Arc Second

The prefix, arc, is used solely to differentiate this angular measurement from a second of time. By convention, it is 1 part in 1,296,000 of a circle; or 1 part in 3,600 of a degree; or 0.0002778 degree.

Error Bandwidth

The total measure of deviation between all readings of an instrument throughout its measuring range.

Error

The difference between a measured value and the true value.

Flatness & Wobble

A composite tolerance applied to a flat surface rotating about an axis, e. g., the faceplate of a rotary table. It is defined such that the entire rotating surface lies between two parallel planes separated by a distance equal to the tolerance specified.

Hysteresis

A bias resulting from the reversal of direction.

Index Accuracy

The agreement of the result of an angle measurement between the actual reading of the instrument and the true value, that is free of other error, at that point.

Latent Motion

The measure of change, as measured at the platen in the axial and radial directions, between the unclamped and clamped condition on a rotary table equipped with axis clamps.

Parallelism

The state of two planes parallel to each other; where two planes are equidistantly spaced in three dimensional space.

Perpendicularity

Condition of a line in which all angles to a reference plane are at right angles.

Repeatability

The measure of accuracy by which an instrument permits the return to a specific point.  In a normal or Gaussian distribution, the results are spread roughly symmetrically about the central value, and small deviations from this central value are more frequently found than the large deviations.  The normal curve can be represented by

The standard deviation, denoted by

is found by taking the difference between each observed particular value and the mean, then squaring the difference, adding all the squares, dividing by the number of readings, and then taking the square root.

Resolution

The smallest increment of measure to which rotary table can respond.

Rotary Axis Definition

A measurement which compares two full consecutive axis rotations to a known standard of roundness.

Roundness

A characteristic that all parts of a circle are identical.  The measurement of roundness is essentially a measurement of the change in radii, and the roundness error is the measurement between the minimum and maximum radii in one lateral plane. 

Runout T.I.R.

Total indicator deflection as measured over one revolution of the rotary table.

Wobble (Axis)

The measure of deviation of a rotating plane to a reference plane.  The measurement is usually performed with a mirror and an autocollimator.