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Re: TIR vs Concentricity

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Posted By<" ">Kelly Bramble on February 18, 2002 at 12:02:24:

In Reply to: TIR vs Concentricity posted byE. Northcutt on February 13, 2002 at 12:41:29:

You did not indicate the particular application for which you and applying TIR (Total Indicator Reading) or Concentricity.

To determine the desired dimensional specification, I recommend the following.

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)?

Hope this helps!

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