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Concentricity again

Concentricity again

Concentricity again

I am having difficulty explaining the difference between gd&t concentricity and t.i.r. to a foreign customer.
The application is a simple straightforward condition of diameter to diameter.

His position is this:

The callout on his drawing is concentricity (doubled circle symbol) = .008.  His inspection shows Total Indicated runout of .010. He claims that this is concentricity.
I have tried to explain the ASTM definition of both, but to no avail.
My position is that in this case, T.I.R. approximates twice the out of concentric condition, and that his T.I.R. result should be divided by two.

Does anyone know of a simple definition for this premise that I can show my customer?

I am obviously not arguing with this guy, but I could use some help on this one.



RE: Concentricity again

I don't know mathematically how TIR and concentricity can be related.  What I do know is that to inspect concentricity, you simply measure OD and ID rings through the length of the cylinder.  The rings have to be concentric.  TIR, on the other hand, states that the entire ID and OD cylinders have to be within the tolerance, not individual rings along the cylinder.


For some pleasure reading, the Round Table recommends FAQ731-376

RE: Concentricity again

Not knowing all the details, I would have to agree with your customer.  A concentricity tolerance of .008 means your tolerance zone is .008 in diameter and that the two axis must be no more than .004 apart.  If the TIR is .010 then the axis are .005 apart.  Of course we have to assume here that the parts are round.  Out of roundness would show up on the indicator and affect the apparent concentricity.

RE: Concentricity again

The fundamental difference between concentricity and “T.I.R” (Total Indicator Reading) is that they are actually looking at completely different elements of the controlled diameter.  Concentricity is a type of location tolerance that looks ONLY at the derived median points of a feature and their relation to a datum axis.  “T.I.R” is essentially the method used in verifying a total runout specification.  Total runout is defined as a composite SURFACE control that controls location, orientation and form all within one single tolerance specification.

Per ASME Y14.5M-1994 concentricity is defined as follows:

5.12.1 Concentricity Tolerancing.  A concentricity tolerance is a cylindrical (or spherical) tolerance zone whose axis (or center point) coincides with the axis (or center point) of the datum feature(s). The median points of all correspondingly-located elements of the feature(s) being controlled, regardless of feature size, must lie within the cylindrical (or spherical) tolerance zones. The specified tolerance and the datum reference can only apply on an RFS basis. See Fig. 5-54. Unlike the control covered by para. 5.1 1.1, where measurements taken along a surface of revolution are made to determine the location (eccentricity) of the axis or center point of the actual mating envelope, a concentricity tolerance requires the establishment and verification of the feature’s median points.

Per ASME Y14.5M-1994 total runout is defined as follows for a “coaxiality” type of control. Total Runout for Composite Control of Surfaces. Total runout provides composite control of all surface elements. The tolerance is applied simultaneously to all circular and profile measuring positions as the part is rotated 360". See Fig. 6-48. Where applied to surface, constructed around a datum axis, total runout is used to control cumulative variations of circularity, straightness, coaxiality, angularity, taper, and profile of a surface. Where applied to surfaces at right angles to a datum axis, total runout controls cumulative variations of perpendicularity (to detect wobble) and flatness (to detect concavity or convexity).

Using a TIR methodology to verify a concentricity specification is technically incorrect based on the standard interpretations.  Yet, I see it done this way time and time again and parts that pass this check seem to work just fine in their intended application.  This tells me that runout, and not concentricity, was probably the specification that should have been applied in the first place.


RE: Concentricity again

In simple terms, runout includes variation in form (roundness) as well as variation in location.  Therefore, runout will always be greater than or equal to concentricity.  For runout to be equal to concentricity the roundness must be perfect (zero).  There is no other relationship between the two.  Most people prefer to measure runout as it is simpler and faster.  If the TIR meets the spec then you can be sure the concentricity is good.  If the TIR is higher that the concentricity then you can not say whether the part is good or not.  In this case it MAY be but you will have to measure concentricity to be sure.

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