Terminology
The quality of machined surface is characterized by the
accuracy of its manufacture with respect to the dimensions specified by
the designer. Every machining operation leaves characteristic evidence
on the machined surface. This evidence in the form of finely spaced micro
irregularities left by the cutting tool. Each type of cutting tool leaves
its own individual pattern which therefore can be identified. This pattern
is known as surface finish or surface roughness.
1. Roughness :
Roughness consists of surface irregularities which result
from the various machining process. These irregularities combine to form
surface texture.
2. Roughness Height :
It is the height of the irregularities with respect to
a reference line. It is measured in millimeters or microns or microinches.
It is also known as the height of unevenness.
3. Roughness Width :
The roughness width is the distance parallel to 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 always be greater than the roughness width
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 which are outside the
roughness width cut off values. Waviness is the widely spaced component
of the surface texture. This may be the result of workpiece or tool deflection
during machining, vibrations or tool runout.
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 from the 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. If X is
the measured value from the profilometer, then the AA value can be calculated
as shown below.
9. Root Mean Square (rms)
The rms value can be calculated as shown below. Its numerical
value is about 11% higher than that of AA.
SURFACE FINISH IN MACHINING
The resultant roughness produced by a machining process
can be thought of as the combination of two independent quantities:
1. Ideal roughness, and
2. Natural roughness
Ideal Roughness
Ideal surface roughness is a function of only feed and
geometry. It represents the best possible finish which can be obtained
for a given tool shape and feed. It can be achieved only if the built-up-edge,
chatter and inaccuracies in the machine tool movements are eliminated completely.
For a sharp tool without nose radius, the maximum height of unevenness
is given by:
The surface roughness value is given by:
Practical cutting tools are usually provided with a rounded
corner, and figure below shows the surface produced by such a tool under
ideal conditions. It can be shown that the roughness value is closely related
to the feed and corner radius by the following expression:
where r is the corner radius.
Natural Roughness
In practice, it is not usually possible to achieve conditions
such as those described above, and normally the natural surface roughness
forms a large proportion of the actual roughness. One of the main factors
contributing to natural roughness is the occurrence of a built-up edge.
Thus, larger the built up edge, the rougher would be the surface produced,
and factors tending to reduce chip-tool friction and to eliminate or reduce
the built-up edge would give improved surface finish.
Factors Affecting the Surface Finish
Whenever two machined surfaces come in contact with one
another the quality of the mating parts plays an important role in the
performance and wear of the mating parts. The height, shape, arrangement
and direction of these surface irregularities on the workpiece depend upon
a number of factors such as:
A) The machining variables which include
a) cutting speed
b) feed, and
c) depth of cut.
B) The tool geometry
The design and geometry of the cutting tool also plays
a vital role in determining the quality of the surface. Some geometric
factors which affect achieved surface finish include:
a) nose radius
b) rake angle
c) side cutting edge angle, and
d) cutting edge.
C) Workpiece and tool material combination and their mechanical properties
D) Quality and type of the machine tool used,
E) Auxiliary tooling, and lubricant used, and
F) Vibrations between the workpiece, machine tool and cutting tool.
