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Re: Concentricity Tolerance
Re: Concentricity Tolerance -- Ron A. Post Reply Top of thread Forum
Posted by: John Paul Belanger

04/12/2003, 09:25:13

Very rarely is concentricity a "practical" thing -- even if used properly, it creates a headache in terms of measuring. It is a problem because the true meaning of concentricity requires us to find the precise mathematical center point of each cross-sectional slice through a given diameter (or as many cross-sections as can be taken). Then each of these points must stay within the specified tolerance cylinder.So you're right -- there are a few things that are funny about your example. First, anytime concentricity is used, it should raise a flag: in some special applications it may make sense, but usually position can be used instead (or runout).Secondly, it probably shouldn't be used on screw threads (what a bear to find the center of each x-section!).And third, if concentricity is used, it is meant to line up a single axis to another axis. You can't really make an entire bolt circle concentric to something (I'd have to see the full drawing; maybe they were just specifying each thread individually to a datum).So many people use the concentricity symbol because it sounds like what they want, but they don't realize the headaches that it creates downstream. They should have just substituted a position symbol (RFS) in its place and we can then use a pitch gage to thread in there and measure the center of the pitch gage. Sorry for the long-winded answer, but yes, your initial instinct was correct!

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