What is Surface Roughness

What is surface roughness ?

It is quantified by the vertical deviations of a real surface from its ideal form. If these deviations are

large, the surface is rough; if they are small the surface is smooth

Lay –this is the direction of the surface pattern and determine by the production method.Roughness height – it is the height of irregularities with respect to reference line.Waviness width – it is the peak to valley distance of surface profile .Waviness - it is the widely space condition of surface texture caused by vibration, deflection, heat treatment.

No surface is perfectly smooth, but the better the surface quality, the longer a product generally lasts, and the better is performs.

Analysis of surface traces

Root mean square value-

It is define as square root of the mean of the squares of the ordinates of the surface measure from a mean line.

Let Sample lenght is (l)Average of surface roughness height (z)

Centre line average or Arthmetic average Ra is the arithmetic average value of the departure of the profile from the centre

line throughout the sampling length L

To calculate the roughness using samples at evenly spaced positions

The roughness can also be calculated by area,

Rz-Ten point peak to valley average height

Rz is the average difference between the five highest peaks and five deepest valleys within the sampling length, the heights being measured from a line parallel to the mean line and not crossing the profile.

R (max) – Maximum Peak-to-Valley Height

• It is a distance between two lines parallel to mean line, one line touching the profile at heighest profile and other lowest point of profile within sample lenght (L).

• This parameter fail to give the true picture ,used only when some technological or functional reason.

Mean Leveling Depth Rpm

• The Mean Leveling Depth Rpm is the mean of five leveling depths of five successive sample lengths l.

R3z

• Base roughness profile depth.--- The height of the 3rd highest peak from the 3rd lowest valley per sampling length. The base roughness depth is found in five sampling lengths and then averaged.

The R parameters do not give any information about surface irregularity shapes or spacing.For that we can use

1 ) SKEWNESS 2) KURTOSIS

Skewness

• Skewness is the degree of departure from symmetry of a distribution. A positively skewed distribution has a "tail" which is pulled in the positive direction. A negatively skewed distribution has a "tail" which is pulled in the negative direction

Kurtosis

• Kurtosis: Measure of the "peakness" compared to a normal distribution

• A sharp amplitude distribution curve indicates large, sharp peaks and valleys; kurtosis > 3.

• A broad, flat amplitude distribution indicates many small, rounded peaks and valleys in the profile; kurtosis < 3.

Turning , shaping ,EDM are having positive skewness , while grinding , honing , milling ,abrasion produce surface

having negative skewness but high kurtosis value.

Measurement of roughness

• Touch inspection• Visual inspection• Scratch inspection• Microscopic inspection• Surface photographs

Stylus probe instrument

A skid is moved over the surface either by hand or by motor. Skid when moved over the surface, follow its general contours which provide a datum for measurement. A recorded device is used to trace the record of surface profile.

Brown & Sharpe Surfcom unitThis technique uses a stylus that tracks small changes in surface height, and a skid that follows large changes in surface height. The use of the two together reduces the effects of non-flat surfaces on the surface roughness measurement. The relative motion between the skid and the stylus is measured with a magnetic circuit and induction coils.

Modern method

• Pneumatic method• Light cross section method• Scanning Laser Microscope

CONCLUSION• Surface roughness is an important factor when dealing with issues such as

friction, lubrication, and wear. It also has a major impact on applications involving thermal or electrical resistance, fluid dynamics, noise and vibration control, dimensional tolerance.

• Additionally, a brief survey of some of the roughness measurement techniques currently available was provided. Stylus profilers are the most common instruments used today for roughness measurement

REFERENCES

• Thomas, T.R., Rough Surfaces, 2nd ed., Imperial College Press, London

• METEROLOGY - By R. K. Jain• Wikipedia