Surface Finish and Surface Roughing

Surface Finish and Surface Roughing

Surface Finish and Surface Roughing - The proper functioning and service life of a machine part depend to a considerable extent on the quality of its surfaces, i,e., its surface finish.
Smooth surface finish is essential in order to

(i) Reduce friction between two parts moving on or in each other.

(ii) Control wear.

(iii) Achieve accuracy and pressure retaining ability in mechanisms such as injectors and high pressure cylinders.

(iv) Increase fatigue strength of highly stressed members which are subjected to load reversals. A smooth surface eliminates the sharp irregularities which are the greatest potential source of fatigue cracks.

(v) For parts such as gears, surface finish control may be necessary to ensure quiet operation.

(vi) Smoothness is often essential for eye appeal M the finished product.

It is not like that, that very smooth surfaces C are always desired. The surface finish must be a compromise between sufficient roughness for proper wearin and sufficient smoothness for expected service life.

For parts where a boundary lubrication condition exists or where surfaces may not be compatible as in two extremely hard surfaces running together, a slightly roughened surface will usually assist in lubrication.

Most new moving parts do not attain a condition of complete lubrication as a result of imperfect geometry, running clearances and thermal distortions. Therefore, the surface must wearin by a process of actual removal of metal

Every machining process leaves its characteristic marking on the workpiece surface. Each kind of cutting tool leaves its own individual pattern that can be easily identified. This pattern (which is in the form of finely spaced irregularities left by the cutting tool on the work surface) is known as surface finish or surface roughness.

Pattern Left by a Shaper Tool and an End Mill Cutter

These surface irregularities are related to the efficiency of the machining operation. The height, shape, arrangement and direction of these surface irregularities of workpiece depend on a number of factors such as :

(i) Speed, feed, and depth of cut in machining.

(ii) Design, geometry and cutting capacity of the cutting tool.

(iii) Cooling and lubrication.

(iv) Composition of workpiece material.

(v) Type and condition of the machine tool.

(vi) Auxiliary tooling and the jig or fixture.

(vii) Vibration of both job and cutting tool.

The lesser are the irregularities on the workpiece surface the better is the surface finish.

Surface. The surface of an object is the boundary which separates that object from another object, substance or space.

Surface texture. Repetitive or random deviations from the nominal surface which form the pattern of the surface. Surface texture includes roughness, waviness, Jay and flaws. Lay IS the direction of the prominent surface pattern.

Surface roughness consists of the finer irregularities in the surface texture usually including those irregularities which result from the inherent action of the production process. These are considered to Include traverse feed marks and other irregularities within the limits of the roughness width cut off.

Roughness width is the distance paral1e1 to the nominal surface between successive peaks or ridges which constitute the predominate pattern of the roughness.

Roughness width cot off is the greatest spacing of repetitive surface irregularities to be included in the measurement of average roughness height. Roughness width cut off must always be greater than the roughness; width in order to obtain the total roughness height rating.

Surface Characteristics

Roughness height. This is rated as the arithmetical average deviation expressed in micrometers, measured normal to an imaginary centre line, running through the roughness profile.The lesser are these irregularities, the better is the surface finish