Need Help! Shake / Jerk / Stutter when changing direction


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Thread: Shake / Jerk / Stutter when changing direction

  1. #1
    Member dempseyrob's Avatar
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    Default Shake / Jerk / Stutter when changing direction

    Hi there,

    I have a PRO4896 Avid CNC with Fusion 360 and the Avid Post Processor. Everything has been set up accurately as per Avid's instructions.

    The project I am running is carving a Terrain Map (3D carving) using 3 passes - Pocket, Parallel roughing and Parallel finish.

    I am trying to reduce the shake / jerk / stutter when the machine changes direction. I have had some success with changing toolpath settings in Fusion 360 and I am experiencing this with low feedrates. It's obviously much worse with higher feedrates.

    In the X/Y plane on the Pocket pass, changing the reduced feed radius and distance has helped, along with leads, transitions, reduced feedrate.
    In the Z direction, the same phenomenon happens but I'm unsure how to help.

    I want to get the machine moving smoothly between directions and instructions, then build up the speed from there. Any help would be much appreciated.


    Example of GCode from finishing pass:
    (1003)
    (MACHINE)
    ( VENDOR AVID)
    ( DESCRIPTION AVID CNC)
    (T1 D=3.175 CR=1.587 - ZMIN=-29.537 - BALL END MILL)
    G90 G94 G91.1 G40 G49 G17
    G21

    (PARALLEL2)
    M5
    T1 M6
    S15000 M3
    G54
    G0 X751.104 Y19.993
    G43 Z15. H1
    G1 Z-24.515 F2000.
    G18 G3 X746.104 Z-29.515 I-5. K0.
    G1 X602.84
    X602.023 Z-28.706
    X600.727 Z-27.2
    G2 X599.896 Z-26.734 I-1.162 K-1.096
    G1 X598.596 Z-26.42
    X598.338 Z-26.384
    X595.354 Z-26.114
    X594.403 Z-26.022
    G2 X586.434 Z-25.589 I-43.21 K-721.79
    G1 X584.968 Z-25.653
    X583.97 Z-25.664
    X582.638 Z-25.483
    X580.205 Z-25.081
    X578.911 Z-24.823
    X577.626 Z-24.527
    X576.332 Z-24.271
    X576.145 Z-24.245
    X574.81 Z-24.143
    X574.519 Z-24.145
    X573.233 Z-24.192
    G2 X566.68 Z-25.286 I3.911 K-43.615
    G1 X565.538 Z-25.361
    X564.212 Z-25.306
    X562.785 Z-25.348
    X561.712
    G2 X557.467 Z-24.939 I-13.886 K-121.646
    G1 X556.296 Z-24.924
    X554.949 Z-24.983
    X553.542 Z-25.153
    X552.235 Z-25.327
    X550.901 Z-25.428
    G2 X545.926 Z-25.731 I1.456 K-64.795
    G1 X545.053 Z-25.747
    G2 X539.174 Z-25.796 I-1.901 K-124.454
    G1 X537.906 Z-25.798
    G3 X529.718 Z-25.89 I-6.773 K239.269
    X524.982 Z-25.781 I-0.816 K67.639
    G1 X523.617 Z-25.753
    X522.13 Z-25.797
    X520.878 Z-25.807
    G3 X511.909 Z-25.472 I3.27 K207.732
    G1 X510.319 Z-25.327
    X509.368 Z-25.235
    X507.979 Z-25.162
    X506.174 Z-25.167
    X499.857 Z-25.15
    X497.528 Z-25.172
    X495.844 Z-25.091
    G2 X489.03 Z-24.988 I-4.531 K-74.331
    G1 X486.786 Z-25.031
    X485.518 Z-25.025
    X484.543 Z-25.004
    X483.415 Z-24.904
    X482.114 Z-24.674
    G2 X479.338 Z-24.145 I-9.779 K-43.743


    Other small issues that may help zero in on the problem:
    - When Rapid Feedrate is at 100% (when it moves to first coordinates) it can sometimes skip and jump along it's way and vibrate.
    - The stutter is not totally confined to a change of direction. It sometimes happens in the middle of a linear move.
    - Mach 4 will not allow be to home or jog the machine if it is already in the home position (from the previous day / emergency stop). I have to manually push it off the limits to get it working.

    If some of this vibration is normal then let me know. Previous experience is on a MPCNC so I'm new to an Avid machine but not a total newbie.

    Many thanks

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  2. #2
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    Default Re: Shake / Jerk / Stutter when changing direction

    Hi,

    - When Rapid Feedrate is at 100% (when it moves to first coordinates) it can sometimes skip and jump along it's way and vibrate.
    - The stutter is not totally confined to a change of direction. It sometimes happens in the middle of a linear move.
    Those are symptoms of the motors being tuned to run too fast or too high acceleration. Go to the motor tuning pages and reduce the max speed and acceleration to 50% of current setting
    and try running the tool path again.

    Craig



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    Default

    Quote Originally Posted by dempseyrob View Post
    Hi there,

    I have a PRO4896 Avid CNC with Fusion 360 and the Avid Post Processor. Everything has been set up accurately as per Avid's instructions.

    Many thanks

    You will find that L shaped bracket that the tensioner screw runs through will turn slightly and catch the threads. Thus the pinion gear will freeze and as it gets yanked around it will slowly ratchet up higher and higher off the rack gear adding slop into the axis and thus make worse and worse cuts.

    If you don't know the screw, it is the screw running through the red spring that puts tension on the pinion gear holding it against the rack. One is present on each axis.

    It was such a pain in my rear end that it was the second thing listed on my checklist until I replaced it with a smooth shank M8.

    All you have to do is turn on the machine. Home it. Put the spindle into the middle of your table, leaving the motors engaged. Then walk around the machine and push gently on each axis both forwards and back (twice for the Y because you have one of those on each side). There should be zero play in the axis.

    Then try to see if you can push each stepper motor straight down, lifting the pinion gear off the rack. If you can't or it starts making crunching sounds as the threats tear through that L bracket, loosen and readjust.

    I got so dern tired of it I purchased a partially threaded M8 bolt and put some grease on it. I have complained to AvidCNC about it and they say I am the only person who has ever had trouble with it.



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Shake / Jerk / Stutter when changing direction

Shake / Jerk / Stutter when changing direction