All ODs on the part need to be concentric within 0.07 TIR - What does this imply

T

True Position

#11
Re: All ODs on the part needs to be concentric within 0.07 TIR - What does this imply

Y14.5 specifies concentricity as median points of the controlled feature relative to a datum axis. ;)

Stijloor.
Right, and a square has exactly the same median point between opposite sides so it is perfectly concentric.
 
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Jim Wynne

Staff member
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#12
Re: All ODs on the part needs to be concentric within 0.07 TIR - What does this imply

Right, and a square has exactly the same median point between opposite sides so it is perfectly concentric.
There was a time, in the deep dark past, when dinosaurs roamed the earth and GD&T was not widely known or used. The callout in question was written in those dark ages, and it was commonly seen in the wild. While "concentricity gage" is certainly a misnomer, that's what those things have always been called. At the time the drawing in question was issued, standard practice was to use something like that (or even the centers on a lathe) to rotate the part while indicating on the target diameter. Standardized GD&T came into being (in large part) because of issues like this.
 
T

True Position

#14
Re: All ODs on the part needs to be concentric within 0.07 TIR - What does this imply

There was a time, in the deep dark past, when dinosaurs roamed the earth and GD&T was not widely known or used. The callout in question was written in those dark ages, and it was commonly seen in the wild. While "concentricity gage" is certainly a misnomer, that's what those things have always been called. At the time the drawing in question was issued, standard practice was to use something like that (or even the centers on a lathe) to rotate the part while indicating on the target diameter. Standardized GD&T came into being (in large part) because of issues like this.
Agreed, the use of concentricity when the intent is runout is very common still now. Occasionally an engineer will see 'concentricity' on his list of GD&T symbols and put it on a drawing intending runout. 'Coaxial' is also used the same way.
 

Stijloor

Staff member
Super Moderator
#15
Re: All ODs on the part needs to be concentric within 0.07 TIR - What does this imply

Agreed, the use of concentricity when the intent is runout is very common still now. Occasionally an engineer will see 'concentricity' on his list of GD&T symbols and put it on a drawing intending runout. 'Coaxial' is also used the same way.
Yes, very much so... Still a lot of (educational) work to do...

Stijloor.
 

bobdoering

Stop X-bar/R Madness!!
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#16
Re: All ODs on the part needs to be concentric within 0.07 TIR - What does this imply

Off topic but interested:
I thought you could not check roundness accurately rotating directly on the part due to the potential of getting a part with lobing that hides some of the out of round. Same reason you can't use a V-block? I was always under the impression that you needed something which moved completely independent of the part. (Like the two types of roundness checkers or a CMM.)
You are correct. If you place the indicator over the rollers, it is roundness. It may contain error by not being correct for the lobing in the part. If you had 3 lobe parts, it would be better than a micrometer (2 point measurement - which is only good for 2 lobe parts). Very similar to the process for choosing the correct number of jets on an air gage.

To capture roundness without influence of lobing error (and better yet, actually determining the lobing) you would have to have a roundness checker (e.g. Talyrond) or CMM with sufficient data points. However, as a lower cost (although they are not cheap!) option for shop floors, it may be adequate to use the "concentricity" gage for roundness if the lobing is known to be 3-lobe.
 

bobdoering

Stop X-bar/R Madness!!
Trusted Information Resource
#17
Re: All ODs on the part needs to be concentric within 0.07 TIR - What does this imply

Agreed, the use of concentricity when the intent is runout is very common still now. Occasionally an engineer will see 'concentricity' on his list of GD&T symbols and put it on a drawing intending runout. 'Coaxial' is also used the same way.
I just saw a print where they called out concentricity instead of roundness (which is what they really meant). I made sure the engineer currently responsible for that part corrected the error - fast. Big difference!!
 
N

naghee

#18
Re: All ODs on the part needs to be concentric within 0.07 TIR - What does this imply

I have to agree with True Position when he/she states that they probably mean runout. Concentricity would apply to the centerlines and not the diameter surface. Measuring the TIR on the surface of the diameter compounds the error by accumulating both the concentricity variation and the roundness varition together into one TIR number.
Not to confuse matters anymore, but has any consideration been given to total runout versus regular (any element) runout?
:confused:
The Impeller is mounted on a tapered shaft with a locking mut hold it in position. Then, the dial gauge was set, and the impeller rotated on its center axis.

No i have not considered the total runout.

I feel that, the note is not clear enough to say " the ODs need to be within a certain tolerance with reference to the OD."

If held by the shaft, and measure TIR of ODs "A", "B" and "C" what is the acceptable TIR as per the note?
Held by OD "A" and measure TIR of OD "B" and "C" what is the acceptable TIR as per the note?
held by OD "B" and measure TIR of OD "B" and "C" what is the acceptable TIR as per the note?

The problem is one of the OD is verymuch offcentric to the centerline, and I am not sure if I need to fix the drawing note.
 
T

True Position

#19
Re: All ODs on the part needs to be concentric within 0.07 TIR - What does this imply

The Impeller is mounted on a tapered shaft with a locking mut hold it in position. Then, the dial gauge was set, and the impeller rotated on its center axis.

No i have not considered the total runout.

I feel that, the note is not clear enough to say " the ODs need to be within a certain tolerance with reference to the OD."

If held by the shaft, and measure TIR of ODs "A", "B" and "C" what is the acceptable TIR as per the note?
Held by OD "A" and measure TIR of OD "B" and "C" what is the acceptable TIR as per the note?
held by OD "B" and measure TIR of OD "B" and "C" what is the acceptable TIR as per the note?

The problem is one of the OD is verymuch offcentric to the centerline, and I am not sure if I need to fix the drawing note.
From how you posted the note, "All ODs on the part needs to be concentric within 0.07 TIR" for each of your examples the answer is 0.07.

I would setup holding on each diameter one at a time and check all of the other OD to 0.07. Rewriting it to say runout is a good idea, but the intent seems clear.
 
A

alspread

#20
Re: All ODs on the part needs to be concentric within 0.07 TIR - What does this imply

The problem is one of the OD is verymuch offcentric to the centerline, and I am not sure if I need to fix the drawing note.
You have the ability to change the print? I must have missed that. I assumed you were working with an old drawing that was going to be difficult if not impossible to correct.
If you have the ability to change the print, then by all means you should. Modern practice would dictate establish ment of an apropriate datum and specifying the relationship requirement of other features to the datum.

If the diameter that is eccentric has no function or impact on the performance of the system, then perhaps you can apply a larger tolerance to that feature.
 
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