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Thread: True Position of ZERO

  1. #1
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    True Position of ZERO

    We have an engineer that is designing parts and he is using a True Position of Zero, then, per his statement "building the tolerance into the hole tolerance, or movement away from MMC"

    In the 40 years of machining, I have NEVER seen Zero True Position. I need some support from someone that is an expert in this area to dispute this method, or I need to be open for change.
    Thanks,

    Brett.....

  2. #2
    Technical Fellow jboggs's Avatar
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    I don't understand what "true position of zero" means. Can you post an example?

  3. #3
    Administrator Kelly Bramble's Avatar
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    If you had posted an image or exact details of the position tolerance specification we could chart it out for you..

    Does the specification include a MMC modifier (Circle M) after the zero tolerance?

    Something like this?

    zero-position.png
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  4. #4
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    Quote Originally Posted by Kelly Bramble View Post
    If you had posted an image or exact details of the position tolerance specification we could chart it out for you..

    Does the specification include a MMC modifier (Circle M) after the zero tolerance?

    Something like this?

    zero-position.png
    Yes, this is how it is written. The True Position Indicator, with a MMC symbol, the Hole size and tolerance. The only difference is he has a +/- of .025" (Inch)
    Brett.....

  5. #5
    Administrator Kelly Bramble's Avatar
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    The MCC modifier means that at that specified size (Maximum Material Condition) which the smallest as-built allowed by tolerance for a hole feature or the largest allowed as-built by tolerance and for a pipe, stud (external feature) that a zero position tolerance is applicable at that specific as-built size

    As the feature of size varies from the stated condition from MMC to LMC (Least Material Condition) an increase in the specified positional tolerance is allowed equal to the amount of size divergence of the as-built feature from MMC.

    This may seem confusing and I'm not prepared to train you on an engineering forum so...

    See these webpages and experiment with the numbers within the calculator.

    For a hole (internal) feature see:

    https://www.engineersedge.com/calcul...-tolerance.htm

    For an external feature see:

    https://www.engineersedge.com/calcul...-tolerance.htm

    I would agree that a zero tolerance at MMC position specification can seem confusing and contradictory, however this is a legitimate specification.

    Also, I’m reasonable sure the person that specified the requirements is formally training in GD&T. You should consider doing that as well.

    Here are some books you might consider studying:

    Geometric Boundaries III per. ASME Y14.5-2009
    https://www.engineersedge.com/catalo...oducts_id/632/

    Geometric Boundaries IV per. ASME Y14.5-2018
    https://www.engineersedge.com/catalo...oducts_id/656/
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  6. #6
    Technical Fellow jboggs's Avatar
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    Drawing standards are great, and valuable, but if they are not clearly understood by both the creator and the user unfortunately they can be just another source of confusion. I don't know the environment we're talking about here but I might point out to the engineer that his true objective is to create a functional part, not a "perfect" drawing. Absent other external standards requirements, he should consider the level of understanding of the users of his drawing in his quest to create a functional part.

  7. #7
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by jboggs View Post
    Drawing standards are great, and valuable, but if they are not clearly understood by both the creator and the user unfortunately they can be just another source of confusion. I don't know the environment we're talking about here but I might point out to the engineer that his true objective is to create a functional part, not a "perfect" drawing. Absent other external standards requirements, he should consider the level of understanding of the users of his drawing in his quest to create a functional part.
    I agree in ~ general with your statement; however engineering, manufacturing, quality and others whom must create or interpret an engineering drawing should have sufficient training and or experience to understand industry standard specifications.

    In this case, zero tolerance at MMC is not that big a deal for one whom understands the basics of tolerance modifiers. When I train or consult with my customers, I do point out the confusion this specification tends to create with some and that they should be prepared to answer any questions that may arise.

    I’m sure that the originator of this topic is aware that they can and should consult with the engineering drawing originator when a specification seems non-normal. The engineering drawing originator should be able to answer with logic and clarity.
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  8. #8
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    All I wanted to know is whether the way the engineer is calling it normal. I always thought that there should be some sort of locational tolerance even at MMC. I have never seen someone make a feature with no tolerance when the hole is at it's smallest diameter. Has anyone else ever seen a no tolerance feature at MMC? We make Rockets, so this is in the Aerospace industry.

    Brett....
    Last edited by brett7506111; 07-16-2020 at 11:21 PM. Reason: typo

  9. #9
    Technical Fellow jboggs's Avatar
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    Kelly,
    Agreed.
    Joe

  10. #10
    Administrator Kelly Bramble's Avatar
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    Quote Originally Posted by brett7506111 View Post
    All I wanted to know is whether the way the engineer is calling it normal. I always thought that there should be some sort of locational tolerance even at MMC. I have never seen someone make a feature with no tolerance when the hole is at it's smallest diameter. Has anyone else ever seen a no tolerance feature at MMC? We make Rockets, so this is in the Aerospace industry.

    Brett....
    It has a tolerance when not produced at MMC size. Ultimately, the value of this specification is that the are more size and as-manufactured position tolerances available for manufacturing.

    It's actually a good thing. Go learn what it means....
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

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