Calibration
A set of operations which establish, under specified conditions, the relationship between values indicated by a measuring instrument and the corresponding known values of a standard.
Calibration Constant
A numerical value which is used to compensate for inaccuracies in an instrument.
Calibration Standard
A test artefact, or master reference item, having known values.
Center Line
A reference line parallel to the general direction of a profile, which divides the profile into two equal parts.
Co-axiality Value
The diameter of a cylinder that is coaxial with the datum axis and will just enclose the axis of a second cylinder.
Co-ordinate System
All measured points on a metrology instrument will have some form of
co-ordinate. For example, a surface measured using a roughness instrument will collect data whose co-ordinates will consist of x (arm movement) and z (gauge movement) values.
On a roundness system a roundness plane’s data may have co-ordinates for radial (arm position and gauge reading), height (column position) and angular (spindle position) values.
Concentricity
Concentricity is twice eccentricity and can be described as the diameter of a circle that is created when the centre of a component is rotated around the datum axis. Two roundness planes can be considered concentric if they have the same centre.
Contact Direction
This relates to the measuring direction of the stylus. If an operator was measuring a component for flatness on a surface facing upwards, then the contact direction would be down.
Contact Speed
The speed at which an axis moves when commanded to move into contact with the component.
Crowning (cresting)
For surface measurement, crowning (sometimes referred to as cresting) is the process of determining the highest point of a component that has a convex form, or the lowest point of a concave form. On some instruments crowning can be done automatically by using the measured data.
The term Cresting is more properly applied to the alignment of the stylus to the axis of rotation of an instrument designed for the measurement of roundness. See section on cosine errors.
Cut-off
In basic terms, a cut-off is a filter and is used as a means of separating or filtering the wavelengths of a component. Cut-offs have a numerical value that when selected will reduce or remove the unwanted wavelengths on the surface. For example, a roughness filter cut-off with a numeric value of 0.8mm will allow wavelengths below 0.8mm to be assessed with wavelengths above and around 0.8mm being reduced in amplitude; the greater the wavelength, the more severe the reduction. For a waviness filter cut-off with a numeric value of 0.8mm, wavelengths above 0.8mm will be assessed with wavelengths below 0.8mm being reduced in amplitude.
Cut-off Length
A term often used to mean the same as Sampling Length.
Cylindricity
The amount by which a part departs from a perfect cylinder
