外文翻譯--高速磨削與cbn砂輪-應(yīng)用程序和未來的技術(shù)

1、<p>　　外文翻譯：英文+中文 16頁 5909字?jǐn)?shù)</p><p>　　High-speed grinding with CBN grinding wheels ---applications and future technology</p><p>　　M.J. Jackson,*, C.J. Davis, M.P. Hitchhiker, B. Mills<

2、/p><p><b>　　Abstract</b></p><p>　　The basic mechanisms and the applications for the technology of high-speed grinding with CBN grinding wheels are presented. In addition to developments

3、 in process technology associated with high-speed machining, the grinding machine, coolant system, and the grinding tool also need to adapt to high-speed machining. Work piece-related factors inurning the results of mach

4、ining are also discussed. The paper concludes with a presentation of current research and future developments in the area of high</p><p>　　1. Introduction</p><p>　　More than 25 years of high-spe

5、ed grinding have expanded the field of application for grinding from classical finish machining to high-performance machining. High-speed grinding offers excellent potential for good component quality combined with high

6、productivity. One factor behind the innovative process has been the need to increase productivity for conventional finishing processes. In the course of process development it has become evident that high-speed grinding

7、in combination with preliminar</p><p>　　2. Theoretical basis of high-speed grinding</p><p>　　In view of the random distribution of cutting edges and cutting-edge shapes, statistical methods are

8、applied to analyses the cutting mechanism in grinding. The mean unreformed chip thickness, hcu, and the mean chip length, lcu, are employed as variables to describe the shape of the chip. The unreformed chip thickness is

9、 dependent on the static density of cutting edges, Cstat, and on the geometric and kinematics variables [1,2]:</p><p><b>　　(1)</b></p><p>　　where Vw is the work piece speed, VS the g

10、rinding wheel speed, ae the depth of cut, deq the equivalent grinding wheel diameter, and α，β，γ are greater than zero. On the basis of this relationship, it can be established that an increase in the cutting speed, assum

11、ing all other conditions are constant, will result in a reduction in the unreformed chip thickness. The work piece material is machined with a larger number of abrasive grain contacts. At the same time, the number of cut

12、ting edges involve</p><p>　　of the work piece to be machined.</p><p>　　As the cutting speed increases, the quantity of thermal energy that is introduced into the work piece also increases. An in

13、crease in cutting speed is not normally accompanied by a proportional reduction in the tangential grinding force, and thus results in an increase in process power. Reducing the length of time the abrasive grain is in con

14、tact with the work piece can reduce the quantity of heat into the work piece. An increase in the machining rate of the process is necessary for this to happe</p><p>　　Experimental results [3] illustrate that

15、 increasing the cutting speed by a factor of two while maintaining the same metal removal rate leads to a reduction in the tangential force but, unfortunately, leads to an increase in the amount of work done. Owing to co

16、nstant grinding time, there is an increase in the process energy per work piece and, subsequently, in the total thermal energy generated. When the material removal rate is also increased the rising tangential force resul

17、ts in a further incr</p><p>　　There are three fields of technology that have become established for high-speed grinding. These are</p><p>　　1. High-speed grinding with CBN grinding wheels.</p

18、><p>　　2. High-speed grinding with aluminum oxide grinding wheels.</p><p>　　3. Grinding with aluminum oxide grinding wheels in conjunction with continuous dressing techniques (CD grinding).</p&g

19、t;<p>　　Material removal rates resulting in a super proportional increase in productivity for component machining have been achieved for each of these fields of technology in industrial applications [4,5] (Fig. 1)

20、. High equivalent chip thickness of between 0.5 and 10 mm are a characteristic feature of high-speed grinding. CBN high-speed grinding is employed for a large proportion of these applications. An essential characteristic

21、 of this technology is that the performance of CBN is utilized when high cu</p><p>　　3. Grinding tools for high-speed grinding</p><p>　　CBN grinding tools for high-speed machining are subject to

22、 special requirements regarding resistance to fracture and wear. Good damping characteristics, high rigidity, and good thermal conductivity are also desirable. Such tools normally consist of a body of high mechanical str

23、ength and a comparably thin coating of abrasive attached to the body using a high-strength adhesive. The suitability of cubic boron nitride as an abrasive material for high-speed machining of ferrous materials is attribu

24、te</p><p>　　High cutting speeds are attainable above all with metal bonding systems (Fig. 2). One method that uses such bonding systems is electroplating, where grinding wheels are produced with a single-lay

25、er coating of abrasive CBN grain material. The electro-deposited nickel bond displays outstanding grain retention properties. This provides a high-level grain projection and large chip spaces. Cutting speeds of 280 m s-1

26、 are possible [6]. The service life ends when the abrasive layer wears out.</p><p>　　The high roughness of the cutting surfaces of electroplated CBN grinding wheels has disadvantageous effects. The high roug

27、hness is accountable to exposed grain tips that result from different grain shapes and grain diameters. Although electroplated CBN grinding wheels are not considered to be dressable in the conventional sense, the resulta

28、nt workpiece surface roughness can nevertheless be influenced within narrow limits by means of a so-called touch-dressing process. This involves removing the </p><p>　　Multi-layer bonding systems for CBN gri

29、nding wheels include sintered metal bonds, resin bonds, and vitrified bonds. Multi-layer metal bonds possess high bond hardness and wear resistance. Profiling and sharpening these tools is a complex process, however, on

30、account of their high mechanical strength. Synthetic resin bonds permit a broad scope of adaptation for bonding characteristics. However, these tools also require a sharpening process after dressing. The potential for pr

31、actical application o</p><p>　　The selection of the appropriate grade of vitrified CBN grinding wheel for high-speed grinding is more complicated than for aluminium oxide grinding wheels. Here, the CBN abras

32、ive grain size is dependent on specific metal removal rate, surface roughness requirement, and the equivalent grinding wheel diameter. As a starting point when specifying vitrified CBN wheels, Fig. 4 shows the relationsh

33、ip between CBN abrasive grain size, equivalent diameter, and specific metal removal rate for outside dia</p><p>　　vitrified bond used in the grinding wheel. Table 2 shows the wheel grade required for a varie

34、ty of workpiece materials that are based on crankshaft and camshaft grinding operations. </p><p>　　The stiffness of the component being ground has a significant effect on the workpiece/wheel speed ratio. Fig

35、. 5 demonstrates the relationship between this ratio and the stiffness of the component. Steels such as AISI 1050 can be ground in the hardened and the soft state. Hardened 1050 steels are in the range 62±68 HRc. Th

36、ey are burn sensitive and as such wheels speeds are limited to 60 m sÿ1. The standard structure contains the standard bonding system up to 23 vol.%. Whereas the abrasive grain v</p><p>　　In addition to

37、the need to select the appropriate bonding system for grinding wheels in accordance with the requirements of the application concerned, the strength of the body of the grinding wheel requires optimization with high cutti

38、ng speeds. In the case of very high cutting speeds, conventional grinding wheel designs involving a rectangular body and a bore often leads to excessive and irregular extensions of the body and cracking of the abrasive c

39、oating. In order to eliminate the possibility</p><p>　　CBN [8,9] and electroplated CBN grinding wheels [10].</p><p>　　4. High-speed machine tool development</p><p>　　The advantages

40、of high-speed CBN grinding can only be realised in an effective manner if the machine tool is adapted to operate at high cutting speeds. In order to attain very high cutting speeds, grinding wheel spindles and bearings a

41、re required to operate at speeds in the order of 20 000 rpm. The grinding wheel/spindle/motor system must run with extreme accuracy and minimum vibration in order to minimise the level of dynamic process forces. Therefor

42、e, a high level of rigidity is required for t</p><p>　　Another important consideration is the level of drive power required when increases in rotational speed become considerable. The required total output i

43、s composed of the cutting power, Pc, and the power loss, Pl:</p><p>　　The cutting power is the product of the tangential grinding force and the cutting speed:</p><p>　　The power loss of the driv

44、e is comprised of the idle power of the spindle, PL, and power losses caused by the coolant, PKSS, and by spray cleaning of the grinding wheel, PSSP, thus</p><p>　　The power measurements shown in Fig. 6 conf

45、irm the influence of the effect of cutting speed on the reduction of cutting power. However, idling power has increased quite significantly. The grinding power, Pc, increases by a relatively small amount when the cutting

46、 speed increases and all other grinding parameters remain constant. However, this means that the substantial power requirement that applies at maximum cutting speeds results from a strong increase in power is due to rota

47、tion of the grind</p><p>　　The quantities and pressures of coolant supplied to the grinding wheel and the wheel cleaning process are the focus of attention by machine tool designers. This is shown in Fig. 7

48、[11]. The power losses associated with the rotation of the grinding wheel are supplemented by losses associated with coolant supply and wheel cleaning. The losses are dependent on machining parameters implying that machi

49、ne settings and coolant supply need to be optimised for high-speed grinding.</p><p>　　In addition to the advantage of effectively reducing the power required for grinding, optimisation of the coolant supply

50、also offers ecological benefits as a result of reducing the quantities of coolant required. Various methods of coolant supply are available such as the free-flow nozzle that is conventionally used, the shoe nozzle that e

51、nsures `reduced quantity lubrication', and the mixture nozzle that ensures `minimum quantity lubrication'. The common task is to ensure that an adequate supply</p><p>　　A shoe nozzle, or supply throu

52、gh the grinding wheel, enables coolant to be directed into the workpiece±wheel contact zone. A substantial reduction in volumetric flow can be achieved in this way. In comparison to the shoe nozzle, supply through t

53、he grinding wheel requires more complex design and production processes for the grinding wheel and fixtures. An advantage of this supply system is that it is independent of a particular grinding process [13]. Both system

54、s involve a drastic reduction in su</p><p>　　5. Factors affecting quality</p><p>　　The aim of high-speed CBN grinding is to substitute conventional machining operations such as milling, turning,

55、 and surface broaching. The high-speed grinding process focuses on machining large volumes of material in the shortest possible time. This may lead to workpiece quality becoming impaired as the equivalent chip thickness

56、increases in proportion to grinding forces [10,15] The machine tool must be able to absorb such large forces. It is possible to reduce the amount of heat in the grinding</p><p>　　高速磨削與CBN砂輪-應(yīng)用程序和未來的技術(shù)</p&

57、gt;<p><b>　　摘要</b></p><p>　　基本原理和應(yīng)用技術(shù)在高速研磨上占有相當(dāng)?shù)牡匚?。除了發(fā)展高速切削的相關(guān)技術(shù)外，磨床、冷卻系統(tǒng)、磨輪也需要適應(yīng)高速加工。文章最后提出了當(dāng)前和今后的研究發(fā)展方面的高速磨削，發(fā)展用立方淡化硼砂輪的高速磨床。</p><p><b>　　引言</b></p><p

58、>　　經(jīng)過多年的高速機(jī)的發(fā)展，其應(yīng)用領(lǐng)域不斷擴(kuò)大，從古典磨削加工工藝，一直到完成高性能機(jī)械加工過程中。高速磨削發(fā)揮了重要作用，而且生產(chǎn)出的產(chǎn)品優(yōu)質(zhì)高產(chǎn)。其中一個(gè)因素是在創(chuàng)新的過程中，必須提高生產(chǎn)力的常規(guī)整理過程。在發(fā)展過程中顯然高速磨削加工工藝過程與初步理論基礎(chǔ)的配合接近完成，使配置過程順序與新的高性能能力提高。同時(shí)采用適當(dāng)?shù)墓ぞ邫C(jī)、研磨機(jī)，還有可能擴(kuò)大到高性能軟材料切削機(jī)的發(fā)展。</p><p>　　首先

59、，在討論原理的基礎(chǔ)研究過程中，磨軟材料的有關(guān)配置工具也需要達(dá)到要求，實(shí)施有效的環(huán)保制冷系統(tǒng)，調(diào)查工作也同時(shí)是適合高速磨削技術(shù)影響的一個(gè)變數(shù)。</p><p><b>　　高速磨削的理論基礎(chǔ)</b></p><p>　　由于隨機(jī)分配切割邊緣和尖銳部分的隨機(jī)分布，得以使統(tǒng)計(jì)分析的方法應(yīng)用于切割機(jī)原理。使得平均厚度改變，hcu ，平均長度的控制，都被認(rèn)為是造型的變數(shù)用來描述

60、的。改變的厚度取決于Cstat和幾何構(gòu)造等變量，相當(dāng)于：</p><p>　　這些的工作是速度vs即砂輪的速度、砂輪直徑deq、另還有大于零α、β、γ等參數(shù)，根據(jù)這種關(guān)系，假設(shè)所有其他條件不變，可以認(rèn)為增加切割速度，將減少切削厚度的轉(zhuǎn)變。機(jī)械與材料片工作大量磨料接觸。同時(shí)，由于一些材料磨損老化，這導(dǎo)致了高速磨削的優(yōu)勢特點(diǎn)得以減少，工作表面粗糙度增加。因此，提高的砂輪速度可以增加工作的質(zhì)量，或者增加了生產(chǎn)力，這一進(jìn)

61、程取決于技術(shù)特點(diǎn)和對質(zhì)量要求。</p><p>　　隨著切削速度的增加，產(chǎn)生的熱量隨之增加。增加切削速度一般不是伴隨著磨削力的比例減少而減少，從而導(dǎo)致磨削過程增加力的大小。 縮短研磨時(shí)間同樣是可以減少熱量的一種方式。切削率增加了必要的過程，同樣是為了實(shí)現(xiàn)這一目標(biāo)，在磨料厚度增加的過程中用降低切割速度來降低超載的砂輪負(fù)荷。</p><p>　　試驗(yàn)結(jié)果表明，切割速度提高了兩個(gè)要素，同時(shí)保持相

62、同的金屬清除率可降低導(dǎo)致的壽命短縮，但同時(shí)會(huì)導(dǎo)致增加過多的工作量。由于經(jīng)常在速度提高時(shí)都伴隨有增加能量的過程，每一次工作完成，隨后產(chǎn)生的總熱能都增加。當(dāng)然金屬清除率提高了，同時(shí)也提高了磨削機(jī)的工作能力。熱能數(shù)量引進(jìn)工作是一件和最初時(shí)的情況一樣的工作，適用于機(jī)器工作片數(shù)量雖然減少速度但保持較高的速度增長的金屬。這些因素表明，產(chǎn)量可提高機(jī)械使用高速磨削熱而不接受不良影響部分。</p><p>　　目前的高速機(jī)有如下三

63、個(gè)方面的技術(shù)，這些是：</p><p>　　1. 高速研磨機(jī)，CBN砂輪機(jī)。</p><p>　　2. 高速磨削與研磨氧化鋁車輪。</p><p>　　3. 氧化鋁研磨機(jī)床。</p><p>　　金屬清除率超比例增加，導(dǎo)致的生產(chǎn)力部分機(jī)械已達(dá)到每個(gè)領(lǐng)域的工業(yè)應(yīng)用技術(shù)標(biāo)準(zhǔn)，如第五章附圖1。芯片相當(dāng)于0.5至10微米厚度的一個(gè)特點(diǎn)是高速磨床。高速

64、磨削大部份就是采用了這樣的應(yīng)用。這項(xiàng)技術(shù)的一個(gè)重要特點(diǎn)是使用時(shí)的表現(xiàn)是切削速度高。</p><p><b>　　3.高速磨削工具機(jī)</b></p><p>　　CBN高速切削阻力方面有特別的要求，需要良好阻尼特性、高硬度，也需要良好的導(dǎo)熱性能。這些工具通常包括一組具有很高的結(jié)構(gòu)強(qiáng)度和薄涂層的磨料用高粘合劑附在剛體上。含有立方氮化硼作為磨料適合于高速磨削黑色物質(zhì)，因?yàn)樗?/p>

65、的硬度高和具有極好的耐磨性和耐熱性。</p><p>　　首先實(shí)現(xiàn)高切割速度與金屬焊接系統(tǒng)(附圖2)。方法之一，這種系統(tǒng)是利用電鍍的方式，使機(jī)輪與生產(chǎn)單一層次的涂層材料鍍在其表面。這就構(gòu)成了一個(gè)高性能的、切削速度可達(dá)到280米每秒的磨具，一直到磨料層磨損完后使用壽命才結(jié)束。</p><p>　　零件表面粗糙度對切割機(jī)磨輪也有不利的影響。粗糙度對產(chǎn)品的質(zhì)量有非常重要的影響，造成產(chǎn)品不同的形位

66、誤差。雖然CBN切削磨輪并不算是傳統(tǒng)意義上的磨削工具，結(jié)果工件表面粗糙度也可能在影響范圍內(nèi)，通過微小切削過程，這將使周邊材料的磨料涂層通過很小的步驟使切削深度減2至4微米，從而有效地減少粗糙度。</p><p>　　CBN切削磨輪包括砂輪金屬連接、樹脂連接、玻璃化連接，多層金屬高硬度連接等特有連接和穿阻力。分析這些工具，更是一個(gè)復(fù)雜的過程，但由于其結(jié)構(gòu)強(qiáng)度高，合成樹脂的連接在允許的范圍內(nèi)具有廣泛的適應(yīng)性相結(jié)合的特

67、點(diǎn)，使得這些工具也需要一個(gè)標(biāo)準(zhǔn)的處理化過程，玻璃化實(shí)際應(yīng)用的潛力還沒有得到充分的發(fā)揮。配合適當(dāng)?shù)脑O(shè)計(jì)機(jī)構(gòu)，保證新發(fā)展的速度超過砂輪允許的速度范圍。相對于其他類型的連接方式，連接的玻璃化形式很容易在同時(shí)擁有高抗性。與樹脂和金屬不透水連接相比，砂輪玻璃化連接的調(diào)整可以應(yīng)用于廣泛的不同制造過程和設(shè)計(jì)過程中。圖 3顯示了一個(gè)典型的微觀結(jié)構(gòu)砂輪白玻璃化。</p><p>　　選擇適當(dāng)?shù)牟ＡЩ拜喐咚倌ハ鞅蠕X基輪氧化更為嚴(yán)重

68、，在這里，所含磨料金屬顆粒大小取決于其具體的金屬清除率、表面粗糙度要求。直徑相當(dāng)?shù)纳拜啠ＡЩ瘜?shí)際上為突破口，圖4顯示的是含有磨料顆粒大小不同的關(guān)系，相當(dāng)于直徑和金屬遷移率的具體行動(dòng)外直徑的關(guān)系。然而，選擇也取決于磨料表面粗糙度的要求，粗糙度是有限制的，具體的金屬清除率也有要求。</p><p>　　除了要按照適當(dāng)?shù)脑磉x擇適當(dāng)機(jī)制的規(guī)定的適用問題外，組織結(jié)構(gòu)，以及優(yōu)化的要求需要滿足砂輪切割速度高的特點(diǎn)。對于高切

69、割速度、砂輪的常規(guī)設(shè)計(jì)和組織結(jié)構(gòu)，都往往導(dǎo)致過度延伸的引起不規(guī)則裂痕及的磨料涂層的磨損。為了消除高速機(jī)床可能出現(xiàn)的問題，工件與磨具形狀必須能配合以達(dá)到高切割速度。</p><p>　　4、高速機(jī)床的發(fā)展前景</p><p>　　高速機(jī)床所具有的優(yōu)勢是能實(shí)現(xiàn)有效調(diào)整，機(jī)床切割速度高。為了達(dá)到很高的速度，砂輪軸承運(yùn)轉(zhuǎn)速度必須達(dá)到一定大小，砂輪轉(zhuǎn)動(dòng)系統(tǒng)必須極度平穩(wěn)，以減少震動(dòng)。因此，必須很好的固

70、定整個(gè)機(jī)床。保持高速磨輪高的切削速度，必須對磨輪進(jìn)行平衡，這些技術(shù)措施，使更多的工件和產(chǎn)品質(zhì)量得到保持。</p><p>　　另一項(xiàng)重要因素是驅(qū)動(dòng)力需要增加時(shí)，轉(zhuǎn)速變得很大。因而必須削減由PC、功耗等引起的功率：</p><p>　　其中由切割速度引起的功率：</p><p><b>　　功率損失，由：</b></p><p

71、>　　測量圖6所示，確認(rèn)了影響切割速度的因素。然而，更多的能耗在空轉(zhuǎn)中。切割機(jī)的電力、PC、相對少量的速度增長，并使所有其他切割機(jī)參數(shù)保持不變。不過，這意味著需要大量的電力，適用于最大切割速度產(chǎn)生強(qiáng)勁的電力需要是因?yàn)檗D(zhuǎn)動(dòng)的砂輪、冷卻液供應(yīng)以及修整砂輪。</p><p>　　制冷量和供應(yīng)的壓力，砂輪清洗過程中，重點(diǎn)是機(jī)床設(shè)計(jì)。如圖所示，除了電力損失與換輪的損失外，砂輪與冷卻劑清洗磨輪以及供應(yīng)都引起能量損失。加

72、工的工藝參數(shù)取決于能耗，需最佳發(fā)揮高速磨床的切削能力。</p><p>　　除了有效降低機(jī)器所需的電力，更要符合理想的生態(tài)環(huán)保效益，同時(shí)供應(yīng)冷卻的系統(tǒng)也需要使其數(shù)量減少。各種方法都可使供應(yīng)的冷卻液等自由從噴頭流通，用傳統(tǒng)的噴頭確保潤滑液流量減少，混合噴頭可保證最低數(shù)量潤滑油浪費(fèi)，共同的任務(wù)是確保有足夠制冷系統(tǒng)的砂輪裝置，如圖7所示。</p><p>　　噴頭或通過砂輪供應(yīng)，也可將其用于與制

73、冷裝置接觸不多的磨輪區(qū)。體積流量大幅減少，這樣才能實(shí)現(xiàn)。通過供應(yīng)砂輪的噴頭設(shè)計(jì)和生產(chǎn)工藝比較復(fù)雜，需要的設(shè)備及砂輪。利用這一系統(tǒng)提供的，是一個(gè)獨(dú)立的過程中特別嚴(yán)重的現(xiàn)象。 兩種方式有減少供應(yīng)砂輪的加速冷卻效果，更要有效地減少制冷劑的數(shù)量及制冷效果。供應(yīng)量很小，達(dá)每小時(shí)幾毫升冷卻液。由于冷卻效果降低，以現(xiàn)代化的接觸方式將噴嘴配到專用區(qū)，。目前國內(nèi)冷卻系統(tǒng)用于高速磨輪的方式已經(jīng)審查通過。</p><p><b&

74、gt;　　5. 品質(zhì)因素</b></p><p>　　以高速切削取代傳統(tǒng)加工等業(yè)務(wù)，高速磨削加工過程可在最短的時(shí)間完成大量零件的加工。這樣可能導(dǎo)致質(zhì)量受損，成為等同機(jī)晶片厚度增加。機(jī)床必須要能夠承受這樣大的力量，可以減小砂輪的工作速度。但是，迄今為止的實(shí)踐經(jīng)驗(yàn)表明，并不是所有的材料在高速磨削時(shí)其機(jī)械特性都好。在極其惡劣的條件下，用耐高溫的材料，如鎳合金的基礎(chǔ)上，增加了工作的進(jìn)程，以至于因此無法避免微觀

75、結(jié)構(gòu)破壞，但是這些材料可以更有效地利用在表面的加工進(jìn)程。</p><p>　　材料加工碩士論文，主要介紹液態(tài)金屬的成型工藝、金屬焊接成型和材料表面處理工程。具體包括：液態(tài)金屬的工藝性能、砂型鑄造、鑄造工藝設(shè)計(jì)、鑄造工藝CAD/CAE、電弧的特性、焊絲的熔滴過渡、焊縫成形、埋弧焊、氬弧焊、二氧化碳?xì)怏w保護(hù)焊、等離子弧焊、熱噴涂、氣相沉積、激光表面工程技術(shù)等。是碩士研究生所撰寫的學(xué)術(shù)論文，具有一定的理論深度和更高的學(xué)

76、術(shù)水平，更加強(qiáng)調(diào)作者思想觀點(diǎn)的獨(dú)創(chuàng)性，以及研究成果應(yīng)具備更強(qiáng)的實(shí)用價(jià)值和更高的科學(xué)價(jià)值。</p><p>　　根據(jù)礦石回采過程中采場管理方法不同，非煤井工礦山采礦方法可分為四大類： 　?。?）空場采礦法，特點(diǎn)是在回采過程中，采空區(qū)主要依靠暫留或永久殘留的礦柱來支撐，采空區(qū)始終是空著的，一般在礦石與圍巖很穩(wěn)固時(shí)采用。 　?。?）崩落采礦法，特點(diǎn)是隨著礦石采出，有計(jì)劃地用崩落礦體的覆蓋巖層和上下盤巖石來充填采空區(qū)，以

77、控制采區(qū)地壓。一般在礦體圍巖不穩(wěn)固，地表允許陷落的條件下采用。 　?。?）充填采礦法，特點(diǎn)是在回采時(shí)，采空區(qū)依靠充填其內(nèi)的充填物來支撐。這種方法能有效地維護(hù)采空區(qū)，對圍巖的穩(wěn)固性要求不高，但生產(chǎn)成本較高。主要用于開采礦石價(jià)值高，充填材料充足，地表不允許陷落和地質(zhì)條件特別復(fù)雜的條件。</p><p>　　原油的成分主要有：油質(zhì)（這是其主要成分）、膠質(zhì)（一種粘性的半固體物質(zhì)）、瀝青質(zhì)（暗褐色或黑色脆性固體物質(zhì)）、碳質(zhì)

78、（一種非碳?xì)浠衔铮Ｊ陀商細(xì)浠衔餅橹骰旌隙傻?，具有特殊氣味的、有色的可燃性油質(zhì)液體！天然氣是以氣態(tài)的碳?xì)浠衔餅橹鞯母鞣N氣體組成的，具有特殊氣味的、無色的易燃性混合氣體。在整個(gè)的石油系統(tǒng)中分工也是比較細(xì)的：構(gòu)成石油的化學(xué)物質(zhì)，用蒸餾能分解。原油作為加工的產(chǎn)品，有煤油、苯、汽油、石蠟、瀝青等。嚴(yán)格地說，石油以氫與碳構(gòu)成的烴類為主要成分。分子量最小的4種烴，全都是煤氣。 </p><p>　　小抄這是一種在考

79、場上常用的作弊工具，也是一種作弊手段，正規(guī)的學(xué)名為“夾帶”。考生將自己需記憶的內(nèi)容抄寫在一張紙上（最初是很小的紙，現(xiàn)在考上比較猖狂了竟有用大紙的。當(dāng)然也有寫在草稿紙上的。），在考試時(shí)趁監(jiān)考老師不注意，偷偷拿出來狂抄。實(shí)際上，這是一種極不明智的方法，很容易被發(fā)現(xiàn)。現(xiàn)在，有許多學(xué)校為了避免此事就規(guī)定不準(zhǔn)考生用自帶的草稿紙。現(xiàn)在比較流行的一種“小抄”是將電子課件或答案以極小的字體打印在小紙片上。 小抄這種方法通常以小六號(hào)字體分兩欄打印，每個(gè)