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Thread: Position tolerance with circularity

  1. #1
    Associate Engineer
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    Sep 2019
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    Position tolerance with circularity

    Hello,

    I am designing a circular flange with a fairly large OD. There are two different ID diameters that need to be held as coaxial as possible. Since the wall of the part is somewhat thin, I am calling out free state circularity. In order to hold them coaxially, I was planning on calling out a true position on the 9.254-9.252 diameter back to datum B in the attachment. Is this a good method? My thinking is that even in the free state, the positional callout should maintain the diameters coaxially. In this way, our CMM operator could measure the circularity and positional tolerance in the free state without the complexity of using an inspection fixture to constrain the part. I would not expect the measured positional tolerance to change much in a free state vs a constrained state.
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  2. #2
    Administrator Kelly Bramble's Avatar
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    By default and regardless of the dimensioning and tolerancing standard specified on the engineering drawing - All dimensional measurements shall be made with the part in a free state condition unless otherwise specified.

    Inspection should NOT be using an "inspection fixture" to restrain the part during measurement unless otherwise specified.

    So, your Free State specification is meaningless.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

  3. #3
    Associate Engineer
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    Understood. So, If I keep the free state modifier for circularity, I could add the positional tolerance with a note specifying how to constrain the part to measure that positional tolerance, correct?

    Another scenario that I thought of was to eliminate the free state modifier for my circularity, add the positional tolerance, but keep the AVG diameter. Is this OK?
    -The average diameter would allow more relief on the circularity based on section 5.5 in the 2009 standard. Although, from section 5.5.3, it appears that AVG is not used unless the free state symbol for circularity is used and then a constraint is specified to measure another feature. If that is true, I am not sure why AVG could not be used alone since measuring in the free state condition is default.

  4. #4
    Principle Engineer
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    Why not tolerance a wall thickness to the O.D. that can be checked with a simple ball or tube micrometer instead of relying on CMM and inspection tables. Think like a machinist.

  5. #5
    Administrator Kelly Bramble's Avatar
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    Function first, it needs to work - then consider manufacture-ability.

    There is not a requirement that "AVG" diameter must include a "Free State" specification - it's just an example.

    ASME Y14.5-2009, para 4.2 overviews specifying a "Retrained Condition" - which is typically a note, drawing illustration, details to simulate install requirements.

    ASME Y14.5-2009, para 5.5.1 - 5.5.3 lots said here but know that it is not uncommon to have dual dimensional requirements for part features. Such as size and tolerance variation specifications for restrained and unrestrained. This is where the Free State specification is helpful on an engineering drawing. We might want our free state variations to not be too large and prevent install or create excessive material stress after install.
    Tell me and I forget. Teach me and I remember. Involve me and I learn.

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