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Concentricity on the thread - does it make sense when related to other holes ?
4

Concentricity on the thread - does it make sense when related to other holes ?

Concentricity on the thread - does it make sense when related to other holes ?

(OP)
Hi,

Can someone help me solve onging discussion wihin Engineering dept. I enclosed the example of Concentricity marked on the thread realted to two smaller diameters located before the thread itself.
I've been trying to find a standard which expressly explains if its according to GD&T rules.

1) Is this annotations by the thread correct to the GD&T rules ?

2)It is possible to measure the minor diameter of the thread and its position to other holes to establish the concentricity, but is it possible somehow to measure the thread concentricity ot it would be enough med minor diameter of the thread ?

Explanations will be more than appreciated as basically there is a war going on ! :)

Bartosz

RE: Concentricity on the thread - does it make sense when related to other holes ?

Is this an ISO GPS or ASME drawing?
ASME: concentricity should be replaced with position.
ISO: (I have not much exposure), but concentricity is a special case for position and seems legitimate callout with the axis derived/extracted from the pitch diameter being the one controlled.

RE: Concentricity on the thread - does it make sense when related to other holes ?

(OP)
Hi,

We do not use one standard internally therefore we have a place for such discussion unfortunately.

I just got a feedback that it is totally stupid which i I belive it is not and trying to justify the usage of it.

I've seen plenty of drawings with the tolerance like this therefore I started search for standard.

How to physically measure the pitch diamter in this case ?

Thx.
Bartosz

RE: Concentricity on the thread - does it make sense when related to other holes ?

Quote (ketrab88)

We do not use one standard internally ........
That's too bad!

Quote (ketrab88)

How to physically measure the pitch diamter in this case ?

You don't measure it. You gage it! (If the thread is called/modified at MMC)

RE: Concentricity on the thread - does it make sense when related to other holes ?

This would be an incorrect use of concentricity if ASME is invoked. The drawing appears to be an ISO one though.

John Acosta, GDTP Senior Level
Manufacturing Engineering Tech

RE: Concentricity on the thread - does it make sense when related to other holes ?

2
Concentricity is a measure that opposing elements are equidistant from a reference. Unless that's a double pitch thread, there are no opposing elements. Directly across from a thread peak will be a thread root. A position tolerance is considered to apply to the theoretical pitch diameter surface. The pitch diameter is where the thread is evenly divided between tooth and tooth-space. This is hugely expensive to determine.

Instead, one uses the concept of MMB (Maximum Material Boundary) and substitutes a precision gage that represents the MMC (Maximum Material Condition) condition of the threaded part. This allows using the Position tolerance which accepts variation from MMB as a modifier.

The expected problem is that if the spiral of the thread isn't exactly coincident with the datum and the thread is not at the MMB boundary, when the thread is tightened it will be pulled off center, so that condition needs to be analyzed to see that there are no deformation related problems.

Since nothing is perfect and detailed inspection of screw threads is unsuitably expensive, this is the best option. If being off-center is a problem, then the design usually separates the alignment function from the threading/load holding function using a separate threaded nut.

RE: Concentricity on the thread - does it make sense when related to other holes ?

(OP)
Hi,
Thank you very much for all your answers :)

RE: Concentricity on the thread - does it make sense when related to other holes ?

ketrab88:

As a side bar to the discussion: From my experience, Concentricity is applied to put the mass of the part on the same center. Conceptually, you can have a square Concentric to a triangle - the features do not need to be surfaces-of-revolution (cylinders, etc). This typically is for rotating parts where dynamic balancing (vibration control) is a design requirement.

Certified Sr. GD&T Professional

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