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runout vs concentricity question

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tactical14

tactical14

Mechanical
Feb 13, 2012
15
If I mount the following object on a V block at point A and put a indicator at point B, what am I measuring? Basically I want to see if the two diameter's center lines are collinear.

k1x4e0.png
 
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You are measuring circularity and coaxiality.
 
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what if I trace up and down the smaller diameter? Is that runout?
 
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If you measure at one point at a time, it is Circular Runout. If you move the indicator longtitudinally while measuring, it is Total Runout. In total runout in this case, you measure circularity, coaxiality, straightness, taper, angularity.
 
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I cannot see your picture (tinypic.com is blocked in my place), but you are always better off placing your datum feature in chuck/collet when measuring either runout or concentricity.
If your feature A is not perfectly round, using V-block to establish axis of rotation is a major pain.
Just an opinion. :)
 
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There are situations where a V-block can give false compliance readings. Keep this in mind.
 
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If you leave the indicator in one position and rotate the part you are measuring circular runout. It is not possible to measure concentricity with a dial indicator. However, since runout will always be greater than concentricity it can be used as a quick check. If the runout is less than the concentricity requirement you are good.

Due note TheTick's statement about v-blocks. If your A feature has significant lobing it can effect the results.

----------------------------------------

The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.
 
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tactical14,

Without touching the aspect of proper datum axis establishment by V-block at the moment, depending on how you will rotate and move the dial indicator or the part during the check, you may verify different things - circular runout, total runout or even ASME's concentricity. (dgallup, are you sure that it is not possible to measure concentricity [per ASME definition] with a dial indicator, of course assuming that datum axis is well established?)

What GD&T standard are you working to - ASME or ISO? Concentricity has different meaning in both, so it would be good to know what "Concentricity" means to you. Additionally, assuming you are in ASME world, "I want to see if the two diameter's center lines are collinear" makes me think that you are very close to mix Concentricity with Coaxiality.
 
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Concentricity vs. runout is a perennial debate in GD&T. The requirements for establishing concentricity are far greater than for runout.

Concentricity simply can NOT be fully verified with a chuck or V-block. It requires more. That is one reason that runout even exists as a spec. Runout is simpler to measure and in most cases is sufficient.

Many people specify concentricity when they really mean runout, and even specify measurement methods for runout. It is "folk GD&T", and not correct.
 
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TheTick,
Let's say that I have a will, time and money to try to inspect ASME's Concentricity of an external cylinder relative to datum axis derived from other external cylinder with the use of single dial indicator and a chuck. I place datum feature cylinder in the chuck - the axis of the chuck becomes my datum axis. Then I start to probe toleranced cylinder surface with the dial indicator, but not in a continuous way, like I would do in case of circular or total runout check. Instead of it, I probe one point, register the indicator reading, and then rotate the part by 180 degress around datum axis and register second reading. Next I analyze magnitude of difference in readings and divide it by 2. This is the distance of single median point from datum axis.

By doing this I inspected and analyzed one pair of diametrically opposed points. And I can/have to do it for other points in the same cross section and in other cross sections along the axis of toleranced cylinder.

Wouldn't it work as a check of actual Concentricity error?
 
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No, mostly because it does not account for form variances. No guarantees on the roundness or cylindricity of the surfaces, thus no guarantees on v-block readings for concentricity.
 
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I mentioned nothing about v-block usage in my example - I want to use a chuck. In my firts comment I also stated that I did not want to touch the aspect of datum axis establishment by v-block.

In the light of that, do you still think that my method of inspection does not lead to proper actual Concentricity error check?
 
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Pmarc,
Is the technique you describe any different from using a chuck and 2 indicators?
As concentricity is described as condition of two opposite points, chuck and 2 indicators should be perfectly fine IMHO.

The Tick,
As concentricity controls TWO OPPOSITE POINTS, and runout controls ALL THE POINTS, runout is more restrictive control than concentricity.
If part passes runout requirement, it automatically passes concentricity reqirement of the same amount, and THAT IS the reason that runout even exists as a spec.
 
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CH,
No, geometrical fundamentals behind my idea is no different than in case of use of 2 dial indicators. The only difference - you will need two of them, I will need just one.
 
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Imagine surface A is an ellipse and surface B is a corkscrew.
 
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The Tick,

pmarc said:
Let's say that I have a will, time and money to try to inspect ASME's Concentricity of an external cylinder relative to datum axis derived from other external cylinder with the use of single dial indicator and a chuck...
Click to expand...

There is no need for envisioning some fancy scenarios with ellipses, corkscrews and v-blocks. I was asking for simple answer to the simplest possible case of all.
 
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Trying to help tactical14 understand why a V-block is not a good idea. Remember him? He's the guy we're trying to help.
 
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So am I.
If corkscrew doesn’t have opposite points, then concentricity doesn’t apply.
Does OP part look anything like a corkscrew? Because if it’s not, you are not helping.
(I won’t post anything else here, I promise)
 
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