Need Help! CNC Accuracy issues


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  1. #1
    Member Angelok's Avatar
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    Default CNC Accuracy issues

    I'm not sure if this is the right place to be posting this, (I'm new here) so if it's not any suggestion on a better location would be appreciated.

    I recently purchased a used machine (Digital Tools RD-903-1) and for the most part it works well but it does give me some accuracy issues. It's have a 5' x 10' cutting area with stepper motor and a Porter Cable router, I'm using Vcarve Pro for the drawing and a older version of Mach3.

    What happens is if i cut a small circle say round 1.5" diameter, it isn't perfectly round, but if i cut that same circle over and over again it is constant. If I cut a larger circle say around 8" diameter it fine. I did a test, where I put a 4' x 8' sheet on the table and cut out 11" x 11" squares and there weren't all the same. It was only a small difference but it seems that the accuracy is different at different placed on the table.

    Does anyone have any ideas on what I can do or what the issue is.

    Thanks,

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  2. #2
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    Default Re: CNC Accuracy issues

    If you set it to do say a 12x12 inch square is it the right size? If its the right size but not repeatable or not square you have some setup issues. If its an older machine it might mean worn parts or adjustments as needed. Start by making the test squares and then look for worn out or loose parts.

    Retired Master Electrician, HVAC/R Commercial. FLA Saturn 2 4x4 CNC Router Mach4 Kimber 1911 45ACP


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    Member Angelok's Avatar
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    Default Re: CNC Accuracy issues

    I've been doing a bit of research and that does sound like the best place to start. this is my plan:
    Set the machine at zero and run an patters to see if it ends at the same starting point, from what I recall from the last tests I did is it was only the X axis. So that would lead me to believe that it's mechanical.
    Then I'm going to do the 12 x 12 test you suggest on different areas on the table and see what that gives me.

    Is it possible its a software problem?

    Thanks for the info.

    Angelo



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    Default Re: CNC Accuracy issues

    Quote Originally Posted by Angelok View Post
    I've been doing a bit of research and that does sound like the best place to start. this is my plan:
    Set the machine at zero and run an patters to see if it ends at the same starting point, from what I recall from the last tests I did is it was only the X axis. So that would lead me to believe that it's mechanical.
    Then I'm going to do the 12 x 12 test you suggest on different areas on the table and see what that gives me.

    Is it possible its a software problem?

    Thanks for the info.

    Angelo
    I can be anything, but its an used machine, The easiest thing you can do right now is a good visual inspection of all the moving parts. If its got belts check them, and so on. It could be combination of things. Rule #1 start with the simplest first. Don't change a lot of adjustments until you see what the last one you did changes. KISS rule. Good Luck!!

    Retired Master Electrician, HVAC/R Commercial. FLA Saturn 2 4x4 CNC Router Mach4 Kimber 1911 45ACP


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    Community Moderator ger21's Avatar
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    Default Re: CNC Accuracy issues

    Is it possible its a software problem?
    Not likely.
    Flex in the machine is far more likely.

    Gerry

    UCCNC 2017 Screenset
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    http://www.thecncwoodworker.com/2010.html

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

    (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: CNC Accuracy issues

    Hi Angelo - Are you cutting a true circle (G2 or G3?) or is the cutting via a dxf file? If its via dxf use a fixed small increment such as 1/16" or smaller on export, some dxf settings use angles and chords and these can make arcs funny shapes. Imperative you need to check all mechanical stuff is OK. I'm not familiar with Mach3 but most CAM software also has accuracy settings via the speed of cut. So accuracy can depend on feed speed. In UCCNC this is called interpolation mode. Its either exact or constant velocity mode. In constant velocity mode the path interpolation is trying to maintain a constant velocity so will try to stay between path parameters set. So if you cut a circle using a slow speed and then a fast speed and they are different then it could be your interpolation settings as well....Cheers Peter



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    Default Re: CNC Accuracy issues

    Thanks



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    Default Re: CNC Accuracy issues

    Quote Originally Posted by peteeng View Post
    Hi Angelo - Are you cutting a true circle (G2 or G3?) or is the cutting via a dxf file? If its via dxf use a fixed small increment such as 1/16" or smaller on export, some dxf settings use angles and chords and these can make arcs funny shapes. Imperative you need to check all mechanical stuff is OK. I'm not familiar with Mach3 but most CAM software also has accuracy settings via the speed of cut. So accuracy can depend on feed speed. In UCCNC this is called interpolation mode. Its either exact or constant velocity mode. In constant velocity mode the path interpolation is trying to maintain a constant velocity so will try to stay between path parameters set. So if you cut a circle using a slow speed and then a fast speed and they are different then it could be your interpolation settings as well....Cheers Peter
    Hey Peter, Thanks for the info. i think what your talking about is still a bit too advanced for me I've found some inaccuracies on my X axis. i'm not sure what it is yet but I feel much better about the machine now that I'm starting to isolate the issues. Once I have that figured out I'll look into what you were talking about.

    Once again thanks for putting in the time and sending the info.



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    Default Re: CNC Accuracy issues

    Hello everyone,
    I want to say thanks to everyone here, this forum is awesome!
    I did some testing on my machine today to see if I could isolate some of the issues and I’m starting to think this may need more work than I anticipated.
    First what I did was set the machine to zero and ran a few patterns. I wanted to make sure that it went back to zero each time. It was perfect!
    The next thing I did was cut out some 10” x 10” squares. I cut out 24 pieces form a sheet 34” x 92” so 3 pieces along the Y axis and 8 along the X axis. (See attached)
    On the X axis they were all the same, not perfect with a bit of a bow but not my concern at this point (baby steps). The Y axis on the other hand… the first row was 9.9375” the middle row was 10” and the last row was 10.03125”. As the machine move to the right the cut get larger.
    I later did another test with 11.5” x 11.5” pieces and got similar results.
    What would cause the measurement to increase?
    Thanks

    Attached Thumbnails Attached Thumbnails CNC Accuracy issues-20190325_151412-jpg  


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    Default Re: CNC Accuracy issues

    Hello, did you check your machine for wear as I suggested and others did also? If not your chasing a rainbow. No amount of questions posted on here will solve your problem until you decide to to the basics.

    Retired Master Electrician, HVAC/R Commercial. FLA Saturn 2 4x4 CNC Router Mach4 Kimber 1911 45ACP


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    Activation process peteeng's Avatar
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    Default Re: CNC Accuracy issues

    There are several mechanical reasons for inconsistent results eg is the gantry orthogonal to the bed axis? Is the gantry mechanically sound in regard to travel on each side. If the bearings or the rack is poor on one side then the gantry "walks" as it runs along. ie one side leads then the other side leads or lags etc. This is a sort of slip/stick mechanism. All mechanical components must be checked and found to be correctly operating. My really big question is how are you measuring objects to 1/10000" ? Peter



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    Default Re: CNC Accuracy issues

    From the picture it looks like you have vertical racks with plenty of chip debris collecting on them. That could be one source of inaccuracy.

    If the two tracks that your gantry run on are not parallel, then it could be squeezing or pulling the legs of your gantry apart. This could cause the gantry to flex more when at one side of the table, and a flex in your gantry would result in the travel of the tip of your end mill increasing or decreasing during Y travel. That could be an explanation to the differing Y measurements of your squares.



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