Mechanical measurement and MetrologySurface finish measurement

Prepared by – Navroz Navodia

Surface texture• The characteristic quality of an actual surface due to

small departures from its general geometrical form a pattern or texture on the surface• The problem is basically geometrical• They are classified as follows

1. First order – due to inaccuracy of machine tool2. Second order-due to vibration of any kind3. Third order-due to characteristic property of

machine4. Fourth order-due to rupture of material

Surface finish terminology1. Roughness2. Waviness3. Surface4. Actual surface5. Nominal surface6. Roughness height7. Roughness width8. Roughness width cut-off9. Waviness height10.Waviness height11.Profile12.Centre line13.Effective profile

Methods of measuring surface finish

1. Surface inspection by comparison method 2. Direct instrument measurement3. Other methods of measuring surface finish

Comparison methods

1. Touch inspection:• In this method finger tip is moved at a speed of

25mm per second and irregularities upto 0.01mm can be measured.

2. Visual inspection:• Means visualizing by naked eyes and is just a

subjective evaluation.

Comparison methods3. Scratch inspection• Lead or babbit or plastic is moved along the surface to

be inspected.

4. Microscopic inspection• Microscope can only measure small surface.• Compared with a master surface.

• 5. Surface photograph• Magnified photograph with different illumination are

taken.

Comparison methods6. Micro interferometer• In this method an optical face is placed on the surface

to be inspected and illuminated by a monochromatic light and then interference band are studied by microscope.

7. Wallace dynamometer• It consists of a pendulum which is lifted to initial position and allowed to swing over testing surface. Surface smoothness is determined by friction as swing time is a direct measure of surface finish.

Comparison methods

8. Reflected light intensity• In this method a beam of light(known qty.) is projected upon surface and is reflected in several directions as beams of lesser intensity and change in length in different directions is measured by photocell.

Direct instrument measuring method

1. Profilometer• Commonly used in

U.S.A. for direct measurement of surface quality• This instrument is best

suited for measuring surface finish of deep bores.

Direct instrument measuring method

2. Tomlinson Surface Tester• Design by Dr. Tomlinson.• It is reliable and cheaper.• It is slow in operation.• Not suitable for continuous use.• Delicate and requires great care.

Direct instrument measuring method

Direct instrument measuring method

3. The Taylor-Hobson Talysurf• The Talysurf is an electronic instrument working on

carrier modulating principle.• Microprocessors have made available complete

statistical multi-trace systems measuring several places over a given area and can provide standard deviations and average over area-type readings and define complete surface characterisation.

Direct instrument measuring method

Direct instrument measuring method

4. Profilograph

Direct instrument measuring method

5. Light cross section method

Other methods for measuring surface roughness

• 1. Pneumatic method• Also known as air

leakage method• Pneumatic comparator

is used• Air discharge is

measured by a special variable air flow meter

Other methods for measuring surface roughness

2 Light interference microscope• It is a non contact

process• It is directly related to

RA value• Good resolution can

give scratch spacing of 0.5 micrometer.

Mecrin instrument• This instrument

assesses the surface through its frictional property and the average slope of the irregularities• Ra values ranges from

3-5 micrometer

Adverse effect of poor surface finish

1. Fatigue strength• High surface finish increase fatigue strength

2. Wear resistance • Decreases with poor surface finish

3. Corrosion resistance• Decreases with poor surface finish

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