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1. MECHANICAL MEASUREMENT AND METROLOGY
MECHANICAL ENGINEERING
SURFACE ROUGHNESS: TERMINOLOGY AND TYPES
1. Roughness:Roughness consists of surface irregularities which result fromthe various machining process. These irregularities combine to formsurface texture.
2. Roughness Height:It is the height of the irregularities w ith respect to a reference line. It is measured in millimeters or microns or micro inches. It is also known
as the height of unevenness.
3. Roughness Width:The roughness w idth is the distance parallelto the nominal surface between successive peaks or ridges which constitute the predominate
pattern of the roughness. It is measured in millimeters.
4. Roughness Width Cut Off: Roughness width cut off is the greatest spacing of respective surface irregularities to be included in the measurement of the
average roughness height. It should alw ays be greater than the roughness w idth in order to obtain the total roughness height rating.
5. Lay: Lay represents the direction of predominant surface pattern produced and it reflects the machining operation used to produce it.
6. Waviness: This refers to the irregularities w hich are outside the roughness width cut off values. Waviness is the w idely spaced component of the surface
texture. This may be the result of w ork piece or tool deflection during machining, vibrations or tool run out.
7. Waviness Width: Waviness height is the peak to valley distance of the surface profile, measured in millimeters.
8. Arithmetic Average (AA): A close approximation of the arithmetic average roughness-height can be calculated fromthe profile chart of the surface. Averaging
from a mean centerline may also be automatically performed by electronic instruments using appropriate circuitry through a meter or chart recorder.
TYPES OF SURFACE ROUGHNESS:
1. FIRST ORDER: This includes the irregularities arising out of inaccuracies in the machine tool itself. This includes the surface irregularities arising due
to deformation of w ork under the action of cutting forces and the w eight of the material itself.
2. SECOND ORDER: It is irregularities caused due to vibrations of any kind such as chatter marks.
3. THIRD ORDER: It is the irregularities caused by a machining it due to characteristics of the process even through the machine w ere perfect and
completely free from vibrations. The characteristics of the process includes speed, feed, depth of cut, etc.
4. FOURTH ORDER: It is the irregularities developed due to the rupture of the material during the separation of the chip fromthe already finished surface
of the w ork piece.
SPECIFICATION OF SURFACE TEXTURE CHARACTERISTICS:
1. a = Roughness value i.e. Ra value in µm.
2. e = Machining allow ance in mm.
3. c/f = Sampling length or instrument cut-off length in mm.
4. b = Machining/production method.
5. d = Direction of lay in the symbol form as =, ┴, X, M, C, R.
GUIDED BY: PROFF. PRAVIN SOLANKI