Economics of Machining

Economics of Machining

Economics of Machining - While a study of the geometry and mechanics of metal cutting is necessary in order to achieve efficiency, metal cutting is primarily an economic activity. When roughing, the aim is to remove a particular volume of metal in minimum time or at minimum cost. When finishing, the area of finished surface produced is the criterion.

The main aim is to produce quality parts at minimum cost.

Machining operations are used in industry to manufacture parts/products.

Since industry is very closely related with economics, hence the importance of studying the economic implications of the machining operations cannot be ignored.

Of vital interest to the manufacturing engineer are two quantities namely

(i) Production costs, and

(2) Production rates.

Both these depend on the choice of cutting parameters e.g.,

(a) Cutting speed

(b) feed, and

(c) depth of cut.

In practice, a high production rate would probably mean low production costs, it should be pointed out that these two factors must be considered separately and that the manufacturing conditions giving maximum production rate will not be identical to those conditions giving minimum cost of production.

Analysis of production costs and production rates can be a complicated subject, and in many cases the analysis will apply only to the particular operation in question. However, experience gained over the years has led to certain empirical rules or guiding principles for choosing the optimum cutting conditions for a given machining operation.

In the following discussion the production time is defined as the average time taken to produce one component, and the production cost is defined as the total average cost of performing the machining operation on a component using one machine tool.

Assuming the appropriate tool and cutting fluid were chosen for the machining of a batch of components, the only cutting conditions to be determined are the cutting speed and feed.

Feed is the distance moved by the tool relative to the workpiece in the feed direction for each revolution of the tool or workpiece or each stroke of the tool or workpiece. Confusion may arise in certain multipoint tool operations, such as milling, where the feed settings or the machine refer to the relative speed between the tool axis and the workpiece in the feed direction (the feed speed).

Cutting speed may be defined as the rate in meters per minute at which the surface of the workpiece moves past the cutting tool.

It may be stated that in any operation, when either the cutting speed or feed are increased while the other condition is held constant, the actual machining time will be reduced, and the tool wear rate will increase.

Thus, very low speeds and feeds will result in a high production time because of the long machining time. Alternatively very high speeds and feeds will also result in a high production time because of the frequent need to change cutting tools. Clearly an optimum condition will exist giving minimum production time. Similarly an optimum condition will arise for minimum production cost. At low speeds and feeds the cost will be high because of the cost of using the machine and operator for the long machining times.

At high speeds and feeds the costs will be high because of the cost of frequent tool replacement. The manufacturing engineer's problem is how to minimize both the production time and the production cost. Since, in general, these objectives cannot be reached simultaneously, a compromise must be sought.