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Thread: Z axis play

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  1. #1
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    Z axis play

    Guys, I have an OpenBuild C-Beam machine that I have modified to give me a little extra depth and I have replaced the dewalt router with a VFD spindle router.

    I have some lag between the bed moving and the spindle cut catching up with the movement. Lag is not noticeable on the X axis but on the Y axis there can be significant lag depending on cut depth.

    The origin of the Lag is on the Z axis where there is flexing of the Cross beam as the cut is being made.
    Is this normal to have some flexing and if it is can it be quantified?

    Not sure I explained this correctly.



    TIA

  2. #2
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    Re: Z axis play

    It sounds like you increased the leverage against your gantry beam when you increased the height, This design https://openbuildspartstore.com/open...-beam-machine/ was minimal to start with, barely rigid enough to cut soft materials. No, it's not normal for gantry beams to twist visibly, but it does happen, especially when design parameters are exceeded. You can try replacing it with a stouter piece of extrusion, but that might just make another part of that machine the weakest link.
    Andrew Werby
    Website

  3. #3
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    Re: Z axis play

    Quote Originally Posted by awerby View Post
    It sounds like you increased the leverage against your gantry beam when you increased the height, This design https://openbuildspartstore.com/open...-beam-machine/ was minimal to start with, barely rigid enough to cut soft materials. No, it's not normal for gantry beams to twist visibly, but it does happen, especially when design parameters are exceeded. You can try replacing it with a stouter piece of extrusion, but that might just make another part of that machine the weakest link.
    Thought that about the gantry height as well but test showed that the twisting of the beam is there at what ever height the z axis is. I replaced the spindle drive today to see if the extra weight was an issue but it made no difference at all to the amount of flexibility the beam has. I guess it is a design issue. I suppose a work around would be to increase the number of passes over a design and limit depth of cut to something like 0.5mm per cut.

    Think I’ll also look for a different machine, not much choice for the hobbyist though all seem to be cheap machines from Amazon.

    Thanks for the input

  4. #4
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    Re: Z axis play

    Hi Lodds - you can fill the extrusion with expanding concrete. Not normal concrete. Concrete is about the same stiffness as aluminium so the fill is a good match and it will considerably stiffen the gantry in torsion which is your current issue. Your Z axis tool plate is not stiff enough as well. if you wobble the spindle with your hand you will see how the machine flexes and where you need to stiffen it. Peter

  5. #5
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    Re: Z axis play

    I'm not sure how the term "lag' is being used in in the top post.
    To measure deflection a dial indicator would be handy to narrow down the source. My guess is the v-wheels with their stand-offs are responsible for most of the flex. Ramp entries and small depth of cut will help.
    Click image for larger version. 

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    Anyone who says "It only goes together one way" has no imagination.

  6. #6
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    Re: Z axis play

    Quote Originally Posted by peteeng View Post
    Hi Lodds - you can fill the extrusion with expanding concrete. Not normal concrete. Concrete is about the same stiffness as aluminium so the fill is a good match and it will considerably stiffen the gantry in torsion which is your current issue. Your Z axis tool plate is not stiff enough as well. if you wobble the spindle with your hand you will see how the machine flexes and where you need to stiffen it. Peter
    Peter, What a great idea, not even heard of expanding concrete before. I'll check it out.

    I'm also considering replacing the crossbeam with 222mm plywood but reflecting on how to attach it to the current structure not sure though how the X axis would function, still looking at all options.

    Tony

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