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1 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(guó) 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(kāi)始 高速切削的試驗(yàn)又開(kāi)始進(jìn)入各種試驗(yàn)研究 高速切削的機(jī)理開(kāi)始被 科學(xué)家們所認(rèn)識(shí) 1979 年開(kāi)始由德國(guó)政府研究技術(shù)部資助 德國(guó) 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(kāi)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(guó)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(guò)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 2 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)減小 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(lái)越高的要求 同時(shí) 解決了三維曲面形狀高效精密加工問(wèn)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(guò)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 3 m 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 3 晶粒尺寸平均為 10 m 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(guó) 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?好地保持刃口形狀 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(lái) Kyocera 公司用 TiN 來(lái)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 1 m 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) 4 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(zhǎng)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 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(guò)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(guò)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(guò) 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 5 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 6 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(zhǎng)度的影響 可見(jiàn)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(jiàn)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(guò)多的刀刃會(huì)減少容 屑空間 容易引起切屑粘刀 為避開(kāi)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)?TiAlN 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(rùn)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?gòu)较蚝洼S向跳動(dòng) 控制動(dòng)平衡精度 與機(jī)床聯(lián)接普遍 7 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(guò)盈 配合夾緊 這種結(jié)構(gòu)刀具的徑向跳動(dòng)在 4 m 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(rùn)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(guò)渡段 盡可能減少速度的急劇變化 粗加 工不是簡(jiǎn)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(guò)仿真 驗(yàn)證 刀位數(shù)據(jù)的正確性 刀具各部位是否與零件發(fā)生干涉 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(wú)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 8 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(rùn)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(guò)程的溫度變化 產(chǎn)生 熱疲勞 降低刀具壽命和可靠性 現(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(wèn)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(rùn)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(yǔ) 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(guó)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(guó)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(zhēng)能力 刀具的 競(jìng)爭(zhēng)力應(yīng)集中在高性能帶來(lái)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(guó)目前的實(shí)際情況 建議重點(diǎn)發(fā)展涂層技術(shù) 如 9 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(kù) 附錄 外文文獻(xiàn) 10 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 11 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 Electric discharge machining Laser cutting 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 12 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 13 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 The actual NC tool 14 The machine control unit MCU The communication interface between the NC machine and the MCU 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 15 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 16 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 machine is needed Pros and Cons in Justifying NC Justifying a conversion from manual to NC machines and processes can be difficult From a strictly business perspective such a conversion entails a great deal of risk primarily centering around the fact that manually operated machines use fewer consumable tools per hour of operation but they also require more hours to produce