The Virtual Condition of a feature is a concept used to describe the worst-case envelope within which either of two features must lie in order to mate acceptably. For a shaft that fits into a hole, the shaft virtual condition must be smaller than the hole virtual condition. We had discussed this topic in the section 2D Virtual Condition. The concepts and illustrations are the same since tolerance zones and virtual condition boundaries now simply become 3D right circular cylinders that project out of the page.
As we had seen for the 2D case, for an external feature of size, such as a shaft, the virtual condition is equal to the size at MMC plus the size of the tolerance zone. For the shaft in Figure VC1.1, the diameter of the virtual condition is the diameter of the MMC shaft plus the diameter of the position tolerance zone.
For an internal feature of size, such as a hole, the virtual condition is equal to the size at MMC minus the size of the tolerance zone. For the hole in Figure VC1.2, the diameter of the virtual condition is the diameter of the MMC hole minus the diameter of the position tolerance zone.
Figure VC1.3 shows the shaft and hole virtual conditions superimposed. Since the shaft virtual condition is smaller than the hole virtual condition, the two parts will always mate.
In summary, the way to calculate virtual condition (VC) for a shaft and hole is:
SHAFT VC = MMC diameter + Position Tolerance Zone Diameter
HOLE VC = MMC diameter - Position Tolerance Zone Diameter
Virtual condition is extremely useful in the design of functional gauges. A functional gauge made to conform to virtual condition will ensure that a part will always mate with its counterpart. See 3D Functional Gauge.