Concentricity vs. Runout - Page 2
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  1. #21
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    Quote Originally Posted by JNieman View Post
    If you only meant for the hexagon to illustrate how concentricity can be perfect while form is horrible (like it was /supposed/ to be a cylinder but you ended up with a hexagon for hypothetical reasons) then sure, you've got it spot on.
    This is what I was going for. I was mainly expanding off Halco's comment earlier.

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    Concentricity is an absolute term, runout is a relative term.

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    Quote Originally Posted by Halcohead View Post
    There are important differences between runout, total runout, and concentricity:

    Concentricity is how well a shape is centered on a theoretical axis, regardless of roundness. So a hexagon can theoretically be perfectly concentric to a datum axis while having huge runout due to the fact that it isn't round.

    This is technically incorrect. Concentric is a geometric term, not a machining term. "Cone Centric" means circles made by slicing a cone with planes perpendicular to a line running through the focus, thus making circles with the same focus, but differing diameters.

    A hexagon cannot be a cone, only a six sided pyramid, so therefore cannot be concentric.

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    Quote Originally Posted by genther View Post
    This is technically incorrect. Concentric is a geometric term, not a machining term. "Cone Centric" means circles made by slicing a cone with planes perpendicular to a line running through the focus, thus making circles with the same focus, but differing diameters.

    A hexagon cannot be a cone, only a six sided pyramid, so therefore cannot be concentric.
    How about I add the term coaxial. Isn't this the same thing as concentric?

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    Quote Originally Posted by genther View Post
    A hexagon cannot be a cone, only a six sided pyramid, so therefore cannot be concentric.
    7.6.4 Concentricity
    Concentricity is that condition where the median
    points of all diametrically opposed elements of a
    surface of revolution (or the median points of correspondingly
    located elements of two or more radially
    disposed features) are congruent with a datum axis (or
    center point).
    "Surface of Revolution" means you're technically correct. However, the hexagonal example given previously was a purely hypothetical deviation from a circular shape. It's just showing how a control on form and location can differ. Nominally it should've been circular. The hexagon was an exaggerated example of deviation-of-form.

    Examples of other exaggerated form variations can be Figures 7-60 and 7-61 in the 2009 standard.

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    Resurrecting this thread after a couple years since it's apparently still being referenced.

    I stand by my original post, and in case it isn't clear I'm referring to the technical use of GD&T terms in engineering and manufacturing. I would offer a clarification that in the actual GD&T concentricity definition, it should only be applied to features of revolution: Concentricity – GD&T Basics. So yes the hexagon was just a theoretical example of something that was meant to be round, but obviously isn't, but is still perfectly concentric.

    However as a design engineer I've never had a shop or inspector complain about using concentricity to locate a hexagonal feature, and it's often faster and simpler than other ways of tolerancing the location of a heaxgonal feature machined into a shaft.

    Another way to think of concentricity is it's "True Position" applied to a cylindrical feature that is coaxial to the referenced datum axis.

    Quote Originally Posted by JNieman View Post
    That hexagon would only have 0.058" runout if it was /supposed to be a cylinder/

    Runout is the measurement of deviation from the nominal condition. Not the measure of deviation from another feature. You also wouldn't wisely use runout on a hexagonal shaft.
    The above is incorrect. This is conflating "runout" with "profile tolerance". "Profile" is deviation from a nominal condition for an arbitrary contour. "Runout" is the same as "profile applied to a cylindrical feature", however runout intrinsically assumes (and can only be applied to) a cylindrical feature. That hexagonal feature can be perfect (meet a .00001" profile tolerance), and it will still have .058" runout, because it still is not round. Runout is concentricity PLUS circularity: Runout – GD&T Basics. However to be fair, a common colloquial (shop floor) use of "runout" is to mean deviation from the nominal intended surface (for example people will say how much indicated runout they saw when measuring the parallelism of two flat faces).

    To rephrase things a different way, "runout" is a special case of "profile of a line" applied to a feature of revolution (cylinder, cone, etc). Similarly, "cylindrical runout" or "total runout" is a special case of "profile of a surface" applied to a cylindrical feature.

    I've also asked questions like this in many many interviews of both engineers and machinists. It's very rare that candidates have a good handle on these nuances, but that's usually less important than whether, when the time arises, they're willing and able to learn and understand.

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    Runout = concentricity + circularity (or roundness)

    Look the individual definitions up here: GD&T Symbols | GD&T Basics

    Doesnt get any simpler than that

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    I considered concentric to be good/ or error to the same center
    So, a certain shaft OD and a bearing fit (end) diameter may have a .0001 difference to exact same center.
    Checking that part with the OD held dead and an indicator at the bearing fit would show the gain side + .0001 and the fall-off side minus .0001 so the TIR (total indicator reading) would be .0002. I also called this run out.
    To have one end bearing fit diameter + .0002. TIR (to shaft center)and the other end bearing fit diameter + .0002 checked between centers, but the high points 180 degrees apart the error would be .0004 one end to the other.

    Axial error would/might be the action/result of running a part on an error live center or held in a chuck or collet...not straight to the center line.

    Also, Out of a chuck indicated a place on the part to dead zero this condition can still be there… I called this wobble and it can change at different checking points along a diameter. Yes one may use highest precision bearings and have .0001 to .0003 error in a shaft end to end and wonder what is wrong.

    Perhaps the best method to achieve a straight and concentric part is to run a part between dead centers for the finish pass.

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    Quote Originally Posted by KristianSilva View Post
    Runout = concentricity + circularity (or roundness)

    Look the individual definitions up here: GD&T Symbols | GD&T Basics

    Doesn't get any simpler than that
    I like it but simple?

    Anything that gives discussions and needs definitions isn't what I call simple

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    Quote Originally Posted by Gordon B. Clarke View Post
    I like it but simple?

    Anything that gives discussions and needs definitions isn't what I call simple
    You may well be right!

    But the best way to end a discussion is with a definitions, you cant argue with them!


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