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專題：
高速切削的刀具材料及切削技術(shù)的應(yīng)用
一、前言
???高速切削的研究歷史，可以追溯到二十世紀(jì)30年代由德國Carl Salomon博士首次提出的有關(guān)高速切削的概念。Salomon博士的研究突破了傳統(tǒng)切削理論對(duì)切削熱的認(rèn)識(shí)，認(rèn)為切削熱只是在傳統(tǒng)切削速度范圍內(nèi)是與切削速度成單調(diào)增函數(shù)關(guān)系。而當(dāng)切削速度突破一定限度以后，切削溫度不再隨切削速度的增加而增加，反而會(huì)隨切削速度的增加而降低，即與切削速度在較高速度的范圍內(nèi)成單調(diào)減函數(shù)。Salomon博士的研究因第二次世界大戰(zhàn)而中斷。50年代后期開始，高速切削的試驗(yàn)又開始進(jìn)入各種試驗(yàn)研究，高速切削的機(jī)理開始被科學(xué)家們所認(rèn)識(shí)。1979年開始由德國政府研究技術(shù)部資助、德國Darmstadt大學(xué)PTW研究所牽頭、由大學(xué)研究機(jī)構(gòu)、機(jī)床制造商、刀具制造商、用戶等多方面共同組成的研究團(tuán)隊(duì)對(duì)高速銑削展開了系統(tǒng)的研究。除了高速切削機(jī)理外，研究團(tuán)隊(duì)同步研究解決高速銑削中機(jī)床、刀具、工藝參數(shù)等多方面的應(yīng)用解決方案，使高速銑削在加工機(jī)理尚未得到完全共識(shí)的情況下首先在鋁合金加工和硬材料加工等領(lǐng)域得到應(yīng)用，解決模具、汽車、航空等領(lǐng)域的加工需求，從而取得了巨大的經(jīng)濟(jì)效益。
??? 根據(jù)1992年國際生產(chǎn)工程研究會(huì)(CIRP)年會(huì)主題報(bào)告的定義，高速切削通常指切削速度超過傳統(tǒng)切削速度5－10倍的切削加工。因此，根據(jù)加工材料的不同和加工方式的不同，高速切削的切削速度范圍也不同。高速切削包括高速銑削、高速車削、高速鉆孔與高速車銑等，但絕大部分應(yīng)用是高速銑削。目前，加工鋁合金已達(dá)到2000－7500m/min；鑄鐵為900－5000m/min；鋼為600－3000m/min；耐熱鎳基合金達(dá)500m/min；鈦合金達(dá)150－1000m/min；纖維增強(qiáng)塑料為2000－9000m/min。
二、高速切削的特點(diǎn)
實(shí)踐表明，高速切削具有以下加工特點(diǎn)：
切削力降低；
工件熱變形減少；
有利于保證零件的尺寸、形位精度；
已加工表面質(zhì)量高；
工藝系統(tǒng)振動(dòng)減??；
顯著提高材料切除率；
加工成本降低；
??? 高速切削的上述特點(diǎn)，反映了在其適用領(lǐng)域內(nèi)，能夠滿足效率、質(zhì)量和成本越來越高的要求，同時(shí)，解決了三維曲面形狀高效精密加工問題，并為硬材料和薄壁件加工提供了新的解決方案。
三、高速切削加工刀具材料選用
??? 鋁合金
??? 易切削鋁合金
??? 該材料在航空航天工業(yè)應(yīng)用較多，適用的刀具有K10、K20、PCD，切削速度在2000～4000m/min，進(jìn)給量在3～12m/min，刀具前角為12°～18°，后角為10°～18°，刃傾角可達(dá)25°。
??? 鑄鋁合金
??? 鑄鋁合金根據(jù)其Si含量的不同，選用的刀具也不同，對(duì)Si含量小于12%的鑄鋁合金可采用K10、Si3N4刀具，當(dāng)Si含量大于12%時(shí)，可采用PKD(人造金剛石)、PCD(聚晶金剛石)及CVD金剛石涂層刀具。對(duì)于Si含量達(dá)16%～18%的過硅鋁合金，最好采用PCD或CVD金剛石涂層刀具，其切削速度可在1100m/min，進(jìn)給量為0.125mm/r。
??? 鑄鐵
??? 對(duì)鑄件，切削速度大于350m/min時(shí)，稱為高速加工，切削速度對(duì)刀具的選用有較大影響。當(dāng)切削速度低于750m/min時(shí)，可選用涂層硬質(zhì)合金、金屬陶瓷；切削速度在510～2000m/min時(shí)，可選用Si3N4陶瓷刀具；切削速度在2000～4500m/min時(shí)，可使用CBN刀具。
鑄件的金相組織對(duì)高速切削刀具的選用有一定影響，加工以珠光體為主的鑄件在切削速度大于500m/min時(shí)，可使用CBN或Si3N4，當(dāng)以鐵素體為主時(shí)，由于擴(kuò)散磨損的原因，使刀具磨損嚴(yán)重，不宜使用CBN，而應(yīng)采用陶瓷刀具。如粘結(jié)相為金屬Co，晶粒尺寸平均為3μm，CBN含量大于90%～95%的BZN6000在V=700m/min時(shí)，宜加工高鐵素體含量的灰鑄鐵。粘結(jié)相為陶瓷(AlN＋AlB2)、晶粒尺寸平均為10μm、CBN含量為90%～95%的Amborite刀片，在加工高珠光體含量的灰鑄鐵時(shí)，在切削速度小于1100m/min時(shí)，隨切削速度的增加，刀具壽命也增加。
??? 普通鋼
??? 切削速度對(duì)鋼的表面質(zhì)量有較大的影響，根據(jù)德國Darmstadt大學(xué)PTW所的研究，其最佳切削速度為500～800m/min。
??? 目前，涂層硬質(zhì)合金、金屬陶瓷、非金屬陶瓷、CBN刀具均可作為高速切削鋼件的刀具材料。其中涂層硬質(zhì)合金可用切削液。用PVD涂層方法生產(chǎn)的TiN涂層刀具其耐磨性能比用CVD涂層法生產(chǎn)的涂層刀具要好，因?yàn)榍罢呖珊芎玫乇３秩锌谛螤?，使加工零件獲得較高的精度和表面質(zhì)量。
金屬陶瓷刀具占日本刀具市場(chǎng)的30%，以TiC-Ni-Mo為基體的金屬陶瓷化學(xué)穩(wěn)定性好，但抗彎強(qiáng)度及導(dǎo)熱性差，適于切削速度在400～800m/min的小進(jìn)給量、小切深的精加工；Carboly公司用TiCN作為基體、結(jié)合劑中少鉬多鎢的金屬陶瓷將強(qiáng)度和耐磨兩者結(jié)合起來，Kyocera公司用TiN來增加金屬陶瓷的韌性，其加工鋼或鑄鐵的切深可達(dá)2～3mm。CBN可用于銑削含有微量或不含鐵素體組織的軸承鋼或淬硬鋼。
??? 高硬度鋼
??? 高硬度鋼(HRC40～70)的高速切削刀具可用金屬陶瓷、陶瓷、TiC涂層硬質(zhì)合金、PCBN等。
??? 金屬陶瓷可用基本成分為TiC添加TiN的金屬陶瓷，其硬度和斷裂韌性與硬質(zhì)合金大致相當(dāng)，而導(dǎo)熱系數(shù)不到硬質(zhì)合金的1/10，并具有優(yōu)異的耐氧化性、抗粘結(jié)性和耐磨性。另外其高溫下機(jī)械性能好，與鋼的親和力小，適合于中高速(在200m/min左右)的模具鋼SKD加工。金屬陶瓷尤其適合于切槽加工。
??? 采用陶瓷刀具可切削硬度達(dá)HRC63的工件材料，如進(jìn)行工件淬火后再切削，實(shí)現(xiàn)“以切代磨”。切削淬火硬度達(dá)HRC48～58的45鋼時(shí)，切削速度可取150～180m/min，進(jìn)給量在0.3～0.4min/r，切深可取2～4mm。粒度在1μm，TiC含量在20%～30%的Al2O3-TiC陶瓷刀具，在切削速度為100m/min左右時(shí)，可用于加工具有較高抗剝落性能的高硬度鋼。
??? 當(dāng)切削速度高于1000m/min時(shí)，PCBN是最佳刀具材料，CBN含量大于90%的PCBN刀具適合加工淬硬工具鋼(如HRC55的H13工具鋼)。
??? 高溫鎳基合金
??? Inconel 718鎳基合金是典型的難加工材料，具有較高的高溫強(qiáng)度、動(dòng)態(tài)剪切強(qiáng)度，熱擴(kuò)散系數(shù)較小，切削時(shí)易產(chǎn)生加工硬化，這將導(dǎo)致刀具切削區(qū)溫度高、磨損速度加快。高速切削該合金時(shí)，主要使用陶瓷和CBN刀具。
??? 碳化硅晶須增強(qiáng)氧化鋁陶瓷在100～300m/min時(shí)可獲得較長的刀具壽命，切削速度高于500m/min時(shí)，添加TiC氧化鋁陶瓷刀具磨損較小，而在100～300m/min時(shí)其缺口磨損較大。氮化硅陶瓷(Si3N4)也可用于Inconel 718合金的加工。
??? 加拿大學(xué)者M(jìn).A.Elbestawi認(rèn)為，SiC晶須增強(qiáng)陶瓷加工Inconel 718的最佳切削條件為：切削速度700m/min，切深為1～2mm，進(jìn)給量為0.1～0.18mm/z。
??? 氮氧化硅鋁(Sialon)陶瓷韌性很高，適合于切削過固溶處理的Inconel 718(HRC45)合金，Al2O3-SiC晶須增強(qiáng)陶瓷適合于加工硬度低的鎳基合金。
??? 鈦合金(Ti6Al6V2Sn)
??? 鈦合金強(qiáng)度、沖擊韌性大，硬度稍低于Inconel 718，但其加工硬化非常嚴(yán)重，故在切削加工時(shí)出現(xiàn)溫度高、刀具磨損嚴(yán)重的現(xiàn)象。日本學(xué)者T.Kitagawa等經(jīng)過大量實(shí)驗(yàn)得出，用直徑10mm的硬質(zhì)合金K10兩刃螺旋銑刀(螺旋角為30°)高速銑削鈦合金，可達(dá)到滿意的刀具壽命，切削速度可高達(dá)628m/min，每齒進(jìn)給量可取0.06～0.12mm/z，連續(xù)高速車削鈦合金的切削速度不宜超過200m/min。
??? 復(fù)合材料
??? 航天用的先進(jìn)復(fù)合材料(如Kevlar和石墨類復(fù)合材料)，以往用硬質(zhì)合金和PCD，硬質(zhì)合金的切削速度受到限制，而在900℃以上高溫下PCD刀片與硬質(zhì)合金或高速鋼刀體焊接處熔化，用陶瓷刀具則可實(shí)現(xiàn)300m/min左右的高速切削。
四、高速切削刀具技術(shù)
??? 高速切削刀具不僅在耐用度和可靠性方面比常規(guī)加工有更高的要求，在刀具系統(tǒng)的安全性方面也有特殊的要求。
圖3 刀具伸出量對(duì)耐用度的影響
從提高耐用度和可靠性角度，需要考慮：
刀具基體與涂層材料
刀尖幾何結(jié)構(gòu)
刀刃數(shù)和刀桿伸出量
切削用量
走刀方式
冷卻條件
刀具與工件材料匹配從提高使用安全性方面，需要考慮：
刀具系統(tǒng)強(qiáng)度與尺寸
刀桿與機(jī)床的夾持方式
刀片夾緊方式
刀具動(dòng)平衡
圖4 球頭鐵刀不同銑削方式對(duì)耐用度的影響
??? 由于高速切削高轉(zhuǎn)速和快進(jìn)給等特點(diǎn)，除了良好的耐磨性和高的強(qiáng)度韌性的先進(jìn)刀具材料，優(yōu)良的刀具涂層技術(shù)，合理的幾何結(jié)構(gòu)參數(shù)和高同心度的刀刃精度質(zhì)量等因素外，還需特別注意其它因素對(duì)刀具耐用度的影響。圖3為不同刀具伸出量對(duì)切削路徑長度的影響，可見伸出量越短，耐用度越高。一般情況下，順銑的耐用度高于逆銑，而往復(fù)銑的耐用度最低(見圖4)。圖4中向下進(jìn)實(shí)際反映刀具頂著進(jìn)給方向進(jìn)刀，而向上進(jìn)反映刀具拖著進(jìn)給方向進(jìn)刀，對(duì)耐用度也有較大影響。鋁合金高速銑削通常用雙刃銑刀，過多的刀刃會(huì)減少容屑空間，容易引起切屑粘刀。為避開共振頻率，也可采用三刃銑刀以增加沖擊頻率。鋁合金加工容易產(chǎn)生積屑瘤，這對(duì)高速銑削非常有害。要減少積屑瘤的產(chǎn)生，刀具表面要平滑；避免采用物理氣相沉積(PVD)涂層刀具，因?yàn)門iAlN涂層很易與鋁產(chǎn)生化學(xué)反應(yīng)，可以選用非涂層刀具，細(xì)晶金剛石涂層或類金剛石涂層刀具；如有可能，盡量采用油霧刀具內(nèi)冷進(jìn)行冷卻潤滑。
??? 高速銑削刀具結(jié)構(gòu)對(duì)刀具耐用度和安全性均有很大影響，關(guān)鍵要點(diǎn)包括刀具系統(tǒng)的平衡設(shè)計(jì)；減少徑向和軸向跳動(dòng)；控制動(dòng)平衡精度；與機(jī)床聯(lián)接普遍采用HSK刀柄或類似雙面接觸短錐刀柄；刀具的夾緊最新趨勢(shì)是采用冷縮式夾緊結(jié)構(gòu)(或稱熱裝式)，裝夾時(shí)利用感應(yīng)或熱風(fēng)加熱使刀桿孔膨脹，取出舊刀具，裝入新刀具，然后采用風(fēng)冷使刀具冷卻到室溫，利用刀桿孔與刀具外徑的過盈配合夾緊，這種結(jié)構(gòu)刀具的徑向跳動(dòng)在4μm，剛性高，動(dòng)平衡性好，夾緊力大，高轉(zhuǎn)速下仍能保持高的夾緊可靠性，特別適用于更高轉(zhuǎn)速的高速銑削加工。
五、高速切削工藝技術(shù)
??? 高速切削工藝主要包括：適合高速切削的加工走刀方式，專門的CAD/CAM編程策略，優(yōu)化的高速加工參數(shù)，充分冷卻潤滑并具有環(huán)保特性的冷卻方式等等。
??? 高速切削的加工方式原則上多采用分層環(huán)切加工。直接垂直向下進(jìn)刀極易出現(xiàn)崩刃現(xiàn)象，不宜采用。斜線軌跡進(jìn)刀方式的銑削力是逐漸加大的，因此對(duì)刀具和主軸的沖擊比垂直下刀小，可明顯減少下刀崩刃的現(xiàn)象。螺旋式軌跡進(jìn)刀方式采用螺旋向下切入，最適合型腔高速加工的需要。
??? CAD/CAM編程原則是盡可能保持恒定的刀具載荷，把進(jìn)結(jié)速率變化降到最低，使程序處理速度最大化。主要方法有：盡可能減少程序塊，提高程序處理速度；在程序段中可加人一些圓弧過渡段，盡可能減少速度的急劇變化；粗加工不是簡單的去除材料，要注意保證本工序和后續(xù)工序加工余量均勻，盡可能減少銑削負(fù)荷的變化；多采用分層順銑方式；切入和切出盡量采用連續(xù)的螺旋和圓弧軌跡進(jìn)行切向進(jìn)刀，以保證恒定的切削條件；充分利用數(shù)控系統(tǒng)提供的仿真驗(yàn)證的功能。零件在加工前必須經(jīng)過仿真，驗(yàn)證①刀位數(shù)據(jù)的正確性，②刀具各部位是否與零件發(fā)生干涉，③刀具與夾具附件是否發(fā)生碰撞，確保產(chǎn)品質(zhì)量和操作安全。
??? 高速銑削加工用量的確定主要考慮加工效率、加工表面質(zhì)量、刀具磨損以及加工成本。不同刀具加工不同工件材料時(shí)，加工用量會(huì)有很大差異，目前尚無完整的加工數(shù)據(jù)。通常，隨著切削速度的提高，加工效率提高，刀具磨損加劇，除較高的每齒進(jìn)給量外，加工表面粗糙度隨切削速度提高而降低。對(duì)于刀具壽命，每齒進(jìn)給量和軸向切深均存在最佳值，而且最佳值的范圍相對(duì)較窄。高速銑削參數(shù)一般的選擇原則是高的切削速度、中等的每齒進(jìn)給量fz、較小的軸向切深ap和適當(dāng)大的徑向切深ae。
??? 在高速銑削時(shí)由于金屬去除率和切削熱的增加，冷削介質(zhì)必須具備將切屑快速?zèng)_離工件、降低切削熱和增加切削界面潤滑的能力。常規(guī)的冷卻液及加注方式很難進(jìn)入加工區(qū)域，反而會(huì)加大銑刀刃在切入切出過程的溫度變化，產(chǎn)生熱疲勞，降低刀具壽命和可靠性?，F(xiàn)代刀具材料，如硬質(zhì)合金、涂層刀具、陶瓷和金屬陶瓷、CBN等具有較高的紅硬性，如果不能解決熱疲勞問題，可不使用冷卻液。
??? 微量油霧冷卻一方面可以減小刀具－切屑－工件之間的摩擦，另一方面細(xì)小的油霧粒子在接觸到刀具表面時(shí)快速氣化的換熱效果較冷卻液熱傳導(dǎo)的換熱效果方式能帶走更多的熱量，目前已成為高速切削首選的冷卻介質(zhì)。
??? 氮?dú)庥挽F冷卻介質(zhì)在鈦合金的高速銑削中取得了很好的效果。氮?dú)庥挽F冷卻介質(zhì)除具有空氣油霧的冷卻潤滑作用外，還具有抗氧化磨損等作用，在33m/min的銑削速度時(shí)，相比較空氣油霧冷卻，刀具耐用度提高60%，銑削力可降低20%－30%。
六、結(jié)語
??? 高速切削是一項(xiàng)先進(jìn)的、正在發(fā)展的綜合技術(shù)，必須將高性能的高速切削機(jī)床、與工件材料相適應(yīng)的刀具和對(duì)于具體加工對(duì)象最佳的加工工藝技術(shù)相結(jié)合，充分發(fā)揮高速切削技術(shù)的優(yōu)勢(shì)。高速切削工具技術(shù)也是一項(xiàng)關(guān)鍵技術(shù)，為了適應(yīng)和推動(dòng)我國高速切削技術(shù)的發(fā)展，我們應(yīng)該充分認(rèn)識(shí)到，工具制造是一個(gè)高技術(shù)含量的行業(yè)，應(yīng)加強(qiáng)該領(lǐng)域的基礎(chǔ)研究、工程研究和應(yīng)用研究；迅速發(fā)展的高速切削技術(shù)極大的刺激高性能刀具的需求，我國工具行業(yè)應(yīng)重點(diǎn)在刀具的耐磨性、精度和可靠性方面加強(qiáng)研發(fā)力度，提高刀具的競(jìng)爭能力；刀具的競(jìng)爭力應(yīng)集中在高性能帶來的整體經(jīng)濟(jì)效益，在應(yīng)用領(lǐng)域推廣使用高性能刀具；提供個(gè)性化技術(shù)服務(wù)；根據(jù)我國目前的實(shí)際情況，建議重點(diǎn)發(fā)展涂層技術(shù)(如耐磨(硬、軟)涂層、復(fù)合涂層、納米結(jié)構(gòu)涂層等)，刀具質(zhì)量保障技術(shù)和刀具數(shù)據(jù)庫。
附錄Ⅰ:外文文獻(xiàn)
Numerical Control
One of the most fundamental concepts in the area of advanced manufacturing technologies is numerical control.
Controlling a machine tool using a punched tape or stored program is known as numerical control (NC). NC has been defined by the Electronic Industries Association (EIA) as “ a system in which actions are controlled by the direct insertion of numerical dada at some point .the system must automatically interpret at least some portion of this data.” the numerical data required to produce a part is known as a part program..
A numerical control machine tool system contains a machine control unit (MCU) and the machine tool itself. The MCU is further divided into two elements: the data processing unit (DPU) and the control loops unit (CLU). The DPU processes the coded data from the tape or other media and passes information on the potions of each axis, required direction of motion, feed rate, and auxiliary function control signals to the CLU. The CLU operates the drive mechanisms of the machine, receives feed back signals concerning the actual position and velocity of each of the axes, and signals the completion of operation. The DPU sequentially reads the data. When each line has completed execution as noted by the CLU, anther line of data is read.
A data processing units consists of some or all of the following parts:
1) Data input device such as a paper tape reader, magnetic tape reader, RS232-C port, etc
2) Data-reading circuits and parity-checking logic
3) Decoding circuits for distributing data among the controlled axes
4) An interpolator, which supplies machine-motion commands between data points for tool motion
A control loops unit, on the other hand consists of the following:
1) Position control loops for all the axes of motion, where each axis has a separate control loop
2) Velocity control loops, where feed control is required
3) Deceleration and backlash take up circuits
4) Auxiliary functions control, such as coolant on/off, gear change, spindle on/off control
Geometric and kinematic data are typically fed from the DPU to the CLU.
The CLU then governs the physical system based on the data from the DPU.
Numerical control was developed to overcome the limitation of human operators, and it has done so. Numerical control machines are more accurate than manually operated machines, they can produce parts more uniformly, they are faster, and the long-run tooling costs are lower. The development of NC led to the development of several other innovations in manufacturing technology:
l Electric discharge machining
l Laser-cutting
l Electron beam welding
Numerical control has also made machine tools more versatile than their manually operated predecessors. An NC machine tool can automatically produce a wide variety of parts, each involving an assortment of widely varied and complex machining processes. Numerical control has allowed manufacturers to undertake the production of products that would not have been feasible from an economic perspective using manually controlled machine tools and processes.
Historical Development of NC
Like so many advanced technologies, NC was born in the laboratories of the Massachusetts Institute of Technology. The concept of NC was developed in the early 1950s with funding provided by the U.S. Air Force. In its earliest stages, NC machines were able to make straight cuts efficiently and effectively.
However, curved paths were a problem because the machine tool had to be programmed to undertake a series of horizontal and vertical steps to produce a curve. The shorter the straight lines making up the steps, the smoother is the curve. Each line segment in the steps shown in the close up in Fig.2.17 had to be calculated. This was a cumbersome approach that had to be overcome if NC was to develop further.
This problem led to the development in 1959 of the Automatically Programmed Tools (APT) language. This is a special programming language for NC that uses statements similar to English language to define the part geometry, describe the cutting tool configuration, and specify the necessary motions. The development of the APT language was a major step forward in the development of NC technology. The original NC systems were vastly different from those used today. The machines had hardwired logic circuits. The instructional programs were written on punched paper, which was later to be replaced by magnetic plastic tape. A tape reader was used to interpret the instructions written on the tape for the machine. Together, all of this represented a giant step forward in the control of machine tools. However, there were a number of problems with NC at this point in its development.
A major problem was the fragility of the punched paper tape medium. It was common for the paper containing the programmed instructions to break or tear during a machining process. This problem was exacerbated by the fact that each successive time a part was produced on a machine tool, the paper tape carrying the programmed instructions had to be rerun through the reader. If it was necessary to produce 100 copies of a given part, it was also necessary to run the paper tape through the reader 100 separate times. Fragile paper tapes simply could not withstand the rigors of a shot floor environment and this kind of repeated use.
This led to the development of a special magnetic plastic tape. Whereas the paper tape carried the programmed instructions as a series of holes punched in the tape, the plastic tape carried the instructions as a series of magnetic dots. The plastic tape was much stronger than the paper tape, which solved the problem of frequent tearing and breakage. However, it still left two other problems.
The most important of those was that it was difficult or impossible to change the instructions entered on the tape. To make even the most minor adjustments in a program of instructions, it was necessary to interrupt machining operations and make a new tape. It was also still necessary to run the tape through the reader as many times as there were parts to be produced. Fortunately, computer technology became a reality and soon solved the problem of NC associated with punched paper and plastic tape.
1) Advent of Direct Numerical Control
The development of a concept known as direct numerical control (DNC) solved the paper and plastic tape problems associated with numerical control by simply eliminating tape as the medium for carrying the programmed instructions. In direct numerical control, machine tools are tied, via a data transmission link, to a host computer (Fig 2.18). Programs for operating the machine tools are stored in the host computer and fed to the machine tool as needed via the data transmission linkage. Direct numerical control represented a major step forward over punched tape and plastic tape. However, it is subject to the same limitations as all technologies that depend on a host computer, the machine tools also experience downtime. This problem led to the development of computer numerical control.
2) Advent of Computer Numerical Control
The development of the microprocessor allowed for the development of programmable logic controllers (PLCs) and microcomputer. These two technologies allowed for the development of computer numerical control (CNC). With CNC, each machine tool has a PLC or a microcomputer that serves the purpose. This allows programs to be input and stored at each individual machine tool. It also allows programs to be developed off-line and downloaded at the individual machine tool. CNC solved the problems associated with downtime of the host computer, but it introduced another problem known as data management. This is a problem all work settings dependent on microcomputers have. The same program might be loaded on ten different microcomputers with no communication among them. This problem is the process of being solved by local area networks that connect microcomputers for better data management. The problem of data management led to the development of distributed numerical control.
3) Advent of Distributed Numerical Control
Distributed numerical control (also called DNC) takes advantage of the best aspects of direct numerical control and computer numerical control. With distributed numerical control there are both host computers and local computers at the individual machine tools (Fig 2.19). This allows the programs to be stored in the host computers and, thereby, better managed. However, it also allows them to be downloaded to local microcomputers or PLCs. It also allows for local input and interaction through microcomputers or PLCs at the machine levels.
NC Machine Components
There are four components in any NC machine:
l The actual NC tool
l The machine control unit (MCU)
l The communication interface between the NC machine and the MCU
l A variety of accessories for performing specific jobs on the NC machine
The actual NC machine may be a milling machine, lathe, drill, or any other type of machine tool.. The MCU is the control unit that holds the programs that instruct the NC machine. The MCU also has various devices available for operator input. Information contained in the MCU is carried to the activators on the NC machine through the communication interface. These activators receive the electronic signals from the MCU and cause the mechanical apparatus of the NC machine to operate.
Less sophisticated NC machines have open-loop activators. An open-loop activator can receive a signal and carry out the instructions contained in that signal, but cannot feed back to the MCU to show that instructions carried in the signal have been properly completed. More sophisticated NC machine use closed-loop activators. A closed-loop activator can receive and carry out a signal and feed data back to the MCU showing that the signal has been carried out and to what extent. The more sophisticated closed-loop systems are been used more and more because they allow closer monitoring and immediate corrective action when problems with executing a program arise. The accessories are special tools required to carry out a specific job.
NC Programming
These are four ways to program an NC machine: manual programming, digitizing, written programs, and graphic programs (Fig 2.20). Manual programming is the most cumbersome of the four. It involves calculating numerical values that identify tool location and specify tool direction. Once these values have been calculated, recorded and feed into the MCU. Digitizing is a process frequently used in computer-aided design and drafting, whereby a drawing of a part is traced electronically. As it is traced, the various points on the drawing are converted into X-Y coordinates and stored in the computer. Once the drawing has been completely traced, the stored X-Y coordinates define the part and can be fed to an NC machine to provide instructions on tool positioning and movement.
Written programs are similar to those developed for use with any computer. With such programs, English language-type statements are written to describe tool positions and movement, as well as speed and feed rates. Such programs are fed into the MCU, where they are translated into machine language and forwarded to the NC machine’s activators.
The most modern, sophisticated method of programming an NC machine is by using a three-dimensional model of the part to provide the data that guide the NC machine in producing the part. As NC technology continues to develop, this programming method will eventually be used more than any other.
Classifications of NC Machines
Numerical control machines are classified in different ways. An early method was to categorize them as being either point-to-point or continues-path machines. Point-to-point machines, as the name implies, move in a series of steps from one point to the next (Fig 2.21). Point-to-point machines are less sophisticated and less precise than continuous-path machines. Continuous-path machines move uniformly and evenly along the cutting path rather than through a series of horizontal and vertical steps. Such machines are more sophisticated and require move memory in the MCU than point-to-point machines. Fig 2.22 illustrates the type of cutting paths performed by continuous-path machine.
Anther way to classify NC machines is as positioning or contouring machines. Point-to-point machines are considered positioning machines. Continuous-path machines are considered contouring machines. Positioning machines have as few as two axes: the X axis and the Y axis. Contouring machines must have at least three axes: the X, Y and Z axes. Fig 2.23 illustrates the movements governed by the X, Y and Z axes. Note that X represents the longitudinal axes, Y the transverse axis, and Z the up-and-down or vertical axis. Fig 2.24 is a simply line diagram of a typical three-axis machine tool showing how movement is accomplished. On some machines, movement is accomplished by positioning the spindle, and thus the tool, longitudinally along the X axis, transversely long the Y axis, and vertically along the Z axis. The work-piece is affixed to the table. With other machines, both the spindle and table (thus the work-piece) can be moved.
Positioning machines work well for drilling applications. Milling operations are more likely to be contouring machines to allow for three-dimensional control.
Some of the more sophisticated positioning machines are able to accomplish angular cuts known as slopes. These are cuts that move across the quadrants formed by the intersection of the X and Y axes at angles other than 90 degree to either the X or Y axis (Fig 2.25). Slopes are generally imprecise and inaccurate. However, there are instances in which the ability to make angular cutting paths is important. In these cases, slopes can be an important feature, particularly where the cut surfaces do not have to mate with anther surface. When precise, accurate angular cutting paths must be made, a contouring