Machining operations are utilised in view of the better surface finish that could be achieved by it compared to other manufacturing operations. Thus, it is important to know what would be the effective surface finish that can be achieved in a machining operation. The surface finish in a given machining operation is a result of two factors:

1. The ideal surface finish, a result of the geometry of the manufacturing process, which can be determined by considering the geometry of the machining operation.
2. The natural component, a result of a number of the uncontrollable factors in machining, which is difficult to predict.

2.1.11 Ideal Surface Finish in Turning

In Fig 2.47, the geometry of the surface produced in turning with a sharp-cornered tool is shown. The situaton shown is only possible with no sharp tool, no BUE, no machine-tool chatter and elmination of all possible machine-tool inaccuracies in movements.

Normally the surface finish is represented by any suitable iindex, such as an arithmetic average, $R_{a}$, centreline average, $R_{cla}$, or any other suitable parameter. For our convenience, let us derive the finish expression for centreline average, $R_{cla}$. It is defined as, "the arithmetic average value (AA) of the departure of the whole of the profile both above and below its centreline throughout the prescribed meter cut-off in a plane substancially normal to the surface."

Referring to Fig. 2.47, the surface roughness value is given by,

$R_{cla}=\cfrac{|\text{Area } abc| + |\text{Area } cde|}{f}$ ----- (69)

where $f$ is the feed rate. Since triangles $abc$ and $cde$ are equal, $R_{cla}=\cfrac{2(\text{Area } abc)}{f}=\cfrac{R_{max}}{4}$ ----- (70) From the geometry, $R_{max}=\cfrac{f}{\cot{\lambda} +\cot{gamma}}$ ----- (71) Substituting this is in the above equation, we get $R_{cla}=\cfrac{f}{a(\cot{\lambda} +\cot{gamma})}$ ----- (72)

de will happen in

the surface rhesus d one a shown in Fig 248

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(74)

However, the actual turning tool used would have a nose radius in place of the sharp tool point, which modifies the surface geometry as shown in Fig. 2.48a. If the feed rate is very small, as it normally happens in finish turning, the surface is produced by the nose radius alone as shown in Fig. 2.48b.

The above are essentially the geometric factors and the values represent an ideal situation. However, the actual surafce finish obtained depends to a great extent upon a number of factors such as

• Cutting process parameters, speed and depth of cut.
• Geometry of the cutting tool.
• Application of cutting fluid.
• Work and tool-material characteristics.
• Rigidity of the machine tool and the consequent vibrations.

The major influence on surface finish is exerted by the feed rate and cutting speed. As the feed decreases, from the above equation distance on surface finish is used by the food and goodAs the foed decreases om the above equation can see that the route de deres Similarly, the cuning speed web The whole choice of ti m es for finish dexirable to hve igh cutting speed and small fans