Virtual Condition vs Tolerance Zone

In our video question line, our GD&T instructors answer questions submitted by our students. The topic of the question line video below is regarding Virtual Condition and Tolerance Zones. In this video, we answer the question, “Is the Virtual Condition the same as the Tolerance Zone?”

 

Is the Virtual Condition the same as the Tolerance Zone?

A student asked us this question regarding the drawing below. In this drawing, we see a cylinder with a maximum and minimum diameter specified. This diameter size dimension has an attached feature control frame indicating that this feature is being controlled by derived median line straightness.

Figure 1: Drawing of a Cylinder with Straightness Form Control

 

We have a size tolerance and a straightness tolerance that must both be met for this part to pass inspection. But are these the same as the Virtual Condition?

Let’s define Virtual Condition: Virtual Condition is the actual maximum envelope that a feature can be within and still maintain specification. It is a calculation taking both size and geometric tolerances into account that determines the worst-case scenario – the envelope boundary that no element can be outside of and pass inspection.

The figure below illustrates the Virtual Condition of this cylinder. You can see that it encompasses both the maximum allowable size and form of the cylinder. If the cylinder is at its maximum size and the central axis of the cylinder falls within the derived median line straightness tolerance zone, it would be within the Virtual Condition boundary.

Figure 2: Virtual Condition of the Cylinder

 

Note that though the Virtual Condition envelope is larger than the Maximum OD of the cylinder, the cylinder is still required to fall within the specified size tolerance to pass inspection.  Therefore, you can see that the answer to our question is “No.” Virtual Condition and Tolerance Zone are not the same thing.

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