Concentricity vs. TIR vs. True Position of circular features
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  1. #1
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    Default Concentricity vs. TIR vs. True Position of circular features

    I think I have a fairly good handle on these three geometric tolerancing features. I often wonder why a true position tolerance is called out on a pair of round features on turned parts, where a runout or total runout callout would be better. Concentricity within one thou between the datum round feature and a second round feature would allow .002 indicator reading, yes? A true position of .001 between the same two surfaces would allow only .001? Is this the reason the true position callout is popular on round features, rather than a smaller concentricity callout. I think the total runout callout of .001 also limits the roundness of each feature, and their shared axis of rotation.

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    Concentricity between one round feature (the datum) and another round feature are where the bearings go.

    Part should be inspected rotating around these features rather than the centers.

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    Quote Originally Posted by aerodark View Post
    I think I have a fairly good handle on these three geometric tolerancing features. I often wonder why a true position tolerance is called out on a pair of round features on turned parts, where a runout or total runout callout would be better. Concentricity within one thou between the datum round feature and a second round feature would allow .002 indicator reading, yes? A true position of .001 between the same two surfaces would allow only .001? Is this the reason the true position callout is popular on round features, rather than a smaller concentricity callout. I think the total runout callout of .001 also limits the roundness of each feature, and their shared axis of rotation.
    Might possibly be a side-effect of the general shift in technology of how things are made lo these many years.

    Take two imaginary parts.

    One akin to a motor armature shaft. Several diameters, perhaps a taper, shoulders for bearings and seals, retention nut and its threads. All concentric to a fare-thee well. Old hat, and traditionally 'tween centres lathe work, cone-head through CNC.

    Second part is a disk, three projections, one round, one square, one hex, each a different distance from centre. Concentric? Well...'Sort of'. As if each were the sole member on its own 'bolt circle'. 'Manually' several re-positionings on a rotab, and far more likely done on a mill, rather than lathe.

    With CNC, however, and the CAD/CAM systems wanted to provide marching orders to it, absolute positions and/or relative offsets are more useful than TIR for that sort of work. Essential, even.

    Once such a toolset is in the hands of the designer, more than just 'habit' entices him to stay with that sort of callout, essential or not.

    If, on the NEXT job the only machine-tool available happens to be a multi-axis machining centre, not a conventional lathe (CNC or not), the next one of those straightforward motor armature shafts may find its shape being 'generated' rather than turned as well.

    Just a SWAG, but it looks like that sort of an 'artifact' of the toolset in-use to me...


    Bill

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    Bill has the answer.

    CNC and CMM are fundamentally cartesian co-ordinate systems so concentricity doesn't exist as a native concept. Has to be derived from positional error. So if you are designing and inspecting in cartesian why bother to complicate matters and probably introduce errors with something non-native. You can also work without a real physical reference as its the relationship of the various surfaces which matters.

    Concentricity is a property of polar co-ordinate systems, such as lathe work and always requires a reference. Such as centres. Even if using true position concentricity tolerances should be provided for turned work as there are some issues with true position in this area primarily due to the partial independence of surface form error generation from axis concentricity error.

    Clive

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    I don't know what it is, but I can't understand a thing Bill writes. Is that English? It's like you go out of your way to use the most obscure verbiage you can find. Put down the Thesaurus. I suppose you think it makes you sound smarter or more educated, but it has the opposite effect for me.


    In our inspection department, concentricity is measured as true position. These are axis to axis measurements, so for the purposes of a CMM, they are the same.

    Runout or TIR has an addition component. Runout also includes the condition of the surface. If you had a bore that was elliptical, it could have good concentricity, but poor runout.

    Because runout is a measurement of a surface, it is hard to measure on a CMM. You would have to take a large number of points, or use a scanning probe.

    Many times you will see both roundness and concentricity on a feature. Those two callouts accomplish the same basic thing as runout.

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    Another reason I'm glad I'm retired

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    Quote Originally Posted by ewlsey View Post
    I don't know what it is, but I can't understand a thing Bill writes. Is that English? It's like you go out of your way to use the most obscure verbiage you can find. Put down the Thesaurus. I suppose you think it makes you sound smarter or more educated, but it has the opposite effect for me.
    Me too.

    -----

    Concentricity is by definition a comparison of two axes of rotation. It has nothing to do with the properties of the surface.
    The deviation of actual from nominal is the concentricity of the feature. Surface characteristics are determined by TIR.

    As was mentioned previously, an elliptical feature can be concentric with a cylindrical feature.

    In fact a rectangle can be concentric with a triangle, providing both axes can be properly identified and located.

    - Leigh

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    I think the criticism of Bill's response is a bit heavy-handed... Made perfect sense to me, but more as a generalized commentary on design and tolerancing practices as driven by the advances in both use of computerized design technology, and the translation of those practices into manufacturing reality. For my part, I agree that the use of true position call-outs has expanded greatly over the last 10-15 years, often as a one-size solution that, in some cases, could be better solved with more germane and specific tolerancing practices. I see this particularly with round vs. rectilinear work, as several have noted.

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    Quote Originally Posted by specfab View Post
    I think the criticism of Bill's response is a bit heavy-handed... Made perfect sense to me

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    Quote Originally Posted by specfab View Post
    I think the criticism of Bill's response is a bit heavy-handed...
    He reminds me of an engineer, and I like it.

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