Their surfaces are lapped and are flat and parallel within a range of 1-5 micro inch. Gage blocks are available in sets of various sizes. The blocks can be assembled in many combinations to obtain desired lengths. The gage block assemblies are used as an accurate reference length to measure the part's length. The micrometer is commonly used for measuring the thickness and inside or outside diameters of parts.
Micrometers are also available for measuring depths.
Micrometers can be equipped with digital readout to reduce errors in reading. The profile projector is used for measuring two-dimensional contours of precision specimens and other work pieces produced. The part to be measured is magnified by an optical system and projected on a screen. The reading on the screen gives the dimension of the part. The following is the photo of a profile projector.