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Sizing a custom lead screw and 4 post motor mount

Discussion in 'General Talk' started by JP Scott, May 25, 2020.

  1. JP Scott

    JP Scott New
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    Hello All,
    Hope you are well and safe.
    I'm converting a belt drive to lead screw and need some info.
    [​IMG]
    My desire is to build something like this. How much of the 8mm lead screw is to be inserted onto the 8mmx1/4in coupler? as an example. let's say the C-Beam is 1000mm. the end caps at 11mm each.
    this gives 1022mm lead screw + the attach length for the lead screw = ? total length of lead screw?
    Please let me know, thanks so much.
    The other question, is there an end plate that will work with a NEMA23 High Torque motor which allows for a 4 post mounting instead of the 2 post shown in the pics I've seen? I'm hoping not to have to build such a plate myself.
    Cheers,
    Jim
     
  2. Alex Chambers

    Alex Chambers Master
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    If it helps, on a workbee from Ooznest (old version) 1000 mm C-beam on Y axis, + 2 x 6mm end plates + flexible coupler, leadscrew = 1040 mm.
    Alex.
     
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  3. JP Scott

    JP Scott New
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    Hi Alex, thanks for the info. Is the flex couple 25 mm long? There should be some space between the lead screw and the motor shaft when inserted in the 25 mm flex couple used in the OB design. The OB motor mount uses 40 mm spacers. This required gap between the lead screw end and the motor shaft end should then allow me to derive the amount of lead screw that should come past the 11 mm end plate that mounts to the C-Beam. Hope someone has the answer to this. Thanks, Jim
    IMG_5180.JPG
     
  4. Giarc

    Giarc Master
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    Are you doing a custom size? Openbuilds lead screws are sized for all of these components if buying a standard sized beam. I believe the screws are sized 40mm longer than the beam. I do not know the exact dimension, but I just split the difference. The couplers are 25mm long. 8mm Metric Acme Lead Screw
     
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  5. JP Scott

    JP Scott New
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    Hi Giarc, thanks for your reply. It seems I'm not communicating well on this issue. I need to know the length of the lead screw that should come out of the end plate where the motor is mounted. The spacer on the OB model shown above is 40 mm. It seems to me that the flex coupler needs some gap inside in order for it to flex. If the lead screw is 20 mm and the motor shaft is 20 mm, then there is no space inside for the flex couple to do it's job, right?
    Jim
     
  6. Rob Taylor

    Rob Taylor Veteran
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    The lead screw and the motor shaft are stationary in translation with respect to each other because the screw is also fixed in place via its bearing mounts. The flex coupler allows for axial misalignments (imperfect motor standoffs) between the motor shaft and the screw. Whatever these are are baked in at assembly, no further flexion necessary, so no gap necessary. Think of it like a less-complex universal joint.

    The screw can't be too long for the motor mountings because it's being contained in 8mm ID bearings- it can just slide forward and re-lock a little further in. The screw only needs to be an approximate size- long enough to sit inside both bearings and reach the motor coupler, but any more is unnecessary. It's not, however, problematic- there'll just be more screw sticking out of the non-motor-end bearing.
     
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  7. JP Scott

    JP Scott New
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    Hi Rob,
    Thanks for the comments. It would seem then, that the Z axis beam is 1066.8mm (42 in) plus two end caps (11 mm each) plus the flex couple amount say 20 mm yields total length of lead screw of 1108.8 mm. I'd like it to be flush on the end opposite of the motor, so if it is too long I supposed it could be cut off? Do you know how hard it is to cut?
    Jim
     
  8. Rob Taylor

    Rob Taylor Veteran
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    screw length = (beam length) + 2*(bearing plate thickness) + (standoff length) - (motor boss depth) - (motor shaft length) +/- 1mm

    Cuts with an angle grinder, I may have cut it with a bandsaw too, I forget now, and the video I did of my lead screw axis doesn't show the work on the screw itself. You're not gonna get it perfect, but it should be possible to get it quite close.

    Standoffs could be adjusted with washers/shims too, if necessary, to pull some length out of the rear bearing. Of course you also only need two functional axes to cut the standoffs down, too, so that's also an option.
     
  9. Mark Carew

    Mark Carew OpenBuilds Team
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    Hello Jim,
    Rob and Giarc and Alex are correct, there is 40mm added to the leads beyond the length of the V-Slot and you can even cut the lead with a hacksaw if you do not have a band-saw handy. Rob has the right idea for adjustments the motor can be moved where you like adding or removing shims and spacers.
    With the thicker C-Beam end mount plates you will have approx 16mm sticking out to clamp the coupling to.
    Did a quick measure from the models in Resources
    Hope this helps
    leadscrew_sizing.PNG
     
  10. JP Scott

    JP Scott New
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    Hello Mark,
    Thanks so much! Okay if my math holds the C-Beam is 42 in or 1066.8mm. Each end plate is 11 mm and the lead screw amount that should be available for the flex couple is 16 mm. So the total lead screw length I need to order is:
    1066.8 mm
    22.0 mm
    16.0 mm
    1104.8 mm So, I'll order 1105 mm.
    I also need advice on all the connecting pieces to mount the end plates to the C-Beam, the lead screw bearings, shims, collars, etc.
    Thanks,
    Jim Scott
     
  11. Mark Carew

    Mark Carew OpenBuilds Team
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    Hello Scott,
    Actually the C-Beam End Mount Plate is 12mm that was a typo that has been corrected.
    The C-Beam lenght is 1000mm
    The Lead screw is 1040mm
    The amount the lead sticks out is 16mm

    Please review the parts list for the C-Beam XL Actuator Build to get an idea of the parts you would like to order HERE on the Build Page. (Part list is a tab at the top)
    You will be able build the parts into an order from the links leaving out the parts you plan to substitute.
    I hope this helps and wish you all the best with your Build.
     
  12. Alex Chambers

    Alex Chambers Master
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    Just to add to what others have said, at the end furthest from the motor the important thing is that the leadscrew goes completely through the bearing - if it's a few mm short and doesn't go completely through the plate that won't matter.
    Alex.
     
  13. jeffmorris

    jeffmorris Well-Known
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    I think that lead screws should be longer so that bearings and collars are outside the end plates instead of inside so that there will be no screw whipping.
     

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