runout

Runout is a metrology term used to describe how much a rotating surface deviates from true concentricity or a perfect geometric form as the part turns. It is commonly discussed in the context of cylindrical and rotary components, where precision depends on minimizing deviation from a true circle or cylinder.

There are two primary concepts:

- Circular runout: the variation in the radius of a surface as the part is rotated around its axis, typically measured at a fixed axial position with a dial indicator. In a perfectly true part, readings should be constant; deviations indicate circular runout.

- Total runout: the maximum deviation of a surface from a true cylinder during a complete rotation, incorporating both radial (out-of-round) and axial (end-to-end) deviations. It reflects overall accuracy of the surface profile through 360 degrees.

Measurement methods involve a dial indicator mounted to a stable stand or a testing fixture. The indicator’s stylus contacts the surface at a chosen radius, and the part is rotated while readings are recorded. Consistent clamping, proper centering, and appropriate gauge length are important to obtain meaningful results. Calibration and repeatability are essential, and multiple test positions may be used to assess consistency along the length of the part.

Causes of runout include eccentric mounting, workpiece misalignment, worn or damaged jaws and chucks, spindle or bearing runout, out-of-round stock, and thermal expansion. Runout can affect bore alignment, surface finish, and fit with mating parts, making it a critical consideration in precision machining, grinding, and assembly.