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Re: Concentricity Vs Positional tolerance (and their relation to TIR)


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Posted by Jacek Palinski (208.51.233.194) on January 06, 2003 at 14:47:03:

In Reply to: Concentricity Vs Positional tolerance (and their relation to TIR) posted by Venkataraghavan on January 03, 2003 at 09:49:09:

: Hi there,

: Seasons greetings and a Happy New year.

: I want some good explanation from any of you experts. I have a cylindrical component with 2 diameters. This is a deep drawn and turned Stainless steel component. The drg calls for a positional tolerance of dia 0.15 for the smaller diameter w.r.t. the larger diameter. A few questions now.

: Measurement: I mount the component on a mandrel such that the larger dia is located and I then measure the TIR on the smaller dia with a dial indicator.

: What does this TIR reading give?
: Would this TIR divided by two give me the concentricity?
: Is this concentricity the same as the positional tolerance that I want ?
: Or is there a specific relation between concentricity and positional tolerance.

: If Concentricity and Positional tolerance in this case is same, then why would one want Positional tolerance?

: Is the above method of measurement correct at all.

: Or should I go for a CMM.

: Would appreciate your quick and detailed responses.

: thanks

: Venkat

Yes, you should go for a CMM measurement.
Your drawing calls for a true position, and CMM is the best tool to do it. True positional tolerance may be explained in either of the following ways:
1 - in terms of the surface of a hole - while maintaining the specified size limits of the hole, no element of the hole surface shall be inside a theoretical boundary located at true position;
2 - in terms of the axis of the hole - where a hole is at MMC (minimum diameter), its axis must fall within a cylindrical tolerance zone whose axis is located at true position.

Positional tolerance is type of a coaxial tolerance, and the parts do not have to be rotating during the CMM measurement, nor in the real life application.

Concentricity is mainly used to tolerance the parts rotating at high speed, where out of balance condition would be a problem.
For concentricity, the locations of the midpoints of diametrically opposed feature elements are controlled relative to the axis of the datum feature. Concentricity can only be used on a RFS basis, so it is more stricter then true position. Measurement setup requires at least two indicators at 180 degree to each other, rotating the part and noting the readings of both indicators at each "clock" position, after that doing some math, as to determine the cloud of the median point and so forth - very long and time consuming process.

The way you were proposing to do a measurement, would be appropriate for measuring the runout, which is a composite of a circularity (another name for roundness) and concentricity, thus you would be trying to do more then asked for. As explained by Randy, this is more complicated that it looks. So, again, go for the CMM and check just the true position.

Jacek



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