机电技术专业英语 教学课件 作者 徐起贺 张庆良pata a lesson four.pptVIP

* 机电技术专业英语 Lesson Four Text In drop forging, a piece of metal, roughly or approximately of the desired shape, is placed between die faces having the exact form of the finished piece, and forced to take this form by drawing the dies together1. This method is widely used for the manufacture of parts both of steel and brass. Large ingots are now almost always forged with hydraulic presses instead of with steam hammers, since the work done by a press goes deeper2. Metal in the form of hotrolled or coldrolled sheets or strips may be formed into many shapes by forcing the sheet into the impressions in metal dies. This process is known as stamping, or pressing of metals, and involves many different operations, such as bending, drawing, punching, etc. These operations are successfully carried out through the proper die design and proper operation of the press in which the dies are placed. Sheets or strips of steel, brass, copper, aluminium, or other metal are placed between dies, and a slow action of the power press brings the dies together, forcing the metal to assume the required shape. Forging, Stamping and Rolling Rolling is the process of shaping metal in a machine called rolling mill. Ingots of metal are rolled by forcing them between two rollers rotating in opposite directions, thus pressing the metal into the required shape3. If the rollers have no grooves on their surfaces, the metal is rolled into the form of a sheet or plate. If the rollers have grooves of a certain shape, the metal will take the form of these grooves, thus it may be shaped into the form of bars or rods. There are two kinds of rolling: hot rolling and cold rolling. Before cold rolling, the scale covering the surface of the hotrolled object should be removed. Cold rolling produces a higher surface finishing sheet and gives it a very exact size. The process has innumerable advantages. Many shapes may be manufactured in quantities at a rapid rate and at a relatively low cost. Le