Economics of Machining:

It is not sufficient to suggest a feasible procedure to manufacture the wanted component, but the process should also be economically justified. This is a difficult problem because there are various variables that affect the economics of a machining operation. These variables are following:

A.    tool geometry and tool material,

B.     machine tool capacity ( force, power, size, etc.), and

C.     Cutting conditions (feed, speed and depth of cut).

Optimum selection of these variables can seldom be achieved by a machine operator, rather it is done by the process planning engineer who has access to all relevant data. Real life problem is difficult to solve because it involves process interactions, variation in anticipated sales, etc. Such problem may be studied using dynamic programming, but the solution becomes complex. Instead of full optimization, a procedure that is usually adopted is to select conditions at each operation to yield a sub-optimized solution. These conditions can then be modified, if necessary, after reviewing the process interactions by inspection of the whole production program. This latter phase is carried out continuously using some form of flow chart or production planning chart to organize the data.

Three objective functions frequently used in sub-optimization of machining operations are :

(i) minimum cost per component,

(ii) maximum production rate, and

(iii) maximum profit rate for the operation.

In general, the lowest cost per component consideration leads to lower production rate. Sometimes, optimization process may give the machining conditions which may be beyond the capabilities of the available machine tool. Hence, in selecting the economic operating conditions, machine tool capacities must be taken into account. If the selected conditions are not available on the machine tool proposed for a particular operation, it is necessary to either change the operating conditions or review the machine tool selection by cost comparison. One should not select the machine tool of the capacity higher than the desired one. The capacity limits of a machine tool include feed, speed, power and maximum allowable cutting force (or thrust force). Further, there may be feed and speed constraints to achieve the desired surface finish on the component.

A component usually requires more than one pass of cutting for completion. For simplicity of analysis, we will analyze only a simple case of single pass turning operation.