Sheet Metal Forming: Metal Forming Processes in Manufacturing

 

 

In a manufacturing process, a given material is transformed into a useful part having a complex geometry with well-defined

(a) shape,

(b) size,

(c) accuracy and tolerances,

(d) appearance, and

(e) properties. 

The material usually begins in a shapeless form (such as liquid metal in casting) or of a simple  geometry (such as a blank sheet metal forming). The various manufacturing processes have advantages and limitations in achieving the desired shape, size, tolerances, appearance, and properties of a part.

 

 

Classification of Manufacturing Processes

Manufacturing processes can be divided, in a simplified manner, into five general areas

1) Primary shaping processes

such as casting, melt extrusion, die casting, and pressing of metal powder. In all these processes, the material initially has no shape but obtains a well-defined geometry through the process. Here the first shape is given to the material.

 

2) Forming processes

such as rolling, extrusion, cold and hot forging, bending, and deep drawing, where metal is formed by plastic deformation, without destroying the cohesion of the material.

3) Material removal processes

 in which excess material is removed from the starting workpiece in order to obtain the desired geometry. Some important processes in this category are turning, milling, drilling, sawing, and electro discharge machining.

 

4) Material treatment processes

aim to change the properties and appearance of the part without changing its shape. Heat treating, anodizing, and surface treatment are commonly used material treatment processes.

 

5) Joining processes

in which two or more parts are joined to form a new component or subassembly. Metallurgical joining processes, such as welding, brazing, and soldering, form a permanent and robust joint between components. Mechanical joining processes, such as riveting and mechanical assembly, bring two or more parts together to build a subassembly that can be disassembled conveniently.

 

 

Characteristics of Manufacturing Processes

 

There are four main characteristics of any manufacturing process: achievable geometry, tolerances, production rate, and environmental factors.

 

Geometry

Each manufacturing process is well suited for producing a particular type of geometry. Other geometries may be produced in some cases, but usually not without considerable expense. For example, manufacturing processes using dies and molds can produce parts that are easily removed from a mold made from two halves. However, by using a “split-die” design, it is possible to manufacture forgings, castings, or injection moldings with undercuts and more complex shapes.

 

Tolerances

When fabricating a given component, it is nearly impossible and very costly to make the part to the exact dimensions specified by the designer. Therefore, dimensions should be associated with a tolerance. By using more sophisticated variations of the process and by means of new developments, the quality of the tolerance, that is, precision, can always be improved.

 

 

Production rate

The rate of production, that is, number of parts produced per unit time, that can be attained with a given manufacturing operation is probably the most significant feature of that operation, because it indicates the economics of and the achievable productivity with that manufacturing operation. In industrialized countries, manufacturing industries represent 15 to 25% of gross national product. Consequently, manufacturing productivity, that is, production of discrete parts, assemblies, and products per unit time, is one of the most important factors influencing the standard of living in a country, as well as that country’s competitive position in international trade in manufactured goods. The rate of production or manufacturing productivity can be increased by improving existing manufacturing processes and by introducing new machines and new processes, all of which require new investments. However, the most important ingredient for improving productivity lies in human and managerial resources, because good decisions regarding investments (when, how much, and in what) are made by people who are well trained and well-motivated. As a result, the present and future manufacturing productivity in a plant, an industry, or a nation depends not only on the level of investment in new plants and machinery but also on the level of training and availability of manufacturing engineers and specialists in that plant, industry, or nation.

 

Environmental factors

Every manufacturing process must be examined in view of

(a) its effects on the environment, that is, in terms of air, water, and noise pollution;

(b) its interfacing with human resources, that is, in terms of human safety, physiological effects, and psychological effects,

(c) its use of energy and material resources, particularly in view of the changing world conditions concerning scarcity of energy and materials. Consequently, the introduction and use of a manufacturing process must also be preceded by a consideration of these environmental factors.

 

Metal Forming Processes in Manufacturing

 

Metal forming includes (a) bulk forming processes such as forging, extrusion, rolling, and drawing and (b) sheet forming processes such as brake forming, deep drawing, and stretch forming. Among the group of manufacturing processes discussed earlier, metal forming represents a highly significant group of processes for producing industrial and military components and consumer goods. A common way of classifying metal forming processes is to consider cold (before the crystallization temperature) and hot (above the recrystallization temperature) forming.

Most materials behave differently under different temperature conditions. Usually, the yield stress of a metal increases with increasing strain (or deformation) during cold forming and with increasing strain rate (or deformation rate) during hot forming. However, the general principles governing the forming of metals at various temper matures are basically the same. Therefore, classification of forming processes based on initial material temperature does not contribute a great deal to the understanding and improvement of these processes. In fact, tool design, machinery, automation, part handling, and lubrication concepts can be best considered by means of a classification based not on temperature but rather on specific input and output geometries and material and production rate conditions. Complex geometries, in both massive and sheet forming processes, can be obtained equally well by hot or cold forming. Of course, because of the lower yield strength of the deforming material at elevated temperatures, tool stresses and machine loads are, in a relative sense, lower in hot forming than in cold forming. However, part accuracy is usually higher in cold-formed parts. Forming is especially attractive in cases where (a) the part geometry is of moderate complexity and the production volumes are large, so that tooling costs per unit product can be kept low-for example, in automotive or appliance applications; and (b) the part properties and metallurgical integrity are extremely important, in examples such as load-carrying aircraft and jet engine and turbine components. The design, analysis and optimization of forming processes require (a) analytical knowledge regard metal flow, stresses and heat transfer as well as (b) technological information related to lubrication, heating and cooling techniques, material handling, die design and manufacture, and forming equipment.

able increase in the area-to-volume ratio occurring in the formed part. The term bulk indicates the low area-to-volume ratio in the starting material. The starting material is in billet, rod, or slab form. Bulk forming processes have the following characteristics:

 

• The workpiece undergoes large plastic de[1]formation, resulting in an appreciable change in shape or cross section.

 

• The portion of the workpiece undergoing permanent (plastic) deformation is generally much larger than the portion undergoing elastic deformation, so elastic recovery after deformation is usually negligible.

 

The characteristics of sheet metal forming processes are:

• The workpiece is a sheet or a part fabricated from a sheet.

• The deformation usually has the objective to cause significant changes in shape, but not in cross section, of the sheet. Reduction in sheet thickness is usually not desirable, but it is an unavoidable consequence of the process.

• In some cases, the magnitudes of permanent (plastic) and recoverable (elastic) deformations are comparable; thus, elastic recovery or spring back may be significant.