Như có thể mong đợi, không phải tất cả các vật liệu có thể xử lý như một điều trị thô đảo ngược trình bày uốn. Hoạt động này giới thiệu một số lượng lớn của sự căng thẳng vào phần hình thành (tức là, chủ yếu là các góc). Điều này có thể không chỉ làm suy yếu các khu vực này, nó cũng sẽ gây ra một cứng lớn hơn công việc bình thường của cùng một. | BENDING AND FORMING OPERATIONS 359 BENDING AND FORMING OPERATIONS As can be expected not all materials can handle such a rough treatment the reversed bending presents. This operation introduces a massive amount of strain into the formed section . mainly the corners . This may not only weaken these areas it will also cause a greater than ordinary work hardening of the same. Tearing of the metal wrinkling and other defects may result. 8-9 FORMING Metal forming is a process totally dependent on the influence of outside tensile forces against the structure of the material. The resulting permanent deformation is called forming. The force-exerting instrument is the punch which by pulling the sheet-metal material along makes it enter the die where it is compelled to take upon itself the impression of the assembly. The decision if the part is to be formed or drawn is usually based on the evaluation of its shape and dimensional requirements. Drawing is utilized for those parts made of thicker materials or for those with vertical or slightly inclined walls and sharp corners at the bottom. Since a forming die may often be instrumental in the formation of wrinkles or cause development of excessive tensile strains in the material which away tear the part in the process drawing is often resorted to in such cases. 8-9-1 Forming of Singular Recesses Singular recesses in the flat metal sheet are usually formed by stretching or drawing. Stretching is reserved for parts with smooth connection of contours without excessively sharp edges. Several samples of stretched parts are shown in Fig. 8-47. The maximum amount of stretch for a given material depends on its distribution over the area of stretch. Naturally the larger such an area is the greater the maximum amount of stretch that can be obtained. To evaluate the strain with respect to the amount of stretch Eq. 8-15 may be used s Ỹ o 8-15 Lo where s stretch All other values are in Fig. 8-48. FIGURE 8-47 Shapes of stretched parts. .
