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The definition of "Roundness or Circularity" is; each circular element of the surface in a plane perpendicular to an axis must lie between two concentric circles, in your case one having a radius .002 larger than the other or inner. You should have a size tolerance associated with the Tube (feature) something like Diameter +/- .005 or larger, the size tolerance should be greater than the Roundness tolerance * 2. Additionally, Roundness cannot be associated with a Datum. Roundness is a form tolerance and is a refinement of size. Now, what you are realy after is how to measure for this specification (Roundness). To understand what you are measuring for you will need a visual aid, start by drawing on a piece of paper (or other) three concentric circles large enough to apply a size dimension. Place a size dimension from the inner-most circle to the outer-most circle this is your given Roundness tolerance ".002". Between these two circles you should have a "between circle" this is your tube surface. This tube surface can vary anyway possible (but not being smaller or larger than your size tolerance) but the tubes surface cannot cross the inner circle or outer circle. The concentric circle can move to a best fit location to keep the middle circle within the outer & inner circles. Think of roundness as 2D tolerance zone in that this specification only applies at a number of cross sections (depends on requirements). Ok, this is how you measure Roundness of your tube ... Using calipers measure a cross section and record, then measure several more cross sections at the same location. Your measuring the diameter of the tube repeatedly at the same cross-section by measuring then rotating. Now get your calculator and take the largest measured diameter and subtract the smallest measured diameter then divide by 2 and this is your roundness of that cross-section and, hopefully is not larger than .002. You will need to repeat this process at several (many) cross-sections. The number of cross sections you do is dependant of the degree of confidence you have with the part as well as the function. Now remember each cross-section measurment should also be within the size tolerance given in addition to the roundness tolerance. Now for my disclaimer! There are more accurate ways to measure Roundness (Circularity) by way of a roundness measuring machine. The technique given here is slow and labor intensize (some may argue not accurate) but, will get you thru the woods.
Now you can use a CMM which will take about 16 points find a normalized curve of the points and calculate the roundness for you (look in your software if you have this capability).
Good Luck!
Kelly Bramble