【導(dǎo)讀】題目給出的零件是汽車變速箱箱體。變速箱箱體的主要作用是支承各傳動(dòng)。軸，保證各軸之間的中心距及平行度，并保證變速箱部件與發(fā)動(dòng)機(jī)正確安裝。運(yùn)動(dòng)精度，而且還會(huì)影響汽車的工作精度、使用性能和壽命。是實(shí)現(xiàn)汽車的變速，改變汽車的運(yùn)動(dòng)速度。的外表面上有五個(gè)平面需要進(jìn)行加工。支承孔系在前后端面上。還需加工一系列螺紋孔。因此可將其分為三組加工表面。它們相互間有一定的。這一組加工表面包括：頂面的銑。螺孔，以及4個(gè)mm15?的孔；還有另外兩個(gè)在同一中心線上與兩。的兩側(cè)窗口面；與兩側(cè)窗口面相垂直的12個(gè)。，12個(gè)螺孔均有位置度要求為。由以上分析可知。之間的相互關(guān)系。中的主要考慮因素。為提高孔的加工精度創(chuàng)造條件，便于對(duì)刀及調(diào)整，也有利于保護(hù)刀具。在滿足精度要求及生產(chǎn)率的條件下，應(yīng)選擇價(jià)格最底的機(jī)床。當(dāng)前應(yīng)選用在組合機(jī)床上用鏜模法鏜孔較為適宜。鏜模夾具是按照工件孔系的加工要求設(shè)計(jì)制造的。品種以及產(chǎn)品更新?lián)Q代所需要的時(shí)間短等要求。

　　

【正文】 sistance. Precipitationhardening stainless steels are strong and abrasive, requiring hard and abrasionresistant tool materials. The Effects of Other Elements in Steels on Machinability. The presence of aluminum and silicon in steels is always harmful because these elements bine with oxygen to form aluminum oxide and 湖南文理學(xué)院 畢業(yè)設(shè)計(jì)（論文） 34 silicates, which are hard and abrasive. These pounds increase tool wear and reduce machinability. It is essential to produce and use clean steels. Carbon and manganese have various effects on the machinability of steels, depending on their position. Plain lowcarbon steels (less than % C) can produce poor surface finish by forming a builtup edge. Cast steels are more abrasive, although their machinability is similar to that of wrought steels. Tool and die steels are very difficult to machine and usually require annealing prior to machining. Machinability of most steels is improved by cold working, which hardens the material and reduces the tendency for builtup edge formation. Other alloying elements, such as nickel, chromium, molybdenum, and vanadium, which improve the properties of steels, generally reduce machinability. The effect of boron is negligible. Gaseous elements such as hydrogen and nitrogen can have particularly detrimental effects on the properties of steel. Oxygen has been shown to have a strong effect on the aspect ratio of the manganese sulfide inclusions。 the higher the oxygen content, the lower the aspect ratio and the higher the machinability. In selecting various elements to improve machinability, we should consider the possible detrimental effects of these elements on the properties and strength of the machined part in service. At elevated 湖南文理學(xué)院 畢業(yè)設(shè)計(jì)（論文） 35 temperatures, for example, lead causes embrittlement of steels (liquidmetal embrittlement, hot shortness。 see Section ), although at room temperature it has no effect on mechanical properties. Sulfur can severely reduce the hot workability of steels, because of the formation of iron sulfide, unless sufficient manganese is present to prevent such formation. At room temperature, the mechanical properties of resulfurized steels depend on the orientation of the deformed manganese sulfide inclusions (anisotropy). Rephosphorized steels are significantly less ductile, and are produced solely to improve machinability. Machinability of Various Other Metals Aluminum is generally very easy to machine, although the softer grades tend to form a builtup edge, resulting in poor surface finish. High cutting speeds, high rake angles, and high relief angles are remended. Wrought aluminum alloys with high silicon content and cast aluminum alloys may be abrasive。 they require harder tool materials. Dimensional tolerance control may be a problem in machining aluminum, since it has a high thermal coefficient of expansion and a relatively low elastic modulus. Beryllium is similar to cast irons. Because it is more abrasive and toxic, though, it requires machining in a controlled environment. 湖南文理學(xué)院 畢業(yè)設(shè)計(jì)（論文） 36 Cast gray irons are generally machinable but are. Free carbides in castings reduce their machinability and cause tool chipping or fracture, necessitating tools with high toughness. Nodular and malleable irons are machinable with hard tool materials. Cobaltbased alloys are abrasive and highly workhardening. They require sharp, abrasionresistant tool materials and low feeds and speeds. Wrought copper can be difficult to machine because of builtup edge formation, although cast copper alloys are easy to machine. Brasses are easy to machine, especially with the addition pf lead (leaded freemachining brass). Bronzes are more difficult to machine than brass. Magnesium is very easy to machine, with good surface finish and prolonged tool life. However care should be exercised because of its high rate of oxidation and the danger of fire (the element is pyrophoric). Molybdenum is ductile and workhardening, so it can produce poor surface finish. Sharp tools are necessary. Nickelbased alloys are workhardening, abrasive, and strong at high temperatures. Their machinability is similar to that of stainless steels. Tantalum is very workhardening, ductile, and soft. It produces a poor surface finish。 tool wear is high. 湖南文理學(xué)院 畢業(yè)設(shè)計(jì)（論文） 37 Titanium and its alloys have poor thermal conductivity (indeed, the lowest of all metals), causing significant temperature rise and builtup edge。 they can be difficult to machine. Tungsten is brittle, strong, and very abrasive, so its machinability is low, although it greatly improves at elevated temperatures. Zirconium has good machinability. It requires a coolanttype cutting fluid, however, because of the explosion and fire. Machinability of Various Materials Graphite is abrasive。 it requires hard, abrasionresistant, sharp tools. Thermoplastics generally have low thermal conductivity, low elastic modulus, and low softening temperature. Consequently, machining them requires tools with positive rake angles (to reduce cutting forces), large relief angles, small depths of cut and feed, relatively high speeds, and proper support of the workpiece. Tools should be sharp. External cooling of the cutting zone may be necessary to keep the chips from being “gummy” and sticking to the tools. Cooling can usually be achieved with a jet of air, vapor mist, or watersoluble oils. Residual stresses may develop during machining. To relieve these stresses, machined parts can be annealed for a period of time at 湖南文理學(xué)院 畢業(yè)設(shè)計(jì)（論文） 38 temperatures ranging from C?80 to C?160 ( F?175 to F?315 ), and then cooled slowly and uniformly to room temperature. Thermosetting plastics are brittle and sensitive to thermal gradients during cutting. Their machinability is generally similar to tha