One common area of confusion within GDT is the differences between the various ways of specifying how true a cylindrical surface or surface of revolution is: roundness, cylindricity, concentricity, circular runout, and total runout. Let us start with roundness. As shown in Figure CS1, roundness applies to individual circular cross sections of a surface of revolution or of a sphere.
Figure CS1 : ROUNDNESS
Cylindricity, on the other hand, applies to all cross-sections of a cylindrical surface simultaneously. The surface must lie between the two cylindrical surface which bound the tolerance zone and are determined by a best-fit nominal cylinder. Figure CS2 illustrates cylindricity.
Figure CS2 : CYLINDRICITY applies to all cross-sectional elements simultaneously.
It is a common misconception that roundness and cylindricity can be checked by taking diametral measurements (as with a micrometer) or by using an indicator and vee block. A diametral measurement does just what the words imply; it measures the diameter. It does not check the shape of the surface which is what roundness and cylindricity control. Since the roundness or cylindricity tolerance is a radial distance between concentric boundaries, a radial method of checking the surface is necessary. However, rotating a part between centers is not an acceptable method since it relates the part surface to an axis, which technically is a check of another geometric tolerance called runout.
To truly check for the roundness or cylindricity of a surface without regard to the axis of the part, the part must be rotated about the ultra-precision spindle of a specialized roundness measuring machine. A probe contacts the surface and transcribes an enlarged profile of the surface onto a polar graph. The profile is then checked against a clear overlay of concentric circles to determine if it falls within the allowable tolerance zone.
Concentricity is the condition in which the axes of all cross-sectional elements of a surface of revolution are common to the axis of a datum feature. Because the location of the datum axis is difficult to find, it is easier to inspect for cylindricity or runout.
Figure CS3 : CONCENTRICITY is based upon the datum axis so that it is difficult to ascertain.
Runout refers to the result of placing a solid of revolution on a spindle such as a lathe, and rotating the part about its central axis while measuring with a dial indicator its surface deviation from perfect roundness. With circular runout, the dial indicator is not moved along the direction of the axis of the part. Circular runout is therefore applied independently at each station along the length of the part as the part is rotated through 360 degrees.
Figure CS4 : CIRCULAR RUNOUT applies to each cross section individually.
Total runout involves moving the dial indicator along the length of the part while the part is rotated, so that it controls the cumulative variations of circularity, cylindricity, straightness, coaxiality, angularity, taper, and profile.
Figure CS5 : TOTAL RUNOUT applies to all cross sections simultaneously.