thoughts on simplifying the Z-axis

Discussion in 'BigBox General Chat' started by R Design, Jun 26, 2016.

  1. R Design

    R Design Well-Known Member

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    Could the Z-axis mechanism be further simplified?

    What if we removed two of the four Z-rods so that the remaining two were diagonally opposite each other, would they provide all the stability that the bed requires? Preventing movements in the XY plane. And preventing the bed from tilting downwards?

    If there is a weakness in this configuration, I could imagine it might be that if you pushed down on an unsupported corner it might sink a little.

    So then, what if we moved the two lead screws and Z axis motors to those unsupported corners (such that they too are diagonally opposite each other).....

    What do you think?
     
  2. mike01hu

    mike01hu Well-Known Member

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    The weakness of the current design is the corners, which means maintaining the bed level relies totally on the length of the buses, their fit to the rods and gravity acting on the lead-screw nuts. The ideal configuration would have two bushes on each rod separated by 50 mm or more or four lead screws belt driven from the existing motors. Your thoughts are good if the bushes were doubled up and separated.
     
    #2 mike01hu, Jun 26, 2016
    Last edited: Jun 27, 2016
  3. R Design

    R Design Well-Known Member

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    Good points Mike.

    I thought there was a problem with a 4 lead screw / no rod approach that it's impossible to set the lead screws up in such a way that they don't wiggle?
     
  4. Old_Tafr

    Old_Tafr Well-Known Member

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    The (agreed) weakness of one bearing is partially negated by fixing the smooth rods at both ends (and adjusting then tightening the upper and lower fixings) and the size of the bed. The smaller the bed the more significant the play n the bearings becomes in tilting the bed.

    Obviously the quality of the bearing comes into play.

    I'm sure there are lots of other designs and approaches that would include a larger (with more stiction) sets of bearings, but this design with floating lead screws, seems a reasonable compromise.

    Having only two supports would lead to lots of problems. The ideal (although impractical on this printer would be three supports. A bit like a three legged stool is always stable, whereas a four legged table has to have legs of exactly equal length. Try sitting on a two legged chair.........it's not very stable.
     
  5. mike01hu

    mike01hu Well-Known Member

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    The lead-screws need bearings at each end and I was surprised that the original design does not have them. My lead-screws are straight but the motor coupling is poor and wobbles, which puts strain on the motor bearings. Also, because the lead-screw, and hence the bed, is only supported by this coupling and the motor bearings, any error in concentricity of the coupling is transferred to the lead-screw and will try to move the bed laterally. Of course, the lead-screw nuts provide some support and the corner rods should give lateral stability and it appears that most people are getting results that support this. It's easy in hindsight to redesign something to do a better job but I don't have the resources to effect those changes:(.
     
  6. Old_Tafr

    Old_Tafr Well-Known Member

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    I had posted a longer (and boring) note on the effects of eccentricity of the motor itself, the holes in the coupling and the possible offset caused by tightening a grub screw on one side only. In the worst case if all these effects were in the same direction then the wobble at the top of the rod, plus tightening the motor mount at one side only as it is done with the bed at the bottom of its travel would cause the threaded rod to wobble a lot at the top.

    Having the top of the threaded rod allowed to wobble a little, not being in a bearing is acceptable.

    The answer, other than meticulously accurately made motors and couplings and threaded rod, is (if there is a lot of wobble at the top) to rotate the threaded rod in the coupling 90o at a time and see the result, first having done the same on the coupling wrt the motor shaft. This could get very tedious, letting the top of the rod float (plus plenty of grease) is probably enough.
     

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