開題報(bào)告
動力刀架設(shè)計(jì)
開題報(bào)告
1、 綜述
1 研究的意義
目前，在世界機(jī)床制造和機(jī)械加工領(lǐng)域，復(fù)合加工技術(shù)是處于領(lǐng)先地位的技術(shù)。實(shí)現(xiàn)車銑復(fù)合加工，有許多可行方案，而臥式車床床身搭載轉(zhuǎn)塔動力刀架就是其多種方案中應(yīng)用最廣泛的一個(gè)。其中，數(shù)控轉(zhuǎn)塔刀架是實(shí)現(xiàn)復(fù)合加工的核心功能部件，不僅可以像普通刀架裝配多把普通車刀，并且能夠同時(shí)提供動力驅(qū)動動力刀具，完成銑、鉆、攻絲、鉸孔等加工。
在當(dāng)今市場單件小批量和快速生產(chǎn)的需求的刺激下，催生出了復(fù)合加工。在眾多領(lǐng)域中，車銑復(fù)合加工發(fā)展目前來說已經(jīng)具有一定規(guī)模。在一般的機(jī)械加工領(lǐng)域中,加工主要分為以下兩種方式：第一，工件轉(zhuǎn)動，這種加工方式是車削加工；第二，刀具轉(zhuǎn)動，這種加工方式是加工中心加工。這兩種加工方式的結(jié)合就是車銑復(fù)合加工,它既能夠完成車削功能,同時(shí)又能夠完成銑、鏜、鉆、鉸、擴(kuò)等功能。車銑復(fù)合加工不僅是機(jī)械加工的發(fā)展方向之一，同樣也是當(dāng)今世界機(jī)床技術(shù)發(fā)展的大潮流。銑刀旋轉(zhuǎn)、工件旋轉(zhuǎn)、銑刀軸向進(jìn)給和徑向進(jìn)給，在這四個(gè)車銑復(fù)合加工機(jī)床的基本運(yùn)動中，要完成這銑刀旋轉(zhuǎn)的動作就需要使用到動力刀架。在引入動力數(shù)控刀架后，車銑復(fù)合加工中心基本實(shí)現(xiàn)了如下幾個(gè)目標(biāo):
⑴縮短制造工藝鏈和物流長度;⑵減少裝夾次數(shù)以及工裝夾具數(shù)量;⑶減小使用占地面積，降低了成本。實(shí)現(xiàn)了在確保質(zhì)量的前提下，提高生產(chǎn)率和自動化的程度。
步入新世紀(jì)以來，我國的數(shù)控機(jī)床行業(yè)發(fā)展勢態(tài)愈發(fā)迅猛，主機(jī)技術(shù)向著高速、智能、復(fù)合、環(huán)保方向不斷發(fā)展。動力刀架作為數(shù)控機(jī)床及加工中心重要部件之一，它的性能優(yōu)劣關(guān)系到整臺機(jī)床的性能，甚至深刻影響著我國制造業(yè)整體水平的高低。當(dāng)前市場中，動力刀架技術(shù)已經(jīng)趨近成熟完善，國外很多著名刀架生產(chǎn)商都已設(shè)計(jì)研發(fā)并推廣出自己的系列化產(chǎn)品，國內(nèi)也已有較為完善的生產(chǎn)機(jī)制。但是絕大部分仍然是模仿國外的產(chǎn)品，缺乏自主創(chuàng)新能力，它與國外的先進(jìn)水平相比較，在精度等方面還存在著一定的差距，在一定程度上這會制約機(jī)床發(fā)展。所以，在動力數(shù)控刀架的設(shè)計(jì)上，提高反復(fù)多次定位的精度和降低刀架出現(xiàn)故障的概率意義重大。
2 研究的現(xiàn)狀
動力刀架最早出現(xiàn)于1980年。經(jīng)過30多年的發(fā)展，隨著伺服刀架的出現(xiàn)，動力刀架由最初的外加動力刀具驅(qū)動模塊的形式發(fā)展為現(xiàn)今伺服動力刀架。伺服刀架又分為雙伺服動力刀架以及單伺服動力刀架。
隨著車削中心和車銑復(fù)合機(jī)床模塊化設(shè)計(jì)的發(fā)展以及對功能部件性能參數(shù)和可靠性要求的逐漸提高，在單伺服動力刀架的基礎(chǔ)上衍生出下列動力刀架產(chǎn)品：
（1） 內(nèi)置電主軸的直驅(qū)動力刀具刀架結(jié)構(gòu)
這種動力刀架采用內(nèi)置的電主軸直接驅(qū)動動力刀具代替伺服電機(jī)通過傳動系統(tǒng)驅(qū)動刀具，是動力刀具的最大轉(zhuǎn)速得到了提升，可達(dá)到10000r/min。同時(shí)刀盤和動力刀座采用了BMT接口，提高了重復(fù)定位精度。此項(xiàng)技術(shù)最先由德國Sauter公司申請專利。
（2） 力矩電機(jī)直驅(qū)式動力刀架
這種動力刀架大幅度減少傳動系統(tǒng)的復(fù)雜性，省去大量零件，使刀架體積變小，性能與功能與原來的伺服電機(jī)驅(qū)動刀盤的動力刀架。但是因?yàn)楹喕藗鲃酉到y(tǒng)，因此可靠性大幅度提升。
（3） 帶Y軸、B軸的動力刀架
目前國外動力刀架的研究均已成熟，德國，日本，意大利，英國等發(fā)達(dá)國家均有成熟的產(chǎn)品運(yùn)用于生產(chǎn)。國外的生產(chǎn)商如德國肖特公司（sauter），意大利巴拉法蒂（baruffaldi）、杜普瑪?shù)峡耍╠uplomatic）等著名的生產(chǎn)商。
世界上最著名數(shù)控轉(zhuǎn)塔刀架生產(chǎn)企業(yè)是德國的Sauter公司和意大利Baruffaldi公司。
這兩家企業(yè)刀架的性能指標(biāo)如表1所示:
表1 國外廠家生產(chǎn)的刀架的技術(shù)參數(shù)
國內(nèi)的動力刀架技術(shù)仍處于發(fā)展階段，目前國內(nèi)動力刀架生產(chǎn)較為著名的生產(chǎn)商有：常州市宏達(dá)機(jī)床數(shù)控設(shè)備有限公司、常州市新墅數(shù)控設(shè)備有限公司、煙臺環(huán)球機(jī)床附件集團(tuán)有限公司、沈陽精誠數(shù)控機(jī)床附件廠。這些廠家中技術(shù)規(guī)格參數(shù)指標(biāo)較高的是沈陽機(jī)床數(shù)控刀架分廠和煙臺環(huán)球機(jī)床附件廠。表2是國內(nèi)兩個(gè)廠家生產(chǎn)的兩種型號的轉(zhuǎn)塔刀架的技術(shù)參數(shù)。
表2 國內(nèi)廠家的刀架技術(shù)參數(shù)
圖1 國內(nèi)產(chǎn)品 圖2 國外產(chǎn)品
圖1，圖2分別為國內(nèi)和國外生產(chǎn)的產(chǎn)品
從上面兩表的數(shù)據(jù)就可以看出國外生產(chǎn)的刀架轉(zhuǎn)位速度快，在實(shí)際加工時(shí)有較高的效率。45°轉(zhuǎn)位并加緊時(shí)間這項(xiàng)種煙臺環(huán)球機(jī)床附件廠生產(chǎn)的刀架的用時(shí)是德國肖特公司的一倍。而且兩家國內(nèi)生產(chǎn)廠家的技術(shù)還是從意大利Baruffaldi公司引進(jìn)的。由此可見，國內(nèi)的刀架生產(chǎn)技術(shù)與國外的先進(jìn)水平仍有很大差距。
3 已有成果
目前煙臺環(huán)球機(jī)床附件廠生產(chǎn)的動力刀架均有不錯(cuò)的性能具體參數(shù)見圖3
圖3 煙臺環(huán)球機(jī)床附件廠的動力刀架參數(shù)
目前，我校研究生在楊慶東教授的指導(dǎo)下也研制出了國內(nèi)首創(chuàng)的力矩電機(jī)直驅(qū)刀架。
力矩電機(jī)直驅(qū)刀架大幅度簡化了刀盤的傳動系統(tǒng)，使刀架體積大幅度減小，由于傳動系統(tǒng)的簡化，刀架的精度以及可靠性也得到了提升。
2、 研究內(nèi)容
1 研究方向
作為大學(xué)生的畢業(yè)設(shè)計(jì)對于動力刀架的設(shè)計(jì)以及研究，要緊跟國內(nèi)外先進(jìn)水平，在了解先進(jìn)動力刀架結(jié)構(gòu)的同時(shí)，設(shè)計(jì)出符合畢業(yè)設(shè)計(jì)要求的動力刀架。根據(jù)導(dǎo)師指導(dǎo)建議，設(shè)計(jì)單伺服電機(jī)驅(qū)動動力刀架。
2 研究內(nèi)容
1、 進(jìn)行數(shù)控車削中心動力刀架總體研究，并進(jìn)行整體布局設(shè)計(jì)；
2、 研究各種電機(jī)，為動力刀架選擇合適的電機(jī)；
3、 研究出一種動力切換的可行方案
4、設(shè)計(jì)刀架結(jié)構(gòu)、動力驅(qū)動，完成關(guān)鍵部件的設(shè)計(jì)計(jì)算；
5、完成動力刀架的二維設(shè)計(jì)和三維設(shè)計(jì) 。
3、 實(shí)現(xiàn)方法及預(yù)期目標(biāo)
基礎(chǔ)參數(shù)
動力刀架中心高 120mm
刀位 12
動力到頭最高速度 6000r/min
相鄰刀具轉(zhuǎn)位時(shí)間 0.2s
1 實(shí)施的初步方案
動力刀架基本結(jié)構(gòu)：（1）轉(zhuǎn)位驅(qū)動系統(tǒng)(2)動力驅(qū)動系統(tǒng)（3）冷卻裝置（4）精定位裝置（5）裝刀裝置（6）數(shù)控刀架換刀動作。
驅(qū)動方式方案
方案一：采用雙伺服電機(jī)驅(qū)動
雙伺服電機(jī)驅(qū)動的動力刀架是最基礎(chǔ)的動力刀架，技術(shù)成熟，成本相對其他刀架較小。簡圖如圖3
圖3 雙伺服動力刀架簡圖
采用兩個(gè)伺服電機(jī)驅(qū)動，伺服電機(jī)1通過液壓機(jī)構(gòu)驅(qū)動刀盤粗分度，再由端齒盤壓緊確定精確分度，伺服電機(jī)2通過齒輪傳動驅(qū)動動力刀具。
方案二：單伺服電機(jī)驅(qū)動動力刀架
簡圖如圖4
圖 4 單伺服電機(jī)驅(qū)動動力刀架簡圖
單伺服動力刀架只采用一個(gè)伺服電機(jī)來驅(qū)動刀盤轉(zhuǎn)動和動力刀具，減小了動力刀架的體積，節(jié)省了空間，減少了功率損失。同時(shí)比起雙伺服電機(jī)驅(qū)動，單電機(jī)驅(qū)動刀盤和動力刀具，有一個(gè)動力切換的問題，難度相對大一些。
方案三：力矩電機(jī)直驅(qū)刀架和電主軸動力模塊
簡圖如圖5
圖 5
力矩電機(jī)直驅(qū)刀架為我校自助研發(fā)的刀架，直驅(qū)刀盤沒有傳動系統(tǒng)精度更高，且電機(jī)在刀盤內(nèi)部，極大減小了刀盤的體積，但是這種技術(shù)還不太成熟，實(shí)現(xiàn)的難度較大。
方案四：力矩電機(jī)外置加上電主軸驅(qū)動動力刀具
簡圖如圖 6
圖 6
力矩電機(jī)外置為安裝動力模塊提供了空間，減小了方案三的難度，但同時(shí)也增大了整個(gè)刀架的體積。
根據(jù)導(dǎo)師的建議以及本人的能力水平?jīng)Q定采用方案二。采用液壓與齒輪的配合切換動力。單伺服電機(jī)驅(qū)動節(jié)省了較多空間，使刀架體積變小，且只采用一個(gè)電機(jī)，提高了功率利用率。
精定位機(jī)構(gòu)
刀具在切削時(shí)需要很高的剛性和定位精度，因此刀架選用端齒盤做精定位元件。如圖 7
圖 7
夾緊機(jī)構(gòu)
壓直接鎖緊機(jī)構(gòu)是由液壓油紅直接控制動齒盤的松開和鎖緊,刀架結(jié)構(gòu)簡單且刀盤鎖緊力大,縮短了傳動鏈,提高了傳動效率。為了完成快速剎緊和得到大的剎緊力,松幵和剎緊一般選用液壓和機(jī)械等來實(shí)現(xiàn)。這種機(jī)液結(jié)合的方法可以簡化刀架的結(jié)構(gòu),能夠迅速換向,傳動平穩(wěn),縮短換刀的時(shí)間,數(shù)控刀架執(zhí)行動作簡單且刀盤鎖緊力大,提高刀架的分度速度外,同時(shí)也使刀具壽命得到提高。
裝刀裝置
裝刀裝置包括刀盤、刀夾及夾刀裝置。目前刀盤有2種模式:歐式VDI刀盤和日式
槽刀盤。
動力刀座選擇
目前在市場上有多種動力刀座如：DIN1809接口類型、DIN5480與DIN5480P接口類型、DIN5482接口類型和梅花式接口類型。
各種類型接口如圖4
圖4各種動力刀座接口
各種接口特點(diǎn)如表3
表3
在此畢業(yè)設(shè)計(jì)中選擇DIN1809接口作為動力刀座的接口。
2 重點(diǎn)、難點(diǎn)
1) 伺服電機(jī)的選擇
2)液壓機(jī)構(gòu)及控制系統(tǒng)的結(jié)構(gòu)設(shè)計(jì)
3)刀盤的旋轉(zhuǎn)定位與精確定位
4）輪系的設(shè)計(jì)
5）整個(gè)刀塔系統(tǒng)的運(yùn)動仿真。
6）單伺服電機(jī)驅(qū)動刀盤和動力刀具的協(xié)調(diào)問題
3 擬解決方案
1.參考機(jī)床動力學(xué)常規(guī)建模方式進(jìn)行建模，然后根據(jù)方程進(jìn)行動力學(xué)仿真。然后根據(jù)仿
真結(jié)果，再重新優(yōu)化建模方式。
2.參考實(shí)驗(yàn)室其他實(shí)驗(yàn)平臺的搭建方法，在此基礎(chǔ)上做合理的方法改進(jìn)與技術(shù)的引用。
3.向刀架生產(chǎn)廠家專業(yè)人員請教、研討測試的相關(guān)事宜。
4.通過網(wǎng)絡(luò)和圖書館以及電子圖書館查閱相關(guān)專業(yè)性文獻(xiàn)資料。
5.定期向?qū)焻R報(bào)階段性的科研成果，提出階段性的問題，與導(dǎo)師共同商討并解決實(shí)際 問題。
四、對進(jìn)度的具體安排
周次 任務(wù)
5-6 根據(jù)老師的意見對自己的方案進(jìn)行修改；選定各種部件型號，
設(shè)計(jì)整體方案計(jì)算主要參數(shù)，畫出草圖；
7-8 關(guān)鍵零件設(shè)計(jì)計(jì)算強(qiáng)度校核，刀塔總裝配草圖設(shè)計(jì)，動力刀部
分設(shè)計(jì)；
9-10 完成動力部分設(shè)計(jì)
11-12 完成總裝配圖，零件圖，及三維模型
13-14 修改改進(jìn)設(shè)計(jì)及其圖紙
15 完成設(shè)計(jì)說明書
16 準(zhǔn)備答辯，上交論文
五、參考文獻(xiàn)
[1] 王家興，馬仕龍.動力刀架的發(fā)展趨勢和應(yīng)用分析[J].機(jī)械工程師，2010，(12).
[2] 趙尚福，郭智春.單伺服動力刀架關(guān)鍵技術(shù)研究[J].機(jī)械工程師.2012年6期
[3] 馬仕龍，李兆維，劉春時(shí).動力刀架的動力刀座接口研究[J].機(jī)械設(shè)計(jì)與制造.2010(12). [4]劉范力旻，劉建功.伺服電機(jī)自動轉(zhuǎn)位刀架應(yīng)用及電機(jī)匹配計(jì)算[J].寧夏機(jī)械,2003,(03). [6]初福春,柳玉民.數(shù)控轉(zhuǎn)塔刀架技術(shù)發(fā)展及其應(yīng)用[J].現(xiàn)代制造,2004(16):88-91
[7]郭永環(huán).數(shù)控車床用轉(zhuǎn)塔動力刀架的發(fā)展方向[J].機(jī)床與液壓,2002(6):25-27.
[8]蘇曉暉.機(jī)床六工位轉(zhuǎn)塔動力刀架的設(shè)計(jì)[J].制造技術(shù)與機(jī)床,2010(3):46.
[9]鄭申,張瑞乾.多工位數(shù)控轉(zhuǎn)塔動力刀架設(shè)計(jì)[J].機(jī)械研究與應(yīng)用,2010(5):84-88.
[10]劉軍山.車銑復(fù)合數(shù)控機(jī)床方案設(shè)計(jì)與運(yùn)動仿真分析[J].西安理工大學(xué),2001
[11]吉濤,劉乘.多工位轉(zhuǎn)塔刀架的數(shù)控實(shí)現(xiàn)[J].機(jī)床及液壓,2006,(9)
[12]張璇; 呂英波; 趙文波. 新型電動刀架的設(shè)計(jì).機(jī)電技術(shù).2009年 04期
[13]郭永環(huán).數(shù)控車床用轉(zhuǎn)塔動力刀架的發(fā)展方向[J].機(jī)床及液壓,2002
[14]蘇曉暉.機(jī)床六工位轉(zhuǎn)塔動力刀架的設(shè)計(jì)[J].制造技術(shù)與機(jī)床,2010(3):46-48.
[15]吳華平,楊俊召.淺談數(shù)控轉(zhuǎn)塔刀架行業(yè)的發(fā)展[J].金屬加工,2010(6):20-21.
[16]蔣宏生.數(shù)控車削中心動力刀架的計(jì)算機(jī)輔助設(shè)計(jì).碩士學(xué)位論文,哈爾濱工業(yè)大學(xué).
[17]駱鳴.數(shù)控機(jī)床刀具夾緊系統(tǒng)的改進(jìn)設(shè)計(jì)[J].天津理上大學(xué)學(xué)報(bào),2009(4):85-88.
[18]王懷棟. 數(shù)控車床自動回轉(zhuǎn)刀架控制系統(tǒng)簡介.數(shù)字技術(shù)與應(yīng)用2011年 03期?
[19]賈德峰. 動力刀架結(jié)構(gòu)參數(shù)優(yōu)化及綜合性能檢測研究.大連理工大學(xué)
[20]吉濤; 劉乘.多工位轉(zhuǎn)塔刀架的數(shù)控實(shí)現(xiàn).機(jī)床與液壓 2006年 09期???
指導(dǎo)教師：（簽署意見并簽字） 年 月 日
督導(dǎo)教師：（簽署意見并簽字） 年 月 日
領(lǐng)導(dǎo)小組審查意見：
審查人簽字： 年 月 日
1
動力刀架設(shè)計(jì)開題報(bào)告 答辯人 導(dǎo)師 動力刀架發(fā)展 外加動力模塊 雙伺服電機(jī)驅(qū)動 中外動力刀架對比 動力刀架基本結(jié)構(gòu) 1 轉(zhuǎn)位驅(qū)動系統(tǒng) 2 動力驅(qū)動系統(tǒng) 3 冷卻裝置 4 精定位裝置 5 裝刀裝置 6 數(shù)控刀架換刀動作 方案一 雙伺服電機(jī)驅(qū)動動力刀架 方案二 單伺服電機(jī)驅(qū)動動力刀架 方案三 內(nèi)置力矩電機(jī)加電主軸 方案四 外置力矩電機(jī)加電主軸 謝謝觀賞 WPSOffice MakePresentationmuchmorefun WPS官方微博 kingsoftwps Abstract
Reliability refers to the ability of a part,device or system to conduct an intended function in a given condition for a certain period of time.Amechanicalsystemorstructuresuchasamachinetoolexercisesthecapacity of the entire system with regard to the various constituent parts that are connected to each other; as such, the reliability of the parts constituting the system determines the reliability of the entire system. A tool post is a device designed to efficiently provide the tools necessary for the processing of a turning machine:the parts used in a hard turning machine which requires higher stiffness must provide greater reliability. For the purposes of this study, the reliability of a tool post, which has the highest failure rate of a turning machine system, was assessed. In order to conduct a reliability assessment of a given tool post,reliability prediction using a failure rate database,weak point analysis, the manufacture of a reliability tester and the calculation of reliability testing and quantitative reliability criteria were also carried out. By so doing, the failure rate, the MTBF (Mean time between failures) and other factors could be calculated. Furthermore,the results can also be applied to otherpartsoftheturningmachineortoareliabilityassessmentofasubsystem by using the suggested assessment method.
Keywords: Reliability assessment; Reliability prediction; Failure rate database; Tool post; Mean time between failures; Failure rate
1. Introduction
A production method to which the concept of reliability is applied has recently been used, rather than simple design and production focusing on the functions in all industrial fields (Saleh, 2006). Reliability refers to the ability of a part, device or system to conduct an intended function in a given condition for a certain period of time. Products that are produced according to such a method meet the customers' requirements in terms of both quality and function. In particular, a mechanical system or structures like a machine tool which exercises the capacity of an entire system in which the many constituent parts are connected to each other; as such, the reliability of the parts constituting the system determines the reliability of the entire system. Therefore the reliability of each part is very important (Lee, 2006). A tool post is a device that efficiently and automatically provides the tools necessary for the processing of a turning machine, and the precision of such a device is the core unit that ultimately determines the precision of a processed product. According to the relevant analyses, a tool post is known to have the highest failure rate among the subsystems that constitute a turning machine system
(RAC, 1991).In particular,a tool post used in relation to processing in a processing system - which requires high stiffness, such as that provided by a hard turning machine-requires higher reliability (Kim, 2005). In general, the reliability assessment of the electronic parts is conducted on the assumption that the failure rate (according to the bath-tub failure rate curve which is generally used) remains the same during the useful life of the electronic parts (Lee, 2001; Lee, 2006). However, although the failure rate of the mechanical parts tends to increase, it is an essential to obtain as much information on the reliability of mechanical parts as possible, all the more so because we don't currently have much information on the failure rate of mechanical parts (Wang, 1999; Lee, 2003). In this study, with regard to the reliability assessment of the tool post used in a hard turning machine, the quantitative calculation of reliability and the calculation of the reliability information of the mechanical parts were conducted by forecasting their reliability and analyzing their weak points using the failure rate of the mechanical parts;manufacturing a reliability tester for the reliability testing of a tool post; carrying out a reliability test for the measurement of such functions as stiffness, repetition and angular resolution; and calculating the quantitative reliability criteria, and so forth.
2. Reliability prediction
Reliability prediction refers to the efforts made to enhance the competitive power of a product in the market and to prevent losses caused by unexpected accidents, mainly by checking the reliability of the product's design according to its development state or by forecasting the reliability of a prototype, thereby enhancing its reliability before production starts (Moasoft Inc., 2002). The reliability prediction methods include FMEA (Failure Mode and Effect Analysis), FTA (Fault Tree Analysis), Worst Case Analysis, performance assessment and the field data method (Customer Service Data), and the failure rate database method, and so on. To conduct reliability prediction effectively,data (information on the failure rate) on the failures of each part is desirable. Unlike electronic parts, there is no clear definition of the failure mode and known reliability data for mechanical parts. Therefore, in this study,we conducted a reliability prediction using the NPRD95 (Nonelectric Part Reliability Data 95), a database containing information on the failure rate of mechanical parts(Lee, 2003). The NPRD95 database, which holds collected and edited data accumulated from 1974 to 1994, is the only source of information on the failure rate of mechanical parts. These failure rates follow an exponential distribution (RAe, 1995). In order to search for information on reliability,modeling of the system is required first of all.The basic data for modeling are the parts list,bills of materials and drawing, and so on. Once modeling has been completed, the reliability information should be entered using the failure rate database. For reliability information, the user chooses the failure rate under the usage environment in the part selection set to part, part sub-type. Figure 1 shows an example of a search of the Connector Pin for the failure rate by using the NPRD95. A tool post, as shown in Fig. 2, is composed of a turret head to which the tools are mounted, a main shaft that supports the turret, a clamping part to fix the rotation of the tool, gears (drive shaft) to transmit power for the rotation of the tool, and electrical sensor parts such as a proximity switch. The turret head has a Fig. 3.Analysis of the sub-assembly and main parts of the tool post.
Time Fig. 4.. Reliability change of the tool post and sub-assembly over time.
drive-gear, whose failure rate is 42.411000 failures/ million hours, while the lowest failure rate is found in the electricity parts, whose failure rate is 19.687800 failures/million hours. The failure rate being in inverse proportion to the MTBF in an exponential distribution, the result means that the mean time between failures of the drive gear is shortest and the failure rate of the electricity parts is longest. Figure 3 illustrates the failure rate of the sub-assembly of the tool post and the weak points and failure rate of the main parts. The percentage of each blank indicates the failure rate of each sub-assembly when it is assumed that the failure rate of the tool post is 100%. The Timing Belt, Pulley and Radial Bearing of the Drive Gear, which has the highest failure rate, have a high failure rate and therefore can be expected to be weak parts. As they also have a high failure rate, the Proximity Switch, Quad-Ring (X-Seal), 3 piece type
holder into which 12 tools can be inserted and installed. Because most mechanical products are composed of components that are linked to each other by rings, bolts and nuts, we classified the composition of the tool post to a single level for reliability prediction. The reliability information should be searched for according to the standards of the specifications, materials, and usage environment of the constituent parts. For the material-related specifications, we referred to the specifications manual of KS 0430I gray cast iron products and KS 03709 nickel chrome molybdenum steel materials, while for parts-related specifications, we referred to the KS specifications and in-house specifications standard. Because the desired usage environment and specifications of the constituent mechanical parts are not always available, we selected the most similar parts (usage environment, materials and specifications) in consultation with the designer. A reliability block diagram is a method for calculating failure rate-related reliability by expressing the flows of energy,matter and information shown by the system (Wang, 2004). In this study, where the tool rotation of the tool post is regarded as the main function, the main parts were put together by series connection. For the prediction results, the MTBF of the tool post was estimated at 8,590 hours and the failure rate at 116.408200 failures/million hours. The reliability prediction conditions involved an operation temperature of 30°C in a GB (Ground Begin) and GC (Ground Controlled) environment.
With regard to the sub-assembly, the highest failure rate is found in drive-gear, whose failure rate is 42.411000 failures/ million hours, while the lowest failure rate is found in the electricity parts, whose failure rate is 19.687800 failures/million hours. The failure rate being in inverse proportion to the MTBF in an exponential distribution, the result means that the mean time between failures of the drive gear is shortest and the failure rate of the electricity parts is longest. Figure 3 illustrates the failure rate of the sub-assembly of the tool post and the weak points and failure rate of the main parts. The percentage of each blank indicates the failure rate of each sub-assembly when it is assumed that the failure rate of the tool post is 100%. The Timing Belt, Pulley and Radial Bearing of the Drive Gear, which has the highest failure rate, have a high failure rate and therefore can be expected to be weak parts. As they also have a high failure rate, the Proximity Switch, Quad-Ring (X-Seal), 3 piece type curvic coupling and the Proximity Sensor of the electric parts are expected to break down during actual operation. Figure 4 illustrates the change in the reliability of the tool post and the constituent sub-assembly over time.The sub-assembly that reliability declines sharply is drive gear because the failure rate of the timing belt of the drive gear has a relatively higher failure rate than the other parts. In addition, we found that the reliability of the sub-assembly was almost equal to the failure distribution rate derived from actual customer service data.
3. Manufacture of a reliability tester and reliability testing
3.1 Tool post reliability tester
The failure of a tool post is the failure of indexing and clamping, which are the most important functions of a tool post.This is thought to be the result of a malfunction of the proximity sensor, which senses clamping or leaks caused by wear and tear of the sealing parts. In addition, damage to the main shaft, the defectiveness of parts assembly,the wear of parts due to the repetition of loads, and the backlash caused by loads asymmetry due to biased tool installation also lead to failure. Therefore, the angular resolution, repetition degree, stiffness and flatness of a tool post are very important elements of function and reliability. Table 1 shows the assessment items for a reliability assessment of a tool post. The reference data are made by machine tools maker. Angular resolution and repetition are measured using an angle encoder, and if the values fall outside the reference value, then curvic coupling wear, 0Ring wear and oil pressure decrease are forecast. In the case of wear of the curvic coupling, the stiffness of the tool post decreases. For this measurement, the wear of the curvic coupling can be measured by inflicting loads with a load cell, measuring the transformation value and the stiffness change. Equally, the proximity sensor bracket vibration and temperature increase caused by continual operation can be measured using an accelerometer sensor and thermocouple. In order to measure the aforementioned items, we made a reliability tester for the structure, as shown in Fig. 5. The tester is divided into a drive part, measurement part, control part and supporting part. The drive part is composed of a servomotor to drive the tool post, a hydraulic device and lubricating device; the measured data are processed in the PC. The supporting part is composed of a surface plate on which the tool post reliability tester is installed, and a bracket to which the sensor is fixed, In the study, we also used a surface plate on which a damper is installed.Figure 6 shows the tool post reliability tester which was actually made for this study.
3.2Assessmentofthe performance of the tool post
We measured the performance of the tool post in order to determine the optimum operational conditions for reliability testing. This was conducted so as to define a failure by consecutively measuring performance in a long operation.
3.2.1 Oil pressure and stiffness/repeatability
The stiffness of the radial direction of the tool post is determined by the change of the oil pressure on the curvic coupling. The oil pressure applied in this study is 20~70 kg/em' and the strength inflicted by the load cell is 400 N. In the test, the stiffness decreased noticeably in oil pressure below 40 kg/em' and remained fixed for oil pressure of more than 40 kg/em'. Figure 7 illustrates the change in stiffness according to the change in oil pressure. The optimum stiffness for maintaining the stiffness of a tool post requires oil pressure ofatleast40kg/em'. Moreover, oil pressure also has a great impact on repeatability. If oil pressure is too low, repeatability declines because of the low clamping force of the curvic coupling. However, the use of too high a level of oil pressure is not desirable in the structural aspect either. Figure 8 illustrates the change of repeatability according to oil pressure. Oil pressure proved most desirable at 50 kg/ern" if we consider repeatability;
because repeatability meets the basic value in oil pressure over 50 kg/ern' and fails to meet the basic value in oil pressure below 50 kg/ern',
3.2.2Angularresolution and thermal expansion Angular resolution, for which the absolute basis is difficult to establish, is harder to assess in comparison with repeatability precision. Figure 9 shows the average value of index errors by measuring the angles of each index after incessant operation for 8 hours. As shown in Fig. 9, the basic index is 9, but the indexing error does not show a consistent tendency. Given the offset of the encoder value by the basic index as a result of measurement, we can see an indexing error of about O.03°s. In addition, given that the error does not occur in only one direction, we can see that the error is not caused by the lopsided curvic coupling. We measured the impact by rotating indexes 3 and 9 repeatedly in order to observe the impact according to the angular resolution. Vibration was measured by an acidometer installed on the bracket used to fix the proximity sensor. As Fig. 10 illustrates, we can see that the impact caused by the clamping of the indexingindexing of3isnearly 10times higherthan that ofthe indexing of 9, which supports the findings of the angular resolution test above. If the indexing is conducted incessantly, thermal expansion occurs in the tool post. Thermal expansion can be measured with a gap sensor installed in the radial axial direction. Thermal expansion was found to be 0.5 um in the radial direction and l.Oum in the axial direction after 72 hours of operation. In the following consecutive operation, no more thermal expansion occurred. Therefore, the failure ofatoolpostcanbedefined as follows; (1) oil pressure isbelow 60 kg/ern', (2) the repeatability is over 0.005°, (3) the thermal expansion is over l.Oum in the radial or axial direction, (4) tool post is stopped. That is, one of the mentioned items leadsto failure ofthetoolpost.
4. Reliability assessment
4.1 Reliability test
Through a reliability test, a variety of results may be obtained depending on the test conditions. In this study, on the basis of the performance assessment conducted above, the reliability test was carried out
under oil pressure of 60 kg/crrr', which is thought to keep the stiffuess and repeatability fixed. Although there are other methods, including the eccentric loading, non-loading and consistent loading methods and so forth, for establishing the load condition for the tool post, we adopted non-loading continuous operation for the reliability testing because it was difficult to select the accelerating force. we conducted the operation repeating the rotation and backlash of the tool post index in a sequence of 1~7-->4->10, where the operation time of a cycle was approximately 8 seconds, in order to obtain the test results more quickly. We measured the encoder data and gap sensor data once after 1,000 cycles (that is, measured after 4,000 times of indexing) in order to quantitatively analyze the test results, and conducted 100,000 cycles of consecutive operation. As a result of the reliability test, three failures occurred in total. In the first failure, the tool post stationed itself completely after about 1.9 million cycles with repeatability rapidly falling after about 1.6 million cycles. We checked the oil pressure of the hydraulic motor supplying the clamping force to identify the cause of the failure, but it proved to be working well, as did the proximity switch.
Although the eause of the first failure was not found,the most probable cause seems to be the vibration of the bracket used to fix the proximity switch. The repeatability of the tool post, which started operation after the first halt, was not as high as in the first operation, but we continued the test because it remained within the basic value. In the second failure, the operation stopped after 1.2 million cycles and normal operation resumed after relocation of the bracket fixing the proximity switch, as in the first failure. In this respect, we conducted a deterioration test of the proximity switch, but the, life and performance of the proximity switch were not affected by 6 million on/offs. Therefore the abnormal operation of clamping is sure to be caused by the transformation of the bracket fixing the proximity switch. The tool post stopped at about the millionth cycle after readjustment of the proximity switch.As a result of the analysis of the cause of the failure, the hydraulic motor, which was intended to supply the clamping force, wasn't working because the oil pressure was 0 kg/ern'. Assuming this to be a problem with the hydraulic system, we disassembled the tool post and found, after analysis, that the O-ring, which was intended to transmit the clamping force, had been damaged, and the quad-ring was worn. Figures II and 12 show the damaged part of the O-ring and wear of the quad-ring.
4.2 Reliability analysis
We operated the tool post for about 4.1 million cycles for a reliability test, during which three failures occurred. From this, we reasoned that the life of a tool post is about 1.8 million cycles, that of a hydraulic one is 4.1 million cycles, and that the most frequent cause of failure is not the life of the proximity switch itself but the displacement of the bracket fixing the
proximity switch. We analyzed the reliability of the tool post on the basis of the data obtained from there liability test.The data that should be collected for a reliability analysis include failed parts, failure time, failure mechanism, failure mode, usage conditions and the measures taken against failure, and so forth. Of these, the failure time, usage conditions and number of failures are the important elements to be used in an analysis of the failure rate. That is why we calculated th