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Thread: size variation of -0.04mm in X axis of every machined part

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    Default size variation of -0.04mm in X axis of every machined part

    Dear All,

    We are facing The following problem :

    Condition: There is a size variation of -0.04mm in X axis of every shaft
    Machined for example size X of 5 pcs differ as follows:

    Sample No. 1 Size X measured size: 13.44
    Sample No. 2 Size X measured size: 13.40
    Sample No. 3 Size X measured size: 13.36
    Sample No. 4 Size X measured size: 13.32
    Sample No. 5 Size X measured size: 13.28

    we alredy check the parameter 1851 and 1852 but the manufacture say they are good.

    Please if you have any Suggestion kindly let me know.

    Thanks

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    Default Re: size variation of -0.04mm in X axis of every machined part

    of sorry i forgot , the control system is :

    Fanuc 0i-Mate TD
    spindle motor 6, 5.5/7.5 kw
    X axis motor 8, 1.2B kw
    Z axis motor 8, 1.2 kw

    Thanks and sorry i am new

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    Default Re: size variation of -0.04mm in X axis of every machined part

    First, you would need to perform a mechanical-play test on that axis, if the resulting readings actually show that a little amount of play exists, you can try to help the machine using the related parameter(s), that process is called backlash compensation.

    But depending on the machine's age/use/abuse, you could find a lot of play, in that case, further mechanical checks would be necessary, ballscrew wear, bearings, couplings, etc.

    http://cnc-professional-forum.com/ga.../DSCF62641.JPG
    Last edited by Sinumeriko; 05-14-12 at 01:12 PM.

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    Default Re: size variation of -0.04mm in X axis of every machined part

    Hi,
    to me it looks like a programming type problem.
    Using incremental commands in a sub program?
    Looping?
    To consistently change size the same amount in the same direction doesn't sound like a backlash problem.

    Regards, Dave.

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    Default Re: size variation of -0.04mm in X axis of every machined part

    With a total variance of .0063" (.16mm) I'd be curious on what tolerance you're trying to hold?
    Are you uni-directionally cutting this diameter? Have you verified your roughing routine and it's insert/cutter?
    What about material? Are you holding better repeatability on other similar machines? Same process? Is your operator changing offsets based off of one sample? Are your operators re-cutting the part?

    ... I think backlash is a bit of a stretch to isolate and troubleshoot the machine as the problem.
    Are you capable of holding tighter tolerances on other features?

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    Default Re: size variation of -0.04mm in X axis of every machined part

    The fact that each subsequent run of the program varies
    by exactly .04 is just too coincidental , and
    points me to a programming/work offset issue.

    Not knowing more about the machine (is this new or old, did
    the problem just appear recently), I would also
    look to see if there are macros being executed for tool
    changes and or sub program calls. If so I'd troubleshoot
    those as well.

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    Default Re: size variation of -0.04mm in X axis of every machined part

    Quote Originally Posted by camsys View Post
    The fact that each subsequent run of the program varies
    by exactly .04 is just too coincidental , and
    points me to a programming/work offset issue.

    Not knowing more about the machine (is this new or old, did
    the problem just appear recently), I would also
    look to see if there are macros being executed for tool
    changes and or sub program calls. If so I'd troubleshoot
    those as well.
    LOL - good catch cam - I totally missed that.
    Yea, that's a smoking gun to the program or a programmed routine. Maybe a variable is being used and not reset?
    It's NOT with your backlash comps if this pattern is that repeatable.

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    Default Re: size variation of -0.04mm in X axis of every machined part

    This is o ften seen when using TNRC (G41\G42) and then having two lines of information in a row without axis motion - this cause unexplained axis accumulation errors. Try checking to see if you are using G42/G42 and if so make sure there are not two line in a row without axis motion prior to a G40
    IE:
    G42X___Z____
    G50S5000
    G96S300

    Tom

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    Default Re: size variation of -0.04mm in X axis of every machined part

    This is often seen when using TNRC (G41\G42) and then having two lines of information in a row without axis motion - this cause unexplained axis accumulation errors. Try checking to see if you are using G42/G42 and if so make sure there are not two line in a row without axis motion prior to a G40
    IE:
    G42X___Z____
    G50S5000
    G96S300

    Tom

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    Default Re: size variation of -0.04mm in X axis of every machined part

    Quote Originally Posted by Tom69 View Post
    This is often seen when using TNRC (G41\G42) and then having two lines of information in a row without axis motion - this cause unexplained axis accumulation errors. Try checking to see if you are using G42/G42 and if so make sure there are not two line in a row without axis motion prior to a G40
    IE:
    G42X___Z____
    G50S5000
    G96S300

    Tom
    Hi Tom,
    I've never know that to be the case, and particularly there is no accumulation of error as stated by the OP when two or more consecutive non motion blocks are programmed in TNR compensation mode. If two or more non motion blocks are specified consecutively, the tool nose center comes to a position vertical to the programmed path of the preceding block at the end of the preceding block, but there is no accumulation of any error.

    I agree with Doug and camsys that its likely to be some programming or Offset issue, but not TNR offset initiated, and Backlash does not present in the way described by the OP. If the OP uses G50 to set the coordinate system, then not cancelling the Tool Offset when the slides are sent back to the tool change position will definitely give the results indicated. In this case there is a continual and consistent drift of the tool change position by the amount of the tool wear offset.

    Unfortunately, the OP is missing in action, so its hard to determine what the issue is without further information. One can only speculate.

    Regards,

    Bill
    Last edited by angelw; 05-19-12 at 12:03 AM.

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    Default Re: size variation of -0.04mm in X axis of every machined part

    Dear Profi,

    Thanks for all your reply, i really do apreciated.

    most have mention that maybe programming problem, therefore i have attached my NC-program for you, please take a look and tell me what you think.

    I didnt take the program in consideration because i have 2 new machine, and 1 machine is runing well with the same programe, but the seconde one have the variation problem.

    i have contact the manufacture and they say is maybe pitch problem, they have send me diffrent pitchs but every time is a variation ,only the value change.

    now the problem is still the same a variation of -0.04 after each piece.

    also when i pitch the home button , the machine go to home X axis light turn on and the X-axis show the value 0.

    Thanks all of you
    Attached Files Attached Files

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    Default Re: size variation of -0.04mm in X axis of every machined part

    The only thing that I found was a double decimal value, "Z-111.45."
    However, most machines will alarm out on this.
    Is this program EXACTLY how it is in the machine?

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    Unhappy Re: size variation of -0.04mm in X axis of every machined part

    yes the program is the same like in the machine.

    and i get this variation, even without inputing the offset value.

    but also wat make me confuse is that the same progrome is running is other machine fine.

    i really dont now what to do ?

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    Default Re: size variation of -0.04mm in X axis of every machined part

    I'm still confused on the "Z-111.45." command alone.
    Also, you mention you're having problems with the size - ???
    Sample No. 1 Size X measured size: 13.44
    Sample No. 2 Size X measured size: 13.40
    Sample No. 3 Size X measured size: 13.36
    Sample No. 4 Size X measured size: 13.32
    Sample No. 5 Size X measured size: 13.28

    Which size is it suppose to be? Tool T0303 @ 13.6? Or 13.55?
    There are quite a few irrelevant moves and some basic bad code but, nothing that would suggest an accumulative error.


    Try this;
    Make a TEST program.
    Put an indicator on the top of your spindle (or some other rigid location).
    Using a tool - doesn't matter what kind, move the tool to the indicator and Zero the indicator and write down your ABSOLUTE position. (we'll call this your work offset)
    G28U0
    G00X25.Z25. (from your indicator Zero)
    G98G01U-23.W-23.F200.
    G01U-2.W-2.F20.
    G04U2.0
    G01U10.
    G28U0
    G28Z0
    M99
    %
    Run this program a few times to see if it repeats.
    If it does, add a tool change at the beginning and end and repeat the test.
    Try to isolate where the error is originating.

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