If you are familiar with GD&T, you’ve probably heard of “virtual condition.” It is a number that represents a worst-case combination of a feature’s size along with its geometric tolerance. For instance, suppose we have the following example of a hole, with a size tolerance of 6 ± 0.2 mm:
The worst case for assembly purposes would be the smallest hole that is also out of position: 5.8 – 0.3 = 5.5. This virtual condition represents the “usable” area that the mating pin must fit within:
Now, virtual condition is usually easy to identify with — ”think of assembling pins and holes together. But sometimes I may be concerned about the outside boundary, created by the largest hole that is also off center:
This outer boundary, called resultant condition, is calculated as: LMC + stated geo tol + any bonus, or: 6.2 + 0.3 + 0.4 = 6.9 mm. This represents the area where any portion of the hole’s edge may possibly fall.
This resultant condition is not of concern when dealing with assembly of holes and pins! But suppose we are simply punching a drainage hole in the bottom of a drip pan. Nothing will be assembling through this hole, but my concern now is that the hole’s edge not be too close to the edge of the sheet metal.
So be aware of both virtual and resultant condition formulas, for internal and external features.