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1、精品文档，仅供学习与交流，如有侵权请联系网站删除编号： 毕业设计(论文)外文翻译（原文）学 院： 机电工程学院 专 业：机械设计制造及其制动化学生姓名： 学 号： 指导教师单位： 机电工程学院 姓 名： 周定礼 职 称： 工程师 2012年 5 月 15 日【精品文档】第 18 页Forming Processes Charles W. Beardsley Abstract:Forming can be defined as a process in which the desired size and shape are ob-tained through the plastic defor

2、mations of a material. The stresses induced during the process are greater than the yield strength, but less than the fracture strength, of the material. The type of loading may be tensile, compressive, bending, or shearing, or a combination of these. This is a very economical process as the desired

3、 shape, size, and finish can be obtained without any significant loss of material. Moreover, a part of the input energy is fruitfully utilized in improving the strength of the product through strain hardening. Keywords: Forming Rolling Forging Extrusion 1.1 Forming The forming processes can be group

4、ed under two broad categories, namely, cold forming, and hot forming. If the working temperature is higher than the recrystalliza-tion temperature of the material, then the process is called hot forming. Otherwise the process is termed as cold forming. The flow stress behavior of a material is entir

5、ely different above and below its recrystallization temperature. During hot working, a large amount of plastic deformation can be imparted without significant strain hard-ening. This is important because a large amount of strain hardening renders the mate-rial brittle. The frictional characteristics

6、 of the two forming processes are also entirely different. For example, the coefficient of friction in cold forming is generally of the order of 0.1, whereas that in hot forming can be as high as 0. 6. Further, hot forming lowers down the material strength so that a machine with a reasonable capacit

7、y can be used even for a product having large dimensions. The typical forming processes are rolling, forging, drawing, deep draining, bending, and extrusion. For a better understanding of the mechanics of various form-ing operations, we shall briefly discuss each of these processes. 1.2 Rolling In t

8、his process, the job is drawn by means of friction through a regulated opening between two power-driven rolls. The shape and size of the product are decided by the gap between the rolls and their contours. This is a very useful process for the produc-tion of sheet metal and various common sections,

9、e. g., rail, channel, angle, and round. 1.3 Forging In forging, the material is squeezed between two or more dies to alter its shape and size. Depending on the situation, the dies may be open or closed.1.4 Drawing In this process, the cross-section of a wire or that of a bar or tube is reduced by pu

10、lling the workpiece through the conical orifice of a die represents the operation schematically. When high reduction is required, it may be necessary to perform the operation in several passes. 1.5 Deep Drawing In deep drawing, a cup-shaped product is obtained from a flat sheet metal with the help o

11、f a punch and a die. The sheet metal is held over the die by means of a blank holder to avoid defects in the product. 1.6 Bending As the name implies, this is a process of bending a metal sheet plastically to obtain the desired shape. This is achieved by a set of suitably designed punch and die. 1.7

12、 Extrusion This is a process basically similar to the closed die forging. But in this operation, the workplace is compressed in a closed space, forcing the material to flow out through a suitable opening, called a die. In this process, only the shapes with constant cross-sections (die outlet cross-s

13、ection) can be produced. 1.8 Advantages and Disadvantages of Hot and Cold Forming Now that we have covered the various types of metal working operations, it would only be appropriate that we provide an overall evaluation of the hot and cold working processes. Such a discussion will help in choosing

14、the proper working condi-tions for a given situation. During hot working, a proper control of the grain size is possible since active grain growth takes place in the range of the working temperature. As a result, there is no strain hardening, and therefore there is no need of expensive and time-cons

15、uming intermediate annealing. Of course, strain hardening is advisable during some opera-tions (viz., drawing) to achieve an improved strength; in such cases, hot working is less advantageous. Apart from this, strain hardening may be essential for a successful completion of some processes 4 (e. g.,

16、in deep drawing, strain hardening prevents the rupture of the material around the bottom circumference where the stress is maxi-mum). Large products and high strength materials can be worked upon under hot con-ditions since the elevated temperature lowers down the strength and, consequently, the wor

17、k load. Moreover, for most materials, the ductility increases with temperature and, as a result, brittle materials can also be worked upon by the hot working operation. It should, however, be remembered that there are certain materials (viz., steels contain-ing sulphur) which become more brittle at

18、elevated temperatures. When a very accu-rate dimensional control is required, hot working is not advised because of shrinkage and loss of surface metal due to scaling. Moreover, surface finish is poor due to oxide formation and scaling. The major advantages of cold working are that it is economical,

19、 quicker, and easier to handle because here no extra arrangements for heating and handling are necessary. Further, the mechanical properties normally get improved during the process due to strain hardening. What is more, the control of grain flow directions adds to the strength characteristics of th

20、e product. However, apart from other limitations of cold working (viz., I difficulty with high strength and brittle materials and large product sizes), the inability of the process to prevent the significant reduction brought about in corrosion resistance is an undesirable feature. 1.9 Roll Forming

21、Roll forming is a continuous process for forming metal from sheet, strip, or coiled stock into shapes of essentially uniform cross section. The material is fed be-tween successive pairs of rolls, which progressively shape it until the desired cross section is produced. During the process, only bendi

22、ng takes place; the material thick-ness is not changed except for a slight thinning at bend radii. 2.0 Roll Forming Methods The two methods used when shaped parts are roll formed are the precut or cut-to- length method and the post-cut method. Method selection is based on the complexity of the cross

23、 section and the production length specification. Precut Method In precut operations, the material is cut to length prior to en-tering the roll forming machine. This process usually incorporates a stacking and feeding system to move the blanks into the roll forming machine, a roll forming ma-chine r

24、unning at a fixed speed of about 15-76m /min, and an exit conveyor and stack-ing system. The cut-to-length process is used primarily for lower volume parts and whenever notching cannot be easily accomplished in a post-cut line; for example, mi-ter cuts in vertical legs. Many times, the material is r

25、un from coil into a shear or blanking press and then mechanically fed into the roll former. Tooling cost is inexpensive with this method because cutting requires only a flat shear die or end notch die. However, end flare is more pronounced and side roll tool-ing is required to obtain a good finished

26、 shape. Post-Cut Method Even though some configurations require the cut-to-length method, the most efficient, most productive, most consistent, and least troublesome is the post-cut method. This method requires an uncoiler, a roll forming machine, a cut-off machine, and runout table. In most segment

27、s of the industry, this is the most widely used method. It can be augmented by various auxiliary operations, including prenotching, punching, embossing, marking, trimming, welding, curving, coiling, and die forming. Any or all of these procedures can be combined to eliminate the need for secondary o

28、perations, resulting in a complete or net shape product. However, the cost of tooling and the tooling changeower time for this method are greater than the tool-ing cost and changeover time for the precut method. 2.1 Advantages and Limitations Roll forming is a high-volume process of producing unifor

29、m, accurately dimen-sioned parts. Production speeds of approximately 15-185m/min are obtained, with 30-55m/min an average. Parts are produced with a minimum of handling, requiring only the loading of coils at the starting end of the machine and removal of finished parts at the exit end, generally ac

30、complished by a minimum of operators. Roll forming can also be used for low-volume production because setup or changeover time from one cross section to another rarely takes more than a few hours, and length changes; gen-erally take only a few minutes on simple shapes. However, considerable time is

31、re-quired for more complex shapes. The process is readily adaptable for combination with other operations and processes to form automatically a broad variety of metal parts. The initial cost of a roll forming line can be compared quite favorably with the cost of a standard stamp-ing line or progress

32、ive die operation. Maintenance costs are generally low. With proper roll design, the right tooling materials, good forming material, and proper lubricant, the form rolls can produce 900, 1000 m of product before shape and tolerance problems develop. If through-hardened steel rolls are used, they can

33、 be eruct or retrofitted, at a fraction of replacement cost, to produce for many more years. The designing of rolls for complicated shapes must be done by experienced roll engineers. Complicated tubular or closed shapes sometimes require mandrels to form the shape properly, and delicate breakable pa

34、rts require frequent replacement when high-production runs are made. 2.2 Materials Roll Formed Any material known today that can withstand bending to a desired radius can be roll formed. The material can be ferrous or nonferrous, cold rolled, hot rolled, polished, prepainted, or plated. Thicknesses

35、of 0.13 to 19mm and material widths of 3 to 1, 830 mm or more can be used in roll forming. Length of the finished part is limited only by the length that can be conveniently handled after it leaves the roll forming machine.In some instances, multiple sections can be formed from a single strip or sev

36、eral strips can be fed simultaneously and combined to produce one composite section. The only absolute requirement for a material, whatever the type, coating, thickness, or width, is that it be capable of being formed at room temperature to the specified radii. Some materials, such as certain titani

37、um alloys, have poor forming characteris-tics at room temperature. Therefore, the material must be heated and then formed on specially designed roll forming machines. 2.3 Roll Forming Machines The roll forming machine (roll former) most commonly used has a number of in-dividual units, each of which

38、is actually a dual-spindle roll forming machine, mounted on a suitable base plate to make a multiple-unit machine. The flexibility of this con-struction permits the user to purchase enough units for immediate needs only. By purchasing additional length of baseplate on the machine, units can be added

39、 at any time for future needs. Some of these machines are provided with machined ends on the baseplates, making it possible to couple several machines together, in tandem, to provide additional units as required. Adjusting screws, for making vertical adjustment of the top rolls, are designed with di

40、als and scales to provide micrometer adjustment and a means of recording the position of the top shaft for each roll pass and each shape being formed. The shaft diameter of most machines is from 25-90mm. Several different types of roll forming machines or roll formers are used. They can be classifie

41、d according to spindle support, station configuration, and drive system. 2.4 Forging Forging is one of the oldest metalworking processes known to man. As early as 2000 B. C., forging was used to produce weapons, implements, and jewelry. The process was performed by hand using simple hammers. Hot for

42、ging is defined as the controlled, plastic deformation or working of metals into predetermined shapes by means of pressure or impact blows, or a combination of both. In hot forging, this plastic deformation is performed above the recrystallization temperature to prevent strain hardening of the metal

43、. During the deformation process, the crystalline structure of the base metal is re-fined and any nonmetallic or alloy segregation is properly oriented. In bar stock, the grain flow is only in one direction. When the contour of the part is changed, the grain flow lines are cut, rendering the metal m

44、ore susceptible to fatigue and stress corrosion. Hot forging develops the grain flow so that it follows the outline of the part being formed. The directional alignment of the grains or fibers helps increase strength, ductility, and resistance to impact and fatigue in the metal.Deformation is affecte

45、d by the stress inherent in the metal, the microstructural characteristics of the starting material, the temperature at which the deformation oc-curs, the rate at which the deformation occurs, and the frictional restraint between the material being forged and the die surface. 2.5 Forging Processes M

46、etal flow during the forging process normally falls into two categories: upset-ting and extrusion. Upsetting occurs when the metal is compressed parallel to the lon-gitudinal axis of the workpiece. This action enables the metal to flow freely in one di-rection as in open-die forging, or it can be re

47、strained as in impression-die forging. Ex-trusion occurs when the metal is compressed parallel to the longitudinal axis of the workpiece and allowed to flow through an orifice in the die cavity. 2.6 Open-Die Forging Open-die forging, also referred to as smith forging, blacksmith forges, hand forging

48、 and flat-die forging, is generally performed without special tooling. The forms obtained and the dimensions maintained are usually dependent upon the skill of the operator and the type of equipment used. However, with the addition of computer control to the equipment, more complex forgings can be p

49、roduced and better dimen-sional control is maintained. This equipment may range from the simple anvil and hammer of the blacksmith to giant, computer-controlled, hydraulic presses capable of delivering up to 75.000 tons of force and producing single forgings weighing several thousand pounds. Most open-die forgings are simple geometric shapes such as