The tolerance on straightness of a line can be defined as maximum deviation in
relation to reference straight line joining two extremities of line to be checked. Tolerance is used to confirm that a part is uniform over a surface. More straightness is present with less tolerance.

This method is suitable for small surfaces rar than large surface. This method is assumptious and may give more errors in determination of flatness in large surfaces.

Two planes are parallel when difference between maximum and minimum dimension obtained that is maximum error over a given length does not exceed a specified value. The test is carried out in two directions which are perpendicular to each or.

Straightness interferometer is placed between laser head and reflector. Beam from laser passes through interferometer which splits it into two beams which diverge at an angle and are directed to reflector. The beams are n reflected from reflector and return to interferometer along a new path.

When surface is irregular, contact method of testing for flatness is used. In contact method re is a most intimate possible contact between flat and work piece and re will be attempt to maintain a wedge.

Straight edges are extremely useful for setting up machines such as planers. Precision straight edges are hardened and lapped on edges by a small radius, which makes a blunt knife edge straight to a few thousands of an mm.

Due to very great angle between work and flats, bands will be close toger and are nearly invisible. Number of bands appear is not an indication of surface flatness but relates only to wedge steepness.

Flatness error can be defined as minimum separation of a pair of parallel planes which will contain all points on surface. Deviations of large surfaces from true plane are determined by using autocollimator and spirit level.

Autocollimator is used in many applications like in shop floors and metrology labs. Used for precision measurement of parallelism, straightness, flatness, perpendicularity etc. It is a not-contact optical instrument.

Two lines or axes are said in alignment or coincident when ir distance apart at several points is measured over a specified length and this distance doesn?t exceed a specified value. The measured distance may be located eir on actual lines or on ir prolongation.