Focusing on the variation on account of size, we can observe that the wall thickness becomes critical when each one of the holes becomes largest within the tolerance of size (dia. 25.3), leading to their LMC.
Further, the holes can shift in every radially outward direction by 0.3. The holes shifting towards each other will lead to a further reduction in the wall thickness. As seen earlier the virtual condition must be evaluated as expanding and reducing envelopes.
The LMC Virtual Condition thus provides the basis of calculations for assuring feature separation and assuring wall thickness protection.
More on Designing with GD&T
The practice of GD&T offers a hoard of other principles for better design. An important principle among these is that of bonus tolerance; where the tolerance of size can be traded off with a geometric tolerance, making the tolerance scheme more flexible and lower in cost. We will visit this topic in a subsequent article on GD&T practice.
In conclusion
The definition of Virtual Condition envelopes in Geometric Tolerancing helps us design products that satisfy function and fit. This is delivered through diligent design calculations that evaluate the combined impact of the several tolerances acting on features.
