cad13992

got this from GDT GUYS fourm (aerospace)

Concentricity is categorized as a tolerance of location, not form. However, surface variations of the controlled diameter can have a direct impact on the end result because a concentricity tolerance invokes a requirement which is much different than what most people realize.

When most people see a concentricity tolerance applied, they interpret the callout just as though it were a positional tolerance. In fact, many people think that concentricity is just another way of specifying position. But, in reality, concentricity is much different.

In ASME Y14.5M-1994, concentricity is defined as “the condition where the median points of all diametrically opposed elements of a figure of revolution (or correspondingly-located elements of two or more radially-disposed features) are congruent with the axis (or center point) of a datum feature.”
To verify "all the median points of all diametrically opposed elements" would require an extensive and time-consuming analysis of the surface in relation to the datum axis or point. Variations and irregularities in the surface will provide a different derived median point at each location measured, resulting in a "cloud" of derived points, all of which must lie within the specified concentricity tolerance zone. If even one of these median points lies outside of this zone, the part is rejected.

I have been working with for a lot of years, and I have never run in to a design situation where I was really interested in knowing where "all the median points of all diametrically opposed elements" resided on a part feature. I have always found that, for coaxial features, the design interest is in controlling either:
(a) the axis of the actual mating envelope to the datum axis,
or
(b) the surface location to the datum axis

Control of the axis location is usually applied when the primary design consideration is location of the feature to assure fit or assembly with a mating part, such as with a counterbored hole, shoulder bolt, etc. Ususally this is accomplished using a positional tolerance; not concentricity.

Control of the surface location is usually applied when the primary design consideration is distribution of the surface about a common center and is usually applied for parts that rotate, such as a pully or drive shaft. Control is accomplished using either a runout tolerance (circular or total) or a profile tolerance; but not concentricity.

In earlier versions of the Y14.5M standard, there was a statement that said that concentricity was used to control dynamic balance of a rotating part. But, they have since removed that statement because there are many factors other than median point distribution that contribute to proper dynamic balance.(such as distribution of material density) In fact, ASME Y14.5M-1994 doesn't provide any clue to the user as where concentricity should be used and even recommends that either a runout or positional tolerance be used instead of concentricity.

Concentricity is not well understood by all, it is expensive and time consuming when it is properly verified, it doesn't provide a control which logically supports most design cnsiderations and the defining industry standard even recommends that you use another type of control. This is pretty compelling evidence to stay away from using a concentricity tolerance if you ask me.