Process Selection and Sequencing

In this, some factors, which have an influence in all processes, are discussed. The factors include size, shape complexity, tolerances and production volume.

After product consideration, selection of sequence of process is greatly influenced by the characteristics of the manufacturing processes. Hence a brief treatment of various manufacturing process right from casting to welding are discussed.

In all casting processes, a metal or alloy is melted, poured into a mold, allowed to solidify and the part thus produced is released from the mold. Very broadly, casting processes fall into two groups: expendable-mold and permanent-mold processes. Choice of process and mold material is greatly influenced by the melting point of the alloy.

All plastic deformation processes exploit the ability of metals and alloys to change shape by plastic flow. The ability to deform without fracture is highly variable. Bulk deformation processes impose material flow in all directions, and the ability to deform is expressed as workability; in sheet metal working processes deformation is essentially two-dimensional, and limits of deformation are referred to as formability. It is important to recognize that the limits of plastic deformation are the outputs of a system and these are greatly affected by the stress state prevailing in the process.

Powder processing involves a sequence of operations. The powder is produced by various techniques: reduction of an oxide, thermal decomposition, electrolysis, hydrometallurgy, or breaking up (atomizing) a melt. Particle size is controlled during powder production or by comminution; size fractions are separated by sizing. After cleaning (and possibly annealing), alloying elements and lubricants are added by blending. There are several process routes to arrive at finished parts.

Parts produced by casting, deformation, and powder processing techniques frequently require some metal removal to create the final shape, meet tolerances, or obtain the requisite surface finish. Machining may also be an economical alternative to create the shaper from a semi fabricated product such as bar or plate. Metal may be removed by chip formation or by less conventional, noncutting techniques.

A good manufacturing sequence is always determined by selecting suitable terminal points. Hence such a sequence must begin and end with part prints. Before formulating any process plan, the process engineer find out what is wanted in the final product. At the end of manufacturing sequence, the process engineer must again make reference to the part print to determine by comparison whether or not the final product can meet all design specifications.