Short Segments Causing Jerky motion


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    Default Short Segments Causing Jerky motion

    I'm hoping to get some help with a general problem I am having that is resulting in jerky motion on a CNC router. I am very new to CNC and CAM, so it is possible I am making some sort of mistake(s) that are easy to correct.

    While using a 3D Pocket Clearing operation to rough out material from the outside of a circular bowl shape, I get very jerky motion, but only on the last passes on each step-down level that are closest to the part. What is odd is that other passes further from the part run nicely for some reason. I have traced this down to the G code and have found that the rough operation happens due to a large number of very short segments, switching between G1 and G2/G3 commands. When the machine runs smoothly, it seems to be running G2/G3 commands that are much longer. I don't understand why the final pass closest to the surface would have such a different movement strategy. When I use a Scallop operation for the final pass with a ball-end, I get a series of very short G1 commands and the machine runs smoothly. So I don't THINK that data starving is the problem, unless a G2/G3 command takes alot more computing than a G1 command would.

    How do I keep Fusion 360 from generating the alternating G1 to G2/G3 steps?

    I have tried turning Smoothing on for both the Pocket Clearing and the Scallop operations, and did not see alot of difference. I have tried air-cutting to make sure I didn't have some feed/speed issue, but the problem still happens.

    Some basic information about my setup:

    UCCNC running with a UC400ETH controller
    200 IPM feed rate (system should be capable of 600 IPM electro-mechanically, not yet fully tested)

    Please let me know what additional information would be helpful, and I will upload it.

    Thanks,
    Robert

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    Community Moderator Jim Dawson's Avatar
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    Default Re: Short Segments Causing Jerky motion

    Do you have a Constant Velocity or Exact Stop setting in your CNC software? If changing that might help. A G2/G3 command does take a lot more computing that a G1 move. There is a lot of trig involved and the controller has to convert the arc into very short line segments.

    Jim Dawson
    Sandy, Oregon, USA


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    Default Re: Short Segments Causing Jerky motion

    What are your CV settings in UCCNC? The default settings are really for metric, and need to be changed if you are running in inches.


    I have tried turning Smoothing on for both the Pocket Clearing and the Scallop operations, and did not see alot of difference.
    Have you played with the tolerances? They can have a big effect when using smoothing.

    Gerry

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    Default Re: Short Segments Causing Jerky motion

    Also, what are your acceleration settings?
    I just spent the last 90 minutes playing with different Fusion Smoothing settings, and UCCNC settings. Acceleration makes the biggest difference by far.

    Gerry

    UCCNC 2017 Screenset
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    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

    JointCAM - CNC Dovetails & Box Joints
    [URL]http://www.g-forcecnc.com/jointcam.html[/URL]

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Default Re: Short Segments Causing Jerky motion

    Jim,

    UCCNC does have a Constant Velocity mode.

    Gerry,
    My CV settings are currently:

    Stop at Degrees: 89
    Look ahead lines count: 200
    Linear Error Max: 0.002
    Linear addition length max: 1
    Linear Unify Length Max: 2
    Corner error max: 0.002
    Arc radius tolerance: 0.1

    Acceleration Settings:
    X and Y Axis: 100 in/s^2
    Z Axis: 200 in/s^2

    I currently run most operations at 200 IPM.

    I have not really played with tolerance settings alot in Fusion 360 or UCCNC. As a general rule, I have been setting Fusion 360 tolerances to between 0.001" and 0.004". For some operations, the smoothing tolerance gets set to 10x less (e.g. 0.0004") and for others, it is the same as the main tolerance. I have a basic understanding of what these tolerances do, but I don't really understand the effect on the machine performance. I recognize that this is probably a pretty tight tolerance for wood, but I am aiming at some fairly precise work in the future, so I want to try to figure out what the system is capable of.

    Thanks,
    Robert



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    Default Re: Short Segments Causing Jerky motion

    Try changing the Linear error max and the Corner Error Max to 0.005, and I think you'll see a decent difference.
    These two settings have a lot to do with how UCCNC blends the short G1 moves together. On the test file I'm running, settings of 0.005 run about 20% faster than 0.002.
    I also found that setting the tolerances tighter in Fusion, to give shorter G1 moves, can actually run faster up to a point, as UCCNC seems to be able to blend the shorter moves faster (smoother?)

    I recognize that this is probably a pretty tight tolerance for wood, but I am aiming at some fairly precise work in the future
    Setting the tolerance larger in UCCNC doesn't necessarily mean than your parts will be less precise.

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

    JointCAM - CNC Dovetails & Box Joints
    [URL]http://www.g-forcecnc.com/jointcam.html[/URL]

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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    Default Re: Short Segments Causing Jerky motion

    Quote Originally Posted by ger21 View Post
    Try changing the Linear error max and the Corner Error Max to 0.005, and I think you'll see a decent difference.
    These two settings have a lot to do with how UCCNC blends the short G1 moves together. On the test file I'm running, settings of 0.005 run about 20% faster than 0.002.
    I also found that setting the tolerances tighter in Fusion, to give shorter G1 moves, can actually run faster up to a point, as UCCNC seems to be able to blend the shorter moves faster (smoother?)
    I will give this a try. Smoothing definitely looks like it will help, also. I ran a program today without it, by accident, and it came out as a bunch of 0.010" G1 moves. The controller definitely had some trouble working through it, and the motion was very slow. It makes me think that data starving is part of the root problem. Is this a limitation of the control software (UCCNC), the computer, or the controller hardware?

    I would still like to figure out if there is some way to prevent having very short alternating G1 and G2/G3 moves? It doesn't look like Smoothing helps this much, but I will play around with it a little more.

    Thanks,
    Robert



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    Default Re: Short Segments Causing Jerky motion

    Quote Originally Posted by DDgitfiddle View Post
    I will give this a try. Smoothing definitely looks like it will help, also. I ran a program today without it, by accident, and it came out as a bunch of 0.010" G1 moves. The controller definitely had some trouble working through it, and the motion was very slow. It makes me think that data starving is part of the root problem. Is this a limitation of the control software (UCCNC), the computer, or the controller hardware?

    I would still like to figure out if there is some way to prevent having very short alternating G1 and G2/G3 moves? It doesn't look like Smoothing helps this much, but I will play around with it a little more.

    Thanks,
    Robert
    It depends on your g-code, but the limitation is likely your settings.
    If the segmens are very short and your look ahead line count is low then it could be data starving, when the controller runs out of data before it reaches the actual point.
    If the circle is small and your axes accelerations are slow then it is possible that the axes can't change the movement direction fast enough. In a circle the movement direction always changes and how fast your axes can change the direction depends on the acceleration you set up.
    How the path is followed is set on the general settings page in the constant velocity settings, those settings also sets how the controller can do the path. It also can be that the linear addition and unify length parameters are too low and again the controller can't look ahead far enough on the path and it runs out of motion data before the new point is reached.



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    Default Re: Short Segments Causing Jerky motion

    Thanks, OlfCNC.
    I need to take some time to try adjusting the CV settings. It certainly SEEMS like data starving, because it really only seems to happen when I get alot of short moves in a row. Based on the answers I have seen so far, here are the things I need to try:

    1) Increase the linear error and corner error maximums to 0.005" (per ger21) and see if that improves things. Try different variables. If I understand correctly, this should help UCCNC find a smooth path a little easier.

    2) Adjust tolerance and smoothing tolerance in Fusion for given test programs to see if that increases the length of individual moves (fewer moves).

    3) Look at the effects of Smoothing. On one program I have, it changes the move distances from about 0.01" to close to 0.25". That is no surprise, but I need to see what other knobs there are that affect this.

    4) Adjust the Look Ahead count, Linear Addition Length, and Linear Unify Length to see how those affect the performance.

    5) Increase the motor acceleration. This doesn't seem like it should be the problem. The motors shouldn't be stopping at each of these points to begin with.

    6) Try to bump the Kernel Frequency to 400kHz. This currently doesn't seem to work with my VFD, but I have one possible way to address that.

    Lots to try.

    Thanks,
    Robert



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    Default Re: Short Segments Causing Jerky motion

    The controller definitely had some trouble working through it, and the motion was very slow. It makes me think that data starving is part of the root problem. Is this a limitation of the control software (UCCNC), the computer, or the controller hardware?
    No, it's entirely in your CV settings.

    I would still like to figure out if there is some way to prevent having very short alternating G1 and G2/G3 moves? It doesn't look like Smoothing helps this much, but I will play around with it a little more.
    It depends on the part in some cases. 3D models, like it sounds like what you are doing, is not going to have many G2/G3 moves in it.


    2) Adjust tolerance and smoothing tolerance in Fusion for given test programs to see if that increases the length of individual moves (fewer moves).
    Not necessarily. The reason it's slow and jerky, is because UCCNC is trying to follow your path with your very tight tolerances. The longer the line segments, then the bigger the angle between them. It's these direction changes that are causing UCCNC to slow down, and speed up, in order to follow those segments as accurately as possible. Increasing the number of segments reduces the angle between them, which can result in UCCNC going faster. There's a point of no returns there, but if the code is going to be mostly G1 moves, I think that shorter moves may work better with UCCNC. More thorough testing is required to conform this, but it looks like it to me.
    Ideally, getting rid of the short segments whenever possible will make the biggest difference.

    4) Adjust the Look Ahead count, Linear Addition Length, and Linear Unify Length to see how those affect the performance.
    I spent about 2 hours testing over the weekend, and found lookahead to not really make any difference at all. But it may in your case. The Linear Addition and Unify can be a bit tricky to wrap your head around. They won't have as big of an affect as the error tolerances.

    5) Increase the motor acceleration. This doesn't seem like it should be the problem.
    No, 100 is a pretty fast acceleration. Increasing it will make it run faster, but I'd leave it alone.


    6) Try to bump the Kernel Frequency to 400kHz. This currently doesn't seem to work with my VFD, but I have one possible way to address that.
    This won't do anything at all. The Kernel frequency just dictates your maximum velocity and acceleration settings. UCCNC won't allow you to set them any higher than the kernel frequency allows.


    My recommendation would be to create a sample model in Fusion, similar to what you've been working with, so that say a scallop toolpath will run in 2-4 minutes. Then use that to play with the different settings to see what makes the biggest difference. Be sure to change one thing at a time.
    From the testing I was doing, acceleration, and error tolerance in UCCNC made the biggest difference. Starting with an error tolerance of .002, and an acceleration of 30, I had a 4 minute run time. With an error tolerance of .005, and acceleration of 100, it dropped to just over 2 minutes. So each of those two settings reduced the run time by roughly 20%-25%.

    Gerry

    UCCNC 2017 Screenset
    [URL]http://www.thecncwoodworker.com/2017.html[/URL]

    Mach3 2010 Screenset
    [URL]http://www.thecncwoodworker.com/2010.html[/URL]

    JointCAM - CNC Dovetails & Box Joints
    [URL]http://www.g-forcecnc.com/jointcam.html[/URL]

    (Note: The opinions expressed in this post are my own and are not necessarily those of CNCzone and its management)


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Short Segments Causing Jerky motion

Short Segments Causing Jerky motion