When a mandrel has represented axis of rotation of spindle etc. It is not necessary that mandrel have been exactly centred and under such conditions axis of rotation of mandrel may describe a conical or hyperboloid.

At many places, it is necessary that surface must be straight e.g. in la machine it is required that tool must move in straight path. Straight line is basis of most methods of measurements.

According to least square method, ?sum of deviations of points from best line is zero and sum of square of deviations from best line is minimum?. First best line is determined and n applies to this method.

Permissible deviations or tolerance of parallelism of two plane surfaces or straight lines is 0.015 mm on 300 mm. Free end of spindle relative to table surface is in upward direction only.

In determining straightness with help of autocollimator instrument is placed at a distance of 0.5-0.75 m on any rigid support and it is completely independent of surface to be tested.

Straight edges can be used to test flatness and straightness of surfaces. Single-edged straight edge can be used to determine flatness by applying it at different places in different directions on tested surface.

Parallelism can be defined as tolerance which controls parallelism between two features. It is possible to find parallelism of cylinder by measuring two sides of a cylinder.

Factors affecting accuracy of straightness measurement by optics are Air turbulence, Mechanical vibrations, Optic errors, Slope errors, Optics not fixed in correct position, Optics not fixed rigidly, Localised heat sources etc.

If distance between two lines is not small or large n V-blocks are used. V- blocks are covered by a straight edge is used. Check for this is made by spirit level. For small distance re is no need of se.

In this method, reading of tilt of reflector is taken in seconds from eyepiece. 1 second of arc is 0.000006 mm/mm. It also depends upon distance between feel centres in millimeter.