Mm Diagrams

Johansson Comparator

Fig 2: The mechanism of sigma comparator 2

Fig 3 The cross strip hinge used in sigma comparator

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Fig 1: Sigma mechanical comparator 4

Back Pressure Type

Flow or Velocity Type

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Definition of straightness of a line in two planes : A line is said to be straight over a given length, if the variation of the distance of its points from two planes perpendicular to each other and parallel to the general direction of the line remains within the specified tolerance limits; the reference planes being so chosen that their intersection is parallel to the straight line joining two points suitably located on the line to be tested and the two points being close to the ends of the lengths to be measured.

STRAIGHTNESS

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Flatness

Squareness

Straight Edge Tool

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Straightness Measurement using Autocollimator

Flatness Measurement

Procedure • Carry out straightness test on all lines AB,BC,AC

etc. Tabulate readings upto the Cumulative error column.

• Let Plane passing through points A, B & D be assumed to be an arbitrary plane, relative to which the heights of all other points may be determined. For it, the ends of line AB, AD & BD are corrected to zero and thus the height of A, B & D are zero.

• The Height of point I is determined relative to plane ABD=000.

• As I is the midpoint of AC also all the points on AC can be fixed relative to the arbitrary plane assuming A=0 and correcting I on AC to coincide with midpoint I on BD.

• Point C is now fixed relative to the arbitrary plane and points B & D are set at zero, all intermediate points on BC and DC can be corrected accordingly.

• The Position of H & G, E & F are known, so it is now possible to fit in lines HG and EF. Midpoint of these lines also coincide with known position of midpoint I.

• In this way, the height of all the points on the surface relative to the arbitrary plane ABD are known.

Flatness error of surface states that the departure from flatness is the minimum separation of a pair of parallel planes which will contain all points on the surface.

• The calculation for final correction to determine the minimum separation of a pair of parallel planes which just contain all the points on the surface is made by “Graphical Method”

• Two points on opposite sides having maximum positive and negative values are selected and joined together by a line XX.

• Let these point be R and S. Draw a line YY parallel to XX to represent the plane ABD as shown in the figure.

• Set off to scale the height of all points relative to YY by taking projections from all the points on the surface.

• By inspection draw a close pair of parallel lines ZZ and ZZ which will contain all of the points.

• The distance between these two lines is a measure of the error in flatness.

Principle of operation of Optical Flat

Fig 1 Flatness testing by interferometry

N.P.L. Flatness Interferometer

The Pitter-NPL Interferometer

Constant Deviation Prism