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concentricity, gd&t (geometric dimensioning & tolerancing), measurement (general)
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  #1  
Old 15th November 2005, 04:33 PM
Mike S. Mike S. is offline
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Question Concentricity Measurement - How to correctly measure concentricity

Can someone please explain in simple terms (for someone who is a mental midget when it comes to geometrical issues) how to correctly measure concentricity? Hopefully using simple tools...

Say I have a part that is 1.000" OD x .500" ID x .400" thick. The .500" hole is supposed to be in the center. Say the concentricity spec. is .020".

How do I measure concentricity on this part?

I hope I won't regret asking, but how does this relate to (or not relate to) TIR?

As always, thanks in advance for the help!

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  #2  
Old 15th November 2005, 05:07 PM
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pilchard pilchard is offline
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Look! Cut & Paste

First, determine the functional requirements of the component in question relative to the assembly, which the component will be installed. With this, review the following application and measurement description for (TIR) total runout and concentricity.

(TIR) Total Runout provides a composite control of all feature elements. Total runout will control variations of circularity, straightness, coaxiality and taper of a feature. Total Runout can be measured with a dial indicator, cmm or other methods. The indicator is located (fixed) relative to the selected datum and the indicator is zero'ed relative to the surface in question. The component is then rotated and the total dial indicator reading is observed while the component is rotated. The indicator reading should not exceed the tolerance given by the specification. So, the total runout specification measures surface deviations relative to the reference axis.

Concentricity is complex and rare as it controls opposed median points to a datum axis. Concentricity will control location and only has some effect on the form and orientation of the feature. Concentricity will not control the form of perfectly oval parts but may have an impact on irregular or "D" shaped features.

Concentricity may be verified with dial indicators, a CMM or by other methods. If dial indicators are used, two diametrically opposed, mastered indicators are placed on either side of the feature and positioned and rotated about the datum. Several readings are obtained at each selected cross section along the entire length of the feature.

To measure for concentricity: Imagine or draw a directional compass on a piece of paper with the north, south, east and west indicated on the compass. Now locate one dial indicator at the east position and one dial indicator at the west position. Lets say that the east dial indicator is reading east and moving east at .010" and the west indicator is reading west moving west at .010". You would subtract the indicator readings from each other to obtain the concentricity tolerance which is .000= .010-.010 at that particular cross section. The indicators have "cancelled" each other out since they moved in opposite directions during measurement. Now rotate the part 90 degrees so the opposed dial indicators are at the north and south positions. Lets say that now you have a reading at the north position and moving south .005" and at the south dial indicator you have a reading of .005 moving in the south direction. You now add the reading together to get .010 = .005+.005. Since both indicators changed in the same direction (south) you would add the indicator readings to obtain the concentricity tolerance. You would repeat this process at many cross section along the entire length of the feature in question.

So the question one needs to ask when deciding on TIR vs. Concentricity is what do you want to control? surface deviations from a datum axis (TIR) or opposed points (sum and diff) relative to a datum axis (Concentricity)?


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  #3  
Old 15th November 2005, 08:46 PM
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JRKH JRKH is offline
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Lets try something simpler

Quote:
In Reply to Parent Post by Mike S.

Can someone please explain in simple terms (for someone who is a mental midget when it comes to geometrical issues) how to correctly measure concentricity? Hopefully using simple tools...

Say I have a part that is 1.000" OD x .500" ID x .400" thick. The .500" hole is supposed to be in the center. Say the concentricity spec. is .020".

How do I measure concentricity on this part?

I hope I won't regret asking, but how does this relate to (or not relate to) TIR?

As always, thanks in advance for the help!
For the part you describe the easiest way to check concentricity is with a set of calipers. Measure the wall thickness (OD to ID) at various points around the part. Look for the largest and smallest readings. (they should be exactly opposite to each other)
On your part the wall should be .250 thick.
Now say that you have found your largest reading is .262 and your smallest is .248.
Subtracting the small from the large yields .014 inch. This would be your TIR (Total Indicated Reading). Concentricity will be half of this.
So - dividing the .014 by 2 = .007 inch.
This is the concentricity reading.

Hope this helps.

James

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  #4  
Old 15th November 2005, 11:17 PM
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Govind Govind is offline
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Let Me Help You Appraisal cost should be in mind

Quote:
In Reply to Parent Post by Mike S.

Can someone please explain in simple terms (for someone who is a mental midget when it comes to geometrical issues) how to correctly measure concentricity? Hopefully using simple tools...
Say I have a part that is 1.000" OD x .500" ID x .400" thick. The .500" hole is supposed to be in the center. Say the concentricity spec. is .020".
How do I measure concentricity on this part?
......!
Mike,
There are several ways to measure concentricity. To suggest a few in the order of precision:
a. Caliper approach (James example)- For wider tolerance.
b. Ball anvil micrometers measured as wall thickness.
c. V-Block, height gauge, level type dial gauge
d. Expansion mandrel in the ID held between centres, height gauge, and level type dial gauge.
e. CMM
f. TALYROUND- roundness, concentricity checker

Additional two useful inputs would be
a.How precise you need your measurement?
b.How many of those you need to measure?
These inputs are for the economics side.

This what will help us decide which method and that will strike a balance between precision, speed, and equipment. This is your “appraisal cost”.

If the concentricity spec is as wide as mentioned in your example, James suggestion is probably the most economical way in terms of speed, precision and equipment choice.

Regards,
Govind.
  #5  
Old 16th November 2005, 08:09 AM
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Jim Wynne Jim Wynne is offline
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Posts: 14,077
Quote:
In Reply to Parent Post by Mike S.

Can someone please explain in simple terms (for someone who is a mental midget when it comes to geometrical issues) how to correctly measure concentricity? Hopefully using simple tools...

Say I have a part that is 1.000" OD x .500" ID x .400" thick. The .500" hole is supposed to be in the center. Say the concentricity spec. is .020".

How do I measure concentricity on this part?

I hope I won't regret asking, but how does this relate to (or not relate to) TIR?

As always, thanks in advance for the help!
You've gotten some good answers here already but I want to point out that in my experience, concentricity is the least understood GD&T specification, and when it's used, it's almost always used incorrectly. In my 200 years of dealing with parts and drawings, I've seen a scant few instances when it was used correctly, and a few more when it wasn't used but should have been.

The source of the confusion is the fact that the word "concentric" in common parlance is synonymous with "coaxial," meaning two or more things sharing a common central axis or point (in the case of plane figures). The meaning is subtly different in GD&T, however (see pilchard's definition in this thread), and therein begins the confusion.

My advice: when you see a concentricity specification, try to make sure you know what the designer intended.

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  #6  
Old 16th November 2005, 08:25 AM
bmccabe - 2006 bmccabe - 2006 is offline
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Posts: 122
Yep!
JRKH is on target.

TIR, run out, and total run out have one thing in common. They might use a dial indicator to measure. Other than that... Well let's not confuse the issue.
  #7  
Old 16th November 2005, 08:39 AM
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Claes Gefvenberg Claes Gefvenberg is offline
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Lightbulb

You may want to have a look at this old thread: Measuring Method - Inside diameter is off set from the outside diameter

/Claes

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  #8  
Old 16th November 2005, 10:12 AM
Mike S. Mike S. is offline
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Y'all are really great. You told me exactly what I needed to know, and more, with special thanks to James.

When things are going really smoothly and you're working inside your sphere of expertise you tend to forget how nice it is to have a place to go for answers with so many experts in so many different things willing to help. I have a feeling I might be soon pestering you again, as life might be getting a good bit more complicated for me.

Thanks again!

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