【正文】 tool wear is high.Titanium and its alloys。 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。 this improves machinability. The size, shape, distribution, and concentration of these inclusions significantly influence machinability. Elements such as tellurium and selenium, which are both chemically similar to sulfur, act as inclusion modifiers in resulfurized steels.Phosphorus in steels has two major effects. It strengthens the ferrite, causing increased hardness. Harder steels result in better chip formation and surface finish. Note that soft steels can be difficult to machine, with 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 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 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。 Force and power requirements。參考文獻[1]濮良貴、紀名剛編.《機械設(shè)計》[M].北京：高等教育出版社，2008[2]機械電子工業(yè)部編.《銑工工藝學(xué)》[M].北京：機械工業(yè)出版社，[3]戴曙編.《金屬切削機床》[M].北京：機械工業(yè)出版社，[4]吳宗澤編.《機械設(shè)計課程設(shè)計手冊（第三版）》[M].北京：高等教育出版社，2007[5]第一機械工業(yè)部編.《金屬切削機床產(chǎn)品樣本 銑床 1977》[M].北京： [6]孫桓、陳作模、葛文杰編.《機械原理》[M].北京：高等教育出版社，[7]趙家奇編.《機械制造工藝學(xué)課程設(shè)計指導(dǎo)書（第二版）》[M].北京：機械工業(yè)出版社,[8]李云主編.《機械制造及設(shè)備指導(dǎo)手冊》[M].北京：機械工業(yè)出版社,原文： MACHINABILITYThe machinability of a material usually defined in terms of four factors: Surface finish and integrity of the machined part。這時，通過扇形齒輪、齒條、撥叉使軸4與孔盤5從右向左移到原位，則孔盤5推動三組齒桿10連同撥叉撥動三個滑移齒輪移動，改變齒輪嚙合對，達到變