Geometric Dimensioning and
Tolerancing
unit 5, product plans & virtual condition
chapter 7, virtual condition
Introduction
• Chapter introduces an important concept related to the
mating of parts in an assembly.
• Interchangeability of parts and assemblies can only be
ensured when parts are accepted at virtual condition or
better.
EML 2023
Department of Mechanical and Aerospace Engineering
2
External Feature
• Suppose we’re making a clearance hole to fit over a peg.
How big should we make the hole
to make sure we have clearance?
Well, what’s the virtual size of the
peg?
EML 2023
Department of Mechanical and Aerospace Engineering
3
Virtual Condition
The virtual condition of a hole is the
region that is guaranteed to be free
space.
The virtual condition of a peg is the
region that might be occupied.
If the virtual condition of the hole is larger than the virtual
condition of the peg, then we are guaranteed that the
hole will clear the peg.
EML 2023
Department of Mechanical and Aerospace Engineering
4
The virtual size of a hole will equal the diameter of the hole
minus any position tolerance. Check the extreme limits.
EML 2023
Department of Mechanical and Aerospace Engineering
5
The virtual size of a peg will equal the diameter of the peg
plus any position tolerance. Check the extreme limits.
EML 2023
Department of Mechanical and Aerospace Engineering
6
Guaranteeing Part Mating
• The hole in the block will always clear the peg if:
the virtual size of the hole (the guaranteed free space)
is greater than
the virtual size of the peg (the largest possible region that may be
occupied by the peg).
EML 2023
Department of Mechanical and Aerospace Engineering
7