Machine Drawing

Surface Roughness 243

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d:\N-Design\Des16-1.pm5

16.2.1 Actual Profile, Af

It is the profile of the actual surface obtained by finishing operation.

16.2.2 Reference Profile, Rf

It is the profile to which the irregularities of the surface are referred to. It passes through the

highest point of the actual profile.

16.2.3 Datum Profile, Df

It is the profile, parallel to the reference profile. It passes through the lowest point B of the

actual profile.

16.2.4 Mean Profile, Mf

It is that profile, within the sampling length chosen (L), such that the sum of the material-

filled areas enclosed above it by the actual profile is equal to the sum of the material-void areas

enclosed below it by the profile.

16.2.5 Peak-to-valley Height, Rt

It is the distance from the datum profile to the reference profile.

16.2.6 Mean Roughness Index, Ra

It is the arithmetic mean of the absolute values of the heights hi between the actual and mean

profiles. It is given by,

Ra = 1/L

x

xL

=

=

z 0 |hi|dx , where L is the sampling length

16.2.7 Surface Roughness Number

The surface roughness number represents the average departure of the surface from perfection

over a prescribed sampling length, usually selected as 0.8 mm and is expressed in microns.

The measurements are usually made along a line, running at right angle to the general direction

of tool marks on the surface. Surface roughness values are usually expressed as the Ra value

in microns, which are determined from (Fig. 16.1),

Ra =

hhh h

n

123 ++++... n

The surface roughness may be measured, using any one of the following :

1. Straight edge

2. Surface guage

3. Optical flat

4. Tool maker’s microscope

5. Profilometer

6. Profilograph

7. Talysurf

Table 16.1 shows the surface roughness expected from various manufacturing processes.