GD&T Concentricity (Don't Use It)

The GD&T standard of Concentricity is not what you might think. The most common fact you can find about concentricity is that it is not what most people expect, and that most people use it wrong.
Concentricity doesn’t actually make sure that circles are concentric. What it does do is make sure that the median points of all diametrically opposed sides of an object fall inside a circular TZ. What does that mean?
It means that if we take a cross section of this pin and control it for concentricity, we would then need to measure all diametrically opposed points and find their median. We can do that by drawing a line through the ideal axis of the pin, measuring the two points that line intersects with the pin’s edge, and calculating a median point, and then repeating that process around the entire pin.
When we finish we would find a large number of median points, and if they all fell within this TZ, than the concentricity control has been met.
If concentricity was called for a pin, this process would then need to be repeated up the entire length of the pin, and every median point of diametrically opposed surface points at every cross section, would need to fall inside the defined cylindrical TZ.
Difficulties
Concentricity is most likely not the control you would want to use for any situation for a few reasons.
Concentricity is difficult and expensive to gauge. A 3D scan of the entire surface, combined with expensive software to calculate every median point of every cross section is the only practical way to gauge.
This is made even harder by the fact that concentricity is in relation to a datum or simulated axis, instead of a real axis.
If a part was made with an attached pin like this, all the median point calculations would need to be compared not to the pin itself, as with cylindricity, but to the datum axis which might not be in the center of the pin.
Concentricity is difficult to gauge because it requires comparing derived median points to a TZ based on datums.
Concentricity might also not be what you want because it doesn’t guarantee the surfaces are round.
For instance, this fly wheel has diametrically opposed high and low points, so all median points of all cross sections fall inside a small TZ, but this is likely not a shape most people would think as having high concentricity.
One of the few times concentricity might be what you want is when rotational balance is important, such as the fly wheel. One thing concentricity does do is guarantee an equal distribution of mass around an axis, which provides balanced rotation. Even this is only true if the material of your part is perfectly uniform.
In most cases, what you should know about concentricity is that you probably want a different control for whatever you’re doing.
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[@jamescarter9147]

Thanks for the video, a suggestion of which different control to use would be helpful.

[@RDTengineered]

total runout. runout. position.

[@lakshmipathi8340]

I have one doubt, for concentricity inspection there is alignment required

[@deadhead7930]

Dont use concentricity? Your statement is ridiculous. Your misleading beginners on a very important subject. A subject that can termine if someone lives or dies - coming from aerospace industry, its the most important aspect - safety. A lot of blueprints in aviation use concentricty. Then again - anyone can create a YT channel and "teach" things.

[@mscscambodia]

If this is so why did ASME do away with concentricity (and symmerty) symbols in the 2018 standard?