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南 京 理 工 大 學(xué) 紫 金 學(xué) 院
畢業(yè)設(shè)計(jì)(論文)外文資料翻譯
系 ： 機(jī)械工程系
專 業(yè)： 機(jī)械工程及自動(dòng)化
姓 名：
學(xué) 號(hào)：
(用外文寫)
外文出處： Journal of Manufacturing Science
and Engineering
附 件： 1.外文資料翻譯譯文；2.外文原文。
指導(dǎo)教師評(píng)語(yǔ)：
通過(guò)指導(dǎo)該生對(duì)翻譯進(jìn)行多次修改以后，該翻譯語(yǔ)句比較正確，全文能夠較好地忠實(shí)于原文原意，全文格式正確，用詞恰當(dāng)，全文的翻譯的工作量符合學(xué)生規(guī)定的基本要求，圖表表達(dá)正確，是一篇較好的外文翻譯。綜合評(píng)定為良好。
簽名：
2010年 3 月 25日
附件1：外文資料翻譯譯文
數(shù)控機(jī)床
Outrata, J. and Jowe, J.
雖然數(shù)控機(jī)床的具體用途和應(yīng)用不盡相同，但所有形式的數(shù)控系統(tǒng)都有共同的優(yōu)點(diǎn)。這里有一些數(shù)控裝備提供的更加重要的好處：
第一個(gè)優(yōu)點(diǎn)是所有的數(shù)控機(jī)床被改進(jìn)成自動(dòng)化。操作員可以減少或消除對(duì)生產(chǎn)工件的干預(yù)。許多數(shù)控機(jī)床可以自動(dòng)運(yùn)行整個(gè)加工循環(huán)，而讓操作員做其他的工作。這給數(shù)控機(jī)床用戶帶來(lái)了幾方面的好處包括減少操作人員的疲勞、減少人為所犯的錯(cuò)誤、可以預(yù)測(cè)每個(gè)工件的加工時(shí)間等。由于機(jī)器是在程序控制下運(yùn)行，降低了數(shù)控機(jī)床操作員傳統(tǒng)機(jī)械加工要求（機(jī)械加工的基本做法）。
第二個(gè)優(yōu)點(diǎn)是一致性和準(zhǔn)確性?，F(xiàn)在的數(shù)控機(jī)床擁有令人難以置信的準(zhǔn)確性和可重復(fù)性。這意味著一旦一個(gè)程序被確認(rèn)，那么兩個(gè)、十個(gè)或一千個(gè)相同的工件可以很容易地達(dá)到規(guī)定精度并保持一致性。
第三個(gè)優(yōu)點(diǎn)是由數(shù)控系統(tǒng)最靈活的形式提供的。由于數(shù)控機(jī)床運(yùn)行的是程序，所以加工不同的零件和加載不同的程序都很簡(jiǎn)單。一旦一個(gè)程序確認(rèn)被用來(lái)加工零件后，下一次要再加工同樣零件時(shí)就很容易拿來(lái)運(yùn)用。這就會(huì)產(chǎn)生另外一個(gè)好處。由于這些設(shè)備非常容易設(shè)置和運(yùn)行，所以它們可以用很短的時(shí)間進(jìn)行設(shè)置，使程序很容易的被加載。這對(duì)現(xiàn)在的實(shí)時(shí)（JIT）的產(chǎn)品需求是非常有必要的。
運(yùn)動(dòng)控制—數(shù)控的核心
任何數(shù)控機(jī)床最基本的功能是自動(dòng)的、精確的、一致的運(yùn)動(dòng)控制。而不像多數(shù)常規(guī)機(jī)床完全運(yùn)用機(jī)械部件運(yùn)動(dòng)，數(shù)控機(jī)床的改革之處在于它用計(jì)算機(jī)控制。數(shù)控設(shè)備兩個(gè)或兩個(gè)以上的方向運(yùn)動(dòng)的形式，稱為軸。這些軸可以用來(lái)精確定位和自動(dòng)沿規(guī)定長(zhǎng)度行走。兩個(gè)最常見(jiàn)的類型是線性軸（驅(qū)動(dòng)沿直線路徑）和旋轉(zhuǎn)軸（沿圓形軌跡驅(qū)動(dòng)）。
與傳統(tǒng)機(jī)床加工中的由曲柄和手輪組成的加工工序相比較，數(shù)控機(jī)床的加工工序是由程序控制的。一般來(lái)說(shuō)，運(yùn)動(dòng)類型（快速，線性和圓形）、軸走動(dòng)、運(yùn)動(dòng)量和運(yùn)動(dòng)速度（進(jìn)給速度）都是用由程序來(lái)控制的。
在運(yùn)行過(guò)程中數(shù)控命令告訴驅(qū)動(dòng)馬達(dá)精確轉(zhuǎn)動(dòng)的次數(shù)。反過(guò)來(lái)驅(qū)動(dòng)馬達(dá)旋轉(zhuǎn)滾珠絲桿，而球螺桿驅(qū)動(dòng)線性軸。反饋設(shè)備（線性比例在幻燈片）是用來(lái)確認(rèn)是否有誤差。參照?qǐng)D1.
圖1. 經(jīng)典的數(shù)控機(jī)床傳動(dòng)系統(tǒng)
用一個(gè)比較簡(jiǎn)單的類比，相同的基本直線運(yùn)動(dòng)可以在一個(gè)普通的老虎鉗上找到，當(dāng)你旋轉(zhuǎn)老虎鉗，絲桿旋轉(zhuǎn)，反過(guò)來(lái)，臺(tái)鉗上的驅(qū)動(dòng)器也可移動(dòng)老虎鉗下顎。相比之下，在數(shù)控機(jī)床直線軸是非常準(zhǔn)確的。驅(qū)動(dòng)電機(jī)的轉(zhuǎn)數(shù)精確地控制著軸直線運(yùn)動(dòng)的量。
如何控制多軸運(yùn)動(dòng) - 理解坐標(biāo)系統(tǒng)
對(duì)于操作員來(lái)說(shuō)，控制定量的直線運(yùn)動(dòng)時(shí)他是不可能告訴每個(gè)驅(qū)動(dòng)電機(jī)旋轉(zhuǎn)多少次來(lái)驅(qū)動(dòng)軸的。相反（這就像不得不搞清楚如何使老虎鉗的下顎精確的移動(dòng)1英寸?。?，如果利用某種形式的坐標(biāo)系，那么所有的多軸運(yùn)動(dòng)將被更為簡(jiǎn)單和合理的方式控制。數(shù)控機(jī)床最常用是直角坐標(biāo)系和極坐標(biāo)系。目前為止，這兩個(gè)中比較常用的是直角坐標(biāo)系。
在一個(gè)數(shù)控程序中程序零點(diǎn)是運(yùn)動(dòng)參考零點(diǎn)，這可以使程序員指定從一個(gè)共同的地點(diǎn)運(yùn)動(dòng)。如果零點(diǎn)被正確的選擇，那么程序采用的坐標(biāo)可以直接顯示出來(lái)。
利用這種技術(shù)，如果程序員想讓機(jī)床移動(dòng)到零點(diǎn)右邊1英寸的地方，那么就輸入X 1.0，如果程序員想讓機(jī)床移動(dòng)到零點(diǎn)上方1英寸的地方，那么就輸入Y 1.0。程序會(huì)自動(dòng)控制驅(qū)動(dòng)電機(jī)運(yùn)轉(zhuǎn)多少次和滾珠絲桿，使軸到達(dá)目標(biāo)位置。這是程序員控制多軸運(yùn)動(dòng)非常常用的方式。參照?qǐng)D2、圖3。
理解絕對(duì)與相對(duì)的概念
所有的討論集中于這一點(diǎn)，假設(shè)編程使用絕對(duì)模式。設(shè)置絕對(duì)模式最常用的指令是G90。在絕對(duì)模式下，所有程序運(yùn)行終點(diǎn)都是以零點(diǎn)為起始點(diǎn)。對(duì)于初學(xué)者來(lái)說(shuō)，運(yùn)動(dòng)指令開(kāi)始通常最好的和最容易的方法是指定端點(diǎn)。但是，還有另一種指定的多軸運(yùn)動(dòng)結(jié)束點(diǎn)的方法。
圖2.從網(wǎng)格圖中觀察X.Y坐標(biāo) 圖3.X.Y和Z坐標(biāo)軸
在增量模式（通常指定G91）下，程序最終點(diǎn)的指定是從程序的當(dāng)前位置，而不是從程序的零點(diǎn)。用這樣的程序指令方法時(shí)，程序員必須時(shí)刻問(wèn)：“我應(yīng)該如何移動(dòng)機(jī)床？”雖然有些時(shí)候的增量模式，可以是非常有幫助，一般來(lái)說(shuō)，這種指令模式對(duì)初學(xué)者來(lái)說(shuō)更麻煩和困難，所以要集中使用絕對(duì)模式。
當(dāng)你做出運(yùn)行指令時(shí)要小心，初學(xué)者要有增量的思維。如果工作在絕對(duì)模式（作為初學(xué)者應(yīng)該使用），程序員應(yīng)該想：“機(jī)床應(yīng)該移到那個(gè)點(diǎn)？”這個(gè)點(diǎn)是程序零點(diǎn)相關(guān)，與機(jī)床當(dāng)前位置無(wú)關(guān)。
對(duì)于任何命令除使它非常容易確定當(dāng)前位置之外，另一個(gè)好處是與工作在絕對(duì)方式下和在行動(dòng)命令期間犯的錯(cuò)誤有關(guān)。在絕對(duì)模式下，如果運(yùn)行錯(cuò)誤由于一個(gè)程序的命令導(dǎo)致，只有一個(gè)運(yùn)動(dòng)是不正確的。從另一方面說(shuō)，如果錯(cuò)誤是在增量運(yùn)動(dòng)模式下發(fā)生，那么從這一點(diǎn)開(kāi)始，接下來(lái)所有的程序運(yùn)行都是錯(cuò)誤的。
分配程序零點(diǎn)
請(qǐng)記住，數(shù)控系統(tǒng)必須通過(guò)一種或多種方式告訴它零點(diǎn)的位置?？刂撇煌臋C(jī)床要做到這一點(diǎn)有很大的差異。一種(或一種以上)的方式是通過(guò)數(shù)控程序來(lái)設(shè)置的。使用這種方法，程序員要計(jì)算程序起始點(diǎn)到程序零點(diǎn)的距離。通常的做法是在程序起始和運(yùn)行開(kāi)始之前時(shí)用G92(或G50)指令設(shè)置。
另外，更新和更好的方法去設(shè)置程序零點(diǎn)是通過(guò)某種形式的偏移。請(qǐng)參照?qǐng)D4，通常機(jī)床制造商通過(guò)利用分配偏移夾具來(lái)抵消程序零點(diǎn)誤差。制造商稱這種方式為利用程序零點(diǎn)分配每個(gè)機(jī)床的幾何偏移。
圖4.通過(guò)G54指令來(lái)設(shè)定系統(tǒng)零點(diǎn)
柔性制造單元
一個(gè)柔性制造單元（FMC）是一個(gè)柔性制造子系統(tǒng)。以下是柔性系統(tǒng)之間存在的差異：
1.一個(gè)柔性制造單元是由一個(gè)中央計(jì)算機(jī)直接控制的。相反，指令從中央電腦傳遞到單元控制器。
2. 該單元是整個(gè)系統(tǒng)的一部分。
柔性制造單元中通常包含以下單元：
. 單元控制器
. 可編程邏輯控制器（PLC）
. 多個(gè)個(gè)機(jī)床
. 一個(gè)材料處理設(shè)備（機(jī)器人或托盤）
高速機(jī)械加工
高速機(jī)械加工是指很高的加工速度和高的表面加工質(zhì)量。例如，具有非常高的材料去除率請(qǐng)參考圖5的切削參數(shù)名稱。在過(guò)去60年來(lái)，高速加工已經(jīng)應(yīng)用于金屬和非金屬等各種材料，包括表面加工質(zhì)量和用機(jī)器制造硬度50HRC以上的堅(jiān)硬材料。大部分鋼鐵部件硬化程度非常高，大約32-42HRC。
圖5.切割數(shù)據(jù)
在粗加工和半精加工時(shí)，用電極加工和電火花加工硬度為63HRC的材料(特別是在加工小半徑和金屬切削難加工到的深孔)。在精加工和超精加工時(shí)表面適當(dāng)添加些硬質(zhì)合金、金屬陶瓷、整體硬質(zhì)合金、混合陶瓷或聚晶立方氮化硼（PCBN）。
對(duì)于許多組件，生產(chǎn)過(guò)程中都需要涉及到這些加工方式，在模具生產(chǎn)時(shí)還包括費(fèi)時(shí)的手工拋光。因此，會(huì)導(dǎo)致生產(chǎn)成本過(guò)高。
這是個(gè)典型的模具生產(chǎn)環(huán)節(jié)或只是一些相同的設(shè)計(jì)工具。這一過(guò)程涉及到設(shè)計(jì)的不斷變化，由于這些變化也可用于測(cè)量和逆向工程相應(yīng)的需要中。
模具的質(zhì)量水平高低的主要標(biāo)準(zhǔn)是三維幾何精度和表面準(zhǔn)確性。如果加工后的質(zhì)量低不能滿足要求，將消耗許多的人工去彌補(bǔ)。而高速機(jī)械加工能產(chǎn)生令人滿意的表面精度，但它會(huì)產(chǎn)生尺寸和幾何精度誤差。
模具工業(yè)發(fā)展的主要目標(biāo)之一是，減少或消除手工拋光的需要，從而提高質(zhì)量，減少了生產(chǎn)成本和交貨時(shí)間。
經(jīng)濟(jì)和技術(shù)因素為高速加工的發(fā)展提供了基礎(chǔ)。
生存
市場(chǎng)競(jìng)爭(zhēng)的不斷加劇也樹立了新的標(biāo)準(zhǔn)。它對(duì)時(shí)間和成本效率的要求越來(lái)越高。這使新的工藝和生產(chǎn)技術(shù)的發(fā)展得以進(jìn)行。高速加工提供為此希望和解決辦法。
材料
新的、更難加工的材料要求我們必須發(fā)展去尋找新的加工方案。航空航天工業(yè)有耐熱合金和不銹鋼；汽車行業(yè)有不同的雙金屬成分，緊湊型石墨鐵和鋁的成分不斷增加；模具工業(yè)的主要面對(duì)硬質(zhì)材料粗加工到精加工的問(wèn)題。
質(zhì)量
持續(xù)競(jìng)爭(zhēng)的結(jié)果是提高了產(chǎn)品的結(jié)構(gòu)和質(zhì)量。如果能正確使用高速加工，就能解決這方面的一些問(wèn)題。代替手工精加工就是一個(gè)很好的例子，特別是在加工精密模具和復(fù)雜三維幾何體時(shí)。
過(guò)程
在這個(gè)快速和高效的時(shí)代要求下，簡(jiǎn)化流程（物流）大多數(shù)時(shí)候可以用高速加工來(lái)解決。在模具和模具工業(yè)中典型的例子就是用設(shè)定好的機(jī)器去制造難加工的工具。隨著高速切削加工的應(yīng)用，昂貴和費(fèi)時(shí)的電火花加工過(guò)程也可以減少或被消除。
設(shè)計(jì)與開(kāi)發(fā)
現(xiàn)代產(chǎn)品競(jìng)爭(zhēng)的主要方法之一就是創(chuàng)新的設(shè)計(jì)。今天產(chǎn)品的平均生命周期是汽車4年，電腦及配件1.5年，手機(jī)3個(gè)月...設(shè)計(jì)快速的發(fā)展和減少產(chǎn)品開(kāi)發(fā)時(shí)間的先決條件是高速加工技術(shù)。
復(fù)雜的產(chǎn)品
一個(gè)有多個(gè)加工表面的部件，如為渦輪葉片設(shè)計(jì)新的優(yōu)化功能和特性。早期的設(shè)計(jì)允許利用手或機(jī)器人進(jìn)行拋光。新的、更加復(fù)雜的渦輪葉片的設(shè)計(jì)和加工必須都通過(guò)高速加工。現(xiàn)在有越來(lái)越多的薄壁工件需要用機(jī)器來(lái)加工如（醫(yī)療設(shè)備，電子，國(guó)防產(chǎn)品，電腦配件）。
生產(chǎn)設(shè)備
大力發(fā)展切削材料，夾具，機(jī)床和數(shù)控系統(tǒng)，特別是CAD/CAM功能和設(shè)備。必須以開(kāi)放的思想去接受新的生產(chǎn)方式和技術(shù)。
高速加工的定義
所羅門的理論，1931年德國(guó)對(duì)機(jī)械高速切削的定義為：假定“在某個(gè)切削速度（是傳統(tǒng)機(jī)械加工的5-10倍），排屑時(shí)切削刃的溫度將減少?！眳⒁?jiàn)圖6.
所以給出結(jié)論:“需提供一個(gè)機(jī)會(huì)以提高加工生產(chǎn)效率相對(duì)傳統(tǒng)切割工具”。
在切削不同材料時(shí)切口溫度相對(duì)減少，不幸的是現(xiàn)代研究未能完全證實(shí)這一理論。
溫度對(duì)于鋼鐵和鑄鐵是逐漸變小的，但對(duì)于鋁和有色金屬是增大的。所以高速加工的定義必須基于其他因素。
鑒于目前的技術(shù)，“高速”概念普遍平均速度是1至10公里每分鐘，約3 300至33 000英尺每分鐘。速度超過(guò)10公里/分鐘是超高速，大部分用在金屬切削實(shí)驗(yàn)領(lǐng)域。顯然，要實(shí)現(xiàn)工件表面的切割速度，主軸旋轉(zhuǎn)速度直接與工件的直徑有關(guān)?，F(xiàn)在一個(gè)很明顯的趨勢(shì)就是運(yùn)用大直徑刀具，這對(duì)模具設(shè)計(jì)有重要意義。
圖6. 芯片根據(jù)切割速度控制溫度
有許多不同的觀點(diǎn)和不同的方法來(lái)定義高速加工。
維護(hù)保養(yǎng)和故障排除
維修管理成員
下面是一個(gè)需要定期保養(yǎng)的臥式加工中心的名單，如圖7所示。列出的是工作頻率、需要的電流的容量和類型。必須遵循這些要求，以保持您的機(jī)器有良好的工作秩序，同時(shí)也使保修安全。
圖7.臥式加工中心
每天要做的事情
頂部冷卻水每8小時(shí)換一次（尤其在重型機(jī)床上要特別注意）
檢查機(jī)油潤(rùn)滑的位置
從封口利用最佳方法清潔芯片
從換刀處清潔芯片
主軸錐度要擦拭干凈
每周要做的事情
請(qǐng)檢查過(guò)濾減壓閥自動(dòng)排水是否正常運(yùn)作。見(jiàn)圖。 8
圖8.潤(rùn)滑和氣動(dòng)方式
在處理TSC機(jī)型時(shí)，要清洗儲(chǔ)存冷卻液的罐子。
打開(kāi)油箱蓋和去除油箱內(nèi)的所有沉積物。斷開(kāi)冷卻泵之前，要小心控制冷卻液并切斷電源。除了TSC機(jī)型每個(gè)月都做做這個(gè)工作。
檢查空氣壓力/安全為85 psi。
為了與TSC機(jī)型相匹配，應(yīng)該把潤(rùn)滑油放在法蘭邊緣。除了TSC機(jī)型每個(gè)月都做這個(gè)工作。
清潔機(jī)器表面要溫和清潔。不要使用溶劑。
根據(jù)機(jī)器的規(guī)格檢查液壓平衡壓力。
用手指涂抹油脂或工具對(duì)對(duì)刀邊緣潤(rùn)滑。
每月要做的事
檢查變速箱機(jī)油的水平。添加機(jī)油，直到油開(kāi)始從廢油管出口流出。
清理托盤底部的清潔墊。
清潔定位軸和負(fù)載箱時(shí)需要拆除托盤。
如果有必要請(qǐng)檢查正常運(yùn)行的方式和輕質(zhì)潤(rùn)滑油。
每半年要做的事
更換冷卻劑和徹底清洗水箱中的冷卻劑。
檢查所有的軟管和潤(rùn)滑線路是否堵塞。
每年要做的事
更換變速箱油。從變速箱油排除廢油，然后慢慢填充2夸脫美孚DTE 25油。
檢查潤(rùn)滑油過(guò)濾器并清除過(guò)濾器底部殘?jiān)?
每2年更換控制箱中的空氣過(guò)濾器。
礦物乳化油將損壞在機(jī)器中的橡膠成分。
疑難解答
本節(jié)的目的是解決使用過(guò)程中已知的問(wèn)題。給出的解決方案是為了給個(gè)人提
供開(kāi)放式CNC模式服務(wù)，首先，確定問(wèn)題的來(lái)源，第二，解決問(wèn)題。
利用常識(shí)
許多問(wèn)題通過(guò)正確的評(píng)估很容易得到解決。所有的機(jī)器都是由程序，機(jī)身與
工作部件構(gòu)成。遇到故障時(shí)你必須檢查這三個(gè)部分中的任何一個(gè)。不要指望機(jī)器能過(guò)改正由于一個(gè)小縫隙而導(dǎo)致的桿抖動(dòng)。
不要懷疑機(jī)床的準(zhǔn)確性在臺(tái)鉗彎曲的部分。如果你不首先中心鉆孔那么孔的中心定位就達(dá)不到要求。
首先發(fā)現(xiàn)問(wèn)題
許多技工在搞清問(wèn)題之前把事情搞砸了，所以他們希望回到過(guò)去。我們所知道的事實(shí)是，超過(guò)一半的零件修理之后仍能保持良好的工作狀態(tài)。如果主軸不轉(zhuǎn)，記住，主軸連接到齒輪箱，它是由主軸驅(qū)動(dòng)器連接到主軸電機(jī)，它連接到I / O板，這是都是由處理器驅(qū)動(dòng)。如果傳送帶是壞的，那么它就不能為主驅(qū)動(dòng)器傳動(dòng)。首先發(fā)現(xiàn)問(wèn)題，不要去只想到簡(jiǎn)單的問(wèn)題。
不要擺弄機(jī)器
你可以在機(jī)器上更改參數(shù)，電線和開(kāi)關(guān)等。不要隨意更改啟動(dòng)部件和參數(shù)。記住，如果你想更改什么，但沒(méi)有正確改動(dòng)那么你會(huì)弄壞其他東西。認(rèn)真考慮改變處理器參數(shù)的時(shí)間。首先，你必須下載所有參數(shù)，刪除12個(gè)連接器，取代主機(jī)，重新連接并重新加載，如果你犯了一個(gè)錯(cuò)誤那么它將無(wú)法工作。不管什么時(shí)候你在操作機(jī)器時(shí)你都應(yīng)該考慮可能發(fā)生意外會(huì)使機(jī)器損壞。檢查你認(rèn)為有問(wèn)題的部位，這是個(gè)廉價(jià)的保險(xiǎn)措施。機(jī)器工作越好那么你做的工作就越少。
附件2：外文原文（復(fù)印件）
CNC machine tools
Outrata, J. and Jowe, J.
While the specific intention and application for CNC machines vary from one machine type to another, all forms of CNC have common benefits. Here are but a few of the more important benefits offered by CNC equipment.
The first benefit offered by all forms of CNC machine tools is improved automation.
The operator intervention related to producing workpieces can be reduced or eliminated.Many CNC machines can run unattended during their entire machining cycle, freeing the operator to do other tasks. This gives the CNC user several side benefits including reduced operator fatigue, fewer mistakes caused by human error, and consistent and predictable machining time for each workpiece. Since the machine will be running under program control, the skill level required of the CNC operator (related to basic machining practice) is also reduced as compared to a machinist producing workpieces with conventional machine tools.
The second major benefit of CNC technology is consistent and accurate workpieces.Today's CNC machines boast almost unbelievable accuracy and repeatability specifications. This means that once a program is verified, two, ten, or one thousand identical workpieces can be easily produced with precision and consistency.
A third benefit offered by most forms of CNC machine tools is flexibility. Since these machines are run from programs, running a different workpiece is almost as easy as loading a different program. Once a program has been verified and executed for one production run, it can be easily recalled the next time the workpiece is to be run. This leads to yet another benefit, fast change over. Since these machines are very easy to set up and run, and since programs can be easily loaded, they allow very short setup time.
This is imperative with today's just-in-time (JIT) product requirements.
Motion control - the heart of CNC
The most basic function of any CNC machine is automatic, precise, and consistent motion control. Rather than applying completely mechanical devices to cause motion as is required on most conventional machine tools, CNC machines allow motion control in a
revolutionary manner2. All forms of CNC equipment have two or more directions of motion,called axes. These axes can be precisely and automatically positioned along their lengths of travel. The two most common axis types are linear (driven along a straight path) and rotary (driven along a circular path).
Instead of causing motion by turning cranks and handwheels as is required on conventional machine tools, CNC machines allow motions to be commanded through programmed commands. Generally speaking, the motion type (rapid, linear, and circular),the axes to move, the amount of motion and the motion rate (feedrate) are programmable with almost all CNC machine tools.
A CNC command executed within the control tells the drive motor to rotate a precise number of times. The rotation of the drive motor in turn rotates the ball screw. And the ball screw drives the linear axis (slide). A feedback device (linear scale) on the slide allows the control to confirm that the commanded number of rotations has taken place3. Refer to fig.1.
Though a rather crude analogy, the same basic linear motion can be found on a common table vise. As you rotate the vise crank, you rotate a lead screw that, in turn,drives the movable jaw on the vise. By comparison, a linear axis on a CNC machine tool is extremely precise. The number of revolutions of the axis drive motor precisely controls the amount of linear motion along the axis.
How axis motion is commanded - understanding coordinate systems
It would be infeasible for the CNC user to cause axis motion by trying to tell each axis drive motor how many times to rotate in order to command a given linear motion amount4.(This would be like having to figure out how many turns of the handle on a table vise will cause the movable jaw to move exactly one inch!) Instead, all CNC controls allow axis motion to be commanded in a much simpler and more logical way by utilizing some form
of coordinate system. The two most popular coordinate systems used with CNC machines are the rectangular coordinate system and the polar coordinate system. By far, the more popular of these two is the rectangular coordinate system.
The program zero point establishes the point of reference for motion commands in a CNC program. This allows the programmer to specify movements from a common location. If program zero is chosen wisely, usually coordinates needed for the program can be taken directly from the print.
With this technique, if the programmer wishes the tool to be sent to a position one inch to the right of the program zero point, X1.0 is commanded. If the programmer wishes the tool to move to a position one inch above the program zero point, Y1.0 is commanded. The control will automatically determine how many times to rotate each axis drive motor and ball screw to make the axis reach the commanded destination point. This lets the programmer command axis motion in a very logical manner. Refer to fig.2, 3.
Understanding absolute versus incremental motion
All discussions to this point assume that the absolute mode of programming is used6.The most common CNC word used to designate the absolute mode is G90. In the absolute mode, the end points for all motions will be specified from the program zero point. For beginners, this is usually the best and easiest method of specifying end points for motion commands. However, there is another way of specifying end points for axis motion.
In the incremental mode (commonly specified by G91), end points for motions are specified from the tool's current position, not from program zero. With this method of commanding motion, the programmer must always be asking "How far should I move the tool?" While there are times when the incremental mode can be very helpful, generally speaking, this is the more cumbersome and difficult method of specifying motion and beginners should concentrate on using the absolute mode.
Be careful when making motion commands. Beginners have the tendency to think incrementally. If working in the absolute mode (as beginners should), the programmer should always be asking "To what position should the tool be moved?" This position is relative to program zero, NOT from the tools current position.
Aside from making it very easy to determine the current position for any command, another benefit of working in the absolute mode has to do with mistakes made during motion commands. In the absolute mode, if a motion mistake is made in one command of the program, only one movement will be incorrect. On the other hand, if a mistake is made during incremental movements, all motions from the point of the mistake will also be
incorrect.
Assigning program zero
Keep in mind that the CNC control must be told the location of the program zero point by one means or another. How this is done varies dramatically from one CNC machine and control to another8. One (older) method is to assign program zero in the program. With this method, the programmer tells the control how far it is from the program zero point to the starting position of the machine. This is commonly done with a G92 (or G50) command at least at the beginning of the program and possibly at the beginning of each tool.
Another, newer and better way to assign program zero is through some form of offset. Refer to fig.4. Commonly machining center control manufacturers call offsets used to assign program zero fixture offsets. Turning center manufacturers commonly call offsets used to assign program zero for each tool geometry offsets.
Flexible manufacturing cells
A flexible manufacturing cell (FMC) can be considered as a flexible manufacturing subsystem. The following differences exist between the FMC and the FMS:
1. An FMC is not under the direct control of the central computer. Instead, instructions from the central computer are passed to the cell controller.
2. The cell is limited in the number of part families it can manufacture.
The following elements are normally found in an FMC:
? Cell controller
? Programmable logic controller (PLC)
? More than one machine tool
? A materials handling device (robot or pallet) The FMC executes fixed machining operations with parts flowing sequentially between operations.
High speed machining
The term High Speed Machining (HSM) commonly refers to end milling at high rotational speeds and high surface feeds. For instance, the routing of pockets in aluminum airframe sections with a very high material removal rate1. Refer to fig.5 for the cutting data designations and for mulas. Over the past 60 years, HSM has been applied to a wide range of metallic and non-metallic workpiece materials, including the production of components with specific surface topography requirements and machining of materials
with hardness of 50 HRC and above. With most steel components hardened to approximately 32-42 HRC, machining options currently include:
rough machining and semi-finish l in its soft (annealed) condition
heat treatment to achieve the final required hardness = 63 HRC machining of electrodes and Electrical Discharge Machining (EDM) of specific parts of dies and moulds (specifically small radii and deep cavities with limited accessibility for metal cutting tools) finishing and super-finishing of cylindrical/flat/cavity surfaces with appropriate cemented carbide, cermet, solid carbide, mixed ceramic or polycrystalline cubic boron nitride (PCBN).
For many components, the production process involves a combination of these options and in the case of dies and moulds it also includes time consuming hand finishing .Consequently, production costs can be high and lead times excessive.
The main criteria is the quality level of the die or mould regarding dimensional geometric and surface accuracy. If the quality level after machining is poor and if it cannot meet the requirements, there will be a varying need of manual finishing work. This work produces satisfactory surface accuracy, but it always has a negative impact on the dimensional and geometric accuracy.
One of the main aims for the die and mould industry has been, and still is, to reduce Or eIiminate the need for manual polishing and thus improve the quality and shorten the production costs and lead times.
Survival
The ever increasing competition in the marketplace is continually setting new standards. The demands on time and cost efficiency is getting higher and higher. This has forced the development of new processes and production techniques to take place. HSM provides hope and solutions...
Materials
The development of new, more difficult to machine materials has underlined the necessity to find new machining solutions. The aerospace industry has its heat resistant and stainless steel alloys. The automotive industry has different bimetal compositions Compact Graphite Iron and an ever increasing volume of aluminum3. The die and mould industry mainly has to face the problem of machining high hardened tool steels, from roughing to finishing.
Processes
The demands on shorter throughput times via fewer setups and simplified flows
(logistics) can in most cases, be solved by HSM. A typical target within the die and mould industry is to completely machine fully hardened small sized tools in one setup. Costly and time consuming EDM processes can also be reduced or eliminated with HSM.
Design & development
One of the main tools in today's competition is to sell products on the value of novelty. The average product life cycle on cars today is 4 years, computers and accessories 1.5 years, hand phones 3 months... One of the prerequisites of this development of fast design changes and rapid product development time is the HSM technique.
Complex products
There is an increase of multi-functional surfaces on components, such as new design of turbine blades giving new and optimized functions and features. Earlier designs allowed polishing by hand or with robots (manipulators). Turbine blades with new, more sophisticated designs have to be finished via machining and preferably by HSM . There are also more and more examples of thin walled workpieces that have to be machined medical equipment, electronics, defence products, computer parts).
Production equipment
The strong development of cutting materials, holding tools, machine tools, controls And especially CAD/CAM features and equipment, has opened possibilities that must be met with new production methods and techniques5.
Definition of HSM
Salomon's theory, Machining with high cutting speeds..." on which, in 1931, took out A German patent, assumes that "at a certain cutting speed (5-10 times higher than in conventional machining), the chip removal temperature at the cutting edge will start to decrease...".See fig.6.
Given the conclusion:" ... seems to give a chance to improve productivity in machining with conventional tools at high cutting speeds..."
Modern research, unfortunately, has not been able to verify this theory totally. There Is a relative decrease of the temperature at the cutting edge that starts at certain cutting speeds for different materials.
The decrease is small for steel and cast iron. But larger for aluminum and other non-ferrous metals. The definition of HSM must be based on other factors.
Given today's technology, "high speed" is generally accepted to mean
Surface speeds between 1 and 10 kilometers perminute, or roughly 3 300 to 33 000 feet per minute. Speeds above 10 km/min are in the ultra-high speed category, and are largely the realm of experimental metal cutting. Obviously, the spindle rotations required to achieve these surface cutting speeds are directly related to the diameter of the tools being used.One trend which i