Runout is a geometric dimensioning and tolerancing (GD&T) term which applies to the process of applying a symbolic language to engineering drawings and computer-generated solid models. The use of runout, and specifically total runout, is to describe the nominal geometry of a part, as well as its allowable variation.
Common Total Runout Terms
Total runout itself is the simultaneous composite control of the entire surface of a part or model. This applies to all circular surfaces and profile measuring positions on the part rotates through one complete circle. There are various ways of specifying how true a cylindrical surface is. Roundness describes a cylinder or cone where all points are perpendicular to a common axis, or a sphere where all points pass through a common center, from which they are equidistant. Cylindricity is a condition of a surface of revolution in which every point on the surface is the same distance from a common axis. Concentricity is the condition of two or more features which share a common axis.
Differences Between Total Runout and Circular Runout
A gauge is used to measure runout on rounded parts. It is placed on a part, which is then rotated, and the total variation across the surface of the part is recorded as the runout. Total runout is measured along the entire specified surface of a part. Total runout requires movement of the gauge or indicator along the rotating part, while circular runout involves the measurement of each cross section individually. In this way, total runout differs from circular runout, which is only measured at one location along the rotating part.
Control of Features
The term "feature" is used in GD&T to describe any physical portion of a part or model. A feature may be the part’s surface. It may also be a hole, tab, pin or slot within the part’s surface. Features are never imaginary portions of a part, such as a line, axis or plane, although these may be visible in engineering drawings. The term feature applies only to portions of a part that physically exist on the finished part.
Straightness and Angularity
A three-dimensional part represented by a two-dimensional drawing may have features which appear as straight lines from certain angles. The tolerance zone in which that line of the feature must lie is known as the straightness tolerance. Angularity tolerance allows the designer to specify a certain extent to which the orientation of a given feature may vary with respect to other portions or features. This is expressed as a tolerance zone at a specified angle from the part’s datum, within which the part’s feature, axis or center plane lies.