Form Gages and Form Gaging Systems

Form gages and form gaging systems are used to inspect parameters such as roundness, angularity, squareness, straightness, flatness, runout, taper and concentricity. Form gages are similar to profilometers, inspection tools used to measure surface profile, roughness, waviness, and other finish parameters. There are two basic types of form gages and form gaging systems: contact and non-contact. Contact or stylus-based instruments measure surface texture by dragging a sharp, pointed tool across the surface. Height variations of the tip are recorded and then used to form a texture profile. Roughness and waviness are also calculated from the profile data. Non-contact form gages and form gaging systems measure surface texture by optically scanning a surface with a light or laser. Optical or light-based instruments may also use triangulation or interferometry to measure or capture a surface profile. Most contact or stylus-based form gages and form gaging systems provide only a two-dimensional (2D) or line profile; however, non-contact devices can also provide three-dimensional (3D) or areal topography measurements. In this way, 2D or 3D roughness and waviness parameters are calculated from profile data. 

Form gages and form gaging systems are used to measure coaxiality, cylindricity, taper, flatness, eccentricity, concentricity, harmonic content, parallelism, roundness, and runout. Coaxiality is a measure of one axis to another axis. Cylindricity is a condition of a surface of revolution in which all points on the surface are equidistant from a common axis. Taper is a narrowing or differential variation in diameter along the axial or length dimension of a cylindrical part. Flatness places all of the points on a part’s surface within a single plane. Eccentricity is a ratio which describes the shape of a conic section. Concentricity is twice the eccentricity.  Harmonic content or shape variations are repeated undulations which occur in a 360° rotation. Typically, harmonic frequencies are described in undulations per revolution (UPR). Parallelism describes the equidistance between two planes or surfaces of a part. Ideal roundness is the condition in which all parts of a circle are identical or equidistant from the center point or axis. Runout is measured by determining the radial difference between two concentric reference circles drawn to just enclose the profile of the part or cylindrical surface under evaluation.  

Squareness, angularity, step height, straightness, thickness, warp and bow are common form parameters for form gages and form gaging systems. Squareness is a measure of variation of the part's surface from a 90° angle to the reference surface. Angularity can be measured by determining the deviation of the part's surface from two enclosing reference planes that are a particular angle to a reference plane or datum axis. Angle gages, angle gage blocks, squares and protractors are used to assess squareness or angularity, but not with the same degree of accuracy as form metrology equipment. Straightness places all of the points of an edge or feature within a single line. It can be measured using either the minimum zone method or least-squares method. Some form gages and form gaging systems can measure warp and blow. Bow in a water or thin sample is a distortion perpendicular to the surface from a straight, flat plane. Warp is a twisting variation of a part's surface from a straight, flat plane.

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