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problem with keeping the Z zero

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  • Greg McFadden
    Guys, I am having trouble keeping Z zero d. I do a part, for example, where the depth in z will cut to 0.5 deep, then I go do something else and come back and
    Message 1 of 7 , Jul 1, 2006
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      Guys, I am having trouble keeping Z zero'd. I do a part, for example,
      where the depth in z will cut to 0.5" deep, then I go do something
      else and come back and do another cut to 0.5" deep and sometimes the
      second cut will be deeper than the first (3-5 mils deeper). I am not
      sure where to start trying to find what is going wrong. I have no
      problems in X or Y doing it. I don't know if it is backlash related
      or missing steps (although I am only moving at ~10ipm in z with the
      269oz xylotex steppers). I am running mach 3 as well, set at 35kHz.
      I will have to try to duplicate it in shallow cuts and see if I can
      readily cause the problem. It seems to be worse the more vertical
      motions there are between cuts which leads me to backlash but I can
      not figure out how to ajust z's backlash.

      Any ideas folks?

      thanks
      greg
    • Doug Stout
      What sort of stepper coupler are you using? Could it be slipping? The most common cause of zero shifting on any vertical CNC, large or small, is slippage of
      Message 2 of 7 , Jul 1, 2006
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        What sort of stepper coupler are you using? Could it be slipping? The most
        common cause of zero shifting on any vertical CNC, large or small, is
        slippage of the coupler.

        On 7/1/06, Greg McFadden <greg.mcfadden@...> wrote:
        >
        > Guys, I am having trouble keeping Z zero'd. I do a part, for example,
        > where the depth in z will cut to 0.5" deep, then I go do something
        > else and come back and do another cut to 0.5" deep and sometimes the
        > second cut will be deeper than the first (3-5 mils deeper). I am not
        > sure where to start trying to find what is going wrong. I have no
        > problems in X or Y doing it. I don't know if it is backlash related
        > or missing steps (although I am only moving at ~10ipm in z with the
        > 269oz xylotex steppers). I am running mach 3 as well, set at 35kHz.
        > I will have to try to duplicate it in shallow cuts and see if I can
        > readily cause the problem. It seems to be worse the more vertical
        > motions there are between cuts which leads me to backlash but I can
        > not figure out how to ajust z's backlash.
        >
        > Any ideas folks?
        >
        > thanks
        > greg
        >
        >


        [Non-text portions of this message have been removed]
      • Greg McFadden
        the coupler is the standard taig pins. -Greg
        Message 3 of 7 , Jul 1, 2006
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          the coupler is the standard taig pins.

          -Greg

          On 7/1/06, Doug Stout <kingshouse41225@...> wrote:
          > What sort of stepper coupler are you using? Could it be slipping? The most
          > common cause of zero shifting on any vertical CNC, large or small, is
          > slippage of the coupler.
          >
          > On 7/1/06, Greg McFadden <greg.mcfadden@...> wrote:
          > >
          > > Guys, I am having trouble keeping Z zero'd. I do a part, for example,
          > > where the depth in z will cut to 0.5" deep, then I go do something
          > > else and come back and do another cut to 0.5" deep and sometimes the
          > > second cut will be deeper than the first (3-5 mils deeper). I am not
          > > sure where to start trying to find what is going wrong. I have no
          > > problems in X or Y doing it. I don't know if it is backlash related
          > > or missing steps (although I am only moving at ~10ipm in z with the
          > > 269oz xylotex steppers). I am running mach 3 as well, set at 35kHz.
          > > I will have to try to duplicate it in shallow cuts and see if I can
          > > readily cause the problem. It seems to be worse the more vertical
          > > motions there are between cuts which leads me to backlash but I can
          > > not figure out how to ajust z's backlash.
          > >
          > > Any ideas folks?
          > >
          > > thanks
          > > greg
          > >
          > >
          >
          >
          > [Non-text portions of this message have been removed]
          >
          >
          >
          >
          > To Post a message, send it to: taigtools@...
          >
          > To Unsubscribe, send a blank message to: taigtools-unsubscribe@...
          >
          >
          >
          > Let the chips fly!
          >
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          >
          >
          >
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        • Doug Stout
          I would check the backlash, but usually a drifting zero is a mechanical problem. It could well be that the stepper is attemting to make upward steps that just
          Message 4 of 7 , Jul 1, 2006
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            I would check the backlash, but usually a drifting zero is a mechanical
            problem. It could well be that the stepper is attemting to make upward
            steps that just simply don't happen, and the controller has no way of
            knowing that. What size is your z axis stepper motor?

            It would be interesting to fashion a counterweight and see if the problem
            goes away. If your Z stepper is small, or the Z gibs are on the tight side,
            or if you are not lubricating enough, the upward load may exceed the force
            of your stepper, and a counterweight will dramatically help this.

            I had a problem on a lathe where the saddle was binding when moving away
            from the chuck, so the zero would drift towards the chuck. The controller
            was sending steps that simply weren't happening, and the controller had no
            way of knowing that.

            Since you indicate that the error is in one direction, and it is opposite
            the direction that would place the greatest load on the stepper, I think you
            are correct that the stepper is "missing steps", but it is likely doing so
            mechanically/physically rather than logically.


            On 7/1/06, Greg McFadden <greg.mcfadden@...> wrote:
            >
            > the coupler is the standard taig pins.
            >
            > -Greg
            >
            >
            > On 7/1/06, Doug Stout <kingshouse41225@...<kingshouse41225%40gmail.com>>
            > wrote:
            > > What sort of stepper coupler are you using? Could it be slipping? The
            > most
            > > common cause of zero shifting on any vertical CNC, large or small, is
            > > slippage of the coupler.
            > >
            > > On 7/1/06, Greg McFadden <greg.mcfadden@...<greg.mcfadden%40gmail.com>>
            > wrote:
            > > >
            > > > Guys, I am having trouble keeping Z zero'd. I do a part, for example,
            > > > where the depth in z will cut to 0.5" deep, then I go do something
            > > > else and come back and do another cut to 0.5" deep and sometimes the
            > > > second cut will be deeper than the first (3-5 mils deeper). I am not
            > > > sure where to start trying to find what is going wrong. I have no
            > > > problems in X or Y doing it. I don't know if it is backlash related
            > > > or missing steps (although I am only moving at ~10ipm in z with the
            > > > 269oz xylotex steppers). I am running mach 3 as well, set at 35kHz.
            > > > I will have to try to duplicate it in shallow cuts and see if I can
            > > > readily cause the problem. It seems to be worse the more vertical
            > > > motions there are between cuts which leads me to backlash but I can
            > > > not figure out how to ajust z's backlash.
            > > >
            > > > Any ideas folks?
            > > >
            > > > thanks
            > > > greg
            > > >
            > > >
            > >
            > >
            > > [Non-text portions of this message have been removed]
            >
            > >
            > >
            > >
            > >
            > > To Post a message, send it to: taigtools@...<taigtools%40eGroups.com>
            > >
            > > To Unsubscribe, send a blank message to:
            > taigtools-unsubscribe@... <taigtools-unsubscribe%40eGroups.com>
            > >
            > >
            > >
            > > Let the chips fly!
            > >
            > > Yahoo! Groups Links
            > >
            > >
            > >
            > >
            > >
            > >
            > >
            >
            >
            >


            [Non-text portions of this message have been removed]
          • Roy
            In my case it s usually the bit slipping in the toolholder. I have to really crank down on the setscrew if there s any vibration.
            Message 5 of 7 , Jul 1, 2006
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              In my case it's usually the bit slipping in the toolholder. I have to
              really crank down on the setscrew if there's any vibration.

              Greg McFadden wrote:
              > Guys, I am having trouble keeping Z zero'd. I do a part, for example,
              > where the depth in z will cut to 0.5" deep, then I go do something
              > else and come back and do another cut to 0.5" deep and sometimes the
              > second cut will be deeper than the first (3-5 mils deeper). I am not
              > sure where to start trying to find what is going wrong. I have no
              > problems in X or Y doing it. I don't know if it is backlash related
              > or missing steps (although I am only moving at ~10ipm in z with the
              > 269oz xylotex steppers). I am running mach 3 as well, set at 35kHz.
              > I will have to try to duplicate it in shallow cuts and see if I can
              > readily cause the problem. It seems to be worse the more vertical
              > motions there are between cuts which leads me to backlash but I can
              > not figure out how to ajust z's backlash.
              >
              > Any ideas folks?
              >
              > thanks
              > greg
              >
              >
            • Greg McFadden
              Well, I just did a little test and it did not do it this time. seems to only happen on long runs with mach (several hours runtime, although I usually see it
              Message 6 of 7 , Jul 1, 2006
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                Well, I just did a little test and it did not do it this time. seems
                to only happen on long runs with mach (several hours runtime, although
                I usually see it on very very long codes... (yes I have checked the
                gcode to ensure that the z call is correct)) this really drives me
                nuts as I can run two parts (identical) and sometimes it will do this
                and sometimes it won't....



                thanks
                Greg

                On 7/1/06, Roy <r@...> wrote:
                > In my case it's usually the bit slipping in the toolholder. I have to
                > really crank down on the setscrew if there's any vibration.
                >
                > Greg McFadden wrote:
                > > Guys, I am having trouble keeping Z zero'd. I do a part, for example,
                > > where the depth in z will cut to 0.5" deep, then I go do something
                > > else and come back and do another cut to 0.5" deep and sometimes the
                > > second cut will be deeper than the first (3-5 mils deeper). I am not
                > > sure where to start trying to find what is going wrong. I have no
                > > problems in X or Y doing it. I don't know if it is backlash related
                > > or missing steps (although I am only moving at ~10ipm in z with the
                > > 269oz xylotex steppers). I am running mach 3 as well, set at 35kHz.
                > > I will have to try to duplicate it in shallow cuts and see if I can
                > > readily cause the problem. It seems to be worse the more vertical
                > > motions there are between cuts which leads me to backlash but I can
                > > not figure out how to ajust z's backlash.
                > >
                > > Any ideas folks?
                > >
                > > thanks
                > > greg
                > >
                > >
                >
                >
                >
                > To Post a message, send it to: taigtools@...
                >
                > To Unsubscribe, send a blank message to: taigtools-unsubscribe@...
                >
                >
                >
                > Let the chips fly!
                >
                > Yahoo! Groups Links
                >
                >
                >
                >
                >
                >
                >
                >
              • Rich Crook
                ... Try slowing the acceleration way down (smaller number) & see if that helps any. I d guess that it s skipping a few steps on each Z++ acceleration due to
                Message 7 of 7 , Jul 1, 2006
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                  >Guys, I am having trouble keeping Z zero'd. I do a part, for example,
                  >where the depth in z will cut to 0.5" deep, then I go do something
                  >else and come back and do another cut to 0.5" deep and sometimes the
                  >second cut will be deeper than the first (3-5 mils deeper). I am not
                  >sure where to start trying to find what is going wrong. I have no
                  >problems in X or Y doing it. I don't know if it is backlash related
                  >or missing steps (although I am only moving at ~10ipm in z with the
                  >269oz xylotex steppers). I am running mach 3 as well, set at 35kHz.
                  >I will have to try to duplicate it in shallow cuts and see if I can
                  >readily cause the problem. It seems to be worse the more vertical
                  >motions there are between cuts which leads me to backlash but I can
                  >not figure out how to ajust z's backlash.
                  >
                  >Any ideas folks?

                  Try slowing the acceleration way down (smaller number) & see if that
                  helps any. I'd guess that it's skipping a few steps on each Z++
                  acceleration due to the motor's weight. That motor is *heavy* - mine
                  (stock 1/4 HP Marathon) weighs about 10 lbs.
                  Take the motor off & hold it in your hand with your arm outstretched.
                  Now, try raising it quickly - that's what you're asking the Z axis motor to
                  do with every Z++ move.
                  As mentioned, adding a counterweight would help this. I'm working on
                  designing one that hangs inside the column tube...

                  If you have a dial indicator or a test indicator, mount it in the spindle
                  (or on the headstock body), & zero it on a block on the table.
                  Then, move away from the block & run a bunch of up & down moves,
                  go back to the block & see if it still reads zero. If the headstock/spindle
                  ends up lower than when you started, you're losing steps on the up moves.
                  If it still reads zero, then your cutter was probably slipping.

                  Note - backlash doesn't accumulate. It only affects the position when
                  reversing direction. On the Z axis, the weight of the head/spindle/motor/etc
                  normally prevents backlash play unless you're doing fast plunge cuts.

                  = Rich =
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