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


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Posted by Randy Kimball (4.46.211.7) on January 03, 2003 at 18:29:16:

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

Good solid theory question worth a solid explanation.

--- I can give the lathe theory portion of the question, sticking to what I am expert at.

Total Indicator Reading is what you will get when you rotate the chuck. Worded differently, ... An indicator reading while a part is in the chuck will tell you only at what TIR the part is in relation to the spindle of the lathe, while in that constrained state. It is NOT the TIR of the part. Because the TIR of the part is also contingent upon the TIR of the mandrel, the fit on the mandrel and the egg/triangle shape of the diameter that is fit over the mandrel is compared to the TIR of the portion you are reading with the indicator.

If the previous operations were cut while in a chuck and the part is at all flexible, it IS NOT ROUND. Although it may be round enough, thus we have concentricity tolerances. Sometimes these tolerances are allowed to be read while the part is in a constrained condition, IF it is called out that way on the face of the drawing. The true concentricity of the two diameters is what they will check to each other while out of the lathe in a non-constrained state.

Be aware that a micrometer will read round when a lathe turned part is actually a triangular shape due to the relaxation of the chuck pressure. This is because the micrometer will measure the high and the low areas at the same time. (this is what makes a wankel engine rotary)

The reason for calling out positional data is to distinguish this phenomena out of the picture.
This said, you can often measure the wall thickness between the two diameters (if this applies) with a ball anvil micrometer and get an accurate concentricity reading. But, you still do not know if the part is round. Rolling the part on a vee block or rollers while using an indicator will tell you if the part is not round, again contingent upon the distance between the rollers and sides of the vee block as they tangent the diameter being rotated. Again, this is a triangle measuring a triangle, but can tell you if something is not round, but not how much it is our of round. ... To truly know you need the mighty CMM......IF your tolerance is that close.

But, yes TIR is twice the concentricity.


Now let's let a tolerancing expert answer the rest of the question.

-randy-



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