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Wouldn't it be better to mount lead screws with the motor on top and the bottom end unconstrained? Compared to the conventional way (motor on the bottom) the lead screw would be under constant tension rather than compression and any misalignment would always be "pulled straight" and in this way minimize z-wobble.

Here is a image for clarification:

Illustration of wobble in both top and bottom mounted stepper motors with lead screws

With the motor on top the load of the bed will help to move the lead screw in line with the mount. In the opposite case the load tends to further increase any misalignment.

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  • $\begingroup$ Are you thinking of buckling of the screws? I don't think the mass of the gantry will be enough to cause the screws to buckle. $\endgroup$
    – 0scar ♦
    Commented Sep 16, 2021 at 21:25
  • $\begingroup$ Is this from the perspective that the lead screw can flex more if the bed connection point is farther from the rigidly fixed motor side and that will allow for more misalignment? If so my answer may not be really valid. $\endgroup$
    – Kezat
    Commented Sep 17, 2021 at 1:47
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    $\begingroup$ See answer below Furthermore, the whole X-gantry is usually constraint in movement by the use of linear guide rods or rails. Basically the movement is limited in X-Y, only the lead screw imperfections can push it sideways, not the buckling of the screw. $\endgroup$
    – 0scar ♦
    Commented Sep 17, 2021 at 17:07
  • $\begingroup$ In CoreXY printers the whole bed rides on the Z screws. In their case the main constraint is space, the corner areas occupied by the X,Y gantries and their motors, no room for extra motors for the table. But in that case they are constrained at the gantry end, no wobble there. Also, you want least wobble nearest to Z=0, as it will impact small, most precise prints. You can usually afford less precision for the top of largest prints occupying the entire build volume. $\endgroup$
    – SF.
    Commented Nov 20, 2023 at 12:04

2 Answers 2

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No, but the answer could depends on how you set up the lead screws, what kind of lead screws are used and how they connect to the stepper. Also it is important how you transfer the linear Z motion into the X gantry.

Note that regularly used TR08 (Trapezoid, 8 mm) lead screws are stiff enough to not flex under the limited load of an X gantry. So, putting them upright or down should not make a difference. Furthermore, the whole X-gantry is usually constraint in movement by the use of linear guide rods or rails.

Note that lead screws are not perfect, certainly the cheaper versions (ball screws are better in that respect). The imperfections can cause the lead screw nut to move in the X-Y direction. If the nut is firmly attached to the X-gantry, this movement can be seen in the print like e.g. waves in Z-direction we call Z-wobble. There are designs like these or these to mitigate that problem. Leaving one end free (so not constraining both ends) of the lead screw is also a mitigation as with constraining both sides, you over-constrain the screw. Note that the spring couplers you see applied frequently are great for misalignment of stepper motor shaft and lead screw, but are detrimental for the Z-axis. When using such couplers for shaft misalignment, you should constrain at least motion in Z-direction.

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The motor is mounted in a fixed position no matter if it's on top or bottom. You can imagine the lead screw as a rod hanging down and supporting the bed in the Z direction only, because all of the XY rigidity comes from the Liner rails the bed is attached to it works just as well if the stiff rod is under compression instead of hanging from the rod.

Any forces that might "pull it straight" are the same forces that can cause Z wobble so you would not want to even try to use the misalignment to straighten a bent lead screw.

The only thing this that really changes with this orientation is if the lead screw is under tension or compression. Even a relatively thin lead screw can easy counter the forces of gravity and the forces is the same in both directions. Its easy and cheap to make a smooth rod or rail straight but not so easy to make a threaded one straight.

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  • $\begingroup$ thanks, I understand. My image above implies that the nut/gantry assembly can slide left to right which in most cases is not true - so, as you already mentioned this will result in the same forces. though if you've had a kinematic coupling this might be a different story $\endgroup$
    – Nagamoto
    Commented Sep 17, 2021 at 10:45

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