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IndustryArena Forum > MetalWorking Machines > Okuma > Okuma lathe C-acis orientation
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  1. #1
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    Okuma lathe C-acis orientation

    We have an Okuma captain 3 axis lathe. When setting up a part we indicate across a flat on the part and calculate that as being C0. When we run the part the holes we drill on the on are always shifted about 1/4 degree. This shift varies a bit from part to part. So, I put in a C-axis variable shift in the program (VZSHC)to try to align it better. Does anyone know why we might have this alignment problem? We also have the same issue on a 3 axis Samsung lathe with a fanuc control.

  2. #2
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    Re: Okuma lathe C-acis orientation

    When setting up a part we indicate across a flat on the part and calculate that as being C0
    hy flast, to indicate much faster, you may use a precision level so to orientate the flat in the horizontal phishical plane, then index the C axis with the angle between the X bed and the horizontal plane ( eq30* ), then start to manualy indicate the flat : at this moment, you should need to adjust only 1-2-a few degress

    another way, that may be faster, is by using X axis feed-skip, but i never tried it on an y lathe when y<>0

    When we run the part the holes we drill on the on are always shifted about 1/4 degree.
    is the live tool coaxial with the spindle ?

    This shift varies a bit from part to part.
    write down the values, observe the tendency, then, if you wish check the coaxility / kindly

    ps : if you wish, i can tell you how to also check the tram, but this is not a must for tools with low diameter
    Ladyhawke - My Delirium, https://www.youtube.com/watch?v=X_bFO1SNRZg

  3. #3
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    Re: Okuma lathe C-acis orientation

    one of the reasons might be that when you indicate the part you rotating one way and when the machine goes to c axis mode there is a difference in positioning
    there is also the positioning error to account for (There is positioning accuracy and there is repeatably accuracy)
    you could try and program it to stop at -90 degrees then rotate back to 0 degrees to keep the spindle rotating in one direction

  4. #4
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    Re: Okuma lathe C-acis orientation

    if machine is in m109, then, when executing m110, it will rotate only in one direction ( thus it is not going always the shortest path ); at the end of this sequence, C axis will be 0 ( this is the common behaviour for lathes, unless changed by parameters )

    after that, when indexing to C<>0, it will always go in one direction ( that may be the shortest path, or not, depending on m15 16 960, etc )

    this way, each C in-position comand is always being achieved by rotating the spindle in a constant direction ( somehow similar to uni-directional-positioning; udp ) ( for example, for a random setup, at 270* is always approaching from 271*, thus from a greater value, but for 10* is always approaching from 9*, thus from a lower value)

    positioning and repetability deviation, are both lower when udp occurs, and they are always within diff-control, and that is << 1/4degrees; even if you use vinpc = 0.9*, cutting will begin at <0.005*, when a normal z-clearance is used / kindly
    Ladyhawke - My Delirium, https://www.youtube.com/watch?v=X_bFO1SNRZg

  5. #5
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    Re: Okuma lathe C-acis orientation

    took the words out of my mouth.

    what you're effectively referring to is backlash in C axis positioning.

    As a test, always rotate the C axis the same direction with the pulse handle to get the flat straight. see if the trend of results improves.

  6. #6
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    Re: Okuma lathe C-acis orientation

    hello again, i just can't stop thinking about this ... i don't believe that there is backlash, especially because 1/4* means 0.554mm circumference length for the od of a 10" chuck, and that is huge, meaning that the C axis should be used only with the break on during drilling, and it would shake during contouring

    vzshc=1/4*means that there is a missalignment in Y direction; there may also exist a missalignment in X direction, but this one can not be corected by using vzshc, so, for now, let's focus only on Y missalignmnet : for example, if there is 0.1mm Y radial excentricity, and drilling occurs at X50, then it will be required to use a C phase corection of 1/4*, and this sounds pretty realistic

    so far so good ...

    This shift varies a bit from part to part
    so, deviation is 1/4* + a bit; 1/4* may be from Y missalingment, and 'a bit' may be from the fixture tir ( because the part is re-clamped ) and/or drills buckling/bending, because of wear/length/specs

    kindly
    Ladyhawke - My Delirium, https://www.youtube.com/watch?v=X_bFO1SNRZg

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    Re: Okuma lathe C-acis orientation

    Quote Originally Posted by deadlykitten View Post
    hello again, i just can't stop thinking about this ... i don't believe that there is backlash, especially because 1/4* means 0.554mm circumference length for the od of a 10" chuck, and that is huge, meaning that the C axis should be used only with the break on during drilling, and it would shake during contouring

    vzshc=1/4*means that there is a missalignment in Y direction; there may also exist a missalignment in X direction, but this one can not be corected by using vzshc, so, for now, let's focus only on Y missalignmnet : for example, if there is 0.1mm Y radial excentricity, and drilling occurs at X50, then it will be required to use a C phase corection of 1/4*, and this sounds pretty realistic

    so far so good ...



    so, deviation is 1/4* + a bit; 1/4* may be from Y missalingment, and 'a bit' may be from the fixture tir ( because the part is re-clamped ) and/or drills buckling/bending, because of wear/length/specs

    kindly
    It is possible that this is a fixturing issue.
    VZSHC would help a Y axis err if the holes were parallel to the Z.

    I still believe it is a backlash during setup issue.
    I have had this issue when setting up to re-machine features on rifle barrels which needed machined in relation to the gas port hole. We indicated across a fixture with a pin in the hole to find C0 for every part.

    Hopefully he finds and shares the answer. I hadn't thought about this again at all until I checked my email and saw the reply notification. Maybe then you can think about something else.

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