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機(jī)床基礎(chǔ)
許多情況下，初步成型加工出來(lái)的工件必須在尺寸和表面光潔度進(jìn)一步精整，以滿(mǎn)足設(shè)計(jì)的技術(shù)要求。為了滿(mǎn)足公差的精度要求，需要從工件上去掉少量材料。通常機(jī)床就是用于這種加工的設(shè)備。
在美國(guó)，材料切削是一個(gè)很大的產(chǎn)業(yè)，其費(fèi)用每年超過(guò)36×109美元，包括材料、勞動(dòng)力、管理費(fèi)、機(jī)床裝運(yùn)費(fèi)等費(fèi)用。通過(guò)銷(xiāo)售、設(shè)計(jì)、在車(chē)間中操作機(jī)器以及在相關(guān)企業(yè)中工作，60%的機(jī)械和工業(yè)工程技術(shù)的大學(xué)畢業(yè)生都跟機(jī)械加工工業(yè)有某些關(guān)系。因此，工科學(xué)學(xué)生在他的學(xué)習(xí)課程中花時(shí)間投身材料切削和機(jī)床的學(xué)習(xí)研究是很明智的。
機(jī)床為切削刀具提供把工件工件加工成所需尺寸的方法，通過(guò)其基本部件的功能，控制刀具和工件的支承關(guān)系。這些基本部件是：
（1） 床身和機(jī)架。這是機(jī)床的主要部件，它支撐主軸和拖板箱并把它們連接在一起。在負(fù)載時(shí)，床身和機(jī)架的變形和振動(dòng)必須保持最小。
（2） 拖板箱和導(dǎo)軌。機(jī)床部件（如拖板箱）的移動(dòng)，通常是在精確的導(dǎo)軌面上的直線(xiàn)運(yùn)動(dòng)。
（3） 主軸和軸承。軸旋轉(zhuǎn)時(shí)產(chǎn)生角位移，軸的位置在機(jī)床中必須極其精確、穩(wěn)定，一般通過(guò)主軸和軸承的精度來(lái)提供保征。
（4） 動(dòng)力裝置。機(jī)床普遍采用的動(dòng)力裝置是電動(dòng)機(jī)。合理布置各個(gè)電機(jī)可以使皮帶和齒輪傳動(dòng)裝置減至最少。
（5） 傳動(dòng)連接機(jī)構(gòu)。連接機(jī)構(gòu)是一般術(shù)語(yǔ)，用來(lái)代表機(jī)械、液壓、氣動(dòng)和電動(dòng)的機(jī)構(gòu)，這些機(jī)構(gòu)和確定的角位移和線(xiàn)性位移相關(guān)聯(lián)。
機(jī)加工可以分成兩大類(lèi)：
（1） 粗加工，其金屬切除率高，因?yàn)榍邢髁Υ?，但尺寸精度較低。
（2） 精加工，其金屬切除率低，因?yàn)榍邢髁π。叽缇群捅砻婀鉂嵍容^高。
因此，與粗加工相比，精加工更應(yīng)該重視靜載荷和動(dòng)載荷（例如由不平衡的砂輪引起的動(dòng)載荷）的影響。此外，加工精度通常不會(huì)受到由于作用力而引起的變形程度的影響。
有是也用鑄鋼或碳鋼制造機(jī)床床身，但一般都用鑄鐵制造機(jī)床床身。因?yàn)殍T鐵便宜，剛性好，耐壓強(qiáng)度高，并且能減弱機(jī)床操作中的振動(dòng)。為了避免床身鑄件截面過(guò)大，可以精心設(shè)計(jì)筋條來(lái)提高床身的抗彎曲應(yīng)力和抗扭轉(zhuǎn)應(yīng)力。筋條的兩種基本類(lèi)型是箱型和片狀斜支承型。箱型結(jié)構(gòu)便于生產(chǎn)，箱壁上的孔口便于型芯的定位和取出。片狀斜支撐筋條有較大的抗扭剛度并能使截面間的碎屑掉落，它常常用于車(chē)床床身。
機(jī)床的拖板箱和導(dǎo)軌可以定位和引導(dǎo)彼此相對(duì)運(yùn)動(dòng)的零部件，通常是改變刀具相對(duì)于工件的位置。運(yùn)動(dòng)一般是直線(xiàn)平移，但也有時(shí)是旋轉(zhuǎn)，例如沿著工件的螺旋角方向把萬(wàn)能螺紋磨床上的砂輪頭轉(zhuǎn)動(dòng)一個(gè)角度。拖板箱的基本幾何結(jié)構(gòu)形狀是平的、V形的、燕尾槽形的和圓柱形的。根據(jù)用途，這些構(gòu)件可以分別使用或以各種方法組合使用。導(dǎo)軌的特性如下：
（1） 運(yùn)動(dòng)精度。當(dāng)拖板沿直線(xiàn)運(yùn)動(dòng)時(shí)，這條直線(xiàn)必須位于兩個(gè)相互垂直的平面內(nèi)，必須沒(méi)有滑動(dòng)旋轉(zhuǎn)。機(jī)床導(dǎo)軌的直線(xiàn)度公差一般是0-0.02毫米每米，在水平面上這個(gè)公差可以使滑軌凸起，以補(bǔ)償滑軌的下陷。
（2） 調(diào)節(jié)方式。為了便于裝配、維持精度和限制由于磨損而引起的滑動(dòng)構(gòu)件之間的“竄動(dòng)”，有時(shí)在拖板內(nèi)裝入扁條，這個(gè)扁條被叫做“鑲條”。通常該鑲條用穿過(guò)長(zhǎng)孔的沉頭螺釘支柱，用平頭螺釘調(diào)整好后再用鎖緊螺母上緊。
（3） 潤(rùn)滑。導(dǎo)軌的潤(rùn)滑方式有以下兩種：（i）間歇潤(rùn)滑，通過(guò)潤(rùn)滑油脂或油壺進(jìn)行，適用于運(yùn)動(dòng)速度低而不頻繁使用的場(chǎng)合。（ii）連續(xù)潤(rùn)滑，例如通過(guò)計(jì)量閥和管道將潤(rùn)滑油送到潤(rùn)滑點(diǎn)，用這種方法形成的油膜應(yīng)該很薄，以避免使拖板“浮起”。如果滑移表面是鏡平面，油就會(huì)被擠出而導(dǎo)致表面粘滯。因而在實(shí)際使用中，拖板滑移表面要經(jīng)過(guò)凹面砂輪的磨削和刮研。這兩種方法產(chǎn)生微小的表面下陷，可以存油，這樣在所有點(diǎn)都不會(huì)發(fā)生零件的完全分離，保持拖板的正確位置。
（4） 防護(hù)。為了維持導(dǎo)軌的良好狀態(tài)，必須滿(mǎn)足以下條件：（i）防止外面的物質(zhì)（如碎屑）的進(jìn)入。如果難以做到，就應(yīng)該選用形狀符合要求的導(dǎo)軌以避免碎屑在導(dǎo)軌上存留，例如倒V形導(dǎo)軌。（ii）潤(rùn)滑油的保持。在垂直或傾斜的導(dǎo)軌面上使用的油應(yīng)該能粘在導(dǎo)軌面上。市場(chǎng)上到處都有這種專(zhuān)用潤(rùn)滑油。油的粘度應(yīng)該足夠高，以避免被切削液沖走。（iii）用防護(hù)罩來(lái)防止意外的損壞。
車(chē)床
一臺(tái)車(chē)床實(shí)現(xiàn)三個(gè)主要功能：（i）牢固地支持工件或刀架和刀具；（ii）提供工件和刀具之間的相對(duì)運(yùn)動(dòng)；（iii）提供一定范圍的走刀和切削速度。
以去除切屑形式來(lái)加工金屬的機(jī)床分類(lèi)如下：
主要使用單點(diǎn)切削刀具的機(jī)床包括：普通車(chē)床，塔式車(chē)床，仿型車(chē)床，單軸自動(dòng)車(chē)床，多軸自動(dòng)車(chē)床，牛頭刨床和龍門(mén)刨床，鏜床。
使用多點(diǎn)切削刀具的機(jī)床包括：鉆床，銑床，拉床，鋸床，齒輪切割機(jī)床。
使用隨機(jī)點(diǎn)切削刀具的機(jī)床包括：化學(xué)蝕刻銑削，電火花加工，超聲波加工。
車(chē)床借助于轉(zhuǎn)動(dòng)的工件對(duì)著刀具來(lái)切去金屬材料，以產(chǎn)生外圓柱面、內(nèi)圓柱面或錐形表面。車(chē)床普遍靠端面切削來(lái)加工工件表面。在端面切削加工中，工件旋轉(zhuǎn)，而刀具作垂直于回轉(zhuǎn)軸線(xiàn)方向的移動(dòng)。
普通車(chē)床（見(jiàn)圖1，略）是最基本的車(chē)床，是研制其他車(chē)床的基礎(chǔ)。驅(qū)動(dòng)電機(jī)裝在床身的基礎(chǔ)上并通過(guò)齒輪、皮帶來(lái)驅(qū)動(dòng)主軸。主軸是一根堅(jiān)固的空心軸，裝在重型軸承之間，其前端用來(lái)安裝驅(qū)動(dòng)盤(pán)（花盤(pán)），以便把確定的運(yùn)動(dòng)傳到工件上。該驅(qū)動(dòng)盤(pán)可以借助螺紋、凸輪鎖緊機(jī)構(gòu)或借助一個(gè)螺紋墊圈和鍵固定在主軸上。
車(chē)床的床身是鑄鐵件，它提供精確磨削過(guò)的滑動(dòng)表面（導(dǎo)軌），上面放有拖板。車(chē)床拖板是H型的鑄件，刀具安裝在拖板的刀架上。溜板箱裝在托板前面，內(nèi)裝有驅(qū)動(dòng)齒輪，可以順著導(dǎo)軌或橫過(guò)導(dǎo)軌移動(dòng)刀具和拖板以提供所希望的刀具運(yùn)動(dòng)。
拖板上面的小刀架能使刀的夾具旋轉(zhuǎn)任意角度。小刀架上的手輪和絲桿可以使刀具做線(xiàn)性運(yùn)動(dòng)，另一個(gè)手輪和進(jìn)給螺紋提供橫向進(jìn)給使小刀架垂直于導(dǎo)軌移動(dòng)。溜板箱中的齒輪可以在拖板沿著導(dǎo)軌和橫跨導(dǎo)軌移動(dòng)時(shí)提供動(dòng)力進(jìn)給。進(jìn)給箱齒輪將運(yùn)動(dòng)傳給拖板并控制刀具相對(duì)與工件的運(yùn)動(dòng)速度。由于進(jìn)給箱的移動(dòng)運(yùn)動(dòng)是由主軸齒輪驅(qū)動(dòng)的，因此進(jìn)給直接與主軸速度有關(guān)。進(jìn)給箱齒輪傳動(dòng)機(jī)構(gòu)也用于加工螺紋并能加工4扣到224扣每英寸的螺紋。
進(jìn)給箱和車(chē)床溜板箱之間的連接軸是光桿和絲桿。許多車(chē)床制造商把這兩種桿合并成一種桿，這樣雖然降低了成本但同時(shí)也降低了精度。進(jìn)給桿（光桿）提供道具的運(yùn)動(dòng)，以便提高工件的精度和表面光潔度。螺紋導(dǎo)桿（絲桿）提供精確的（螺紋）導(dǎo)程，是螺紋切削所必須的。光桿通過(guò)摩擦離合器驅(qū)動(dòng)，在刀具切削超載時(shí)可以進(jìn)行打滑保護(hù)。但絲桿里沒(méi)有這種安全裝置，因?yàn)槁菁y加工不允許打滑。由于螺紋全深很難一次走刀加工完成，因此裝設(shè)一個(gè)螺紋指示盤(pán)以便于下次走刀時(shí)用以重新對(duì)刀。
車(chē)床的尾座裝有一根很精確的軸，軸上的錐孔用來(lái)安裝鉆頭、鉆夾、鉸刀和車(chē)床頂尖。尾座可以沿著車(chē)床導(dǎo)軌移動(dòng)以適應(yīng)工件的不同長(zhǎng)度，也可以把刀具推向工件。尾座可以相對(duì)導(dǎo)軌偏移以加工錐體或錐形表面。
轉(zhuǎn)塔車(chē)床基本上具有某種附加特性的普通車(chē)床，用于半自動(dòng)加工和減少人工操作誤差。轉(zhuǎn)塔車(chē)床的拖板設(shè)有T形槽以便在車(chē)床導(dǎo)軌兩側(cè)安裝夾刀裝置。這樣，當(dāng)轉(zhuǎn)塔轉(zhuǎn)入到合適的位置，可以正確地安裝刀具以便進(jìn)行切削。拖板裝有自動(dòng)擋鉄以便控制刀具行程，提供良好的重復(fù)切削。轉(zhuǎn)塔車(chē)床的尾座是六角形結(jié)構(gòu)，可以安裝六把刀具。雖然操作前刀具和擋鉄的安裝要花大量時(shí)間，但一旦裝刀完成，稍微熟練的工人就可以連續(xù)地重復(fù)操作，直到刀具變鈍需要更換為止。這樣，為加工所做的準(zhǔn)備時(shí)間相對(duì)于所制造的零件數(shù)量是合理的時(shí)候，使用轉(zhuǎn)塔車(chē)床在經(jīng)濟(jì)上才算可行的。
多軸自動(dòng)車(chē)床裝有四、五、六或八根主軸，在每根主軸中裝一個(gè)工件。各主軸可以圍繞著一根中心主軸來(lái)轉(zhuǎn)換位置，而主刀具溜板可以接近所有的主軸。每根軸位上都裝有一側(cè)向可以獨(dú)立操作的刀具滑板。由于各刀具滑板都是靠凸輪操作的，因此加工準(zhǔn)備可能要花幾天時(shí)間，因而至少需要5000件的批量生產(chǎn)，它的使用才是合理的。這種機(jī)床的主要優(yōu)點(diǎn)是所有的刀具能同時(shí)工作，因而一個(gè)工人可以看管幾部機(jī)床。對(duì)于相對(duì)簡(jiǎn)單的零件，多軸自動(dòng)車(chē)床可以在五秒鐘內(nèi)生產(chǎn)加工出一件產(chǎn)品。
牛頭刨、龍門(mén)刨、鉆床和銑床
牛頭刨床使用的是裝在滑枕一端的夾具上的單點(diǎn)刀具。切削加工通常在向前的行程中進(jìn)行。刀具被抬刀架稍稍抬舉，以避免工件表面被嚴(yán)重拖刮。在返回行程中，刀具下面的工作進(jìn)給，為下一次切削做準(zhǔn)備。立式床身內(nèi)裝有牛頭刨的操作機(jī)構(gòu)，床身上裝有支持工件的工作臺(tái)。工作臺(tái)可在滑枕互相垂直的兩個(gè)方向上移動(dòng)。刀具滑塊用來(lái)控制切割深度并依靠手動(dòng)進(jìn)給。它可以在其法向垂直位置的兩側(cè)回轉(zhuǎn)900角，這樣可以相對(duì)于工作臺(tái)表面成一個(gè)角度進(jìn)給刀具。
牛頭刨床有兩種類(lèi)型的驅(qū)動(dòng)機(jī)構(gòu)：改進(jìn)的惠氏快回機(jī)構(gòu)和液壓驅(qū)動(dòng)機(jī)構(gòu)。對(duì)于惠氏機(jī)構(gòu)，電動(dòng)機(jī)驅(qū)動(dòng)大齒輪，大齒輪驅(qū)動(dòng)曲柄，行程長(zhǎng)度通過(guò)可調(diào)節(jié)的曲柄銷(xiāo)來(lái)控制。當(dāng)大齒輪旋轉(zhuǎn)時(shí)，搖臂受力而往復(fù)運(yùn)動(dòng)，并把運(yùn)動(dòng)傳遞給牛頭刨滑枕。
液壓牛頭刨的電動(dòng)機(jī)僅僅用來(lái)驅(qū)動(dòng)液壓泵。牛頭刨的運(yùn)動(dòng)靠液壓油的流向控制。機(jī)械驅(qū)動(dòng)的牛頭刨的切削行程只利用了大齒輪旋轉(zhuǎn)的2200，而返回行程則用了1400.這樣切削行程與返回行程之速度比為1.6比1。液壓牛頭刨有一個(gè)優(yōu)點(diǎn)，即切削速度可無(wú)級(jí)變速。這類(lèi)機(jī)床的主要缺點(diǎn)是在滑枕沖程的終端缺乏確定的限制，可能有千分之幾英寸的行程程度誤差。
龍門(mén)刨非常類(lèi)似于牛頭刨，因?yàn)檫@兩種刨床都主要加工平面和斜面，然而龍門(mén)刨更適合于加工大型工件。龍門(mén)刨的加工場(chǎng)所在工作臺(tái)上，能夠在水平面上往復(fù)運(yùn)動(dòng)，提供直線(xiàn)的切削和進(jìn)給運(yùn)動(dòng)。刨刀裝在橫向懸梁上，并受到垂直方向的立柱的支承。刨刀可以在水平和垂直方向進(jìn)刀和退刀，提供四個(gè)方向的直線(xiàn)進(jìn)給運(yùn)動(dòng)。
由于使用的刀具的類(lèi)型和加工場(chǎng)所的尺寸，龍門(mén)刨的切削速度較低。為了提高龍門(mén)刨床的生產(chǎn)能力，可以使用多刀切削加工中心。另一個(gè)增加產(chǎn)量的方法是同時(shí)在工作臺(tái)上安放多個(gè)工件。龍門(mén)刨的大小取決于加工場(chǎng)所的最大尺寸，也就是說(shuō)加工場(chǎng)所的長(zhǎng)度、寬度和高度要適應(yīng)各種龍門(mén)刨的工作臺(tái)的類(lèi)型。
立式鉆床或手搖鉆床有各種規(guī)格和類(lèi)型，主軸的調(diào)速范圍大并能根據(jù)多數(shù)行業(yè)的要求自動(dòng)進(jìn)給。搖臂鉆床用來(lái)鉆削那種很笨重、不便搬動(dòng)的工作。搖臂上安裝的主軸能夠進(jìn)行速度調(diào)節(jié)并裝有進(jìn)給調(diào)節(jié)機(jī)構(gòu)。通過(guò)轉(zhuǎn)臂繞立柱的轉(zhuǎn)動(dòng)和主軸組件沿?fù)u臂的移動(dòng)，可以把主軸和鉆頭移到機(jī)器所能達(dá)到的任何位置。
普通搖臂鉆床只能使主軸垂直運(yùn)動(dòng)，而萬(wàn)能搖臂鉆床允許主軸圍繞垂直于搖臂的軸線(xiàn)旋轉(zhuǎn)，搖臂繞著水平軸線(xiàn)旋轉(zhuǎn)，這樣可以在任何角度下鉆削。
多軸鉆床具有一個(gè)或多個(gè)通過(guò)萬(wàn)向接頭和可伸縮的花鍵軸來(lái)驅(qū)動(dòng)主軸的裝置。所有的主軸通常都由同一部電動(dòng)機(jī)驅(qū)動(dòng)并同時(shí)進(jìn)給以便鉆削出所要求的孔數(shù)。
銑削工藝用旋轉(zhuǎn)的刀具來(lái)切去金屬。它包括從工件表面切去金屬、擴(kuò)孔和成型切削。例如螺紋加工和齒輪加工。
升降臺(tái)式的的銑床內(nèi)，立柱是其它零件的主要支承構(gòu)件，其基座包括驅(qū)動(dòng)電機(jī)、主軸和刀具。刀具裝于主軸的刀桿上，并且通過(guò)懸臂內(nèi)的軸承支承在其外端上。升降臺(tái)裝在立柱的燕尾槽里，鞍座在燕尾槽里與升降臺(tái)固定在一起，工作臺(tái)被連接在鞍座上。這樣，升降臺(tái)和立柱可以提供相對(duì)于刀具的三個(gè)運(yùn)動(dòng)。借助工作臺(tái)繞著鞍座上的垂直軸線(xiàn)旋轉(zhuǎn)，可以提供第四個(gè)運(yùn)動(dòng)。
床身固定的銑床比升降式和立柱式的剛性更好。工作臺(tái)被直接安裝在機(jī)床基座上，提供了吸收重大切削載荷所需要的剛性，而且僅允許相對(duì)于工作臺(tái)的縱向運(yùn)動(dòng)。垂直運(yùn)動(dòng)靠移動(dòng)整個(gè)刀頭獲得。
仿型銑床的特點(diǎn)或者是刀具與仿型元件的運(yùn)動(dòng)軌跡同步，垂直運(yùn)動(dòng)靠移動(dòng)整個(gè)刀頭獲得。仿型模型的形狀輪廓運(yùn)動(dòng)，而刀頭重復(fù)仿型運(yùn)動(dòng)。
銑削工藝設(shè)計(jì)所考慮的一般原則是：
（1） 只要有可能，將零件設(shè)計(jì)成安裝一次能銑削表面的數(shù)量最多：
（2） 為了能夠使多刀銑削，將零件設(shè)計(jì)成能同時(shí)銑削幾個(gè)表面；
（3） 應(yīng)當(dāng)首先銑最大的平面，這樣，所有尺寸都能同時(shí)銑削幾個(gè)平面為基準(zhǔn)；
（4） 由于銑刀是回轉(zhuǎn)的，所以不可能銑削方形內(nèi)角。
磨床
磨床把磨料粘接成各種形狀和大小的輪子和帶子作為切削介質(zhì)。磨削工藝可以加工出表面光潔度質(zhì)量很高的零件，并提高零件的尺寸精度，因?yàn)槟ハ鞯墓羁梢赃_(dá)到0.00025mm。磨床根據(jù)加工出的表面的形狀分類(lèi)。根據(jù)加工一般表面分類(lèi)，磨床有表面磨床、外圓磨床和專(zhuān)用磨床。
磨削加工在工件生產(chǎn)中顯得極為重要，有如下幾個(gè)原因：
（1） 磨削加工是切削淬硬過(guò)的工具鋼或其它熱處理過(guò)的鋼的最通用的方法。零件可以先加工再熱處理，最后磨削到所要求的尺寸和表面光潔度。
（2） 磨削加工能磨出的表面光潔度為0.5微米，而費(fèi)用并不昂貴。
（3） 磨削加工可在相對(duì)短的時(shí)間內(nèi)保證尺寸精度，因?yàn)槟ゴ材芴峁┰隽繛槿f(wàn)分之一英寸的進(jìn)刀運(yùn)動(dòng)，而不是像其它機(jī)床那樣僅有千分之幾英寸。
（4） 可加工極小而薄的零件。由于施加的磨削壓力很小，減小了工件偏離刀具的趨勢(shì)。
在外圓磨床上，吃刀深度靠移動(dòng)砂輪頭來(lái)控制，包括控制砂輪及其驅(qū)動(dòng)電機(jī)。冷卻劑用來(lái)減少熱變形，帶走切屑和磨削粉塵。
7
Fundamentals of Machine Tools
In many cases products from the primanry forming processes must undergo further refinements in size and surface finish to meet their design specifications. To meet such precise tolerances the removal of small amounts of material is needed. Usually machine tools are used for such operation.
In the United States material removal is a big business-in excess of $36×10 per year, including material, labor, overhead, and machine-tool shipments. Since 60 percent of themechanical and industrial engineering and technology graduates have something connected with the machining industry either through sale, design, or operation of machine shops,or working in related industry, it is wise for an engineering studet to devote some time in his curriculum to studying material removal and machine tools.
A machine tool provides the means for cutting tools to shape a workpiece to required dimensions; the machine supports the tool and the workpiece in a controlled relationship through the functioning of its basic member,which are as follows:
(1) Bed, Structure or Fram. This is the main member which provides a basis for, and a connection between, the spindles and slides; the distortion and vibration under load must be kept to a minimum.
(2) Slides and Slideways. The translation of a machine element(e.g. the slide) is normally achieved by straight-line motion under the constraint of accurate guiding surfaces(the slideway).
(3) Spindles and Bearings. Angular displacements take place an axis of rotation; the position of this axis must be constant within extremely fine limits in machine tools, and is ensured by the provision of precision spindles and bearings.
(4) Power Unit. The electric motor is the universally adopted power unit for machine tools. By suitably positioning individual motors, belt and gear transmissions are reduced to minimum.
(5) Transmission Linkage. Linkage is the general term used to denote the mechanical, hydraulic, pneumatic or electric mechanisms which connect angular and linear displacements in defined relationship.
There are two broad divisions of machining operations:
(1) Roughing, for which the mental removal rate, and consequently the cutting force, is high, but the required dimensional accuracy relatively low.
(2) Finishing, for which the metal removal rate, and consequently the cutting force, is low, but the required dimensional accuracy and surface finish relatively high.
It follows that static loads and dynamic loads, such as result from an unbalanced grindingwheel, are more significant in finishing operations than in roughing operations. The degree of precision achieved in any machining process will usually be influenced by the magnitude of the deflections, which occur as a result of the force acting.
Machine tool frames are generally made in cast iron, although some may be steel casting or mild-steel fabrications. Cast iron is chosen because of its cheapness, rigidity, compressive strength and capacity for damping the vibrations set-up in machine operations. To avoid massive resistance to bending and torsional stresses. Tow basic types of ribbing are box and diagonal. The box formation is convenient to produce, apertures in walls permitting the positioning and extraction of cores. Diagonal ribbing provides greater torsional stiffness and yet permits swarf to fall between the sections; it is frequently used for lathe beds.
The slides and slideways of a machine tool locate and guide members which move relative to each other, usually changing the position of the tool relative to the workpiece. The movement generally takes the from of translation in a straight line, but is sometime angular rotation, e.g. tilting the wheel-head of a universal thread-grinding machine to an angle corresponding with the helix angle of the work piece threat. The basic geometric elements of slides are flat, vee, dovetail and cylinder. These elements may used separately or combined in various ways according to the applications. Features of slideways are as follows:
(1) Accuracy of Movement. Where a slide is to be displaced in a straight line, this line must lie in two mutually perpendicular planes and there must be no slide rotation. The general tolerance for straightness of machine tool slideways is 0~0.02 mm per 1000mm; on horizontal surfaces this tolerance may be disposed so that a convex surface results, thus countering the effect of “sag” of the slideways.
(2) Means of Adjustment. To facilitate assembly, maintain accuracy and eliminate “play” between sliding members after wear has taken place, a strip is sometimes inserted in the slides. This is called a gib-strip. Usually, the gib is retained by socket-head screws passing through elongated slots; and is adjusted by grub-screws secured by lock nuts.
(3) Lubrication. Slideways may be lubricated by either of the following systems:
(i) Intermittently, through grease or oil nipples, a method suitable where movements are infrequent and speed low.
(ii) Continuously, e.g. by pumping through a metering valve and pipe-work to the point of application; the film of oil introduce between surface by these means must be extremely thin to avoid the slide “floating”. If sliding surfaces were optically flat oil would be squeezed out, resulting in the surface sticking. Hence in practice slide surface are either ground using the edge of a cup wheel, or scraped. Both processes produce depressions which retain “pocket” of oil, and complete separation of the parts may not occur at all points; positive location of the slides is thus retained.
(4) Protection. To maintain slideways in good order, the following conditions must be met:
(i) Ingress of foreign matter, e.g. swarf, must be prevented. Where this is no possible, it is desirable to have a form of slideway, which does not retain swarf, e.g. the inverted vee.
(ii) Lubricating oil must be retained. The adhesive property of oil for use on vertical or inclined slide surface is important; oil are available which have been specially developed for this purpose. The adhesiveness of oil also prevents it being washed away by cutting fluids.
(iii) Accidental damage must be prevented by protective guards.
Lathes
A machine tool performs three major functions: (i) it rigidly supports the work piece or its holder and the cutting tool; (ii) it provide relative motion between the work piece and the cutting tool; (iii) it provides a range of feeds and speeds.
Machines used to remove metal in the form of chips are classified as follows:
Machines using basically the single-point cutting tools include: engine lathes, turret lathes, tracing and duplicating lathes, single-spindle automatic lathes, multi-spindle automatic lathes, shapers and planers, boring machines.
Machines using multipoint cutting tools include: drilling machines, milling machines, broaching machines, sawing machine, gear-cutting machines.
Machines using random-point cutting tools (abrasive) include: cylindrical grinder, centreless grinders, surface grinders.
Special metal removal methods include: chemical milling, electrical discharge machining, ultrasonic machining.
The lathe removes material by rotating the work piece against a cutter to produce external or internal cylindrical or conical surfaces. It is commonly used for the production of surfaces by facing, in which the work piece is rotated while the cutting tool is moved perpendicularly to the axis of rotation.
The engine lathe, shown in Fig.1, is the basic turning machine from which other turning machines have been developed. The drive motor is located in the base and drives the spindle through a combination of belts and gears. The spindle is a sturdy hollow shaft, mounted between heavy-duty bearings, with the forward end used for mounting a drive plate to impart positive motion to the work piece. The drive plate may be fastened to the spindle by threads, by a cam lock mechanism, or by a threaded collar and key.
The lathe bed is cast iron and provides accurately ground sliding surfaces (way) on which the carriage rides. The lathe carriage is an H-shaped casting on which the cutting tool is mounted in a tool holder. The apron hangs from the front of the carriage and contains the driving gears that move the tool and carriage along or across the way to provide the desired tool motion.
A compound rest, located above the carriage, provides for rotation of the tool holder through any desired angle. A hand wheel and feed screw are provided on the compound rest for linear motions of the tool. The cross feed is provided with another hand wheel and feed screw for moving the compound rest perpendicular to the lathe way. A gear train in the apron provides power feed for the carriage both along and across the way. The feed box contains gears to impart motion to the carriage and control the rate at which the tool moves relative to the work piece. Since the transmission in the feed box is driven from the spindle gears, the feeds are directly related to the spindle speed. The feed box gearing is also used in thread cutting and provides from 4 to 224 threads per in.
The connecting shaft between the feed box and the lathe apron are the feed rod and the lead screw. Many lathe manufacturers combine these two rods in one, a practice that reduces the cost of the machine at the expense of accuracy. The feed rod is used to provide the accurate lead necessary for the thread cutting. The feed rod is driven through a friction clutch that allows slippage in case the tool is overloaded. This safety device is not provided in the lead screw, since thread cutting cannot tolerate slippage. Since the full depth of the thread is seldom cut in one pass, a chasing dial is provided to realign the tool for subsequent passes.
The lathe tailstock is fitted with an accurate spindle that has a tapered hole for mounting drill, drill chucks, reamers, and lathe centers. The tailstock can be moved along the lathe ways to accommodate various lengths of work pieces as well as to advance a tool into contact with the work piece. The tailstock can be offset relative to the lathe ways to cut tapers or conical surfaces.
The turret lathe is basically an engine lathe with certain additional features to provide for semiautomatic operation and to reduce the opportunity for human error. The carriage of the turret lathe is provided with T-slots for mounting a tool-holding device on both sides of the lathe ways with tools properly set for cutting when rotated into position. The carriage is also equipped with automatic stops that control the tool travel and provide good reproduction of cuts. The tailstock of the turret lathe is of hexagonal design, in which six tools can be mounted. Although a large amount of time is consumed in setting up the tools and stops for operation, the turret lathe ,once set, can continue to duplicate operations with a minimum of operator skill until the tools become dulled and need replacing. Thus , the turret is economically feasible only for production work, where the amount of time necessary to prepare the machine for operation is justifiable in terms of the number of part to be made.
The multi-spindle automatic lathe is provided with four, five, six, or eight spindles, with one workpiece mounted in each spindles. The spindles index around a central shaft, with the main tool slide accessible to all spindles. Each spindle position is provided with a side tool-slide operated independently. Since all of the slides are operated by cams, the preparation of this machine may take several days, and a production run of at least 5000 parts is needed to justify its use. The principal advantage of this machine is that all tools work simultaneously, and one operator can handle several machines. For relatively simple parts, multi-spindle automatic lathes can turn out finished products at the rate of 1 every 5 sec.
Shapers, Planers, Drilling and Milling Machines
A shaper utilizes a single-point tool in a tool holder mounted on the end of the ram. Cutting is generally done on the forward stroke. The tool is lifted slightly by the clapper box to prevent excessive drag across the work, which is fed under the tool during the return stroke in preparation for the next cut. The column houses the operating mechanisms of the shaper and also serves as a mounting unit for the work-supporting table. The table can be moved in two directions mutually perpendicular to the ram. The tool slide is used to control the depth of cut and is manually fed. It can be rotated through 90 deg. on either side of its normal vertical position, which allows feeding the tool at an angle to the surface of the table.
Two types of driving mechanisms for shapers are a modified whitworth quick-return mechanism and a hydraulic drive. For the whitworth mechanism, the motor drives the bull gear, which drives a crank arm with an adjustable crank pin to control the length of stroke. As the bull gear rotates, the rocker arm is forced to reciprocate, imparting this motion to the shaper ram.
The motor on a hydraulic shaper is used only to drive the hydraulic pump. The remainder of the shaper motions are controlled by the direction of the flow of the hydraulic oil. The cutting stroke of the mechanically driven shaper uses 220 deg. Of rotation of the bull gear, while the return stroke uses 140 deg. This gives a cutting stroke to return stroke ratio of 1.6 to 1. The hydraulic shaper has an advantage of infinitely variable cutting speeds. The principal disadvantage of this type of machine is the lack of a definite limit at the end of the ram stroke, While may allow a few thousandths of an inch variation in stroke length.
Planers are similar to shapers because both machines are primarily used to produce flat and angular surfaces. However ,planers are capable of accommodating much larger workpieces than horizontal plane providing a straight-line cutting and feed action. Single-point cutting tools are mounted on an overhead cross rail and along the vertically supported columns. The cutting tools are fed into or away from the workplace on either the horizontal or vertical plane, thus being capable of four straight-line feed motions.
Cutting speeds are slow on the planer because of the workplace size and type of cutting tool being used. In order to increase the production of the planer, multiple tooling stations are employed. Another method of increasing production is to mount a number of workpieces on the table at the same time. The planer size is designated by the maximum workplace capacity of the machine. The height, width, and length of the workplace that can be accommodated on the planer’s worktable varies with the type of planer.
Upright drilling machines or drill presses are available in a variety of sizes and types, and are equipped with a sufficient range of spindle speeds and automatic feeds to fit the needs of most industries. Radial drilling machines are used to drill workpieces that are too large or cumbersome to conveniently move. The spindle with the speed and feed changing mechanism is mounted on the radial arm; by combining the movement of the radial arm around column and the movement of the spindle assembly along the arm, it is possible to align the spindle and the drill to any position within reach of the machine.
Plain radial drilling machines provide only for vertical movement of the spindle; universal machines allow the spindle to swivel about an axis normal to the radial arm to rotate about a horizontal axis, thus permitting drilling at any angle.
A multispindle drilling machine has one or more heads that drive the spindles through universal joints and telescoping splined shafts. All spindles are usually driven by the same motor and fed simultaneously to drill the desired number of holes.
The milling operation involves metal removal with a rotating cutter. It includes removal of metal from the surface of a workpiece, enlarging holes, and form cutting, such as threads and gear teeth.
Within a knee and column type of milling machine the column is the main supporting member for the other components, and includes the base containing the drive motor, the spindle, extremity by a bearing in the overarm. The knee is held on the column in dovetail slots, the saddle is fastened to the knee in dovetail slots, and the table is attached to the saddle. Thus, the build-up of the knee and column machine provides three motions relative to the cutter. The fourth motion may be provided by swiveling the table around a vertical axis provided on the saddle.
Fixed-bed milling machines are designed to provide more rigidity than the knee and column type. The table is mounted directly on the machine base, which provides the rigidity necessary for absorbing heavy cutting load, and allows only longitudinal motion to the table. Vertical motion is obtained by moving the entire cutting head.
Tracer milling is characterized by coordinated or synchronized movements of either the paths of the cutter and tracing elements, or the paths of the workpiece and model. The tracing finger follow the shape of the master pattern, and the cutter heads duplicate the tracer motion.
The following are general design considerations for milling:
(1) Wherever possible, the part should be designed so that a maximum number of surfaces can be milled from one setting.
(2) Design for the use of multiple cutters to mill several surfaces simultaneously.
(3) The largest flat surface will be milled first, so that all dimensions are best referred to such surface.
(4) Square inside corners are not possible, since the cutter rotates.
Grinding Machines
Grinding machines utilize abrasive grains, bonded into various shapes and sizes of wheels and belts to be used as the cutting agent. Grinding operations are used to impart a high-quality surface finish on the workpiece is improved since tolerances of 0.00025 mm are possible in grinding operations. Grinding machines are classified according to the type of surface produced. Common surfaces and classifications of grinding machines are surface, cylindrical, and special machines.
The grinding process is of extreme importance in production work for several reasons.
(1) It is the most common method for cutting hardened tool steel of other heat-treated steel. Parts are first machined in the un-heat-treated condition, and then ground to the desired dimensions and surface finish.
(2) It can provide surface finish to 0.5 um without extreme cost.
(3) The grinding operation can assure dimensions in a relatively short time, since machines are built to provide motions in incremente of ten-thousandths of an inch, instead of thousandths as is common in s can be other machines.
(4) Extremely small and thin parts can be finished by this method, since light pressure is used and the tendency for the part to deflect away from the cutter is miniminzed.
On a cylindrical grinding machine the depth of cut is controlled by moving the wheel head, which includes both the wheel and its drive motor. Coolants are provided to reduce heat distortion and to remove chips and abrasive dust.
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