Geometric dimensioning and tolerances

[60rock]

I want to discuss what actual part symmetry means and if symmetry can also be called true position. Concentricity from one surface to another as well. I would like all opinions, questions, ect. to be discussed. For instance, if a blue print has a datum set on the O.D of a part, and two bores on a shaft are to be concentric to the O.D, does this also mean they must be concentric to each other or that they will be concetric to each other since they are both to be concentric to the O.D?

[laszlozoltan]

I want to discuss what actual part symmetry means and if symmetry can also be called true position. Concentricity from one surface to another as well. I would like all opinions, questions, ect. to be discussed. For instance, if a blue print has a datum set on the O.D of a part, and two bores on a shaft are to be concentric to the O.D, does this also mean they must be concentric to each other or that they will be concetric to each other since they are both to be concentric to the O.D?

Although the bores may be concentric to each other, it is incidental as the datum is THE refrence point to be used, otherwise you will have shops putting out garbage using whatever reference point that fits their work.

GD&T is quite clear, and perfectly logical in my experience.

[billystein]

symmetry means that the section that is dimensioned is opposite the section that is on the other side of the symmetry line. symmetry is not used for true position. symmetry is a form tolerance and not a positional tolerance.