Symmetry vs. Position - Symmetry uses the same theory as Position, right?

[NetScorpium]

Originally Posted by JSW05

Because of the difficulties involved in actually measuring concentricity error, I'm not sure where your data is coming from. In my experience, in almost every instance I've ever seen where concentricity was specified it was really control of position or runout that was needed. The input values for your formula must be the result of measurement for the median points of diametrically opposed elements of the feature being controlled. In practical terms, this involves using two indicators positioned 180 degrees apart on the controlled feature, and determing the net difference in the readings at different points along the feature, and then identifying the worst case, which is the greatest net difference between any two readings. Thus it appears that perhaps your formula may be as simple as finding the difference between two (perhaps differently signed) values.

If this isn't clear, or if I haven't answered your question, please post back and I'll give it another go.

Yes, I understand your explanation. To measure concentricity, I use a Coordinate Measuring Machine. For example, I measure a diameter and the median points of diametrically are placed to 0,0 (X=0,Y=0). Than I measure the other diameter and I calculate the distance between the two median points of diametrically. The result is my concentricity value.

[Bill Ryan]

Originally Posted by NetScorpium

The Pythagorean Theorem (and multiply by 2) is the same to calculate these kind of positions? How? Where can I find more information about position and concentricity calculations?

Yes, the calculation works for one, two, or three variables.

Just about any book on Geometric Dimensioning and Tolerancing will go into detail about Concentricity and Position, as well as all other symbols. A few I'll mention are "ANSI Y14.5", "Geometrics" by Lowell Foster, "Geometric Dimensioning and Tolerancing" by James Meadows, "Geometric Dimensioning and Tolerancing" by Al Neumann. You also might try "Googling" the term GD&T (or spelled out) and see what pops up.

[Jim Wynne]

Originally Posted by NetScorpium

Yes, I understand your explanation. To measure concentricity, I use a Coordinate Measuring Machine. For example, I measure a diameter and the median points of diametrically are placed to 0,0 (X=0,Y=0). Than I measure the other diameter and I calculate the distance between the two median points of diametrically. The result is my concentricity value.

It sounds like you're measuring runout or position, not concentricity. In order to accurately measure concentricity you have to be able to rotate the part about the datum axis using diametrically opposed indicators as I advised before. It could very well be that the result your're getting will satisfy design intent, however. As I also said earlier, it's been my experience that when concentricity is specified it's usually by someone who understands the everyday definition of the word (two things that are coaxial) but isn't aware that it isn't that simple in the GD&T sense.