Diameter Size and Cylindricity

Example: I have a cylinder 5.0 +/-0.001" and I take 3 circle measurements across the cylinder and I get 4.9882, 4.9989, and 4.9990. Eng wants to add a cylindricity callout of 0.001". Does being out of tolerance on the diameter affect they cylindricity? He want to know if the cylinder diameter can vary 0.002" and still have good cylindricity?

Description:

The Cylindricity symbol is used to describe how close an object conforms to a true cylinder. Cylindricity is a 3-Dimensional tolerance that controls the overall form of a cylindrical feature to ensure that it is round enough and straight enough along its axis. Cylindricity is independent of any datum feature the tolerance needs to be less than the diameter dimensional tolerance of the part. Cylindricity essentially forms a perfect cylindrical boundary around the object that the entire 3-Dimensional part must lie in.

GD&T Tolerance Zone:

Two concentric cylinders that run the entire length of the feature – one inner and one outer, in which all the points on the entire surface of the cylindrical feature must fall into. The entire length of the called out feature would be controlled.

That is where I get confused. It says all the points need to fall into the tolerance zone. What if I have a huge taper in my part? The tolerance for cylindricity must be smaller that the tolerance for the diameter.

You're correct in that aspect. What I meant is that the size is not a consideration in the calculation of the cylindricity. The feature is best fit. The callout being less than the diametric tolerance is because if it's larger, it controls nothing. But you can put it on a drawing. It just won't control anything. 

Zen Cat  Thank you so much for helping with this. 

So if I had a cylinder that was super round and straight but was oversized diametrically, I could still have good cylindricity but just out of tolerance on the diameter. What is the tolerance zone based on? The size of the physical part or the cad model?

The tolerance zone of what? 

The tolerance zone of the diameter is based on whatever means it's communicated. It can be controlled by the blueprint callout, the CAD model if that model is used as the master definition, etc. That's drawing/company specific. 

If you're talking about the cylindricity tolerance zone, see the image above that Malgorzata posted. It's 2 concentric cylinders spaced apart by the tolerance called out, best fit around the cylinder releasing all 6 degrees of freedom, which all points must lie within. 

The tolerance zone of what? 

The tolerance zone of the diameter is based on whatever means it's communicated. It can be controlled by the blueprint callout, the CAD model if that model is used as the master definition, etc. That's drawing/company specific. 

If you're talking about the cylindricity tolerance zone, see the image above that Malgorzata posted. It's 2 concentric cylinders spaced apart by the tolerance called out, best fit around the cylinder releasing all 6 degrees of freedom, which all points must lie within. 

Sorry, I meant tolerance zone for cylindricity. Thank you for all of you help.