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ОглавлениеClassification Of Shaping Processes
Production Equipment And Tooling
Manufacturing is the procedure by which materials are transformed into desired shapes. Materials are first formed into preliminary shapes, and then refined into more precise shapes, after which the final shaping and finishing operations are performed. The shaping process is one broad category of manufacturing processes. In the shaping process, a component or product can be created from a solid, granular, particle state, or liquid state, meaning the state of the work material in the shaping phase. In the design of a shaping process, the only factors that are known at the outset are the final product shape and the material with which it is to be made. It is the engineer who must design a process to make a defect-free product; the engineer always operates under constraints due to the shape of the desired object, the material’s properties, the cost of production, the time available, and many other factors. Shaping processes influence the vast majority of products bought or used by consumers. This is the reason why in this book so much space will be devoted to discussing particular shaping processes. These are processes that you, as a designer or manufacturing engineer, will most likely use or encounter while engaged in the practice of mechanical or industrial engineering.
CLASSIFICATION OF SHAPING PROCESSES
Classification of technological shaping processes may be based on many different criteria, depending on the purpose of the processes. In general, shaping process can be classified into two main categories: primary and secondary shaping processes. The primary shaping processes form the overall shape of the product or the component that will, with other parts make up the final product’s shape. The purpose of secondary processes is to provide the final, precisely shaped surfaces that will meet product requirements, such as surface or dimensional tolerances.
The classification used in this book refers to primary shaping processes and is based on the state of the starting material. There are four broad categories of primary shaping processes for metals (Fig. I.1):
Fig. I.1 Classification of primary metal shaping processes.
1.Casting, molding, and other processes, in which the starting material is in the form of a heated liquid.
2.Particulate processing (powder metallurgy), in which the starting material is a powder.
3.Deformation processes, in which the starting material is a solid that is deformed to shape the components.
4.Metal removal processes, in which the starting material is a solid whose size is sufficiently large for the final geometry of parts to be circumscribed by it; in these processes, the unwanted material is removed as chips, particles, and so on.
Casting and molding. In this category starting material is heated to transform it into a liquid state; after that, the material is poured or in some other way forced to flow into a die cavity and allowed there to solidify, thus taking the shape of the cavity, which is nearly the shape, or the “net” shape, of the part. This type of primary shaping process is called casting. Casting processes use two types of mold: expandable mold (sand casting, shell mold casting, investment casting, and ceramic mold casting); and permanent mold (permanent-mold casting, die casting, centrifugal casting, and squeeze casting).
Particulate processing. In this category the starting materials are metals or ceramics in powder form. In this process, metal powders are compressed into desired shapes (often complex) and sintered (heated without melting) to form a solid component. This type of primary shaping process is called powder metallurgy.
Deformation processes. In this category the starting material is in a solid state. The initial shaping of the workpiece is accomplished through the application of external forces to the solid work material, with these forces being in equilibrium. Wit the application of load to the workpiece, internal stress and displacements are generated, causing shape distortions. This type of shaping process is called “deformation”.
Deformation processes can be conveniently classified into bulk deformation processes (rolling, extrusion, and forging) and sheet metal processes (shearing, bending and drawing, and forming). In both cases the surfaces of the deforming material and of the tools are usually in contact, and the friction between them has a major influence. In bulk forming the input material is in billet, rod, or slab form, and a considerable increase in the surface-to-volume ratio occurs in the formed part. In sheet metal processes a sheet blank is plastically deformed into a complex three-dimensional configuration, usually without any significant change in sheet thickness and surface characteristics.
Bulk deformation processes have the following characteristics:
•The workpiece undergoes large plastic deformation, 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. Therefore, elastic recovery or spring-back after deformation is negligible.
The characteristics of sheet metal processes are as follows:
•The workpiece is a sheet or a part fabricated from a sheet. The deformation usually causes significant changes in shape but not in the cross-section of the sheet.
•In some cases the magnitudes of permanent plastic and recoverable elastic deformations are comparable; therefore, elastic recovery or springback may be significant.
Material removal processes. Machining processes are frequently used as primary or secondary processes. In these processes the size of the original workpiece is large enough so that the final geometry of the finished piece can be achieved by employing one or more removal operations. The chips or scrap are necessary to obtain the desired geometry, tolerance, and surface finish. The amount of scrap may vary from a few percent to more than 70% of the volume of the starting workpiece material. Machining as a primary process used for low-production volume parts, for the production of prototypes, and for production of the tooling used in processes such as stamping, injection molding, and other processes, generates a part’s shape by changing the volume of the workpiece through the removal of material.
Machining is also used as a secondary process. Thus, machining is used to improve a basic shape that has been produced by casting or deformation processing, as well as to produce the basic shape itself.
PRODUCTION EQUIPMENT AND TOOLING
Equipment. The type of equipment and especially the tools used in manufacturing depends on the manufacturing process. Machine tools are among the most versatile of all production equipment. They are used not only for producing tools and dies, but also for producing components for other products and production equipment, such as presses and hammers for deformation processes, rolling mills for rolling sheet metal, and so on.
The name of the equipment usually follows from the name of the process. The productivity, reliability, and cost of equipment used for shaping processes are extremely important factors, since they determine the economics and practical application of a given process. For example, in both sheet and bulk forming the stroking rate of forming machines continues to get faster. Because of this machine dynamics and machine rigidity and strength are of increasing concern. As in other unit processes, the use of sensors for process monitoring and control is essential and continues to increase. Sensors are also being increasingly used to monitor the condition of the tooling during operations. Such monitoring systems cannot only improve part quality but can also enable longer tool life.
Tooling. The design and manufacture of tooling are essential factors determining the performance of shaping processes. The key to a successful shaping process lies in the tool design, which generally, to a very large extent, is based on the experience of the designer(s). Innovative multi-action tool designs have recently been developed that are capable of near-net shaping of increasingly complex parts, such as gears and universal joint components. These tooling approaches can be expanded. Many companies are already using computer-aided engineering and computer-aided manufacturing to design and fabricate process tooling. Advanced heat treatment and coating techniques can extend tool life. Based on a developing understanding of the mechanisms of erosive tool wear, studies are being conducted to measure and predict lubricant behevior and heat transfer at the tool material interface. This is an extremely important area, since tool life directly influences the economics of deformation processes.
