Strange threading problem...please help


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    Default Strange threading problem...please help

    Hi, my old green screen Okuma LB15 is behaving strangely. I started her up yesterday to cut a couple of threads on a few hydraulic cylinder rods...something I did last week without issue, & discovered that it is no longer cutting the programmed thread pitch.

    The control is a OSP5000L &I have cut many metric & imperial thread on it over the years.

    This one was a M30 x 2mm pitch external thread. My threading line looks like this

    G97S400M42M3T0303
    G0X40Z20M8
    G71X25.26Z-50H2.44D.3U.07F2M22M33M74

    After cutting the first thread I checked it with the mating nut & it wouldn't go...no worries I thought & adjusted my tool offset & ran it through again. When it still wouldn't fit (but felt a bit loose at the start) I grabbed my thread pitch gauges & to my surprise it was out.

    I ran a few more test pieces at various pitch, including a 12TPI thread using F25.4J12 with the same result. A 2mm pitch over 20 threads looks to be about 0.7mm out of wack & the 12TPI over 1 inch was about 12&1/2 TPI.

    I pulled the cover off & checked the encoder running off the spindle & while the belt looked good & had good tension, I pulled it off anyway & found a small piece of swarf under one tooth. After a good clean, I put it back on, checked the cable & ran another test part...no change.

    The piece of swarf has left a small bump in the belt but I don't think that would cause my problem??

    What do you guys think...encoder problem? The machine tracks (chases) the thread spot on each pass (I re-cut a couple to check) & the error appears to be progressively consistent along the length of the thread...its just not cutting the programmed pitch.

    Is there a way of checking the encoder or any other checks I can do?

    Regards Bruce

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    Did you try to shut it down and open it again?
    Maybe a servo parameter file is wrong.
    First try reinstall software.

    https://www.facebook.com/okuma.tuning


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    Quote Originally Posted by kurmay View Post
    Did you try to shut it down and open it again?
    Maybe a servo parameter file is wrong.
    First try reinstall software.
    Thanks kurmay, yes I did shut it down & it doesn't make a difference.



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    Default Re: Strange threading problem...please help

    try a simple test .... see if travel distance is reported
    correctly. could show encoder roblem or some leadscrew pitch type error.

    use a 200mm slip gauge against chuck face . set dial indicater on it,set to zero ... record Z
    position.
    replace 200mm slip with 50mm block.
    bring dial to block... is movement exactly 150mm ?

    wind Z past zero. then wind Z+ to zero... is both Z positions the same ? ... different means possible backlash error.



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    Quote Originally Posted by Superman View Post
    try a simple test .... see if travel distance is reported
    correctly. could show encoder roblem or some leadscrew pitch type error.

    use a 200mm slip gauge against chuck face . set dial indicater on it,set to zero ... record Z
    position.
    replace 200mm slip with 50mm block.
    bring dial to block... is movement exactly 150mm ?

    wind Z past zero. then wind Z+ to zero... is both Z positions the same ? ... different means possible backlash error.
    Thanks superman,will try this in the morning & post results



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    Default Re: Strange threading problem...please help

    hi i don't know, try this :

    Code:
        G97 S400 M41 M03 T0303 M08 G64
        G00 X25.26+2.44+5 Z20
        G71 X25.26 Z-50 H2.44 D0.3 U0.07 F2 M22 M33 M74 E0 L0 Q0 G95
    

    try also some spring pases, low doc ( 0.10 - 0.15 ), with both G71 & G33

    put rapid key on 25% / kindly


    we are merely at the start of " Internet of Things / Industrial Revolution 4.0 " era : a mix of AI, plastics, human estrangement, powerful non-state actors ...


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    Default Re: Strange threading problem...please help

    Thanks for your input deadlykitten. I ran the program as you suggested & got the same results. Spring cuts are exactly aligned with the previous cuts.

    I checked backlash as Superman suggested & all is well...less than 0.005mm backlash & spot on using gauge blocks) Although this is an old machine it has done very little work so I don't think wear & tear are an issue, more likely electrical components or belts??

    Is it possible for an encoder to be faulty in this way...threading tracking is fine but pitch is wrong?

    Also, I have found a new old stock encoder for sale but the codes are slightly different. Can anyone tell me if it would be ok?

    Original encoder is a Sumtak LF-102.4BM-S11 12VDC 150ma

    Encoder for sale is Sumtak LF-102.4BM-S11B(A) 12VDC 150ma

    Identical apart from the B(A), any idea what the difference is?

    Any other checks I can do?

    Thanks



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    Default Re: Strange threading problem...please help

    Thinking about it a bit more, wouldn't a faulty spindle encoder on these machines effect tracking of the thread more so than pitch?

    cheers Bruce



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    Default Re: Strange threading problem...please help

    Yes...that would cause wrong RPM / feed ratio.

    So look at spindle RPM displayed verses actual



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    Quote Originally Posted by Superman View Post
    Yes...that would cause wrong RPM / feed ratio.

    So look at spindle RPM displayed verses actual
    Ok, perhaps this is starting to make some sense. In the past I have had to replace the speed sensor on the spindle motor...I will check again in the morning, but I think when I ran it today the needle was showing a little under programmed speed. These things need to be tuned in with a scope, so perhaps it needs doing again. Thanks Superman, gives me something else to check out.



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    Default Re: Strange threading problem...please help

    hello again, i was also going to suggest checking the axis data page, more exactly for Z (diff)erences

    if Z diff is small, then it means that S may have a problem with keeping it constant ( check somehow if S actual value is changing )

    if S is not constant, also normal turning operations are affected / kindly

    we are merely at the start of " Internet of Things / Industrial Revolution 4.0 " era : a mix of AI, plastics, human estrangement, powerful non-state actors ...


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    Default Re: Strange threading problem...please help

    Ok, this morning I checked the spindle speed with a digital tachometer & got the following results:

    Programmed spindle speed 500 - actual speed 493.3
    1000 - actual 992
    1500 - actual 1489
    2000 - actual 1987

    All reading seemed constant over a minute or two of reading.

    So, if the spindle is turning slower than what the control thinks it is the thread pitch should be larger than programmed but in my case it is smaller...as if the spindle was going faster than programmed.



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    Quote Originally Posted by WC56 View Post
    So, if the spindle is turning slower than what the control thinks it is the thread pitch should be larger than programmed but in my case it is smaller...as if the spindle was going faster than programmed.
    I sort of agree with your thoughts....
    but I'm not sure if threading actually locks onto progammed RPM or to it's own actual RPM.

    Kitty suggested the check data page... how do your results compare to the control's checking?



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    Default Re: Strange threading problem...please help

    hello again

    All reading seemed constant over a minute or two of reading
    threading lasts less then 1 minute, so ... hmm, try this :
    ... thread with a bit of delay, like 1-2 minutes after spindle started spining ( maybe it will work, until the specialist will come )
    ... thread normally, without delay, without touching the part ( like above or in front ), and use the tachometer during this period, thus to check the rpm during threading ( try to cut a long thread, like almost full Z travel, so to have time to observe spindle fluctuations; also, try a low rpm, so to increase the threading time )

    So, if the spindle is turning slower than what the control thinks it is the thread pitch should be larger than programmed but in my case it is smaller...as if the spindle was going faster than programmed
    it is possible that the controller has a messed up confirmation signal, and it detects that it is too slow, so he tries to accelerate, but it does not receive in time the answer that desired speed is reached, so it is simply continuing to accelerate ?!?! just like you said " error appears to be progressively consistent "


    is there a specialist in your area ? however, i have no clue with such electronics, i can barely change a light bulb, but here is my advice : you need to check 3rd & 7th phase of the red-yellow twisted cable, between ucs and nsx boards, while the green led is on; if the led is off, you need to wait few seconds, so to discharge the voltage ... use a screwdriver; clean your encoder with spirit ? kindly

    we are merely at the start of " Internet of Things / Industrial Revolution 4.0 " era : a mix of AI, plastics, human estrangement, powerful non-state actors ...


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    Default Re: Strange threading problem...please help

    Ok, checked block data page while threading at 300RPM & it is displaying 288RPM. I am wondering if this could be a problem. I had issues with the spindle motor speed sensor on this machine a while back & replaced it with another used one...perhaps something is going on there.

    Just for a test I programed a 3MM pitch (single start) with a feed of 3.07 & it was pretty close to being right. I just don't know what inputs the control uses to establish the correct feedrate.



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    Default Re: Strange threading problem...please help

    I had issues with the spindle motor speed sensor on this machine a while back & replaced it with another used one...perhaps something is going on there
    you can say that again

    Just for a test I programed a 3MM pitch (single start) with a feed of 3.07 & it was pretty close to being right
    why did you used 3.07 instead of 3 ? i supose your feedrate is ok, thus Z is moving fine

    I just don't know what inputs the control uses to establish the correct feedrate
    do you believe that there is a problem with the feedrate ? check Z axis feed value & diff during cutting / kindly

    we are merely at the start of " Internet of Things / Industrial Revolution 4.0 " era : a mix of AI, plastics, human estrangement, powerful non-state actors ...


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    Default Re: Strange threading problem...please help

    Just as a test I divided the error by the number of threads & got approximately 3.07 so ran a little test thread at 3.07...looked pretty close. Yes I believe Z is moving as it should but not when threading.

    Not sure how to check 'Z axis feed value & diff' during cutting. The block data page displays the programed feed while running, if that is what you mean.



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    Default Re: Strange threading problem...please help

    pls, try to contact a specialist

    Yes I believe Z is moving as it should but not when threading
    why do you believe that ? about this behaviour, i made a wild guess in post 14 ... i may be wrong

    Not sure how to check 'Z axis feed value & diff' during cutting
    when all is ok, if diff value is small, it means that desired motion diagram ( that should reach programmed feed value ), is reproduced well in reality

    diff can be checked inside " axis data " interface, but i don't know for your machine

    on newer machines, i can check ( somehow ) this motion diagram, but for you, even if i could, you still need a specialist / kindly

    we are merely at the start of " Internet of Things / Industrial Revolution 4.0 " era : a mix of AI, plastics, human estrangement, powerful non-state actors ...


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    maybe lathe is shy & it got intimidated by your local wifi

    they have a wild effect, on both machine & machinist

    we are merely at the start of " Internet of Things / Industrial Revolution 4.0 " era : a mix of AI, plastics, human estrangement, powerful non-state actors ...


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    Default Re: Strange threading problem...please help

    ?? any how, I ordered a new spindle encoder...very lucky to fie a NOS one close by at a very reasonable price. Will let you know if this fixes the problem.



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Strange threading problem...please help

Strange threading problem...please help