Z banding and the wobblies

Discussion in 'Calibration, Help, and Troubleshooting' started by Digi2life, Jul 21, 2017.

  1. Digi2life

    Digi2life Member

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    I thought I had my Z banding cured after installing steppers with integrated leadscrews and anti-backlash nuts, but ran into this with a tall print: https://photos.app.goo.gl/Ba2Kwrr98WkhrnOM2

    Most of the pics show the 300mm tall tap handle that experienced some bad banding in the last 25% or so of the print. I had the part printing in upper left corner of the bed near the ooze wiper(to minimize travel distances). I think what I'm seeing is movement due mainly to the height of the part and its distance from the center of the bed. I added some weight on the bed opposite the part to fix the weight imbalance but that didn't seem to help. I think what I'm seeing is the slop/clearance in the linear bearings which allows for considerable angular play in the bed. It's easy enough to see by just pushing down on y=0 edge of the bed.

    This print was done with the Prometheus dual extrusion system and was about 32hr total with all the tool changes, so I wasn't happy to see these lines so far into the print.

    I printed a couple tall test prints immediately after the handle finished and they looked OK, so I guess the lesson learned was to center my prints. The part had a small base with brim, but the flex appeared to be mostly in the bed and not in the part itself. In the album above, you can see a grey colored build. That came out very nice and didn't show banding anywhere near what I saw on the tap handle. I also printed a 100mm diameter cylinder at 300mm height in vase mode and it printed beautifully.

    So... I was just curious what others' experiences were and hoping to understand the limitations of the BigBox design a bit better. I'm helping a local Makerspace build a CoreXY extrusion frame printer that will have 3 or 4 leadscrews driven by one stepper and hoping the extra leadscrews will help stabilize the bed better at the higher z positions. I realize we'll have to be quite accurate with the frame to avoid binding but it should be possible.
     
  2. Ephemeris

    Ephemeris Well-Known Member

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    First of all, your color changes look pretty sharp! :)

    If you run the bed down near the bottom, say Z=280, do the tops of the leadscrews start wobbling? If they do, I'm not going to be surprised.

    Contrary to what seems to be the conventional wisdom here, I expect Z-steppers with integral leadscrews to make banding worse not better. Assuming the leadscrew/motor shafts are dead straight (which is not likely) and you've done your tramming well, you are left with a major problem: The steppers are not likely to be vertically aligned.

    The 'lower support' panel the Z-steppers are mounted to is NOT a precision flat plate and the steppers are unlikely to be exactly vertical and are likewise not likely to be parallel to each other. This is aggravated by the laser cut supports between the base and the lower support hold them apart by 68.627 mm (according to Greg's DXF files) but the slots in the front, back, left, and right panels hold them 70 mm apart. This warps the lower support.

    This is not a theoretical point. I just walked over to my Bigbox and used a precision inclinometer to measure the tilt of the lower support where the Z-steppers are mounted. The front-to-back (pitch) tilt is different by about 0.2 degrees. The left-to-right tilt (roll) is different by 0.75 degrees. This is a huge error by 3D printing standards when you project it 300 mm. If my steppers rigidly drove the leadscrews, the leadscrew or the z-platform would have to deflect by about 3 mm :oops:

    Bottom line, I'm afraid the best solution is buy the straightest leadscrews you can find, and install good quality elastomeric couplers (I recommend Ruland ones. Their small ones fit without modifying anything)

    Sorry :(
     
    #2 Ephemeris, Jul 22, 2017
    Last edited: Jul 22, 2017
    mike01hu likes this.
  3. Old_Tafr

    Old_Tafr Well-Known Member

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    This is a slightly contentious area, please take my comments as simply adding to the mix.

    I suspect that there are even more factors at work "some" of which may be reduced by following the commissioning process.

    I believe that if the position of the bed is determined by the four vertical smooth rods, (rigidly held at top and bottom?) assuming these were vertical and exactly equally spaced from each other, and if the bed was level and flat and.... AND if the lead screws were free to move where they go through the bed and at the top a little in a horizontal plane so not exerting a sideways (x and y direction) effect on the bed, that the effect of the lower support not being level (that surface with the trap door above the Rumba) would be "small"

    i.e. the lead screws would be only determining the vertical movement, so a slight angle (assuming the rods were straight with no variation in the pitch) would only mean the distance moved in say one turn would be less. Imagine the rod at 45o, one turn would move the bed an exact distance vertically just less vertically than if the rod was vertical and this distance or a multiple of it would be exact if the pitch were constant. I don't think (but I have not done the sums) that a variation of say 1o would make much difference.

    Equally if the rod were offset in the coupling, (just above the motor) and the coupling was not vertical (both of which I think is the case without exaggerated care) then this offset would not change the distance moved vertically as this would depend mostly on the pitch of the lead screw being accurate and constant throughout its length.

    The fact that even with commissioning and careful assembly and adjustment that plenty of grease is needed on the smooth rods and lead screws to prevent loud graunching and groaning noises when the bed is first moved up and down, which seemed to be the case when most people built their BigBox, is testament that the design isn't perfect.

    I think allowing the top of the lead screw to "float" and also allowing it to flaot where it passes through the bed so the lead screws just provide an exact vertical movement and relying on the four vertical smooth rods to keep the bed level and square means that the effect of the lower support not being flat is "small"

    The fact that (from memory) the lead screw does not float and is just adjusted and fixed during commissioning, means that a none flat (i.e. not straight) lead screw is exerting sideways forces on the four vertical smooth rods (not good).
     
  4. gonzalo

    gonzalo Well-Known Member

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    Adding my 2 cents here, your part is thin, tall, and top heavy, which will exacerbate any mechanical issues and make print speed a big factor. If I had no choice but to print that way I would go VERY slow.
     
  5. Old_Tafr

    Old_Tafr Well-Known Member

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    Another thought.......... Let's say a lead screw is bent, taken to the extreme over one revolution (of the lead screw) the bed would (may?) move up different amounts for each half turn. The total distance would be the same, i.e. whatever the pitch of the thread of the screw was, but each half (assuming you start at the highest or lowest part) would move a different distance so for example it would move not 0.5 and 0.5 of the distance but 0.4 and 0.6.

    If the bend was the full length of the lead screw, then the effect would be lowest at the top and bottom of the print and at its max in the middle. Assuming a print the full height possible.

    PS if it's top heavy? maybe print it the other way up?
     

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