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RE: [taigtools] Weight lifted by Taig Mill Z axis?

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  • Bertho Boman
    Hi Ken, First the easy one: Moving downwards, the mass has increased to twice so max acceleration will be only half which would significantly slow down the
    Message 1 of 13 , May 5 1:44 AM
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      Hi Ken,

      First the easy one:

      Moving downwards, the mass has increased to twice so max acceleration will
      be only half which would significantly slow down the pecking since that is a
      lot of acceleration back and forth.



      Moving up:

      I agree that we probably do not exceed 1G but that makes it an interesting
      mental exercise:

      The connecting cable will always be in tension since there is gravity
      preload on it by that definition. That preload exceeds the acceleration
      forces so we can consider the two masses to be rigidly connected and it
      ought to slow down the up movement of acceleration by the same amount.



      Another way to think about it that the gravity preload exists on both sides
      so the forces cancel out.



      Resonance:

      Since we are not talking about very fast movements, the spring resonance is
      a lot higher than our mills movements so I do not expect a problem with
      that.



      Of course an even better solution is to use a gas spring as Tony suggests.



      An interesting thought:

      If up and down acceleration could be set independently, a standard
      configuration could be made to go down really fast with the help of gravity.
      Of course going up is no improvement.

      Bertho





      From: taigtools@yahoogroups.com [mailto:taigtools@yahoogroups.com] On Behalf
      Of Ken Cline
      Sent: Monday, May 05, 2008 01:54
      To: taigtools@yahoogroups.com
      Subject: Re: [taigtools] Weight lifted by Taig Mill Z axis?




      On 4 May 2008, at 10:35 PM, Bertho Boman wrote:

      > A weight and pulley system will balance out the weight of the Z-axis
      > head
      > which is commonly done. There is a drawback though: The mass of the
      > Z-axis
      > doubles and the acceleration will be less for the same available motor
      > torque.

      Is this a real drawback for a tabletop mill? The added mass will only
      affect downward acceleration (you can't push a mass at the far end of
      a cable) and then only for values greater than 1g (below that rate
      gravity provides the force to move the motor and spindle downwards).
      Is there a need for acceleration faster than 1g on the Z axis of a
      Taig mill? For a graphic demonstration, drop something (I used my
      pencil) a few inches to see how fast 1g is. If my calculations are
      correct, the time it takes to go from 0 to 100 in/min at 1g is less
      than 5 milliseconds. I don't see this affecting the sort of up and
      down operations I do (peck drilling is probably the most demanding).

      > There is a simple alternative that avoids this drawback: A long and
      > strong
      > spring connected to the ceiling or to some bracket above the head
      > that just
      > balance out the weight of the head. In other words, the head hangs
      > from the
      > spring. The added mass is insignificant.

      What prevents a spring like this from developing large vibrations?

      [Non-text portions of this message have been removed]





      [Non-text portions of this message have been removed]
    • Ken Cline
      ... That misses the fact that adding the counterweight reduces the force needed for upward movement by an amount equal to mg. Thus, while mass has indeed
      Message 2 of 13 , May 5 7:50 AM
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        On 5 May 2008, at 2:44 AM, Bertho Boman wrote:

        > First the easy one:
        >
        > Moving downwards, the mass has increased to twice so max
        > acceleration will
        > be only half which would significantly slow down the pecking since
        > that is a
        > lot of acceleration back and forth.

        That misses the fact that adding the counterweight reduces the force
        needed for upward movement by an amount equal to mg. Thus, while mass
        has indeed increased, the force required for upward movement with has
        not (assuming acceleration of no more than 1g). If you could
        accelerate at 1g before adding the counterweight, you should still be
        able to.

        I did another quick calculation. It turns out that the axis will
        travel .0036" accelerating from 0 to 100 in/min at 1g. That works out
        to .005 sec to ramp up to speed (in a distance of .005"), or .02
        seconds per peck. A cycle with 50 pecks takes a while on my mill, and
        only 1 second of that would be acceleration time if I had really fast
        rapids. In practice, I run the Z axis at 15 in/min, resulting in a
        negligible .0023 seconds to reach maximum speed (0.1 second of accel
        for the entire 50 peck cycle).

        By the way, 1g is the value I actually use for acceleration.



        [Non-text portions of this message have been removed]
      • Don Rogers
        Has anyone considered that the Z lead screw nut doesn t have any backlash take up and I was under the impression that the weight on the Z effectively
        Message 3 of 13 , May 5 7:11 PM
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          Has anyone considered that the Z lead screw nut doesn't have any backlash
          take up and I was under the impression that the weight on the Z effectively
          eliminated any backlash. Right/Wrong??? If it is right, then wouldn't the
          balanced head result in some backlash?



          Don



          [Non-text portions of this message have been removed]
        • Monte
          ... backlash ... effectively ... wouldn t the ... Yes, you are correct. The trade offs as far as I know are if you counterweight the Z axis you should be able
          Message 4 of 13 , May 6 12:18 AM
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            >
            > Has anyone considered that the Z lead screw nut doesn't have any
            backlash
            > take up and I was under the impression that the weight on the Z
            effectively
            > eliminated any backlash. Right/Wrong??? If it is right, then
            wouldn't the
            > balanced head result in some backlash?
            >
            >
            >
            > Don

            Yes, you are correct. The trade offs as far as I know are if you
            counterweight the Z axis you should be able to get some more speed, if
            that's what you want. Doing that with the "stock" lead screw nut will
            most likely introduce backlash. To remove some of the backlash you
            can install an adjustable lead screw nut on the Z axis. I believe
            it's the same nut as the CNC Y axis. I bought one from Nick Carter,
            haven't yet installed it. I haven't done any counterbalance work on
            the Z axis, hadn't thought about doing any.
          • eferg2001
            The 3 phase AC, high frequency, high speed replacement spindle assembly I m considering weighs 13 pounds, so counter-weights and springs are not needed.
            Message 5 of 13 , May 6 5:46 PM
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              The 3 phase AC, high frequency, high speed replacement spindle
              assembly I'm considering weighs 13 pounds, so counter-weights and
              springs are not needed.

              http://www.damencnc.com/damencnc.php?dir=cpt&PHPSESSID=fb4caa87a14b409e34432e7bce0fe697&langId=EN&idType=9&PHPSESSID=fb4caa87a14b409e34432e7bce0fe697


              Thanks for the replies,
              Ed
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