Concentricity

Two or more surfaces are concentric when they have a common axis. The shared axis must lie within a cylinder which has the diameter of the tolerance specified for the controlled surface.

As shown in Drawing #27, diameters A, B and D have a common axis within a cylinder of .xxx diameter. The critical concentric portion of diameter A is designated as distance C.

The tolerance .xxx specifies the Full Indicator Movement (FIM) for diameters B and D when the part is rotated on the axis of diameter A. This tolerance should include tolerance allowances for roundness, runout and perpendicularity.

Note: Total Indicator Reading (TIR) and Full Indicator Reading (FIR) have been replaced by the internationally accepted Full Indicator Movement (FIM). When the indicator is in contact with the surface being measured, the full movement of the indicator is the surface runout.

Often, only that portion of a surface which is necessary for the function of the part is required to be within the FIM limits; length C in this example.

Concentricity drawing