converting our X axis round rods to square profile rail should be the biggest improvement yet, by a large margin.

I will make a new thread shortly sharing the progress we have made & include the files to convert the x axis on other lulzbot taz pro printers if anyone wants to do so. The parts are not expensive thankfully, and the biggest investment so far has been just the time it’s taken to modify the existing x axis end holder pieces to accept square aluminum extrusion & hiwin square rails (mgn12h)