In this article we will discuss about the classification of metal forming processes.

1. Compressive Forming Processes:
In these processes the plastic state is reached by application of uni-axial or multi-axial compressive forces. Examples are forging, extrusion, rolling, indenting, coining, etc. (Fig. 2.2) shows the stress states in the material during rolling, forging, coining and extrusion processes. In the rolling process in Fig. 2.2(a) the metal ingot or sheet is pressed between two rotating rolls.

Besides the compression by rolls, the friction between the work metal and surfaces of rolls also gives rise to compressive forces in the longitudinal and lateral directions. Thus, there are compressive forces all around though of different magnitudes at different locations and in different directions.

Similarly, in open die forging in Fig. 2.2(b), the metal body is pressed between two flat or curved dies, while the friction between metal and dies gives rise to compressive forces in the lateral directions. In closed die forging and coining processes also there are compressive forces all around in Fig. 2.2(c).

In direct extrusion in Fig. 2.2(d) the metal is compressed in a container, generally a cylinder, and made to flow though a small opening in the die. The force applied by press ram and the constraint of cylinder walls produce compressive forces all around on the work piece.

Rolling Processes:

Rolling, forging and extrusion are in fact groups of processes. For example, rolling comprises the following processes.

Figure 2.4 illustrates four types of longitudinal rolling processes. Figure 2.4(a) is the conventional longitudinal rolling, which is used for rolling sheets, plates, strips, bars, angle sections, beams, rails, etc. For rolling of each of these sections a number of rolling machines generally called rolling stands are employed. Each reduction is called roll pass. A number of passes are required for rolling any one section.

Most of these products are produced on large scale and in various sizes, therefore, groups of rolling machines called rolling mills are devoted for producing a range of similar profiles. These mills are called by the name of product they produce. Thus we have billet rolling mill for rolling billets which are rectangular sections. Other names are rail and heavy section mill for rolling rails, large size angles, beams, etc.

Similarly plate rolling mill for rolling plates. Other names are hot sheet rolling mills, cold sheet rolling mills, etc. Small sections like bars, small angle sections, strips, etc., are generally rolled in re-rolling mills which are also known as Merchant section rolling mills.

Ring rolling is illustrated in Fig. 2.4(b). The process is used for rolling of ball bearing races, rail-wheel-tyres and other thick rings used in many different applications. The machine consists of a small diameter inner roll and a big diameter outer roll. Besides, two conical rolls are provided for rolling the edges flat.

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