【正文】 e, with 湖南文理學(xué)院 畢業(yè)設(shè)計（論文） 32 builtup edge formation and poor surface finish. The second effect is that increased hardness causes the formation of short chips instead of continuous stringy ones, thereby improving machinability. Leaded Steels. A high percentage of lead in steels solidifies at the tip of manganese sulfide inclusions. In nonresulfurized grades of steel, lead takes the form of dispersed fine particles. Lead is insoluble in iron, copper, and aluminum and their alloys. Because of its low shear strength, therefore, lead acts as a solid lubricant (Section ) and is smeared over the toolchip interface during cutting. This behavior has been verified by the presence of high concentrations of lead on the toolside face of chips when machining leaded steels. When the temperature is sufficiently highfor instance, at high cutting speeds and feeds (Section )—the lead melts directly in front of the tool, acting as a liquid lubricant. In addition to this effect, lead lowers the shear stress in the primary shear zone, reducing cutting forces and power consumption. Lead can be used in every grade of steel, such as 10xx, 11xx, 12xx, 41xx, etc. Leaded steels are identified by the letter L between the second and third numerals (for example, 10L45). (Note that in stainless steels, similar use of the letter L means “l(fā)ow carbon,” a condition that improves their corrosion resistance.) However, because lead is a wellknown toxin and a pollutant, there 湖南文理學(xué)院 畢業(yè)設(shè)計（論文） 33 are serious environmental concerns about its use in steels (estimated at 4500 tons of lead consumption every year in the production of steels). Consequently, there is a continuing trend toward eliminating the use of lead in steels (leadfree steels). Bismuth and tin are now being investigated as possible substitutes for lead in steels. CalciumDeoxidized Steels. An important development is calciumdeoxidized steels, in which oxide flakes of calcium silicates (CaSo) are formed. These flakes, in turn, reduce the strength of the secondary shear zone, decreasing toolchip interface and wear. Temperature is correspondingly reduced. Consequently, these steels produce less crater wear, especially at high cutting speeds. Stainless Steels. Austenitic (300 series) steels are generally difficult to machine. Chatter can be s problem, necessitating machine tools with high stiffness. However, ferritic stainless steels (also 300 series) have good machinability. Martensitic (400 series) steels are abrasive, tend to form a builtup edge, and require tool materials with high hot hardness and craterwear resistance. 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（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。 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（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 that o。 Force and power requirements。機械加工余量與公差手冊。 北京：機械工業(yè)出版社， 1991。 [11] 孫麗媛。機械制造 技術(shù) 。 北京：機械工業(yè)出版社， 1986。 [3] 雙元制培訓(xùn)機械專業(yè)理論教材編委會。由于本夾具用于變速箱體端面的粗加工，對其進行精度分析無太大意義。這些主要元件設(shè)計好后即可畫出夾具的設(shè)計裝配草圖。工件在夾具上安裝好后，氣缸活塞帶動壓塊從上往下移動夾緊工件。 ???? 定向鍵與對刀裝置設(shè)計 定向鍵安裝在夾具底面的縱向槽中 ，一般使用兩個。 （ 6）、補償值 ? mmLxLg m i n1 ???????? ??? 定位誤差分析 本夾具選用的定位元件為一面兩銷定位。其中一面為支承板，兩銷為一短圓柱銷和一削邊銷 。因此在本道工序加工時，主要應(yīng)考慮提高勞動生產(chǎn)率，降低勞動強度。心軸裝在滾針軸承及推力軸承上，即使水平方向已夾緊工件，但工件仍可繞心軸的中心轉(zhuǎn)動。 夾具設(shè)計及操作的簡要說明 鉆鉸工藝孔的夾具如夾具裝配圖 2 所示。即要求：（ 1）、各孔的實際輪廓受最大實體實效邊界的控制即受直徑為 ??? ?? 的理想圓柱面的控制。 夾具精度分析 利用夾具在機床上加工時，機床、夾具、工件、刀具等形成一個封閉的加工系統(tǒng)。即先用 mm10? 的麻花鉆鉆孔，根據(jù) GB1141— 84 的規(guī)定鉆頭上偏差為零，故鉆套孔徑為 mmF811? 即 ??? 。采用氣動夾緊，原始夾緊力可以連續(xù)作用，夾緊可靠，機構(gòu)可以不必自鎖。由《切削手冊》得： 鉆削力 HBDfF ? 鉆削力矩 HBfDT ? 式中： mmD 12? ? ? ? ? 2321872553125