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Any thoughts on rail arrangement?

Discussion in 'Concepts and Ideas' started by Captain Barnacles, Oct 12, 2018.

  1. Captain Barnacles

    Builder

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    Hi All,

    I'll write a proper introduction in the appropriate forum but the brief back-story is that I have been in the 'design' phase of my CNC router build for about two years, slow progress due to time constraints etc. However I now find myself with a few weeks on my hands and I thought I would get properly stuck into the project. Here goes...

    Attached is a sketch of an idea that I had for the X axis gantry beam. Apologies if some of my mechanical terminology is incorrect, it isn't my forte; I was thinking about the forces acting on the X axis carriage once the Z axis assembly is mounted to it. I haven't worked it out, but the mass of the spindle and the Z assembly will be quite heavy and hanging it off the side of the gantry will produce a considerable rotational force (torque?) around some point in the cross section of the gantry profile (moment??).

    Screen Shot 2018-10-12 at 10.13.17.png

    I was wondering whether there is anything to be gained by using the pictured arrangement over the more conventional arrangement of running both linear rails along the same side of the gantry beam. I have also seen designs that run the rails along the top and bottom of the beam.

    It seems to me that using this arrangement the linear rail carriages will be similarly loaded rather than one being pushed inwards whilst the other is pulled outwards.

    Screen Shot 2018-10-12 at 11.20.51.png

    This is where my thought process runs out of steam. Firstly, am I over thinking this? Or, do you think that there anything to be gained from this idea?

    Paul.
     
  2. Rick 2.0

    Rick 2.0 OpenBuilds Team
    Staff Member Moderator Resident Builder Builder

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    Torsion on the beam is a function of how far out the mass hangs off the front of the beam so moving one rail to the back of the beam really has no impact. The only way to really make an impact would be moving both rails to the back side which would allow you to move the mass on the front side closer to the centroid of the beam.

    Basic theory is shown below. Simply, torsion = mass x eccentricity and total torsion is the sum of all masses times their respective eccentricities.

    eccentricity.jpg
    The best solution for resolving torsion due to a heavy spindle is using dual rails. Keeping the X-axis rails as short as possible also helps reduce flexure.
     
    crispin, MaryD and GrayUK like this.
  3. Captain Barnacles

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    Thanks for clarifying that Rick. Now that you have explained it, it seems very logical.

    Another question; If using the Hiwin type linear guides do they have any preference for being loaded in a certain direction? I have looked at diagrams of the guides and it looks as though they should be OK in any orientation but has anyone found that they perform better (or worse) in any particular arrangement?
     

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