Straightness and Flatness

StraightnessStraightness is a condition in which an element of a surface or an axis is a straight lineStraight edge:It is the most commonly used tool for establishing and checking straightness of a surface.It consists of a rectangular steel bar with a narrow cross section and the edge is precisely machined.

Straightness Control

Separation of Roughness, Waviness and Form

Straight edgesUse of straight edge in conjunction with feeler gages

Straightness testing Wedge method

Historical Surface Metrology Measurement Spectrum

Current Metrology Measurement spectrum

Straightness Profile - Stylus Instrument A

Straightness Profile - Stylus Instrument B

Straightness Profile - Cylindricity Instrument

Straightness Profile - Scanning CMM

Comparison of Straightness values

Wavelength Domain and Functional Performance

Auto-collimatorAuto-collimator or level test of surface straightnessAuto-collimator used for checking the straightness of lathe-bed guide-ways

Optical InstrumentsPoint source of light in focal plane of a collimating lensProjection of a point source being reflected from an inclined reflector

Electronic Levels

Electronic Level

Electronic Level

Auto-collimatorAuto-collimator or level test of surface straightnessAuto-collimator used for checking the straightness of lathe-bed guide-ways

Auto-collimator test results

Auto-collimator test resultsGraphs of cumulative error and actual error in a machine bed, determined using an auto-collimator

FlatnessA surface is said to be flat if the following conditions are satisfied: All generators must be straight. All generators must lie in the same plane.A surface, all of whose generators(lines) parallel to the sides are straight, but which is not flat.

Flatness Control

Interference fringes on a flat surfaceFormation of interference fringes on a flat surface viewed under an optical flat in a parallel beam of monochromatic light

Convex surface

Measurement GridsA generalized Union Jack, or rectangular measurement grid with seven horizontal and seven vertical generators

Measurement GridsA Union Jack measurement grid with the same node spacing along all generators

Measurement GridsA triangular, or hexagonal, measurement grid with five rows, seven vertices on each odd-numbered row and eight vertices on each even-numbered row.

Flatness test Procedure for performing flatness test on a surface:Carry out a normal straightness test on each generator.Tabulate each set of results only as far as the cumulative error column.Correct the ends of AC, AG and CG to zero. This gives the heights of points A, C and G as zero and these points then constitute an arbitrary plane relative to which the heights of all other points may be determined.

Flatness testFrom the above step, the height of O is known relative to the arbitrary plane ACG = OOO. As O is the common mid-point of AC, CG, BF and HD all points on AE are now fixed. This is done by leaving A = O and correcting O on AE to coincide with the mid-point O on CG.Correct all other points on AE by an amount proportionate to the movement of its mid-point. Note that E is twice as far from A as the mid-point, its correction is double that of O, the mid-point.As E is now fixed and C and G are set at zero, it is possible to put in CE and GE, proportionally correcting all intermediate points on these generators.The positions of H and D, and B and F, are known so it is now possible to fit in lines HD and BF. This provides a check on previous evaluation since the mid-point of these lines should coincide with the known position of O, the mid-point of the surface.

Cumulative Errors of Lines of test

Flatness test on a sample surface Three corners of a surface adjusted to zero to enable the height of the mid-point to be fixed relative to a plane through the corners. This enables the height of the other corner to be determined

Test - Continued

Test - ContinuedHeight of all measured points related to an arbitrary plane ACG

Test - Continued

Test - Continued

Flatness error estimated by tilting

Flatness error obtained by projection

Flatness error by graphical methods