Symbols for Geometric Tolerance
The followings are the normal symbols for
geometric tolerance shown on the drawings of iron and steel
castings. We just briefly introduce their meanings, and hope they
could be useful for buyers and suppliers to better understand the
Straight – straightness, indicating the tolerance for
straightness to one edge or to one line.
Flat – flatness, indicating the flatness to one surface.
Higher flat will mean the surface must be very flat. Usually this
tolerance is for the rough surface of metal castings.
Round – ovality, indicating how round for one round surface.
If round tolerance is high, then it will mean the ellipse and
irregular circle will not be allowable.
Cylinder – it is for the tolerance to the cylinder, which is
similar with the round, but just for the how cylinder, not just for
Circle Runout – this is one simple tolerance for indicating
the tolerance when round part rotates by one standard axle. High
circle runout means the part needs to rotate perfectly by the
Total Runout – this is the higher tolerance for indicating
the tolerance when round part rotates by one standard axle. It is
hard to explain by English, but it will mean this part needs to
Perpendicular – this indicates two lines or two surfaces need
to be vertical.
Parallel – it means two lines or two surfaces should be
True Position – it means how far for one axle line or center
position to deviate their theoretical positions.
Concentric – it means how the axles of two circles deviate
Profile of Surface – it means the real surface deviate the
Profile of Line – it means the line deviate the theoretical
Angle – it means the allowable angle tolerance.
Diameter – it means the max. dimensional tolerance for one
Radius – it means the max. radius deviation.
Datum – reference, standard, or reference face, standard