cobb, Ansi y14.5 deals with issues of geometric tolerancing. This is a tolerance of form. As has been continually repeated, but seems to fall on deaf ears, geometric tolerances or tolerance of form is one of the most expensive of tolerancing methods, and should not be specified except when established shop procedures cannot be depended on to produce the necessary accuracy.
With that said, a concentric tolerance, controls a part form, take for example a shaft with two different diameters, the concentricity tolerance tries to hold the two centerlines of the two diameters to a given eccentricity. Essentially it means that the deviattion from a single centerline is controlled by x amount.
saxon, thanks for the info. Maybe I should have asked my question more clearly, I understand what concentricity is and how to inspect it. Im looking for someone that has applied it to a component and can tell me what the component was and the function of this component upon final assembly.
The only time I have used Concentricity is in a weldment drawing. I had a plate with a hole, and a another plat/bushing that was being welded to it. I indicated that both holes should be Concentric with each other.
In the example that Cobb listed, and for most single part drawings, I use Runout or Total Runout over Concentricity, especially when I know the part can be inspected for Runout. If I know it can't be inspected correctly, then I resort to Concentricity. "The attempt and not the deed confounds us."
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.
GDTGUY thanks for your imput, I agree with what you said 100% I have not yet seen a pratical application to use concentricity, thats why I asked if anyone else has. Scott Cobb
CSM Manufacturing, Inc
csm-mfg.com
cobb,
As saxon stated, two different shaft diameters. Two bore diameters. A circular OD relative to a bore. Back in the day, this was described as TIR, Total Indicator Runout. I just completed a special hydraulic cylinder with at least 16 concentricity callouts.
