Parallelism Error
Parallelism refers to a particular condition of either an axis, a line, or a surface that is equidistant at all from a datum plane or axis. Parallelism mandates that a feature of the line, axis, or surface is being compared to a datum. This is different from flatness in which a feature of the line, axis, or surface is being compared to itself. Parallelism errors may occur when using a disk micrometer due to the flat, disk-like shape of the measurement faces. A parallelism error occurs between the spindle axis and the linear axis of the measurement tool and can be assessed using a double ball-bar.