【正文】 h, it is normally produced in the form of tips which are either brazed to a steel shank or mechanically clamped in a specially designed holder. Mechanically clamped tool tips are frequently made as throwaway inserts. When all the cutting edges have been used the inserts are discarded, ad regrinding would cost more than a new tip. The high hardness of carbide tools at elevated temperatures enables them to be used at much faster cutting speeds than highspeed steel (of 34m/s（ 600800ft/min） when cutting mild steel). They are manufactured in several grades, enabling them to be used for most machining applications. Their earlier brittleness has been largely overe by the introduction of tougher grades, which are frequently used for interrupted cuts including many arduous facemilling operations. Recently, improvements have been claimed by using tungsten carbide tools coated with titanium carbide or titanium nitride (about coating thickness). These tools are more resistant to wear than conventional tungsten carbide tools, and the reduction in interface friction using titanium nitride results in a reduction in cutting forces and in tool temperatures. Hence, higher metal removal rates are possible without detriment to tool life or alternatively longer tool lives could be achieved at unchanged metal removal rates. The uses of other forms of coating with aluminum oxide and polycrystalline cubic boron nitride are still in an experimental stage, but it is likely that they will have important applications when machining cast iron, hardened steels and high melting point alloys. Ceramics. The socalled ceramic group of cutting tools represents the most recent development in cutting tool materials. They consist mainly of sintered oxides, usually aluminum oxide, and are almost invariably in the form of clamped tips. Because of the parative cheapness of ceramic tips and the difficulty of grinding them without causing thermal cracking, they are made as throwaway inserts. Ceramic tools are a postwar introduction and are mot yet in general factory use. Their most likely application is in cutting metal at very high speeds, beyond the limits possible with carbide tools. Cramics resist the formation of a builtup edge and in consequence produce good surface finishes. Since the present generation of machine tools is designed with only sufficient power to exploit carbide tooling, it is likely that, for the time being, ceramics will be restricted to highspeed finish machining where is sufficient power available for the light cuts taken. The extreme brittleness of ceramic tools has largely limited their use to continuous cuts, although their use in milling is now possible. As they are poorer conductors of heat than carbides, temperatures at the rake face are higher than in carbide tools, although the friction force is usually lower. To strengthen the cutting edge, and consequently improve the life of the ceramic tool, a small chamfer or radius is often stoned on the cutting edge, although this increases the power consumption. Diamonds. For producing very fine finishes of （ 23um) on nonferrous metals such as copper and aluminum, diamond tools are often used. The diamond is brazed to a steel shank. Diamond turning and boring are essentially finishing operations, as the forces imposed by any but the smallest cuts cause the diamond to fracture or be torn from its mounting. Under suitable conditions diamonds have exceptionally long cutting lives. Synthetic polycrystalline diamonds are now available as mechanically clamped cutting tips. Due to their high cost they have very limited applications, but are sometimes used for machining abrasive aluminumsilicon alloys, fused silica and reinforced plastics. The random orientation of their crystals gives them improved impact resistance, making them suitable for interrupted cutting. 車床 用于車外圓、端面和鏜孔等加工的機(jī)床稱作車床。這位聰明的英國人還發(fā)明了一種把主軸和絲杠相連的變速裝置，這樣就可以切削螺紋。導(dǎo)軌上的任何誤差，常常會使整個(gè)機(jī)床的精度遭到破壞。 由于車床的精度在很大程度上取決于主軸，因此主軸的結(jié)構(gòu)尺寸較大，通常安裝在緊密配合的重型圓錐滾子軸承或球軸承中。大多數(shù)現(xiàn)代車床都裝置有 5～ 15 馬 力的電動(dòng)機(jī)，為硬質(zhì)合金和金屬陶瓷合金刀具提供足夠的動(dòng)力，進(jìn)行高速切削。鎖定裝置可以使套筒在所需的位置上夾緊。通過轉(zhuǎn)動(dòng)溜板箱前的手輪，可以手動(dòng)操作拖板沿床身移動(dòng)。而絲杠產(chǎn)生的運(yùn)動(dòng)是通過滑板箱與絲杠之間的直接機(jī)械連接來實(shí)現(xiàn)的，對于螺母可以實(shí)現(xiàn)這種連接。 金屬切削刀具 刀具的形狀（特別是其角度）和材料是刀具的兩個(gè)非常重要的因素。 單尖刀具是指只有一個(gè)前刀面和一條連續(xù)切削刃的刀具。為了節(jié)省磨刀時(shí)間，刀具段后刀面的一部分有時(shí)可以直接鍛造成形，而不需要進(jìn)行磨削。 角度 7 為側(cè)切削刃角，它是測切削刃和刀柄側(cè)面之間的夾角。下面討論最重要的幾種材料和它們對刀具設(shè)計(jì)的影響。實(shí)驗(yàn)表明：高速鋼切削低碳鋼時(shí)，如速度超過(350ft/min),就會很快失效，而且許多高速鋼材料在速度超過 (150ft/min)時(shí)，就不能成功切削低碳鋼。因此，不降低刀具壽命就可得到較高的金屬切削率，或不改變金屬切削率而延長刀具壽命。陶瓷刀具特別脆，盡管目前已能進(jìn)行銑削，陶瓷刀具主要還是限于連續(xù)切削。