tailieunhanh - Manufacturing Processes phần 8

Không đồng nhất (răng cưa) chip bao gồm các khu vực của sự căng thẳng lớn và nhỏ. Chip như vậy là đặc trưng của kim loại với kim loại dẫn nhiệt thấp hoặc có sức mạnh năng suất giảm mạnh với nhiệt độ. Chip từ hợp kim titan thường xuyên của loại n | ADVANCED MACHINING PROCESSES 13-71 Fig. Schematic diagram of the electrochemical machining process. a grinding wheel. Removal rates are up to in3 h 25 cm3 h with practical tolerances on the order of in mm . A graphite or brass electrode wheel is operated around 100 to 600 surface ft min 30 to 180 m min to minimize splashing of the dielectric fluid. Typical applications of this process are in grinding of carbide tools and dies thin slots in hard materials and production grinding of intricate forms. The electrochemical machining ECM process Fig. uses electrolytes which dissolve the reaction products formed on the workpiece by electrochemical action it is similar to a reverse electroplating process. The electrolyte is pumped at high velocities through the tool. A gap of to in to mm is maintained. A dc power supply maintains very high current densities between the tool and the workpiece. In most applications a current density of 1 000 to 5 000 A is required per in2 of active cutting area. The rate of metal removal is proportional to the amount of current passing between the tool and the workpiece. Removal rates up to 1 in3 min 16 cm3 min can be obtained with a 10 000-A power supply. The penetration rate is proportional to the current density for a given workpiece material. The process leaves a burr-free surface. It is also a cold machining process and does no thermal damage to the surface of the workpiece. Electrodes are normally made of brass or copper stainless steel titanium sintered copper-tungsten aluminum and graphite have also been used. The electrolyte is usually a sodium chloride solution up to lb gal 300 g L other solutions and proprietary mixtures are also available. The amount of overcut defined as the difference between hole diameter and tool diameter depends upon cutting conditions. For production applications the average overcut is around in mm . The rate of penetration is up to in